You recall discussions of Black Smokers, the placement of Sulfide deposits at the mid-ocean ridges. The same thing happens at volcanoes when water and magma are in proximity For example, at an active Caldera http://vulcan.wr.usgs.gov/Glossary/Caldera /description_caldera.html Petrology Field Trip to Bemco Mining District Ores from weathered Sulfide deposits • Mineral deposits containing sulfide minerals, e.g. copper sulfides, are subjected to weathering, can go into solution and trickle down to the reducing conditions below the water table, where native metals or rich concentrations of ores are precipitated. e.g. black smokers, hydrothermal circulations Gossan Intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the classic gossan or iron cap all that remains is iron oxides and quartz often in the form of boxworks, quartz lined cavities retaining the shape of the dissolved ore minerals . Solubility in water The Solubility Rules 1. Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing the ammonium ion (NH 4+) are also soluble. 2. Salts containing nitrate ion (NO3-) are generally soluble. 3. Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble. 4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are common soluble salts of silver; virtually anything else is insoluble. 5. Most sulfate salts are soluble, for example FeSO4 is soluble. Important exceptions to this rule include BaSO , PbSO , 4 4 Ag2SO4 and SrSO4 . 6. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are not soluble. 7. Most sulfides of transition metals are highly insoluble. Thus, CuS, FeS, FeS2, ZnS, Ag2S are all insoluble. Arsenic, antimony, bismuth, and lead sulfides are also insoluble. 8. Carbonates are frequently insoluble. Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include FeCO3 and PbCO3. 9. Chromates are frequently insoluble. Examples: PbCrO4, BaCrO4 10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag3PO4 11. Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2. Changing insoluble metal sulfides into soluble sulfates • Oxidizing Zone above the water table • Sulfide minerals, for example ferrous and copper sulfides, are subject to weathering. • Sulfide minerals are oxidized near the surface and produce sulfuric acid. For example: • FeS2 (s) + 7O + H2O →FeSO4 (aq) + H2SO4 Formation of the solvent Ferrous Sulfate The part played by ferric sulfate Fe2(SO4)3 as a solvent can be seen by the following reactions: Pyrite FeS2 + Fe2(SO4)3 → 3FeSO4 + 2S Chalcopyrite CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S Chalcocite Cu2S + Fe2(SO4)3 →CuSO4 + 2FeSO4 + CuS Covellite CuS + Fe2(SO4)3 →2FeSO4 + S + CuSO4 Sphalerite ZnS + 4Fe2(SO4)3 + H2O →ZnSO4 + 8FeSO4 + 4H2SO4 Galena PbS + Fe2(SO4)3 + H2O + 3O →PbSO4 + 2FeSO4 + H2SO4 Silver 2Ag + Fe2(SO4)3 → Ag2SO4 + 2FeSO4 · Most of the sulfates are readily soluble, and these cold dilute solutions slowly trickle downwards through the deposit until the proper Eh-pH conditions are met to cause deposition of their metallic content. Reaction and Trickling Down • Iron sulfate reacts with sulfides, they go into solution as sulfates, acid rainwater then carries, for example copper, as copper sulfate, down to the water table. • CuS(s) + Fe2(SO4)3 (aq) →2FeSO4 (aq) + S(s) + CuSO4 (aq) • The net result is that dissolved copper sulfide trickles down from the oxidizing upper portion of the deposit to that portion at and just below the water table. Reducing Zone below the water table • Below the water table, where additional sulfide minerals remain solid and unoxidized (e.g. Pyrite FeS2), any iron sulfide grains present will react with the copper sulfate solution, putting iron into solution and precipitating copper. • FeS2 (s) + CuSO4 (aq) → FeSO4 (aq) + Cu(s) + 2S(s) • This process is called Supergene Enrichment Hydrothermal Deposit, Bemco Mine Ch. 10 Origin of Basaltic Magma Seismic evidence in the mantle … -> basalts are generated by partial melting of mantle material Probably can derive most other magmas from this primary magma by fractional crystallization, assimilation, etc. Basalt is the most common magma If we are going to understand the origin of igneous rocks, it’s best to start with the generation of basalt from the mantle Chapter 9 has one figure which we should look at for today’s topic: Here is one corner of that figure: Basalts in different Plate Tectonic settings are chemically different Two principal types of basalt in the ocean basins Tholeiitic Basalt and Alkaline Basalt Ocean Islands such as Hawaii have both Tholeiitic AND Alkaline Basalts Common petrographic differences between tholeiitic and alkaline basalts Table 10-1 Subalkaline: Tholeiitic-type Basalt Groundmass Usually fine-grained, intergranular Usually fairly coarse, intergranular to ophitic No olivine Olivine common Clinopyroxene = augite (plus possibly pigeonite) Titaniferous augite (reddish) Orthopyroxene (hypersthene) common, may rim ol. Orthopyroxene absent No alkali feldspar Interstitial alkali feldspar or feldspathoid may occur Interstitial glass and/or quartz common Interstitial glass rare, and quartz absent Olivine rare, unzoned, and may be partially resorbed Phenocrysts Alkaline Basalt Olivine common and zoned or show reaction rims of orthopyroxene Orthopyroxene uncommon Orthopyroxene absent Early plagioclase common Plagioclase later in sequence, uncommon Clinopyroxene is pale brown augite Clinopyroxene is titaniferous augite, reddish rims after Hughes (1982) and McBirney (1993). A third is hi-Al, or calc-alkaline basalt, usually continental Subalkaline: Tholeiites and Calc-alkaline Basalts Example: AFM diagram (alkalis-Fe-Mg) A (Na2O + K2O) , F( FeO + Fe2O3) and M ( MgO ) http://petrology.oxfordjournals.org/cgi/cont ent/abstract/45/3/507 Figure 8-3. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication). Silica content variation in two famous Igneous localities Notice: Skaergard has a Ferrobasalt member, Crater Lake does not. AFM diagram: Tilley: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series Figure 8-14. AFM diagram showing the distinction between selected tholeiitic rocks from Iceland, the MidAtlantic Ridge, the Columbia River Basalts, and Hawaii (solid circles) plus the calc-alkaline rocks of the Cascade volcanics (open circles). From Irving and Baragar (1971). After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548. Above subduction zones MORs and Flood Basalts, and above Plumes AFM diagram showing “typical” areas for various extents of evolution from primitive magma types. Tholeites go through a Ferro-Basalt stage before continuing towards Rhyolite. Recall Skaergard and Mt. Mazama Ophiolite Suite Some Serpentine is formed due to hot water (called Hydrothermal) circulation Samples of mantle material Ophiolites – Slabs of oceanic crust and upper mantle – Thrust faulted onto edge of continent Dredge samples from oceanic fracture zones Nodules and xenoliths in some basalts Kimberlite xenoliths Plume passes through a subduction zone’s carbon – Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth Lherzolite, Harzburgite and Dunite Lherzolite is probably fertile unaltered mantle Harzburgite typically forms by the extraction of partial melts from the more pyroxene-rich peridotite called lherzolite. The molten magma extracted from harzburgite may then erupt on the surface as basalt. If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-poor peridotite called dunite Lherzolite is probably fertile unaltered mantle Dunite and Harzburgite are refractory residuum after basalt has been extracted by partial melting Tholeiitic basalt 15 10 Figure 10-1 Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer. 5 Lherzolite Harzburgite Dunite 0 0.0 0.2 Residuum 0.4 Wt.% TiO2 0.6 0.8 Lherzolite: A type of peridotite with Olivine > Opx + Cpx Olivine Dunite 90 Peridotites Lherzolite 40 Pyroxenites Olivine Websterite Orthopyroxenite 10 10 Orthopyroxene Websterite Clinopyroxenite Figure 2-2 C After IUGS Clinopyroxene Phase diagram for aluminous Notice the mantle will 4-phase Lherzolite: not melt under normal Last was Olivive & Pyroxene, now look at Al mineralss. Al-phase = Ca++ Plagioclase shallow ocean geotherm! CaAl2Si2O8 . (< 50 km) Spinel Lherzolite Spinel is MgAl2O4 50-80 km Garnet Lherzolite 80-400 km Si [4] => Si [6] Mg3Al2(SiO4)3 Si[4] Si[6] coord. > 400 km Figure 10-2 Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153. How does the mantle melt? 1) Increase the temperature Figure 10-3. Melting by raising the temperature. No realistic mechanism for the general case because Temps hotter than the Geothermal Gradient are needed. Maybe accumulate radioactive decay heat? Local hot spots OK; very limited area 2) Lower the pressure: MOR and Rifts – Adiabatic rise of mantle (no conductive heat loss) – Rise to low pressure, lower MP, “decompression melting” Steeper than solidus Intersects solidus D slope = heat of fusion as mantle melts Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting. Basalt origin 1, at the MOR 3) Add volatiles (especially H2O) lowers Melt Pt changes slope Amphibolite Serpentinite Eclogite Eclogite: red to pink garnet (almandinepyrope) in a green matrix of sodiumrich pyroxene (omphacite) Figure 10-4 or 10-5 (2nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites. Basalt origin 2 So, basaltic melts can be created under several circumstances We saw: Plates separate and mantle rises at mid-ocean ridges Adiabatic rise decompression Melting Subduction zones dewatering Third way: Hot spots melting plumes, also basaltic Melting depths vary w\ volcanic province Most within upper few hundred kilometers The melts can mix • There is evidence that plume and MOR can mix (following slides) • Certainly a plume rising through a subduction surface is the favorite model for diamond transport to the surface. E-MORBs • Enriched MORBs (called E-MORBS) have, for example, higher Lanthanum La, Cerium Ce, and also higher Strontium Sr than normal N-MORBs “With increasing depths, the aluminous phase in the upper mantle changes from plagioclase to spinel to garnet The transition from spinel lherzolite (olivine +orthopyroxene + clinopyroxene + spinel) to garnet lherzolite (olivine + orthopyroxene + clinopyroxene+ garnet) could potentially influence the characteristics of some kinds of basalts, particularly mid-ocean ridge basalts (MORB), since this transition is thought to occur at about the same depths at which MORB may originate. There is evidence from trace element and isotope geo-chemistry that [some, the E-MORBs] MORB are generated in the presence of garnet (Klein and Langmuir 1987; Hirschmann and Stolper 1996). The evidence includes the depletion of heavier rare earth elements relative to lighter rare earth elements (Kay and Gast 1973), depletion in 177Lu/176Hf (Salters and Hart 1989) and elevated 230Th/238U ratios (Beattie 1993a, b; LaTourrette et al. 1993). This is generally referred to as the `garnet signature' in MORB. However, if melting started in the garnet lherzolite stability field, simple melting models (e.g. Klein and Langmuir 1987; McKenzie and Bickle 1988; Iwamori et al. 1995) predict a thickness of the oceanic crust much greater than the average crust at 7 +/- 1 km inferred from seismological studies (e.g. White et al. 1992). Several possible solutions have been put forward to resolve this apparent conflict, including: (1) reduced melt productivity of upwelling peridotite (Asimov et al. 1995); (2) variations in depth of melting on the top of the melting zone beneath ridges (Shen and Forsyth 1995); or (3) partial melting of small amounts of garnetbearing assemblages in veins such as garnet pyroxenites or eclogites (among others: Wood 1979; AlleÁgre et al. 1984; Hirschmann and Stolper 1996).” Klemme and O’Neill (2000) Lu = Lutetium Hf = Hafnium Plume and MOR interactions Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11°20′N Yaoling Niu, Ken D. Collerson, Rodey Batiza, J. Immo Wendt, Marcel Regelous Journal of Geophysical Research: Solid Earth (1978–2012) Volume 104, Issue B4, pages 7067–7087, 10 April 1999 The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid-ocean ridge basalts (E-MORBs) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053– 0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from melting of a two-component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched [E-MORB source] domains, whereas the recycled mantle/peridotitic residues are the most depleted [N-MORB source] matrix. On Pb-Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal plume-ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume-ridge interactions” seen along the Mid-Atlantic Ridge. The so-called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic [enriched] lithologies. The positive Na8Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation. Stable Isotopes of Strontium The ratio 87Sr/86Sr is a parameter often reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because Strontium has an electron configuration similar to that of calcium, it readily substitutes for Ca in minerals. 87Sr/86Sr is used, for example, to distinguish enriched E-MORBs from depleted source NMORBs. From a plume From a plume Can we generate both tholeiitic and alkaline basalts from a chemically uniform mantle? Variables (other than X) – Temperature – Pressure Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153. Pressure effects: Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field Alkaline basalts are thus favored by greater depth of melting Figure 10-8 Change in the eutectic (first melt) composition with increasing pressure from 1 to 3 GPa projected onto the base of the basalt tetrahedron. After Kushiro (1968), J. Geophys. Res., 73, 619-634. Crystal Fractionation of magmas as they rise Tholeiite alkaline by Fractionation at medium to high Pressure Recall not at low Pressure, due Albite Thermal Divide Thermal divide, they cannot evolve into one another, separate sources at low Pressure, but fractionation at med to high P does allow evolution of a magma from Tholeiite to Alkaline Initial Conclusions: Tholeiites favored by shallower melting – 25% melting at <30 km tholeiite – 25% melting at 60 km alkaline basalt Tholeiites favored by greater % partial melting – 20 % melting at 60 km alkaline basalt incompatibles (alkalis) initial melts – 30 % melting at 60 km tholeiite Initial Conclusions A chemically homogeneous mantle can yield a variety of basalt types Alkaline basalts are favored over tholeiites by deeper melting Fractionation at moderate to high depths can also create alkaline basalts from tholeiites At low P there is a thermal divide that separates the two series Experiments on melting enriched vs. depleted mantle samples: 1. Depleted Mantle Tholeiite easily created by 10-30% Partial Melting More silica saturated at lower P Figure 10-17a. Results of partial melting experiments on depleted Mantle. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310. Experiments on melting enriched vs. depleted mantle (DM) samples: 2. Enriched Mantle Tholeiites extend to higher P than for Depleted Mantle Alkaline basalt field (purple) at higher P yet Figure 10-17b. Results of partial melting experiments on fertile Lherzolites. Dashed lines are contours representing percent partial melt produced. Strongly curved lines are contours of the normative olivine content of the melt. “Opx out” and “Cpx out” represent the degree of melting at which these phases are completely consumed in the melt. The shaded area represents the conditions required for the generation of alkaline basaltic magmas. After Jaques and Green (1980). Contrib. Mineral. Petrol., 73, 287-310. At a depth of about 670 – 700 km g Olivine ((Mg,Fe)SiO4 )decomposes into silicate Perovskite (FeSiO3) and Periclase (MgO) + silica SiO2 in an endothermic reaction. Endothermic systems cool, contract, are less buoyant. This leads some workers to believe that the 670- 700 km boundary blocks convection from the core mantle boundary, and upper mantle convection cells are distinct. Mantle model circa 1975 Figure 10-16a After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. Newer mantle model Upper depleted mantle = MORB source - Tholeites Lower undepleted & enriched OIB source - Alkaline Boundary = 670 km phase transition Sufficient D density to impede convection so they convect independently Figure 10-16b After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.