The lanthanide tetrad effect and its correlation with K/Rb, Eu/Eu*, Sr

Geochimica et Cosmochimica Acta, Vol. 63, No. 3/4, pp. 489 –508, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/21801 $20.00 1 .00 Pergamon PII S0016-7037(99)00027-7 The lanthanide tetrad effect and its correlation with K/Rb, Eu/Eu*, Sr/Eu, Y/Ho, and Zr/Hf of evolving peraluminous granite suites WOLFGANG IRBER* Technische Universität München, Lehrstuhl für Angewandte Mineralogie und Geochemie, Lichtenbergstrasse 4, D-85747 Garching, Germany (Received June 25, 1998; accepted in revised form January 8, 1999) Abstract—Lanthanide tetrad effects are often observed in REE patterns of more highly evolved Variscan peraluminous granites of mid-eastern Germany (Central Erzgebirge, Western Erzgebirge, Fichtelgebirge, and Northern Oberpfalz). The degree of the tetrad effect (TE1,3) is estimated and plotted vs. K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, and Zr/Hf. The diagrams reveal that the tetrad effect develops parallel to granite evolution, and significant tetrad effects are strictly confined to highly differentiated samples. Mineral fractionation as a cause for the tetrad effect is not supported by a calculated Rayleigh fractionation, which also could not explain the fractionation trends of Sr/Eu and Eu/Eu*. The strong decrease of Eu concentrations in highly evolved rocks suggests that Eu fractionates between the residual melt and a coexisting aqueous high-temperature fluid. Mineral fractionation as a reason for the tetrad effect is even more unlikely as REE patterns of accessory minerals display similar tetrad effects as the respective host rocks. The accessory minerals inherit the REE signature of the melt and do not contribute to the bulk-rock tetrad effect via mineral fractionation. These results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts: ionic radius and charge, which commonly control the element distribution between mineral and melt, are no longer the exclusive control. The tetrad effect and the highly fractionated trace element ratios of Y/Ho and Zr/Hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence. This distinct trace element behavior and the common features of magmatic-hydrothermal alteration suggest the increasing importance of an aqueous-like fluid system during the final stages of granite crystallization. The positive correlation of TE1,3 with bulk-rock fluorine contents hints at the importance of REE fluorine complexation in generating the tetrad effect. As the evolution of a REE pattern with tetrad effect (M-type) implies the removal of a respective mirroring REE pattern (W-type), the tetrad effect identifies open system conditions during granite crystallization. Copyright © 1999 Elsevier Science Ltd the evolution of granitic melts. A simple mathematical formula for the tetrad effect is introduced enabling the correlation of the tetrad effect with geochemical parameters such as K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, and Zr/Hf. Of these, in particular, K/Rb, Y/Ho, and Zr/Hf are known to indicate magmatic-hydrothermal transitional environments (Taylor, 1965; Bau 1996, 1997). Additionally, a Rayleigh REE fractionation is calculated to examine whether mineral fractionation can cause the gradual evolution of the tetrad effect or not. In order to describe those alteration processes that are linked to the release of a high-temperature hydrothermal fluid at the late-stage of granite crystallization, the term “magmatic-hydrothermal alteration” is used in this contribution. This term comprises pervasive albitization, sericitization, topazation, fluoritization, and/or tourmalinization (cf. Strong, 1985; Taylor and Pollard, 1985; Hannah and Stein, 1990). 1. INTRODUCTION In recent years, an increasing number of publications have addressed a rare type of rare earth element (REE) fractionation, which is known in geosciences as the lanthanide “tetrad effect.” In non-geological disciplines it is also described as “doubledouble effect” (Mioduski, 1997), “nephelauxetic effect,” (Jørgenson, 1970) or “inclined W effect” (Sinha, 1978), but regardless of the term used, chondrite-normalized REE patterns with tetrad effect are generally characterized by the subdivision into four segments called tetrads (Masuda et al., 1987: first tetrad 5 La-Nd, second tetrad 5 (Pm)Sm-Gd, third tetrad 5 Gd-Ho, fourth tetrad 5 Er-Lu; Fig. 1). In geosciences, tetrad effect-like REE patterns are reported both in magmatic rocks and in precipitates from hydrothermal fluids (Masuda and Ikeuchi, 1978; Masuda and Akagi, 1990; Akagi et al., 1993; Lee et al., 1994; Kawabe, 1995; Akagi et al., 1996; Bau 1996). Recent discussions about the tetrad effect focus on highly evolved igneous rocks (Bau, 1996, 1997; Pan, 1997), which are often interpreted as transitional between the end-members of magmatic and high-temperature hydrothermal systems (e.g., Bau, 1996; Irber et al., 1997). The objective of this contribution is to examine if the intensity of the tetrad effect correlates with parameters that reflect 2. TETRAD EFFECT Fidelis and Siekierski (1966) and Peppard et al. (1969) initially observed the tetrad effect in patterns of liquid-liquid REE distribution coefficients. Since then, the tetrad effect is well recognized in chemistry as affecting the REE complexing behavior, which is assumed to be influenced by variations in the exchange interactions of unpaired 4f-electrons, spin-orbit coupling or crystal field stabilization (e.g., Fidelis and Siekierski, 1966; Nugent, 1970; Fidelis and Siekierski, 1971; Siekierski, 1971; Sinha, 1978; Dzhurinskii, 1980; Mioduski, 1997). Al- *Author to whom correspondence should be addressed (wolfgang.irber @geo.tum.de). 489 490 W. Irber Fig. 1. Chondrite-normalized REE patterns with and without the tetrad effect (biotite granite 5 G4, He-4194, Fichtelgebirge, cf. Table 1; albitised granite 5 AD45, Abu Dabbab, Egypt, cf. Fig. 6b). Note the upward-curved segments and the internal minima at La, Nd-Pm, Gd, Ho-Er, and Lu (except for Eu). The minima refer to theoretical filling stages of the 4f-electron shell with maximal 14 electrons: 0, 3.5, 7, 10.5, and 14. This leaves Gd in an unique position as it marks the change from unpaired to paired electrons in the filling stages of 4f orbitals. Gd is shared between the second and the third tetrad. Pm is unstable in natural environments. The distinctive behavior of Eu is due to its largely double charged oxidation stage in magmatic systems. though well confirmed by laboratory experiments (e.g., Peppard et al., 1969; Kagi et al., 1993; Yaita and Tachimori, 1996; Litvina et al., 1996), the existence of tetrad effects in geological samples is barely accepted and subject of many pro and contra discussions (e.g., McLennan, 1994; Bau, 1996, 1997; Pan, 1997). The major question of these discussions is if these earlier mentioned physichochemical parameters, that only weakly contribute to the complexing behavior of REE (felements), are able to affect REE abundances in natural systems. Based on theoretical considerations, Masuda et al. (1987) proposed the existence of two different types of tetrad effects. Both types are derived from each other and mirror themselves by definition (M-type in solid samples as residue and W-type in the interacting fluids as extract). The labels “M” and “W” refer to REE patterns with upwards or downwards curved tetrads, respectively. In highly evolved granites, only the M-type is known, and in extreme cases, such as the granites of the Abu Dabbab massif in Egypt (Fig. 1), the chondrite-normalized REE concentrations vary within one tetrad in the range of half a logarithmic unit and are far beyond any criticism of analytical inaccuracy. 3. SAMPLE DESCRIPTION AND GEOLOGICAL BACKGROUND The granites studied are part of the north-western edge of the Bohemian Massif in the mid-eastern part of Germany. Most of these granitic plutons intruded at the end of the Variscan Orogenesis during late Carboniferous to early Permian (Carl and Wendt, 1993; Gerstenberger et al., 1995). The granites are found in three distinct tectonic and metamorphic units of the Bohemian Massif, which are the Saxothuringian, the Moldanubian, and the Zone of Erbendorf-Vohenstrauss (ZEV). Four geographically distinct complexes have been distinguished, each comprising a number of granitic stocks: Central Erzgebirge, Western Erzgebirge, Fichtelgebirge, and Northern Oberpfalz (Fig. 2). All samples were carefully selected to represent the range in chemical and textural evolution of each intrusive as was currently available in outcrop, drilling, or underground mining. The granites are classified as monzo- to syenogranites and comprise less evolved biotite-bearing to highly evolved topaz-bearing Li-mica granites reflecting a wide range in K/Rb ratios (Table 1). They belong to crustally derived peraluminous igneous rocks (A/CNK .1.1) and can be addressed either as I-type (G1, Leuchtenberg) or S-type (Ehrenfriedersdorf, Eibenstock, Bergen, G2-G4). Detailed descriptions of the granites of the Erzgebirge are published by Förster and Tischendorf (1994), of the granites of the Fichtelgebirge and the Northern Oberpfalz by Richter and Stettner (1979), Siebel (1993, 1995), Siebel et al. (1995), Hecht et al. (1997), and Siebel et al. (1997). 3.1. Central Erzgebirge The samples from the Central Erzgebirge belong to the stock-, cupola-, and ridge-shaped apical parts of the Central Erzgebirge pluton (Fig. 2). The granites are rarely exposed in surface outcrops and were generally sampled from recently closed underground tin mines and from drill cores of earlier extensive exploration drilling programs (Tischendorf et al., 1987). The tin-granite of Ehrenfriedersdorf comprises a series of differently evolved subtypes ranging from a fine-grained biotite to an aplitic topaz-albite-Li-mica granite, the latter with intense magmatic-hydrothermal alteration in the apical parts (Lehman and Seltmann, 1995; Lanthanide tetrad effect 491 Fig. 2. Simplified map of sample locations in the Erzgebirge-Fichtelgebirge region including the Northern Oberpfalz. A: Overview showing the outlines of granitic intrusions in the Saxothuringian, the Moldanubian, and the ZEV (Zone von Erbendorf-Vohenstrauss) including sample numbers of the Central Erzgebirge pluton. The little insert displays the working area at the border line of Germany and the Czech Republic. Sample locations and sample numbers are shown in detail in (B) (Fichtelgebirge), (C) (Leuchtenberg massif), and (D) (Western Erzgebirge). A: Austria; Aue: Aue; Auh: Auerhammer; Brg: Bergen; CR: Czech Republic; D: Germany; Efd: Ehrenfriedersdorf; Eib: Eibenstock; Erl: Erla; F: France; Fal: Falkenberg; Flo: Flossenburg; Fri: Friedenfels; Gey: Geyer; GKb: Grober Kornberg; Grf: Greifenstein; Hub: Huberstock; Kib: Kirchberg; KKb: Kleiner Kornberg; Kö: Kösseine; Kv: Kynzvart; Lau: Lauter; Leu: Leuchtenberg; Lie: Liebenstein; Ly: Lysina; Mit: Mitterteich; Nej: Neidek; Nwt: Neuwelt; Pbh: Pobershau; Pl: Poland; Slm: Schlema; Snb: Schneeberg; Stei: Steinwald; Stz: Satzung; Swb: Schwarzenberg; Ws: Weissenstadt; ZM: Zentral massif. Seltmann et al., 1995). Pegmatites form the roof zone of the granitic cupola and pegmatite-aplite dikes cut the granite contact to Cambrian schists. The samples originate from the underground mining levels at 365 m (Se-e7, Se-e9) and 415 m (Se-e23) RSL and represent the equigranular subtype C (equigranular topaz-albite-Li-mica granite) according to the subdivision of Hösel et al. (1994). The Pobershau granite is only known from drilling. It is related to the Central Erzgebirge Pluton and geochemically comparable to the granite of Ehrenfriedersdorf. The samples are taken from exploration drill Pobershau 1/78 at 570 m depth. They represent the common porphyritic subtype (Ir-Pob-1, biotite-muscovite granite) and a more highly evolved aplitic dike (Ir-Pob-4, topaz-bearing muscovite-albite granite). 3.2. Western Erzgebirge The Eibenstock massif forms the largest granite body of the Erzgebirge and comprises a range of coarse-grained, often tourmaline-bearing biotite rock (Eib1) to fine-grained topaz-bearing albite Li-mica granites (Eib3) with abundant aplitic dikes (Eib4). The most common subtype is Eib2, a reddish medium-grained topaz-bearing Li-micamuscovite granite. Petrographic features of magmatic-hydrothermal alteration increase from Eib1 to Eib3 with abundant topaz, fluorite, and late apatite in joints and along grain boundaries (Kühne et al., 1972). Cassiterite is a common accessory mineral in all rock types. Apatiterich quartz veins throughout the massif testify a P-rich late-stage residual fluid. The samples comprise all mentioned subtypes throughout the outcrop area in the German part of the Eibenstock massif. The circular intrusion of Bergen north-west of the Eibenstock massif is formed by biotite-muscovite granites in three major subtypes: coarse (Brg1), medium (Brg2), and fine-grained (Brg3), the latter with abundant tourmaline. Significant tourmalinization of the host rock indicates extensive boron mobilization in the granitic aureole. A significant enrichment in phosphorous in some of the Brg3 subfacies is explained 492 W. Irber Table 1. Summary of all samples studied with characteristic petrographic features as was determined by thin section examination. Also given are the element ratios of A/CNK, Na/K, K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, Zr/Hf, as well as TE1,3. For bulk-rock analyses see Table 2. Texture: fg, mg, cg 5 fine-, medium-, corase-grained; type of alteration: M 5 magmatic hydrothermal (metasomatism), H 5 low-temperature hydrothermal, W 5 weathering; A 5 degree of alteration: 1 5 fresh (,10%); 2 5 weakly altered (10 –30%); 3 5 medium altered (30 –70%); 4 5 strongly altered (70 –100%); (sample sources: Fö 5 Förster and Tischendorf, He - Hecht and Morteani, Ir 5 Irber, Se 5 Seltmann, Sie 5 Siebel). Massif Sample number Texture Brg1 Brg2 Brg3 Brg3 Eib1 Eib1 Eib2 Fö-524 Fö-478 Fö-480 Fö-521 Fö-820 Fö-509 Fö-507 mg-p mg fg-mg fg cg-p cg-p mg 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Eib3 Eib-A Efd Efd Efd Efd Pbh Pbh-A G1 G1 G2 G2 G2 G2 G2 G3 G3 G4 Fö-508 Fö-800 Se-e5 Se-e7 Se-e9 Se-e23 Ir-Pob-1 Ir-Pob-4 He-4095 He-4082 He-9656 He-9654 He-9663 He-9104 He-9149 He-9655 He-9101 He-9143 26 27 28 29 30 31 32 G4 G4 Leu Leu Leu(g) Leu(g) Leu(g) He-4194 He-9025 Sie-L17 Sie-L2 Sie-L1 Sie-L14 Sie-L15 Nr. 1 2 3 4 5 6 7 Type of alterationt A A/CNK Na/K K/Rb Eu/Eu* Sr/Eu H M,H M,H M,H M,H M.H M.H 2-3 2 3 2 2 2-3 3 1.13 1.21 1.18 1.29 1.18 1.24 1.27 1.1 1.2 1.4 1.3 1.0 1.0 1.1 fg-mg fg fg mg mg mg-cg mg-p fg mg-p mg-p mg-p mg-p mg-p mg-p mg-p mg-p mg-p mg-p M,H M,H M,H M,H M,H M,H M M H H H,W H,W M,H M M H,W M M 3-4 3-4 4 3 3 3 2 3 1 1-2 1-2 2 2-3 3 3 2 2 3 1.35 1.36 1.37 1.25 1.26 1.30 1.12 1.26 1.10 1.05 1.12 1.10 1.25 1.11 1.17 1.13 1.22 1.19 mg-p mg-p cg cg cg mg mg M,H M,H H H M M,H M,H,W 3 3-4 1 2 1 2-3 3 1.32 1.18 1.06 1.06 1.19 1.23 1.26 Y/Ho Zr/Hf TE1,3 168 106 114 58 61 49 45 0.45 0.42 0.22 0.13 0.23 0.17 0.11 206 177 204 190 101 96 350 35 30 29 33 31 32 39 32 29 21 17 32 26 19 1.06 1.07 1.13 1.14 1.22 1.19 1.26 1.0 1.5 1.5 1.4 1.3 1.3 1.2 2.9 1.0 1.0 0.8 0.8 1.0 0.9 1.0 0.8 1.0 0.8 37 24 34 28 32 32 62 32 179 170 143 138 102 92 70 116 84 87 0.15 0.09 0.03 0.02 0.06 0.04 0.16 0.09 0.51 0.59 0.23 0.20 0.30 0.17 0.05 0.25 0.24 0.06 425 3708 1780 5140 931 1000 125 249 187 185 75 81 131 69 116 87 92 93 41 37 49 38 41 37 36 43 29 29 29 29 31 30 35 32 30 35 16 16 13 17 17 16 27 13 38 39 33 34 30 26 22 32 26 25 1.24 1.40 1.48 1.41 1.45 1.35 1.22 1.46 1.03 1.01 1.06 1.05 1.10 1.09 1.27 1.10 1.08 1.24 1.2 1.1 1.0 0.9 1.3 1.9 1.7 35 68 239 210 117 56 57 ,0.01 0.06 0.46 0.26 0.13 ,0.01 ,0.01 .562 103 166 200 104 .575 .277 37 34 26 32 30 35 36 16 23 35 31 19 14 13 1.40 1.26 1.00 1.05 1.14 1.30 1.29 Typical secondary features Musc. Musc. Musc., sag., Fe-hydr. Tourm. Musc. Musc. Top., fluo., musc. Top., seric, hem., apatite Top., fluo., apatite Top., sericite, Fe-hydr. Top., fluo., apatite Top., fluo., apatite Top., fluo. Musc. Top., apatite Myrmekite Myrmekite Sericite Musc. Tourm. Top., fluo. Top., musc. Top., Fe-hydr. Top., musc., albite Top., musc., albite Top., fluo., albite, musc. Top., fluo. Sutured quartz Sutured quartz Top., musc. Top., albite, Fe-hydr. Top., albite, Fe-hydr. Fe-hydr.: Fe-hydroxides; fluo.: fluorite; hem.: hematite; musc.: muscovite; sag.: sagenite; seric.: sericite; top.: topaz; tourm.: tourmaline. by the interaction with a P-rich late-stage fluid of unknown origin (Förster and Tischendorf, 1994). 3.3. Fichtelgebirge The granites in the Fichtelgebirge have been divided into four petrographically distinct varieties (G1 to G4) by Stettner (1958), and Richter and Stettner (1979). The voluminous coarse-grained G1 biotite-granite of WeissenstadtMarktleuthen with porphyric K-feldspar is homogeneous in mineralogical composition and shows only minor internal geochemical variation. It lacks any signs of magmatic-hydrothermal alteration as is common for the granite types G2-G4. The two G1 samples are representative for the known range in chemical composition of the so-called older granites. Those granites of the younger group possessing a porphyritic fabric are considered to be the rapidly cooled marginal facies of the younger granite G3 and are collectively referred to the G2 granite type (Richter and Stettner, 1979; Hecht et al., 1997). In geochemical evolution, members of this type span nearly the whole range from a more evolved G1 to the highly evolved G4, a fact which has not been convincingly explained yet (cf. discussion in Hecht et al., 1997). Sericitization, bleaching of biotite and growth of muscovite rims around biotite, albitization, topazation, fluoritization, and/or tourmalinization are the expression of the high-T, hydrothermal overprinting, which is partic- ularly strong in the more fractionated members of the G2 type (Richter and Stettner, 1979). The G3 granite is interpreted to represent a slowly cooled central facies of the younger intrusive group (Richter and Stettner, 1979). Samples from more highly differentiated varieties of the G3 show alteration phenomena similar to those typical of the G2 type. Inclusions of the G2 type in G3 are common. The much smaller G4 granite is texturally similar to the G3. Chemically, it can be distinguished from the G3 only by higher contents of rare alkaline and other volatile elements such as Li, F, and P. Late formed accessories such as cassiterite and arsenopyrite are characteristic (Richter and Stettner, 1979). The abundant development of chessboard albite, topaz, fluorite, and late apatite on fractures point to an intensive magmatic-hydrothermal overprint. All samples of the G2-G4 group reflect the typical range in textural and chemical variation as is observed in the Fichtelgebirge area. 3.4. Northern Oberpfalz The Northern Oberpfalz pluton is composed of a variety of differently evolved granite intrusions of which the Leuchtenberg granite forms the largest intrusive body. The major part of the Leuchtenberg granite represents a coarse-grained, porphyritic biotite granite which is comparable to the G1 granite of the Fichtelgebirge (Siebel, 1995). In the southernmost part, the Leuchtenberg granite grades into a highly Lanthanide tetrad effect differentiated medium- to fine-grained garnet-bearing albite-muscovite granite with locally abundant topaz, fluorite, cassiterite and xenotime. The Mn-rich garnet is considered as being magmatic (Siebel, 1995) and separated garnet fractions show similar tetrad effects as the hosting granite (cf. section 7.1 and Fig. 8). mean of both values for the first (t1) and the third tetrad (t3) yields the overall value of the tetrad effect (Eqn. 3: TE1,3). 4. ANALYTICAL METHODS Some critics claim that the analytical accuracy of the currently available data is insufficient to prove the existence of the tetrad effect in geological samples. And reported tetrad effects are supposed to be the result of analytical uncertainties rather than of natural fractionation processes (cf. McLennan, 1994). Analytical aspects are indeed of critical importance, and due to widely used techniques such as INAA and IDMS, which yield incomplete REE patterns, the tetrad effect can be only suspected at best, e.g., in patterns described by Goad and Cerny (1981), Muecke and Clarke (1981), Walker et al. (1986), Jolliff et al. (1989), Corey and Chatterjee (1990), Kontak (1994), or Williamson et al. (1996). However, a clear recognition of the tetrad effect is confined to complete REE patterns obtained by ICP-MS, ICP-AES, or SSMS. This unfortunately excludes determination by IDMS, which would provide the highest accuracy available, but misses the monoisotopes Pr, Tb, Ho, and Tm. In this contribution, the major element concentrations were analyzed by XRF, while the trace elements Rb, Cs, Ba, Pb Sr, Y, REE, U, Th, Zr, and Hf have been obtained by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Perkin Elmer Elan 500; Table 2). For wholerock ICP-MS analyses, the samples were crushed and powdered in an agate mortar, decomposed with HF/HClO4 in pressure vessels, evaporated to incipient dryness, and taken up in HCl. A detailed description of the ICP-MS method and the correction of element interferences is given by Dulski (1994) who performed the analyses of the samples studied. Precision and accuracy of the ICP-MS data applied are usually better than 610% (Tables 3 and 4). The analytical quality is frequently evaluated and checked by analyses of international reference standards (Dulski, 1994) and by comparison to results of other laboratories using different analytical techniques (e.g., Bau and Dulski, 1995). More information about comparisons of the ICP-MS method to ICP-AES and SSMS is reported in Bau (1996). 5. QUANTIFICATION OF THE TETRAD EFFECT The proposed quantification method determines the deviation of a REE pattern with tetrad effect from a hypothetical tetrad effect-free REE pattern (Fig. 3). The method is especially developed for granitic rocks, and a more general usage requires a careful pre-evaluation if the method is applicable to the type of REE pattern investigated. For the calculation, only those REE pattern were selected that do not show Ce anomalies or erroneous zig-zag patterns due to insufficient analytical accuracy. In general, all REE patterns have to be evaluated prior to calculation of TE1,3 in order to exclude single curved or parabolic REE patterns which would result in positive or negative values of TE1,3 not based on the tetrad effect. From the four tetrads, only the first and the third tetrad can be used for quantification of the tetrad effect. The second tetrad (Pm to Gd) is camouflaged both by the in nature missing Pm and the distinctive behavior of Eu21 at low oxygen fugacities and high temperatures in magmatic systems. The fourth tetrad (Er to Lu) is mostly poorly developed (a point which is later discussed). To determine the hypothetical tetrad effect-free REE pattern, the corner points of the single tetrads La-Nd (and Gd-Ho; Fig. 3) serve as a respective reference. A virtual line is drawn in between these corner points, and the mean deviation of Ce and Pr (and Tb, Dy) from this line expresses the contribution to the respective tetrad (Eqns. 1 and 2). The geometric 493 with t1 5 (Ce/Cet 3 Pr/Prt)0.5 (1) t3 5 (Tb/Tbt 3 Dy/Dyt)0.5 (2) 1/3 Ce/Cet 5 Cecn/(La2/3 cn 3 Ndcn ) 2/3 Pr/Prt 5 Prcn/(La1/3 cn 3 Ndcn ) 1/3 Tb/Tbt 5 Tbcn/(Gd2/3 cn 3 Hocn ) 2/3 Dy/Dyt 5 Dycn/(Gd1/3 cn 3 Hocn ) Lncn 5 chondrite-normalized lanthanide concentration degree of the tetrad effect 5 TE1,3 5 (t1 3 t3)0,5 (3) The calculated values of the tetrad effect (Eqn. 3: TE1,3) range from 1.00 for a REE pattern without tetrad effect, e.g., the C1-chondrite from Anders and Grevesse (1989), toward higher values (TE1,3 .. 1) for REE patterns with tetrad effects. The reproducibility of the TE1,3 values is about 12% and was determined from granites with and without tetrad effect (Irber, 1996). Some criticism of this calculation method may arise from the fact that the fixed corner points (La, Nd, Gd, Ho) of the tetrads already implement the tetrad effect before quantification. The author is well aware of this, but as the parameter leading to the tetrad effect control by definition every REE concentration (see Discussion), a non-influenced reference point is unavailable. This clearly increases the uncertainty in the values calculated and only samples with values of TE1,3 . 1.10 are considered to show the tetrad effect. This value corresponds to an optical control of chondrite-normalized REE patterns where at TE1,3 . 1.10 the tetrad effect becomes well visible. An earlier method to quantify the tetrad effect was proposed by Masuda et al. (1994) who fitted the observed tetrads by a quadratic function and used the resultant quadratic coefficients as a measure of the tetrad effect. This method is of special advantage for IDMS analyses as it evades the problem of the missing monoisotopes Pr, Tb, Ho, and Tm, which are of crucial importance for the first and the third tetrad. However, the fitting procedure idealizes the actually existing tetrad effect by the elimination of analytical deficiencies. And it additionally complicates the data processing in commonly used spread sheets, especially if large sample numbers have to be evaluated. Therefore, the simpler method, as is proposed here, was developed, but requires complete ICP-MS, ICP-AES, or SSMS analyses of the REE. 6. PLOT OF THE TETRAD EFFECT VS. RATIOS OF K/RB, SR/EU, EU/EU*, Y/HO, AND ZR/HF The fractionation of elements which are similar to each other in terms of ionic radius and charge is regarded to be sensitive to changes in melt composition during magma differentiation (Bau, 1996, 1997; Irber et al., 1997). Therefore, the ratios of K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, and Zr/Hf are plotted vs. the tetrad effect to search for common underlying processes in trace element behavior. 494 W. Irber Table 2. Geochemical composition of the granites of the Erzgebirge and the Leuchtenberg granite studied here (XRF and ICP-MS analyses). Geochemical data of the granites of the Fichtelgebirge are reported in Irber et al. (1997). XRF data of the samples of the Leuchtenberg granite are from Siebel (1993). SiO2 (wt.%) TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O O2O5 LOI Total F (ppm) Ba Cs Hf Li Pb Rb Sr Th U V Zn Zr Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Brg1 Fö-524 Brg2 Fö-478 Brg3 Fö-480 Brg3 Fö-521 Eib1 Fö-820 Eib1 Fö-509 Eib2 Fö-507 Eib3 Fö-508 Eib-A Fö-800 70.6 0.37 14.5 2.3 0.044 0.81 1.10 3.41 4.87 0.23 1.36 99.6 924 458 20 4.5 111 43 241 159 21 4 23 47 142 22.3 42.4 80.9 9.01 32.1 5.99 0.770 4.49 0.69 3.93 0.64 1.88 0.26 1.93 0.29 73.5 0.16 14.2 1.3 0.042 0.34 0.52 3.61 4.53 0.16 1.02 99.3 868 229 18 2.4 n.a. 33 356 53 9 13 8 25 71 13.2 14.1 28.8 3.31 11.5 2.38 0.300 1.95 0.36 2.13 0.44 1.46 0.24 1.79 0.26 75.8 0.11 13.6 0.7 0.010 0.18 0.40 3.86 4.12 0.13 0.81 99.7 n.a. 77 16 2.1 n.a. 27 300 20 6 22 6 13 45 11.0 7.21 15.3 1.85 6.4 1.39 0.097 1.30 0.29 1.78 0.38 1.25 0.20 1.71 0.25 75.3 0.06 14.2 0.7 0.054 0.17 0.34 3.60 4.16 0.19 1.00 99.8 1620 26 88 1.6 292 9 595 7 3 12 n.a. 25 27 6.35 3.23 7.07 0.91 3.4 0.97 0.039 0.82 0.17 1.04 0.19 0.60 0.12 0.95 0.15 75.3 0.13 13.8 1.5 0.028 0.18 0.49 3.27 5.01 0.23 0.59 100.6 3520 128 83 2.6 338 16 683 23 14 24 ,5 47 81 16.0 12.9 29.7 3.54 12.3 2.99 0.230 2.98 0.62 3.28 0.51 1.33 0.15 1.04 0.13 74.5 0.11 13.6 1.5 0.023 0.16 0.43 3.13 4.71 0.25 0.82 99.2 4710 52 91 2.7 490 12 801 13 12 22 14 50 70 14.4 9.30 20.9 2.62 9.7 2.62 0.140 2.48 0.52 2.80 0.45 1.12 0.14 0.92 0.11 74.2 0.06 14.1 1.2 0.022 0.11 0.40 3.35 4.49 0.33 0.67 98.9 n.a. 20 127 2.0 574 12 835 14 7 11 ,5 41 37 10.5 3.32 7.90 1.00 3.8 1.18 0.040 1.11 0.28 1.68 0.27 0.74 0.10 0.66 0.11 74.0 0.06 14.3 1.2 0.027 0.17 0.40 3.08 4.48 0.33 1.04 99.1 n.a. 17 77 1.6 609 11 992 17 4 4 ,5 52 26 7.29 2.09 4.91 0.63 2.5 0.77 0.040 0.85 0.21 1.18 0.18 0.53 0.07 0.46 0.09 73.4 0.03 15.4 1.2 0.023 0.08 0.67 3.69 3.77 0.40 0.93 99.6 12300 33 63 1.5 870 5 1325 48 1.5 5 ,5 68 24 2.95 0.65 1.75 0.24 0.9 0.49 0.013 0.41 0.12 0.59 0.08 0.20 0.04 0.25 0.04 6.1. K/Rb In the granites studied, the K/Rb ratio ranges from 24 to 240, and the tetrad effect negatively correlates with K/Rb (Fig. 4a). Samples with K/Rb ratios ,100 show significant tetrad effects (TE1,3 .1.10) and petrographic examinations of these rocks in thin sections document the increase in sericitic alteration of biotite, K-feldspar and plagioclase. The chondritic K/Rb ratio is 242 (cf. Anders and Grevesse, 1989), and the average for magmatic rocks is given as 230, with most of the crustal rocks ranging from 150 to 350 (Taylor, 1965). With increasing degree of differentiation, Rb fractionates preferentially into the residual melt and the K/Rb-ratios decrease in highly evolved magmatic systems below 50. The use of K/Rb as a petrogenetic indicator was discussed by Taylor (1965) and Shaw (1968) in detail. Shaw (1968) explained the extremely low ratios (,50) in pegmatitic-hydrothermal systems by assuming the fractionation between a silicate melt and either biotite or an aqueous phase. The latter was regarded as more likely in highly evolved magmatic systems. In the literature, K/Rb is commonly used to characterize the evolution of granitic melts. Ratios ,100 are regarded to indicate the interaction with an aqueous fluid phase (Clarke, 1992) or mineral growth in the presence of aqueous fluids (e.g., Shearer et al., 1985). 6.2. Y/Ho The chondritic ratio of Y/Ho is 28 (cf. Anders and Grevesse, 1989), and the ratios range from 26 to 50 in the granites studied (Fig. 4b). Paralleled by the increasing degree of the tetrad effect the ratios shift to values .28. The Y/Ho ratio was reviewed by Bau (1996) and proposed as a tool to identify non-charge and non-ionic size controlled magmatic trace element behavior such as found in aqueous systems. There, the fractionation behavior of highly charged ions, which form strong chemical complexes, is additionally influenced by their electron configuration and the character of Lanthanide tetrad effect 495 Table 2 (Continued) Efd Se-e5 Efd Se-e7 Efd Se-e9 Efd Se-e23 Pbh Ir-Pob-1 Pbh Ir-Pob-4 Leu Sie-L17 Leu Sie-L2 Leu(g) Sie-L1 Leu(g) Sie-L14 Leu(g) Sie-L15 74.7 0.01 15.0 1.0 0.029 ,0.02 0.28 3.79 3.92 0.23 1.10 100.0 1770 16 26 1.7 91 2 971 9 4 3 ,10 44 22 3.64 0.90 2.20 0.28 0.9 0.50 0.005 0.54 0.15 0.73 0.07 0.15 0.02 0.19 0.02 73.3 0.04 14.7 1.1 0.025 ,0.02 0.51 3.86 4.18 0.44 1.22 99.4 4845 19 71 1.8 734 8 1230 25 6 25 ,10 44 30 5.69 1.30 3.21 0.43 1.4 0.65 0.005 0.72 0.19 1.18 0.15 0.38 0.05 0.30 0.04 73.1 0.03 14.4 0.9 0.023 ,0.02 0.46 3.62 4.30 0.37 0.80 98.0 4575 24 53 1.9 588 7 1115 15 5 26 ,10 35 33 7.54 1.44 3.67 0.48 1.6 0.81 0.016 0.87 0.25 1.53 0.18 0.42 0.05 0.41 0.05 74.1 0.03 14.8 1.1 0.025 .0.05 0.46 3.59 4.34 0.42 0.80 99.6 4545 20 96 2.0 651 11 1130 12 6 17 ,15 56 33 7.56 1.62 4.43 0.61 2.1 0.91 0.012 1.06 0.24 1.43 0.20 0.46 0.06 0.40 0.05 75.2 0.10 13.4 1.5 0.024 0.13 0.57 3.62 4.62 0.25 0.58 100.0 n.a. 89 40 2.7 320 18 622 20 15 28 10 75 72 18.2 10.6 24.7 3.06 11.0 3.03 0.164 3.16 0.62 3.51 0.51 1.12 0.14 0.80 0.10 72.7 0.03 15.9 1.6 0.051 0.14 0.49 5.30 2.79 0.36 1.05 100.4 n.a. 13 54 2.0 362 2 723 7 5 9 ,10 36 26 7.36 1.29 3.79 0.54 1.9 0.89 0.028 0.91 0.25 1.38 0.17 0.39 0.05 0.31 0.04 68.9 0.47 15.1 2.9 0.043 0.89 0.91 3.30 4.96 0.15 0.77 99.5 n.a. 816 7 5.1 72 47 172 207 35 6 ,10 60 181 20.4 70.2 142 16.6 59.5 9.74 1.25 6.95 0.88 4.40 0.78 1.99 0.26 1.67 0.26 74.8 0.13 13.1 1.1 0.030 0.21 0.63 3.14 5.40 0.04 1.00 99.6 n.a. 180 8 3.1 61 56 214 69 20 4 ,10 32 97 22.0 20.6 45.6 5.53 19.8 4.41 0.347 3.70 0.59 3.45 0.68 2.05 0.29 1.92 0.28 74.7 0.06 13.8 1.1 0.072 0.09 0.41 3.71 4.42 0.08 1.23 99.6 n.a. 37 14 2.3 150 31 313 12 12 6 ,10 37 44 20.0 8.67 20.9 2.60 9.3 2.70 0.112 2.64 0.53 3.39 0.67 1.96 0.33 2.09 0.31 74.7 0.02 14.5 0.6 0.085 0.13 0.20 4.56 3.64 0.13 1.04 99.6 n.a. 12 13 2.1 86 15 544 3 10 9 ,10 15 30 12.9 2.60 7.42 0.97 3.5 1.49 0.006 1.46 0.35 2.19 0.37 1.09 0.19 1.49 0.21 74.3 0.02 14.7 0.7 0.134 0.05 0.21 4.29 3.92 0.16 1.05 99.5 n.a. 8 14 2.2 103 15 573 2 9 9 ,10 33 28 11.8 2.04 5.81 0.81 2.9 1.41 0.006 1.38 0.31 2.01 0.33 0.99 0.17 1.37 0.19 chemical bonding between a central ion and a ligand. Bau and Dulski (1995) suggest the complexation with fluorine as major cause for values .28, while the complexing with bicarbonate is assumed to generate values ,28. 6.3. Zr/Hf In the granites studied, the Zr/Hf-ratios vary between 39 and 9 (Fig. 4c), and granites with lower ratios (,20) are affected by strong magmatic-hydrothermal alteration. The plot of Zr/Hf vs. the tetrad effect shows a negative correlation and only granites with Zr/Hf ,25 display significant tetrad effects. The Zr/Hf ratios in common granites average at 39 (n 5 327; Erlank et al., 1978) and are mostly close to the chondritic ratio of 38 (Anders and Grevesse, 1989). The average for pegmatites is at about 25 (n 5 107; Erlank et al., 1978) and Zr/Hf ratios shift toward smaller ratios with increasing evolution of the silicate melt. Single minerals such as cassiterite from pegmatites and from high-temperature hydrothermal veins show strongly fractionated ratios between 28 to 4 (Möller and Dulski, 1983; Möller 1986). Highly fractionated ratios are, contrary to silicate systems, a characteristic feature of aqueous systems such as seawater, hydrogenetic Fe-Mn crusts, and hydrothermal fluorite (Bau, 1996). Data from Alaux-Negral et al. (1993) allow the calculation of a ratio of 88 for groundwater in granite joints. Extraordinary high Zr/Hf up to 87 in magmatic systems is reported from Dupuy et al. (1992) for intraplate basalts and related to heterogeneities in the upper mantle and/or the interaction of CO2-rich fluids. 6.4. Sr/Eu and Eu/Eu* The Sr/Eu ratio is not commonly used in literature as a parameter to describe magma differentiation. However, this trace element pair displays a distinctive behavior during magma evolution and bears information relevant to trace element behavior in general. The samples in this study range in Sr/Eu from 70 to 5000, the majority with ratios between 100 to 300 (Fig. 5a). With respect to the range observed, the majority of ratios from 100 to 300 is still close to the chondritic value of 139 (Anders and Grevesse, 1989) of the in charge and ionic radius (Sr21:121VI, Eu21: 496 W. Irber Table 3. Comparison of repeated digestions and analyses of a highly evolved granite sample of the Western Erzgebirge (Schwarzenberg granite). The Schwarzenberg granite east of the Eibenstock massif is not treated in this contribution but similar in composition to many of the samples studied here. The analyses reflect the analytical precision of the time when most of the analyses shown here were performed. 803-1a (1. digestion) was analysed in 1993, 803-2a to 803-2e (2. digestion) in 1994. The relative standard deviation for the tetrad-effect-critical REE is smaller than 10% (except for Eu which is at a concentration of 0.02 ppm close to the limit of detection). Analyses were performed at the Geoforschungszentrum Potsdam, Germany, by P. Dulski. SD 5 standard deviation, RSD 5 relative standard deviation (%) Rb (ppm) Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hr Pb Th U 803-1a 803-2a 803-2b 803-2c 803-2d 803-2e Mean SD RSD (%) 484 6.5 7.3 22.9 30.8 15.7 2.28 5.60 0.78 2.56 0.85 0.017 0.77 0.20 1.16 0.20 0.68 0.13 1.18 0.18 1.62 6.60 4.46 9.76 441 6.1 7.2 22.0 30.2 15.0 2.28 5.13 0.72 2.50 0.80 0.012 0.85 0.17 1.11 0.19 0.64 0.12 1.15 0.16 1.73 9.00 4.31 10.11 437 6.0 7.1 21.8 30.0 14.8 2.30 5.20 0.71 2.64 0.78 0.019 0.80 0.17 1.09 0.21 0.67 0.13 1.15 0.18 1.77 9.10 4.42 10.14 417 5.8 6.9 21.4 29.6 14.8 2.31 5.25 0.70 2.51 0.85 0.021 0.71 0.17 1.12 0.20 0.67 0.13 1.18 0.17 1.73 8.90 4.42 10.10 429 5.8 7.1 21.6 30.1 14.8 2.21 5.09 0.73 2.63 0.80 0.021 0.80 0.18 1.10 0.21 0.66 0.13 1.17 0.17 1.70 9.10 4.52 10.33 435 6.0 7.0 21.8 30.8 15.2 2.20 5.26 0.71 2.55 0.82 0.012 0.75 0.17 1.04 0.20 0.66 0.12 1.15 0.16 1.69 9.00 4.34 10.02 441 6.03 7.1 22.0 30.3 15.1 2.28 5.26 0.73 2.57 0.82 0.02 0.78 0.18 1.10 0.20 0.66 0.13 1.16 0.17 1.71 8.62 4.41 10.1 20.8 0.24 0.12 0.48 0.43 0.33 0.03 0.17 0.03 0.05 0.03 0.00 0.04 0.01 0.04 0.01 0.01 0.00 0.01 0.01 0.05 0.90 0.07 0.17 5 4 2 2 1 2 1 3 4 2 3 22 6 6 3 3 2 4 1 5 3 10 2 2 Table 4. Comparison of five individual digestions (9.2.1994) of the georeference sample PM-S (micro gabbro) to the international reference values as are reported in Govindaraju (1994). The REE concentrations of the sample PM-S are similar to those of the Schwarzenberg granite in Table 3. The accuracy of the critical elements for the tetrad effect is better than 610%. Analyses were performed at the Geoforschungszentrum Potsdam, Germany, by P. Dulski. SD 5 standard deviation, RSD (%) 5 relative standard deviation, Diff. (%) 5 relative deviation of the mean analysis to the reference standard value in per cent. Rb (ppm) Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Pb Th U PM-S-1 PM-S-2 PM-S-3 PM-S-4 PM-S-5 Mean SD RSD (%) 1.02 301 11.6 39.5 0.33 136 2.61 6.29 1.05 5.39 1.76 1.09 2.13 0.34 2.13 0.451 1.21 0.166 1.02 0.149 1.16 2.07 ,0.05 ,0.02 1.05 290 11.2 39.5 0.36 140 2.64 6.53 1.09 5.63 1.81 1.11 2.21 0.34 2.26 0.456 1.22 0.167 1.06 0.153 1.25 1.44 ,0.05 ,0.02 1.01 309 12.0 39.2 0.36 139 2.58 6.63 1.07 5.57 1.78 1.06 2.08 0.327 2.15 0.407 1.23 0.164 1.02 0.152 1.12 2.06 ,0.05 ,0.02 0.92 296 11.5 39.5 0.36 141 2.65 6.79 1.07 5.74 1.9 1.11 2.17 0.367 2.2 0.46 1.2 0.161 1.03 0.148 1.18 3.63 ,0.05 ,0.02 0.82 282 11.0 38.1 0.37 143 2.74 6.79 1.07 5.91 1.89 1.1 2.16 0.336 2.25 0.428 1.23 0.173 1.07 0.151 1.21 1.51 ,0.05 ,0.02 1.0 296 11.5 39.2 0.4 139 2.6 6.6 1.1 5.6 1.8 1.1 2.2 0.3 2.2 0.4 1.2 0.2 1.0 0.2 1.2 2.1 0.08 9.22 0.34 0.54 0.01 2.32 0.05 0.19 0.01 0.17 0.06 0.02 0.04 0.01 0.05 0.02 0.01 0.00 0.02 0.00 0.04 0.79 8.7 3.1 3.0 1.4 3.8 1.7 2.0 2.8 1.2 3.1 3.1 1.7 2.0 3.9 2.4 4.6 1.0 2.4 2.0 1.2 3.7 36.9 PM-S reference 1 280 11 39 0.35 148 2.6 6.8 1.08 5.5 1.75 1.07 2 0.36 2 0.42 1.1 0.17 1 0.15 1.12 2.5 0.05 0.03 Diff. (%) 23.6 5.6 4.2 0.4 1.7 25.5 1.7 22.9 20.9 2.7 4.5 2.2 7.5 25.0 9.9 4.9 10.7 22.2 4.0 0.4 5.7 214.3 Lanthanide tetrad effect 497 0.01 (or even lower as Eu/Eu* was calculated using the detection limit for Eu of 0.006 ppm, cf. Table 2). 7. DISCUSSION Fig. 3. Schematic diagram displaying the principles for the calculation of the degree of the tetrad effect (TE1,3). TE1,3 is the geometric mean of the deviations of Ce, Pr, Tb, and Dy from their respective interpolated counterparts (Cet, Prt, Tbt, Dyt). For details see the text. 125VI; Whittacker and Muntus, 1970) similar elements during magmatic conditions. (A small fraction of Eu31 at these conditions is of minor importance and neglected here.) The similarity of Sr and Eu is confirmed by an almost coherent behavior in the granitic systems studied, although Eu is known to be sensitive in ionic size to oxygen fugacity and temperature (Bau, 1991). As known mineral/melt partition coefficients for Sr are slightly higher than those for Eu (cf. Rollinson, 1993), Eu is somewhat increased in the residual melt and should become enriched with respect to Sr during granite differentiation (decrease in Sr/Eu). To demonstrate this, the trend for Sr/Eu is calculated via Rayleigh fractionation. As a starting composition a representative sample of the G2 granite in the Fichtelgebirge is chosen which consists of quartz:plagioclase:K-feldspar:biotite:apatite in proportions of 34:28:29:9:0.44 (wt.%; Table 5). The calculated ratios of Sr/Eu (Table 6) demonstrate the anticipated decrease from 83 to 15 with increasing degree of differentiation. (More details to the Rayleigh fractionation are given in section 7.1.) Contrary to the calculated trend, granites with beginning tetrad effects (TE1,3 .1.10) increase in Sr/Eu beyond 300 up to 5000, which is best seen in the highlighted trend for the Fichtelgebirge granites G1-G4 (Fig. 5a). The increase in Sr/Eu is caused by a significant decrease in Eu concentrations, often below the detection limit of the ICP-MS, and is not followed by the neighboring REE. This de-coupled behavior results in pronounced negative Eu anomalies or Eu/Eu* ratios ,0.05. When the Eu/Eu* ratio is plotted against the tetrad effect (Fig. 5b) all values for Eu/Eu* ,0.2 belong to granite samples with significant tetrad effects (TE1,3 .1.10). The Eu/Eu* values, calculated via Rayleigh fractionation, show that mineral fractionation decreases the Eu/Eu* ratios only down to about 0.06 for F 5 0.08, while the actual ratio of the G4 sample (He-4194) is The correlation of TE1,3 with ratios of K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, and Zr/Hf proves that the tetrad effect develops parallel to granite differentiation, and significant tetrad effects are clearly restricted to the more highly differentiated granite samples. This is also confirmed by published REE bulk-rock data, where well-developed tetrad effects are exclusively related to highly evolved granites with late-stage minerals such as albite, Li-mica, tourmaline, topaz, and/or fluorite (see Fig. 6). The frequent occurrence of topaz and fluorite proves the dominance of fluorine as a major complexing agent during the late-magmatic stage, and widespread late-stage mica formation with occasional tourmalinization is evidence for late-stage fluids enriched in water and boron. As mostly incomplete whole-rock data are given in literature, a systematic comparison of the tetrad effect to these mentioned trace element ratios is impossible. Therefore, the element ratios are only supplemented in the caption of Fig. 6 where possible. If geochemical data are reported, they often show similar values as the granites studied such as Sr/Eu . 200, Eu/Eu* , 0.1, fractionated Y/Ho away from 28, Zr/Hf,38, and significant enrichments of Rb (K/Rb , 100). Some of the granites listed in Fig. 6 show highly fractionated Y/Ho ratios ,28 rather than .28 as the granites from this study. Preliminary investigations (Irber, 1996) have found that peraluminous A-type granites show Y/Ho , 28 while peraluminous S-type granites shift to Y/Ho ratios .28. The reason for this opposite fractionation behavior is not yet known. But regardless of the direction of Y/Ho fractionation, tetrad effects are only observed together with significantly fractionated Y/Ho ratios. 7.1. Mineral fractionation as reason for the tetrad effect? Mineral fractionation is often discussed to generate REE patterns showing a tetrad effect, e.g., the fractionation of apatite (Jolliff et al., 1989; McLennan, 1994), monazite (Yurimoto et al., 1990; Zhao and Cooper, 1992) or garnet (Pan, 1997). The pronounced deep discontinuity in REE patterns at Nd, which appears to be one of the most striking features of the tetrad effect, was successfully modeled by Yurimoto et al. (1990), Zhao and Cooper (1992), and Pan (1997) using monazite fractionation. An examination of the modelled REE patterns reported by Yurimoto et al. (1990), however, reveals that these patterns only display the discontinuity at Nd, but miss the basic characteristic of the tetrad effect. The calculated patterns display no gradual change for all chondrite-normalized REE positions and no smoothly curved tetrads. If the method of calculation of TE1,3 is applied, the resulting values remain at about 1 (5 no tetrad effect) despite the significant fractionated Nd/Sm ratio (cf. Table 6, and Figs. 7a and b). To examine the decoupling of Nd/Sm and the tetrad effect in detail, a REE Rayleigh fractionation (cf. Rollinson, 1993) was calculated between two representative samples of the Fichtelgebirge pluton without (G2, He-9654, TE1,3 5 1.05) and with tetrad effect (G4, He-9149, TE1,3 5 1.40). Both samples 498 W. Irber Fig. 4. Tetrad effect (TE1,3) vs. (a) K/Rb, (b) Y/Ho, and (c) Z/Hf. The straight lines mark the chondritic values, the dotted line defines the boundary to clearly visible tetrad effects (TE1,3 .1.10). G1-G4: Fichtelgebirge; L and Lg: Leuchtenberg (g: garnet-bearing); B1-B3: Bergen; E1-E4: Eibenstock; P1 and P4: Pobershau; e5-e23: Ehrenfriedersdorf. are related to each other by crystal fractionation and mark the known end-members of a differentiation suite of the so-called younger granite group (G2 to G4, cf. Hecht et al., 1997). The author is aware of the criticism by Bea (1998) about modeling of trace elements and especially of the REE. The small grain size of ,0.2 mm, which is typical for many of the REE bearing accessory minerals, may prevent an effective settling in viscous granitic melts. Evidence for more than only gravitative fractionation of accessory minerals, on which a Rayleigh fractionation is based, is supported by the common observation that biotite is strongly enriched in accessory minerals. Also, crystal fractionation does not necessarily involve crystal settling, but also can take place by melt removal from a mush (Tait and Jaupart, 1996) and affects the results of a Lanthanide tetrad effect 499 Fig. 5. Tetrad effect vs. (a) Sr/Eu* and (b) Eu/Eu. For the samples L14, L15, and G4-2 the detection limits of 0.01 and 0.006 ppm Eu, respectively, are used. Therefore, the actual Eu/Eu* may be lower as well as the Sr/Eu higher. The thick arrow shows the trend in Sr/Eu with increasing degree of differentiation as is calculated by a Rayleigh fractionation starting with sample G2 (He-9654), cf. Table 6. Especially highlighted is the actual trend for the Fichtelgebirge granites G1-G4. The straight lines mark the chondritic values, the dotted line defines the boundary to tetrad effects (TE1,3 .1.10). The samples are labeled as in Fig. 4. Rayleigh calculation. Another critical assumption involved in Rayleigh fractionation is that the partition coefficients used and the amount of fractionating minerals remain constant during the evolution of the granitic melt. This assumption is certainly not true (cf. the problems with fractionating zircon below), but necessary, given the uncertainty of how the coefficients might change with the various physicochemical parameters (Pan, 1997). But even if the modelling itself bears a large error, the Rayleigh fractionation should be able to reveal whether tetrad effect-like REE patterns can theoretically be generated by fractionation of minerals with differently shaped REE patterns or not. It is here not primarily attempted to match the trace element concentrations in the G4 sample rather than to match the tetrad effect-like appearance of the G4-REE pattern. To involve all important REE fractionating phases, the major minerals K-feldspar, plagioclase, quartz, biotite, and the accessory minerals apatite, monazite, zircon, and xenotime were included (Table 5). As no partition coefficient is published for xenotime, it was determined by the mineral/bulk-rock ratio using a representative microprobe analysis of xenotime of the G2 granite sample He-9654 (Förster, 1998b). The REE concentrations of the G2 xenotime are similar to recently published LA-ICP-MS data for xenotime in granitic rocks (Bea, 1998). The relative portions of fractionating minerals were derived from petrographic examination and normative mineral calculation. The determination of abundances for the rare but strongly REE-enriched accessory minerals monazite and xenotime is very critical and was supported by results of leaching experiments (Irber, 1996). Under the conditions of these leaching experiments (cf. Irber et al., 1997), monazite and xenotime are rather insoluble whereas apatite dissolves rapidly. The nonleached Ce- (93%) and Y- (77%) fractions after 20 h were used as maximum concentration to calculate monazite and xenotime abundances, respectively. The apatite fraction was determined 500 W. Irber Table 5. Normative mineral composition (wt.%) of the G2 sample (He-9654) and the respective mineral partition coefficients as were used for the Rayleigh fractionation. Missing REE partition coefficients in the original data sets were inter- or extrapolated and are given in brackets (see text for more details). Kdmineral/melt Kdmineral/melt Kdmineral/melt Kdmineral/melt Kdmineral/melt Kdmineral/melt Kdmineral/rock Kdmineral/rock REE Biotite K-fsp Plagioclase Quartz Apatite Zircon Monazite Xenotime wt. % 9 0.44 0.22 (0.044) 16.90 16.75 (15.02) 13.30 14.40 16.00 12.00 37.00 101.5 (187.3) (292.7) (408.7) 527.0 641.5 — — — — La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sr Ba Rb Cs 3.180 2.803 (2.518) 2.233 1.550 0.867 (1.198) 1.053 0.823 (0.680) (0.599) (0.558) (0.537) 0.505 — 5.356 4.2 2.3 28 28 34 0.080 0.037 (0.036) 0.035 0.025 4.450 (0.025) 0.025 0.025 (0.025) (0.026) (0.027) 0.030 0.033 5.4 11.45 1.75 0.195 0.270 0.270 (0.24)) 0.210 0.130 2.150 0.097 (0.081) 0.064 (0.060) 0.055 (0.052) 0.049 0.046 4.4 0.308 0.041 — 0.015 0.014 (0.015) 0.016 0.014 0.056 (0.016) 0.017 0.017 (0.018) 0.018 (0.018) (0.017) (0.014) — 0.022 0.041 0.029 (19.37) 34.70 (45.90) 57.10 62.80 30.40 56.30 (53.50) 50.70 (43.95) 37.20 (30.55) 23.90 20.20 (30.4001) — — — 0.044 3200 3413 3569 3726 2859 — 2144 1786 1429 920 595 395 273 174 — — — — 0.01 2 12 34 89 404 — 4052 5503 6820 6974 6106 5444 3634 2019 — — — — Kd (biotite): Mahood and Hidreth (1993); Kd (K-feldspar): Nash and Crecraft (1985); Kd (plagioclase): Arth (1976); Kd (quartz): Nash and Crecraft (1985); Kd (apatite): Arth (1976); Kd (Zircon): Mahood and Hildreth (1983); Kd (monazite): Yurimoto et al. (1990); Kd (xenotime): mineral/rock after Förster (in press). 1 Derived from Kd Eu. by the amount of phosphate not related to monazite and xenotime. Missing values in the published REE partition coefficients were either linearly interpolated (for one missing value) or by a 3rd order polynomial fit. The 3rd order polynomial fit is necessary for more than one missing REE value (or by extrapolation) as a linear inter- or extrapolation would break the smooth appearance of REE distribution coefficients. The com- Table 6. Bulk partition coefficient (P), the starting composition (G2, He-9654, REE chondrite-normalized, F 5 1) and the calculated values for F 5 0.8, 0.4, 0.2 and 0.08. The chondrite-normalized G4 (He-4194) values are shown for comparison on the last column. Chondrite concentrations are after Anders and Grevesse (1989). P 5 bulk-rock partition coefficient, F 5 amount of residual melt phase, Cl 5 weight concentration of a trace element in the residual melt, Co 5 weight concentration of a trace element in the residual solid. P La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sr Ba Rb Cs Tetrad effect Eu/Eu* Sr/Eu 1.92 2.04 2.12 2.20 1.79 2.26 1.77 1.78 1.86 1.80 1.76 1.83 1.82 1.85 2.93 3.90 0.91 0.27 G2 He-9654 analys. F 5 80% calc. F 5 40% calc. D 5 20% calc. F 5 8% calc. G4 He-4194 analys. 221 183 137 103 68.0 11.5 45.8 40.4 34.9 29.4 28.1 24.2 23.3 21.9 54.9 411.0 341 16.8 1.04 0.21 83 180 145 106 78.6 57.0 8.7 38.6 34.0 28.8 24.6 23.7 20.1 19.4 18.1 35.7 215.3 348 19.7 1.03 0.18 71 94.8 70.8 49.0 34.1 32.9 3.6 22.6 19.7 15.8 14.1 14.0 11.3 10.9 10.0 9.4 28.9 370 32.7 1.01 0.13 45 50.1 34.5 22.6 14.8 18.9 1.5 13.3 11.5 8.7 8.1 8.2 6.3 6.2 5.6 2.5 3.9 393 54.0 0.99 0.10 28 21.5 13.4 8.1 4.9 9.2 0.5 6.5 5.6 4.0 3.9 4.1 3.0 2.9 2.5 0.4 0.3 427 105 0.97 0.06 15 7.1 7.4 6.2 4.3 6.1 0.09 5.5 8.0 7.2 4.4 3.5 3.1 2.9 2.3 2.8 2.0 1073 113 1.37 0.01 562 Formula for Rayleigh fractionation: Cl 5 F(P-1) 3 Co. Lanthanide tetrad effect 501 Fig. 6. Chondrite-normalized REE patterns with marked tetrad effects of selected granites (a) from this study and (b) from literature data (see the text for a detailed reference). The REE pattern with the best developed tetrad effect is highlighted by a thick line in both diagrams. (a) L14: Leuchtenberg; G4: Fichtelgebirge (He-4194); Pob-4: Pobershau, Erzgebirge (aplitic granite); E4: Eibenstock, Erzgebirge (aplitic granite, Fö-800); e5: Ehrenfriedersdorf, Erzgebirge (Se-e5); all analyses GFZ Potsdam, for element ratios see Table 1. (b) HP 36: tourmaline granite of Harney Peak, Black Hills, South Dakota, USA: Eu/Eu*: 0.19, TE1,3: 1.26 (Yurimoto et al., 1990); Nr. 12 : albitised Li-mica granites from Linwu, Hunan Province, China: TE1,3: 1.78 (Masuda and Akagi, 1990); 35.5: albitised Li-mica granite of Cinovec, Erzgebirge, Czech Republic: K/Rb: 22, Sr/Eu: 2429, Eu/Eu*: 0.021, Y/Ho: 13, Zr/Hf: 5, TE1,3: 1.37 (Cocherie et al., 1991); 82-4b: topaz-bearing Li-mica granite from Pleasant Ridge, Southern New Brunswick, Canada: K/Rb 5 20; Sr/Eu $120; Eu/Eu* # 0.013; Y/Ho 5 18; Zr/Hf 5 9; TE1,3 5 1.21 (Taylor, 1992); 3318WR 5 garnet-bearing leucogranitic gneiss, Sobaegsan Massif, South Korea: Eu/Eu* 5 0.05; Sr/Eu 5 654; TE1,3 5 1.14 (Lee et al., 1994); MGx : garnet-bearing albitised granite of the Preissac pluton in the Preissac-Lacorne batholith, Quebec, Canada: K/Rb 5 62, Sr/Eu 5 391, Eu/Eu*5 0.045, Y/Ho 5 34, Zr/Hf 5 6, TE1,3 5 1.29 (Mulja et al., 1995); AD 45 5 fluorite-rich albitised leucogranite of Abu Dabbab, Eastern Desert of Egypt: K/Rb 5 37, Sr/Eu 5 656, Eu/Eu* 5 0.054, Y/Ho 5 7, Zr/Hf 5 2, TE1,3 5 2.24 (Mohamed, 1994; Bau, 1997; analysis GFZ Potsdam). plete set of partition coefficients is shown in Table 5, where all values calculated are given in brackets. The missing partition coefficient for Sr in apatite was substituted by that of Eu, which is only of little effect for the resulting values and does not affect the resulting trend. It has to be noted that an initial Rayleigh calculation did not show a decrease of the HREE as seen from the G2 to the G4 sample. To resolve this, the fraction of differentiating zircon was increased from 0.044% to 0.22%. The reason for this unexpected behavior is not yet known, but other possible options like the increase in fractionating xenotime did not have the desired effect. The results of the Rayleigh fractionation demonstrate the increasing discontinuity at Nd with decreasing F (Fig. 7a). At a value of 8% remaining melt, the REE concentrations are near to those of the target G4 granite. However, the Rayleigh fractionation cannot explain the low Eu concentration of the G4 granite and does not generate a tetrad effect-like REE pattern. The TE1,3 values in the modeled REE patterns remain at about 1 during the different steps despite the drastic degree of differentiation (F 5 0.8 to 0.08; Table 6). In a second approach, an attempt was made to find any random combination of fractionating minerals that would result in a tetrad effect-like REE pattern similar to that of the G4 granite. The calculation was performed by stepwise iteration using the Microsoft EXCEL solver. The starting mineral assemblage was as used for the Rayleigh fractionation above. Free variables in this iteration were the amounts of fractionating minerals. Fixed side parameters determined the value for F (residual melt) #10%, for Eu #0.5 ppm and the sum of fractionating minerals to 100%. The resulting REE pattern roughly matches the G4 pattern but still does not show a tetrad effect (Fig. 7b). During the iteration, the amount of fractionating plagioclase was increased to 99.5%, while the fractions of biotite, quartz, and K-feldspar were lowered toward 0.1% (Fig. 7b). The abundance of apatite was reduced to about 50% of the starting concentration while the fractions of zircon, monazite, and xenotime remained at 502 W. Irber Fig. 7. (a) Chondrite-normalized REE patterns of the residual melt calculated at varying degrees of fractionation. Also given are the REE patterns of the REE composition at start (G2, He-9654) and of the target highly evolved G4 granite (He-4194). (b) Iterative determined REE pattern that is closest to that of the G4 granite at F # 10%, Eu # 0.5, and the sum of fractionating minerals 5 100% (see text). The REE patterns at start (G2, He-9654) and of the target G4 granite (He-9149) are shown for comparison. The iterative determined mineral assemblage is made up of biotite (0.01 wt.%), K-feldspar (0.01 wt.%), plagioclase (99.45 wt.%), quartz (0.01 wt.%), apatite (0.22 wt.%), zircon (0.23 wt.%), monazite (0.06 wt.%), and xenotime (0.01 wt.%). about the pre-set level given at start. The resulting mineral assemblage is unrealistic and demonstrates that even under allowance of random mineral combinations the known partition coefficients do not lead to a tetrad effect-like REE pattern. Additional arguments against common mineral fractionation causing the tetrad effect are provided by separated minerals which show similar tetrad effects as the host rock: fluorite (Höhndorf et al., 1994), apatite, garnet (both Fig. 8), monazite (Förster, 1998a), and xenotime (Förster, 1998b). For instance, separated garnet fractions from highly evolved granite samples of the Leuchtenberg granite increase in TE1,3 with increasing degree of differentiation (TE1,3 garnet/host rock: L1 5 1.24/ 1.14; L14 5 1.40/1.30; L15 5 1.41/1.29). The garnet already crystallizes in a late-stage melt with tetrad effect and inherits the given REE signature. The garnet does not contribute to the generation of the tetrad effect via mineral fractionation. This is opposite to Pan (1997) who notes that all published REE patterns with tetrad effect and a significant discontinuity at Er contain garnet, and, therefore, suggests that garnet fractionation effectively contributes to the tetrad effect. But to simplify the tetrad effect to discontinuities at certain REE positions, only, does not meet the basic principles of the tetrad effect. This simplification is similar to the discontinuity at Nd, which is easily modeled by monazite fractionation, but is not related to the tetrad effect sensu strictu. Although the simultaneous existence of a discontinuity at Er and of garnet is doubtless true also for the Leuchtenberg granite, and although garnet fractionation might indeed cause an enhanced Er discontinuity, this is not necessarily related to the tetrad effect (see also the comments in Bau, 1997). McLennan (1994) suggested fractionating fluorapatite as possibly causing the tetrad effect during granite differentiation. However, fluorapatite does not show any REE fractionation in silicate melts (Table 5) or in aqueous fluids (Ayers and Watson, 1993), which would resemble the tetrad effect. Recent experiments of Fleet and Pan (1995) on REE partitioning of fluorapatite display one smooth upward-curved pattern from La to Lu with a maximum at Nd (at Gd in Ayers and Watson, 1993). Separated apatite (TE1,3 5 1.28) from a Eibenstock granite sample (Fig. 8) clearly adopted the REE pattern of the residual fluid with a tetrad effect similar to the host rock (TE1,3 5 1.26). Incomplete REE pattern from apatites in the Bob Ingersoll pegmatite (Jolliff et al., 1989) show large variations in REE patterns, even within one apatite crystal. This once more suggests that changes in fluid composition control the REE pattern in apatite rather than a possible selective REE fractionation between apatite and fluid. Jolliff et al. (1989) also noted that the “kinked” REE patterns in apatite correlate in degree with the Lanthanide tetrad effect 503 Fig. 8. Chondrite-normalized REE analyses of separated garnet fractions from the Leuchtenberg granite and from separated apatite of the Eibenstock granite (Eib2, Fö-507). The numbers below the pattern display the calculated degree of the tetrad effect (TE1,3). vertical position within the pegmatite. This indicates a relationship, where vertical REE differentiation is caused by upward migration of an aqueous fluid or volatile complexes. If mineral fractionation would generally result in bulk-rock and mineral REE patterns displaying the M-type tetrad effect, the respective W-type pattern would be typical for residual late stage melts (e.g., aplites) and late-stage minerals. This, however, is in strong opposition to the common observation of pronounced M-type patterns, exclusively. In summary, the observations do not support arguments in favor of mineral fractionation causing the tetrad effect. Despite the fact that uncertainties in partition coefficients of highly evolved melt systems limit the Rayleigh fractionation, it seems rather unlikely that a simple process of mineral fractionation is able to generate REE patterns displaying the tetrad effect. 7.2. Tetrad effect and Eu depletion Significant tetrad effects were found together with extremely low Eu concentrations which could not be modeled by a Rayleigh fractionation (Table 6), although Eu anomalies (Eu/Eu*) are commonly explained by feldspar fractionation (e.g., Möller and Muecke, 1984). The Rayleigh fractionation has shown that common mineral fractionation is able to account for Eu/Eu* down to a minimum of 0.06. Ratios ,0.06 (cf. Fig. 5b), however, were only achieved in the unrealistic case that feldspar is the exclusive fractionating mineral phase (Fig. 7b). The strong decrease in Eu concentrations is also seen in Sr/Eu ratios which, at TE1,3 .1.10, show a clear turn to high values which is opposite to the trend if mineral fractionation is dominant (thick arrow in Fig. 5a, according to the calculated Sr/Eu ratios in Table 6). A possible change in the oxidation stage from Eu21 to Eu31 can not explain this de-coupled behavior. A trivalent charged Eu would behave similar to the other REE and would more likely be retained in the melt (not increasing the Eu anomaly) rather than removed (increasing the Eu anomaly). A possible explanation was proposed by Muecke and Clarke (1981) who suggested that the strong Eu depletion in the late-stage of granite crystallization may indicate a preferential Eu fractionation into a co-existing aqueous fluid phase rather than into feldspar. Candela (1990) derived from theoretical constraints that the REE fractionation between a silicate melt and a Cl-rich fluid phase could easily account for the strong separation of the divalent Eu from the trivalent REE. This, however, would not explain the separation of Eu and Sr. The trend in Sr/Eu ratios toward high values might be explained by the different complexing behaviors especially for Eu. The reason for the de-coupling of Sr and Eu, however, remains unresolved as trace element speciation in highly evolved melts and magmatic fluids is not sufficiently understood. 504 W. Irber 7.3. Tetrad effect due to chemical complexation? Although less evolved granitic systems (e.g., biotite granites from Fichtelgebirge or the Oberpfalz) represent fractionated melt with respect to chondrites, they still show chondritic or nearly chondritic K/Rb, Sr/Eu, Y/Ho, and Zr/Hf ratios (Figs. 5 and 6). Therefore, common element differentiation between fractionating minerals and a silicate melt is unlikely to cause major fractionation trends as are observed in highly evolved granitic rocks. Based on geostandards in Govindaraju (1994), Bau (1996) defined a CHARAC-field (5 element behavior is charge and radius controlled) in which nearly chondritic ratios of the geochemical twins Y/Ho and Zr/Hf comprise silicate rocks with SiO2-contents ,70 wt.%. In this diagram, strongly fractionated ratios, far beyond the range observed here, are strictly confined to aqueous systems (e.g., hydrothermal fluorites, hydro-genetic Fe-Mn-crusts and seawater; Bau, 1996). Gradual shifts away from the CHARAC field are shown by evolving granitic systems (.70 wt.%) similar to the granites studied here (cf. Irber et al., 1997). While silicate or aqueous systems obviously represent the end-members of a possible fractionation range, the trace element behavior in evolving granitic systems reflects a continuous transition from one into the other system. This is also supported by investigations on melt inclusions from the Ehrenfriedersdorf granite (Central Erzgebirge) indicating a continuous development from a magmatic into a high-temperature (T , 450°C) hydrothermal stage (Thomas, 1994), which is paralleled by increasing features of magmatic-hydrothermal alteration. However, whether the silicate melt grades into a high-temperature aqueous fluid (c.f. London, 1992) or a co-existing exsolved aqueous fluid increased in volume and importance (c.f. Burnham and Ohmoto, 1980) cannot be distinguished. Bau (1996) proposed as a major cause for the pronounced fractionation the influence of element specific electron configurations, which affect the stability of chemical complexation. The latter is of major importance in aqueous systems, but of minimal influence in pure silicate melts where ionic radius and charge largely control the trace element behavior (Goldschmidt, 1937). The correlation of the tetrad effect with the examined trace element ratios, and in particular with those of the geochemical twins Y/Ho and Zr/Hf, hints to similar underlying physicochemical principles being responsible for the trace element fractionation in highly evolved melt systems. The origin of the tetrad effect is commonly assumed to be based on interactions of 4f-electrons with the valence electrons of the complexing agent (Sinha, 1978; Dzhurinskii, 1980; Kawabe, 1992; Akagi et al., 1993). The degree of these electron interactions is described by the Racah parameters which change in dependence on (a) the number of 4f-electrons, and (b) the type of complexing ligand. The smooth curved tetrads within a REE pattern suggest that 4f-electron interactions slightly increase or decrease the complex stability relative to the neighboring lanthanides. This superimposes the commonly gradually changing complexing stability from La toward Lu (cf. Wood, 1990a and 1990b; Byrne and Biqiong, 1995; Haas et al., 1995), and could also provide an explanation for the generally underdeveloped fourth tetrad. Higher filling stages of the 4f-electron orbitals of the HREE and shorter distances of the 4f-electrons to the atomic core reduce the possibilities of 4f-orbitals to interfere with potential ligands and to influence the complexing behavior (Dzhurinskii, 1980). This is supported by laboratory experiments of Yaita and Tachimori (1996), which showed systematic weaker developed tetrads from La to Lu, accompanied by more covalent bonds of the HREE with the complexing ligand, and by differences in the ligand number (LREE: 4, HREE: 3). Even if these experiments are far away from conditions in crystallizing granite magmas, they well demonstrate the possible influence of REE complexing properties on the appearance of the tetrad effect. In summary, parallel developing fractionation trends of geochemical twins (Y/Ho, Zr/Hf) and of the REE (tetrad effect) suggest a common underlying cause such as the increasing influence of strong chemical complexation as an important control on element fractionation in “transitional” silicate-aqueous systems like evolving granite melts. 7.4. Fluorine complexation as a factor for the tetrad effect? Kawabe (1992) published a detailed work based on the RFSPT (refined spin pairing energy theory) after which liganddependent differences in the ionic radii of REE would cause the tetrad effect. The variable differences of the ionic radii are based on the variable expansion of the electron cloud (nephelauxetic effect) and the type of complexing as well as the inner atomic structure. The nephelauxetic effect of a lanthanide bond in a fluorine aquo-complex differs from that of a lanthanide bond in a crystalline oxygen complex. The difference results in the tetrad effect. Kawabe (1992) illustrates this with examples of the standard enthalpies of LnF3 (rhomb.) and LnO1.5 (cub.), whose difference shows a tetrad effect, while the single data sets do not. If adopted to geological systems, the tetrad effect may be generated during REE fractionation (a) at the transition from a silicate melt to a high-temperature hydrothermal system or (b) between coexisting silicate melt, aqueous high-temperature late stage fluid and crystallizing minerals, in both cases with strong fluorine complexing of the REE. The possible importance of fluorine complexation for the element fractionation is indicated by the correlation of the tetrad effect with bulk-rock fluorine concentrations (Fig. 9). According to Wood (1990b) the presence of topaz or fluorite, as is observed in the granites studied, indicates that fluorine was the most important complexing agent of the late-stage fluids (see also Keppler, 1993). A possible Cl-complexation can be neglected as the temperature-dependent increase in complexing constants is far more significant for fluorine than for chlorine, and Wood (1990b) suggests that in those systems the Clcomplexation is insignificant. In contrast to chlorine, fluorine partitions selectively into a granitic magma rather than into an exsolved aqueous fluid phase (Manning, 1981; Webster and Holloway, 1990). This would retain those REE in the melt that build stronger complexes with fluorine. However, as available REE complexing constants with fluorine are either consistent with the tetrad effect (Becker and Bilal, 1985) or are not (Wood, 1990b), the question of fluorine complexation as supporting factor to the tetrad effect remains unresolved, even if it appears to be reasonable from the available data. Lanthanide tetrad effect 505 Fig. 9. Tetrad effect (TE1,3) vs. fluorine (ppm) in bulk-rock. Fluorine data were only available from 21 samples but represent most of the granitic massifs studied except for the granites of Leuchtenberg and Pobershau. The dotted line marks the boundary to clearly visible tetrad effects (TE1,3 .1.10). E1-E4: Eibenstock; G1-G4: Fichtelgebirge; B1-B3: Bergen; e5-e23: Ehrenfriedersdorf. 8. CONCLUSION The quantification of the tetrad effect (TE1,3) allows to plot it vs. geochemical parameters known to be sensitive to granitic melt differentiation and magmatic-hydrothermal transitional environments. The strong correlation of the tetrad effect with K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, and Zr/Hf reveals the gradual development of the tetrad effect parallel to granite evolution. Significant tetrad effects are only seen in highly evolved granitic rocks (here peraluminous S-type) with highly fractionated ratios of K/Rb, Sr/Eu, Eu/Eu*, Y/Ho, and Zr/Hf. A Rayleigh fractionation was performed in order to see whether mineral fractionation can cause the tetrad effect and the highly fractionated Sr/Eu and Eu/Eu* ratios. However, even under allowance of random mineral combinations during fractionation no tetrad effect could be generated. Also, the fractionation trends of Sr/Eu and Eu/Eu* are only partly explained by mineral fractionation (e.g., feldspar). The strong decrease of Eu concentrations in highly evolved granitic rocks is more likely to indicate Eu fractionation between a residual melt and a coexisting aqueous high-temperature fluid. The results point to significant changes in behavior of elements in highly evolved granitic melts, where classic mineral/melt element fractionation, based on ionic radius and charge, is no longer the exclusive control. Analyses of accessory minerals such as garnet, apatite (both this study), fluorite, monazite, and xenotime (other studies) display similar tetrad effects as the respective host rocks. The accessory minerals inherit the REE signature of the melt, but do not contribute to the tetrad effect by mineral fractionation. Highly fractionated element ratios of Y/Ho and Zr/Hf indicate more similarities of the trace element behavior to aqueous systems rather than to silicate melts. This and the strong features of magmatic-hydrothermal alteration suggest either (i) a gradual transition from the silicate melt into a high-temperature hydrothermal fluid during granite crystallization or (ii) the increasing importance of a co-existing exsolved aqueous fluid phase. The positive correlation of the tetrad effect with fluorine concentrations of the bulk rock hints to fluorine complexation as a possible factor contributing to the tetrad effect. As the generation of the REE tetrad effect (M-type) implies the removal of a respective mirroring REE pattern (W-type), the tetrad effect identifies open system conditions during granite crystallization. This would additionally support the idea of a significant Eu fractionation into a coexisting aqueous fluid out of the silicate melt system as is indicated by the pronounced negative Eu-anomalies in combination with significant tetrad effects. Acknowledgment:—The author is very grateful to Hans-Jürgen Förster, Lutz Hecht, Reimar Seltmann, Wolfgang Siebel, and Gerhard Tischendorf for providing sample material and analytical data. Peter Möller is warmly thanked for support and discussion throughout my time in Potsdam. Special thanks go to Peter Dulski for the ICP-MS analyses, and to Michael Bau who first noticed the tetrad effect in the sample suite shown here. Numerous helpful comments of Fabio Ramos Dias de Andrade and of three anonymous reviewers significantly improved the manuscript. The lively Tasha Black is acknowledged for the correction of the written English of an early version. 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