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APPENDIX A
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1. Detailed sample description and background
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The majority of the xenoliths in this study can be classified into one of two
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groups, commonly referred to as Groups I and II (Frey and Prinz, 1978). Menzies (1983)
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refined the classification but referred to Types I and II; in this paper we employ the
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original terminology of Frey and Prinz (1978). Group I inclusions are mainly olivine-rich
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peridotites, but pyroxene-rich lithologies are also present. The olivine-rich lithologies
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include spinel lherzolite, spinel harzburgite, and spinel dunite. Pyroxenes and spinels in
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Group I rocks are typically Cr-rich and TiO2 poor, and silicates generally have Mg
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numbers (Mg#) >0.85. Bulk and mineral compositions and trace-element and isotope
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geochemistry indicate that the olivine-rich Group I rocks are the products of extraction of
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basaltic liquid during partial melting in the mantle lithosphere (Frey and Prinz, 1978).
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Some of these rocks show trace-element and isotopic evidence, chiefly in clinopyroxenes,
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of subsequent metasomatism by enriched magmas and/or CO2-H2O fluids (Frey and
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Prinz, 1978; Menzies, 1983; Galer and O’Nions, 1989). Group I xenoliths are subdivided
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into IA and IB to distinguish between those that respectively do not or do record evidence
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for later metasomatism. Within IB xenoliths, the metasomatic signature is most
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widespread in the most refractory, clinopyroxene poor samples. The pyroxene-rich
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lithologies include orthopyroxenites, websterites, and clinopyroxenites with varying
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olivine contents. The orthopyroxene-rich Group I rocks are interpreted as either residual
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tectonic layers in peridotites, or as cumulates from the extracted basaltic liquid (Frey and
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Prinz, 1978). Clinopyroxene-rich rocks may occur as layers or veins in lherzolites, or as
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discrete xenoliths. They possess geochemical signatures of equilibration with alkaline
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mafic magma. The common occurrence of exsolution lamellae in clinopyroxene implies
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that this event predated entrainment in the basanitic magma that carried the xenoliths to
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the surface (Frey and Prinz, 1978).
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Group II xenoliths are highly variable in terms of mineral proportions and
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compositions, and include spinel clinopyroxenites, spinel olivine websterites, and many
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others. Kaersutitic amphibole is common. Pyroxenes in Group II xenoliths are usually
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Al2O3- and TiO2-rich, but Cr-poor. This is reflected in Group II spinels and
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clinopyroxenes, which are generally more Al-rich and Cr-poor than their Group I
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counterparts. The Mg# of silicates ranges widely, but is typically <0.85. Group II
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xenoliths are interpreted as cumulates from an SiO2-undersaturated magma, most likely
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the host basanite (Frey and Prinz, 1978). Samples transitional between the two groups
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have also been identified (Wilshire and Shervais, 1975; Wilshire and Jackson, 1975; Frey
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and Prinz, 1978).
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In this study, we focused on Group I xenoliths to assess Fe isotope systematics of
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the local mantle lithosphere prior to modification by the magmatic event that brought
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them to the surface. Five lithologically distinct xenoliths were studied, including spinel
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lherzolite, spinel harzburgite, spinel dunite, clinopyroxenite, and olivine websterite. The
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spinel lherzolite came from our private collection; the four other samples were obtained
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from the Smithsonian Museum.
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The mineral identities and textures in the Ol- and Cpx-rich xenoliths are
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consistent with our classification of them as Group I (c.f., Frey and Prinz, 1978, their
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table 1). The olivine websterite is more similar to Group II websterites studied by Frey
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and Prinz, but it lacks amphibole and the pyroxene and spinel textures are more similar to
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Group I samples.
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Mineral compositional characteristics can also be used to distinguish Group I and
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II samples (Table 1). Figure 3a shows Al2O3 content of pyroxenes vs. Cr number in
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spinel. Using data from Frey and Prinz (1978) and Galer and O’Nions (1987), it can be
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seen that these compositional parameters clearly distinguish xenoliths from Groups I and
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II. Also shown are samples interpreted by Frey and Prinz to be Group I transitional. The
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olivine- and clinopyroxene-rich xenoliths all lie in the Group I field. In contrast, the
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olivine websterite appears to show transitional characteristics, in that the Cpx Al2O3
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content is similar to the most aluminous Group I Cpx, but the spinel has low Cr# more
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typical of Group II. Figure 3b shows a similar result: the Al2O3 contents of coexisting
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Opx and spinel from Group I define a linear, positively correlated array. A transitional
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Group I sample plots at the most aluminous end of this trend. In contrast, Group II
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samples plot together at high but roughly constant Al2O3 spinel. Again, our olivine- and
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clinopyroxene-rich samples lie along the Group I array. In contrast, the olivine websterite
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sample displays characteristics of both groups, in that it lies along the array but is at the
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highest Al2O3 contents of the data shown in the figure.
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Thus, the clinopyroxenite and spinel-bearing lherzolite, harzburgite and dunite
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possess characteristics that identify them clearly as Group I xenoliths. In contrast the
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olivine websterite is more similar to xenoliths interpreted as transitional to Group II. In
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the following, we refer to the websterite as transitional and treat it separately from the
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other xenoliths.
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The three peridotite xenoliths chosen for this study represent a wide range of
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olivine abundance, including a spinel lherzolite (CEM1-3), a harzburgite (111-312-37),
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and a dunite (111-312-26). Spinel lherzolite CEM1-3 is composed of 62% olivine (Ol),
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24% orthopyroxene (Opx), 13% clinopyroxene (Cpx), and <1% chromian spinel (Spl).
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The average spinel composition for this xenolith is
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3+ VI
IV
(Mg 0.73 Fe2+
0.27 ) (Al1.19 Cr0.74 Fe0.06 ) O4 as determined by charge balance assuming no
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vacancies using the method described in Droop (1987). The spinels in this sample are
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notable in having relatively high concentration of Cr as well as relatively small amounts
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of Fe3+ assigned to the octahedrally coordinated site. Therefore, the iron in the spinel of
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CEM1-3 is dominantly ferrous and tetrahedrally coordinated. Olivine in this sample is
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Fo91 and contains negligible amounts of ferric iron (Fe3+/∑Fe = 0.04 ± 0.12), with all
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iron being assigned to octahedrally coordinated sites. Iron in pyroxenes is also
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octahedrally coordinated, although in most samples has higher ferric/ferrous ratios than
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that of olivine. Clinopyroxene in CEM1-3 has Fe3+/∑Fe = 0.22 ± 0.08, and Opx has
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Fe3+/∑Fe = 0.15 ± 0.03. BSE images of CEM1-3 reveal no petrographic signs of
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metasomatism or interaction with a melt (Supp. Fig. 1a). Young et al. (2009) found that,
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in this same xenolith, heavy Mg isotopes are concentrated in spinel compared to the other
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minerals, and Mg isotope thermometry records a spinel-olivine equilibration temperature
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of 815 ± 11 °C with no corrections for cation substitutions.
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Sample 111-312-37 is a harzburgite consisting of 69% Ol, 27% Opx, 4% Cpx,
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and <1% chromian Spl. The calculated average spinel formula is
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3+ VI
IV
(Mg 0.72 Fe2+
0.28 ) (Al1.04 Cr0.90 Fe0.05 ) O4 . These spinels also have high chromium content
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and relatively low trivalent iron in octahedral coordination versus divalent iron in
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tetrahedral coordination (Fe3+/∑Fe = 0.16 ± 0.01). Olivine, Cpx, and Opx have Fe3+/∑Fe
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of 0.07 ± 0.08, 0.21 ± 0.54, and 0.12 ± 0.06, respectively. BSE images of this harzburgite
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show partially altered regions near the basalt-xenolith contact (Supp. Fig. 1b).
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Sample 111-312-26 is a dunite with 69% Ol, 27% Opx, 4% Cpx, and <1% Spl.
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3+ VI
IV
The average spinel formula is (Mg 0.79 Fe2+
0.21 ) (Al1.54 Cr0.42 Fe0.04 ) O4 ; the spinel in this
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dunite is more than 50% poorer in chromium than spinels from the other two peridotite
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xenoliths in this study. Most of the iron is divalent and in the tetrahedral site (Fe3+/∑Fe =
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0.16 ± 0.02). Olivine, Cpx, and Opx have Fe3+/∑Fe of 0.03 ± 0.08, 0.33 ± 0.55, and 0.10
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± 0.04, respectively. BSE images of this xenolith also show some alteration by the host
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basaltic magma (Supp. Fig. 1c), similar to that seen in the harzburgite.
Two pyroxene-rich xenoliths were analyzed for this study: a clinopyroxenite (SC-
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1-66), and an olivine websterite (SC-1-70). The clinopyroxenite contains 95% Cpx, 5%
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Opx, <1% Spl, and no Ol. The average spinel composition in this rock is
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3+ VI
IV
(Mg 0.72 Fe2+
0.28 ) (Al1.44 Cr0.50 Fe0.04 ) O4 . These spinels are similar in chromium content
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to those in the dunite. Clinopyroxene and Opx in this sample have Fe3+/∑Fe of 0.12 ±
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0.07 and 0.09 ± 0.03, respectively. The websterite is composed of 58% Opx, 32% Ol,
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10% Cpx, and <1% Spl, and the spinels have a calculated average mineral formula of
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3+ VI
IV
(Mg 0.78 Fe2+
0.22 ) (Al1.91 Cr0.06 Fe0.03 ) O4 . Unlike all of the other xenoliths, the spinels in
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this websterite have very little chromium. As in every peridotite xenolith, most of the
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iron in the spinel structure from the clinopyroxenite and the websterite exists as
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tetrahedrally coordinated Fe2+. Olivine, Cpx, and Opx in this websterite have Fe3+/∑Fe
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of 0.08 ± 0.04, 0.29 ± 0.05, and 0.08 ± 0.04, respectively. BSE images of both
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pyroxenite xenoliths show alteration along grain boundaries, which may be textural
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indicators of metasomatism (Supp. Fig. 1d,e).
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2. Methods
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2.1. Electron probe analyses and mineral formulae recalculation
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Major element analyses were carried out on thin sections of each xenolith with a
JEOL Superprobe microanalyzer with five spectrometers at UCLA. We used an
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accelerating voltage of 15 kv, a 15 nA current, 20 seconds counting time, and a focused
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beam. Mineral Fe3+/∑Fe values were calculated from electron microprobe analyses
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following Droop (1987). Uncertainties on the Fe3+/∑Fe ratios were calculated using
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Monte Carlo techniques and the 1 sigma uncertainties associated with the oxides in each
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average mineral composition -- note that variations in SiO2 and FeO* have the largest
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effect on calculated Fe3+ values (Canil and O’Neill, 1996). The reported Fe3+/∑Fe
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errors (Table 1) are 2 standard deviations and are based on 100000 iterations per
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calculation.
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2.2. Column Chemistry
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We used pre-filled Poly-Prep® columns measuring 4 cm x 0.8 cm with 10 ml
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reservoirs for the two purification steps. These columns contain 0.3 ml (wet) of Bio-
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Rad™ AG 1-X8 analytical grade resin in 200 to 400 mesh chloride form. Columns were
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washed initially 3 times with 10 ml of 7N HCl alternating with ~18 MΩ cm2/cm water
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and conditioned with 0.5N HCl and 7N HCl. A typical load on the column consists of
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between 10 and 50 g of Fe in 300 l of 7N HCl. Matrix elements, including Ca, Mg,
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Al, and Cr, are eluted by passing 9 ml of 7N HCl through the column, leaving Fe adhered
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to the resin. Iron is then recovered by passing 1.5 ml of 0.5N HCl through the column.
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The pure iron eluent is subsequently evaporated on a 120 °C hotplate until just dry, then
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immediately picked up in 1 ml 2% HNO3 for mass spectrometry. Columns are used only
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once, then discarded.
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Elution curves were determined from synthetic dissolved rock solutions
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containing various concentrations of Al, Mg, Ca, Fe, Cr, Mn, Mg and Ti. These rock and
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mineral “analogues” were made by mixing varying amounts of Spex Certi-Prep standard
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solutions to simulate the differing proportions of each element in typical Ol, Cpx, Opx,
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and Spl from San Carlos xenoliths as determined by EMPA analyses of the xenoliths
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from this study. The most reliable indicator of complete recovery of Fe in the presence
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of matrix elements on the columns was the absence of measurable shifts in 56Fe/54Fe and
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57
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during this study. In other words, every time we carried out the column chemistry
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procedure, one or more of the columns was loaded with a mineral analogue solution
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instead of an actual dissolved mineral in order to test the column chemistry procedure for
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the particular composition(s) of the mineral being purified. These “column tests” allow
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for a high degree of confidence with regard to the isotope-ratio fidelity of our column
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chemistry procedure.
Fe/54Fe following Fe recovery. These “zero enrichments” were checked routinely
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Tests show that even extremely small amounts of Cr will cause interferences and
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matrix effects, resulting in erroneously low 57Fe and 56Fe values. This problem was
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solved by repeating the column chemistry procedure with the Fe + Cr eluents. Repeating
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the Fe column chemistry procedure for a single sample results in complete recovery of Fe
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with no measurable shifts in 56Fe/54Fe and 57Fe/54Fe and elimination of Cr. Therefore, we
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repeated the column separation procedure for spinel and whole rock samples in this
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study.
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References
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Canil D. and O'Neill, H. S. C. (1996) Distribution of ferric iron in some upper-mantle
assemblages. Journal of Petrology 37(3), 609-635.
Droop G. T. R. (1987) A general equation for estimating Fe3+ concentrations in
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ferromagnesian silicates and oxides from microprobe analyses, using
15
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Frey F. A. and Prinz M. (1978) Ultramafic inclusions from San Carlos, Arizona:
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Galer S. J. G. and O’Nions R. K. (1988) Chemical and isotopic studies of ultramafic
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