Gordon McKay

Clinopyroxene REE distribution coefficients for shergottites: The REE content of the Shergotty melt☆

Abstract REE distribution coefficients were measured between synthetic pyroxenes and melts similar in composition to the inferred Shergotty intercumulus liquid, by in situ analysis with the electron microprobe, using samples doped to per cent concentration levels. These coefficients exhibit a strong positive correlation with pyroxene wollastonite content. Recommended values for augites similar in composition to the Shergotty cumulus augite cores (Wo32En48) are D(La) = 0.023, D(Ce) = 0.039, D(Nd) = 0.10, D(Sm) = 0.17, D(Eu) = 0.16, D(Gd) = 0.20, D(Yb) = 0.29, and D(Lu) = 0.30. Recommended values for pigeonites similar in composition to the Shergotty cumulus pigeonite cores (Wo12En58) are D(La) = 0.002, D(Ce) = 0.004, D(Nd) = 0.019, D(Sm) = 0.031, D(Yb) = 0.13, and D(Lu) = 0.13. These values resemble distribution coefficients measured for terrestrial basalts of higher Mg Fe than the Shergotty melt, rather than those of more evolved siliceous rocks of similar Mg Fe to the Shergotty melt. Thus it appears that Mg Fe is considerably less important than other factors in determining distribution coefficient values for pyroxenes. REE abundances were computed for the Shergotty intercumulus melt using distribution coefficients measured for the natural phase compositions. These computed abundances display LREE depletion. Such LREE-depleted abundance patterns cannot be generated by simple partial melting of a mafic mineral assemblage having the LREE-enriched patterns required for a two-stage Sm-Nd evolution history. This result implies that either (1) the melting process was complex (e.g., continuous melting), (2) the source region had a more complex history than that of the simple 2-stage model, or (3) Shergotty does not satisfy the closed-system assumption of the calculation, and has suffered post-magmatic LREE-enrichment

Crystal/liquid partitioning of REE in basaltic systems Extreme fractionation of REE in olivine

Partition coefficients for Nd, Sm, Gd, and Yb between olivine and synthetic basaltic melts were determined using the percent level doping technique and special microprobe analytical techniques for REE in olivine. These techniques permit much more accurate background measurement and thus greater sensitivity than conventional techniques. Spurious results were obtained if REE contents were measured in olivine at locations closer than about 100 micrometers to crystal margins, because of detection of REE x-rays generated in surrounding glass. For results that avoid this “edge” effect, average olivine/liquid distribution coefficients are Nd, 0.00007 ± 3; Sm, 0.00058 ± 7; Gd, 0.00102 ± 8; and Yb, 0.0194 ± 3 for synthetic lunar mare basaltic melts; and Sm, 0.00079 ± 9; Gd, 0.00144 ± 13; Yb, 0.027 ± 3 for synthetic lunar highlands basaltic melts. These values for the light REE are much lower than those of previous experimental or phenocryst/matrix studies. The higher LREE values, and consequently flatter patterns obtained in earlier experimental studies probably resulted from measurement of spuriously high LREE contents in olivines, because of small crystal size. Flatter patterns from phenocryst/matrix studies probably result from small amounts of contamination in olivine mineral separates. The steep slope of the new olivine/liquid pattern is consistent with olivine/clinopyroxene patterns previously obtained from inclusion-free olivines separated from peridotite nodules.

A simple inorganic process for formation of carbonates, magnetite, and sulfides in Martian meteorite ALH84001

Abstract We show experimental evidence that the zoned Mg-Fe-Ca carbonates, magnetite, and Fe sulfides in Martian meteorite ALH84001 may have formed by simple, inorganic processes. Chemically zoned carbonate globules and Fe sulfides were rapidly precipitated under low-temperature (150 °C), hydrothermal, and non-equilibrium conditions from multiple fluxes of Ca-Mg-Fe-CO2-S-H2O solutions that have different compositions. Chemically pure, single-domain, defect-free magnetite crystals were formed by subsequent decomposition of previously precipitated Fe-rich carbonates by brief heating to 470 °C. The sequence of hydrothermal precipitation of carbonates from flowing CO2-rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules, magnetite, and Fe sulfides in ALH84001. In separate experiments, kinetically controlled 13C enrichment was observed in synthetic carbonates that is similar in magnitude to the 13C enrichment in ALH84001 carbonates.

Experimental partitioning of rare earth elements and strontium: Constraints on petrogenesis and redox conditions during crystallization of Antarctic angrite Lewis Cliff 86010

The partitioning of REE and Sr among anorthite, fassaitic pyroxene, and synthetic melts similar in bulk composition to angrite LEW 86010 was studied experimentally at 1 atm and 1175–1210°C using the percent level doping technique. Most experiments were at an oxygen fugacity 1 log unit above the iron-wustite buffer, but Eu and Gd partitioning were studied from iron-wustite to just above quartz-fayalite-magnetite. Pyroxene partition coefficients are correlated with Al content of the pyroxene. Despite the fassaitic nature of the synthetic pyroxenes, partition coefficients are not dramatically different from those for diopside. Pyroxene/melt REE partition coefficients range from .08 for La to .45 for Yb. Plagioclase/ melt coefficients (except for Eu) range from .022 for La to .004 for Yb. DEu varies by nearly a factor of 2 for pyroxene and nearly a factor of 5 for plagioclase over the four log unit range fO2 studied. Parent melts calculated by inverting natural pyroxene and anorthite cores from LEW 86010 using partition coefficients from this study are in excellent agreement with one another. This agreement is strong evidence for (1) equilibrium between the natural mineral cores at the time the meteorite crystallized, and (2) lack of subsequent subsolidus diffusive modification of REE abundances in the cores. The overall levels of the computed REE patterns agree well with REE abundances in bulk samples of LEW 86010, supporting the idea that this sample formed through a process approximating closed-system fractional crystallization. The observed variation of DEu/DGd with fO2 for pyroxene and plagioclase was combined with Eu and Gd abundances from mineral cores in LEW 86010 to estimate the oxygen fugacity under which this sample crystallized. Results indicate crystallization at about 1 log unit above iron-wustite, considerably more oxidizing than conditions under which common basaltic achondrites such as eucrites are thought to have formed.

Terminology for trace-element partitioning

A self-consistent terminology for partitioning data is presented. Ratios of the concentration of a component in two phases are termed partition coefficients and given the symbol D. Ratios of partition coefficients are termed exchange coefficients and given the symbol KD. The prefix “bulk” implies that these coefficients are weighted according to the proportions of coexisting phases. Bulk partition and bulk exchange coefficients are denoted by D and KD, respectively.

Evidence for exclusively inorganic formation of magnetite in Martian meteorite ALH84001

Abstract Magnetite crystals produced by terrestrial magnetotactic bacterium MV-1 are elongated on a [111] crystallographic axis, in a so-called “truncated hexa-octahedral” shape. This morphology has been proposed to constitute a biomarker (i.e., formed only in biogenic processes). A subpopulation of magnetite crystals associated with carbonate globules in Martian meteorite ALH84001 is reported to have this morphology, and the observation has been taken as evidence for biological activity on Mars. In this study, we present evidence for the exclusively inorganic origin of [111]-elongated magnetite crystals in ALH84001. We report three-dimensional (3-D) morphologies for ~1000 magnetite crystals extracted from: (1) thermal decomposition products of Fe-rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments; (2) carbonate globules in Martian meteorite ALH84001; and (3) cells of magnetotactic bacterial strain MV-1. The 3-D morphologies were derived by fitting 3-D shape models to two-dimensional bright-field transmission-electron microscope (TEM) images obtained at a series of viewing angles. The view down the {110} axes closest to the [111] elongation axis of magnetite crystals ([111]⋅{110} ≠ 0) provides a 2-D projection that uniquely discriminates among the three [111]-elongated magnetite morphologies found in these samples: [111]-elongated truncated hexaoctahedron ([111]-THO), [111]-elongated cubo-octahedron ([111]-ECO), and [111]-elongated simple octahedron ([111]-ESO). All [111]-elongated morphologies are present in the three types of sample, but in different proportions. In the ALH84001 Martian meteorite and in our inorganic laboratory products, the most common [111]-elongated magnetite crystal morphology is [111]-ECO. In contrast, the most common morphology for magnetotactic bacterial strain MV-1 is [111]-THO. These results show that: (1) the morphology of [111]-elongated magnetite crystals associated with the carbonate globules in Martian meteorite ALH84001 is replicated by an inorganic process; and (2) the most common crystal morphology for biogenic (MV-1) magnetite is distinctly different from that in both ALH84001 and our inorganic laboratory products. Therefore, [111]-elongated magnetite crystals in ALH84001 do not constitute, as previously claimed, a “robust biosignature” and, in fact, an exclusively inorganic origin for the magnetite is fully consistent with our results. Furthermore, the inorganic synthesis method, i.e., the thermal decomposition of hydrothermally precipitated Fe-rich carbonate, is a process analogue for formation of the magnetite on Mars. Namely, precipitation of carbonate globules from carbonate-rich hydrothermal solutions followed at some later time by a thermal pulse, perhaps in association with meteoritic impact or volcanic processes on the Martian surface

Rare earth element carriers in the Shergotty meteorite and implications for its chronology

Ion probe measurements of the rare earth element (REE) concentrations of individual grains of the Shergotty meteorite are reported. Phases analyzed include whitlockite, apatite, baddeleyite, augite, pigeonite, maskelynite and K-rich glass. U concentrations of whitlockite and apatite crystals were also obtained. The whole rock REE pattern is dominated by whitlockite, which contains over 95% of the light rare earth elements (LREE). REE concentrations in apatite are much lower than estimated by Laulet al. (1986). All the whitlockites (whether intergrown with pyroxene, equant or interstitial) have the same relative abundances of LREE (i.e. patterns are almost flat from La to Sm). The observation, by Joneset al. (1985), of a skeletal whitlockite with LREE enrichment is not confirmed by analyses of the same grain. Pyroxene rims are not enriched in LREE. No leachable carrier, enriched in LREE and associated with pyroxene (Laulet al., 1986; Jagoutz and Wanke, 1986), has been found. Instead, either a laboratory contamination or a petrographically cryptic “phase” such as a film on grain boundaries is suspected as the carrier of LREE enrichments. If a grain boundary film carries the enrichments, it would not be resistant to the metamorphic resetting which has affected other isotopic reservoirs in this sample. Thus, there is no compelling reason to consider the Sm-Nd pyroxene/leachates line (Jagoutz and Wanke, 1986) as a 360 m.y. isochron. Estimates of REE abundances in the Shergotty intercumulus melt indicate that a complex petrogenesis is required, in agreement with the conclusions of McKayet al. (1986a). Pyroxene distribution coefficients measured experimentally (McKayet al., 1986a) are compared with estimates from measured REE abundances in augite and pigeonite. Evolution of REE abundances in the Shergotty late-stage interstitial melt, as inferred from analyses of whitlockite, conforms with trends predicted from partitioning considerations, and requires no special processes such as metasomatism. The average U concentrations of whitlockite and apatite are respectively 540 and 1,550 ppb. Although the calcium phosphates are enriched in U, they contain less than 20% of the U in Shergotty.

Re-evaluation of intercumulus liquid composition and oxidation state for the Shergotty meteorite

Abstract The Shergotty Martian meteorite contains pigeonite and augite with homogeneous magnesian cores, thought to be cumulus crystals. Using elemental maps of thin sections, we estimate the abundance of cumulus pyroxene cores at 13 vol%. This estimate is roughly half that determined from prior melting experiments (28%), and coincides with the observation of a similar discrepancy between the proportions of cumulus pyroxenes estimated from modal observations and melting experiments in the Zagami shergottite. An intercumulus liquid composition calculated using this new cumulus pyroxene estimate is virtually identical to that calculated for Zagami. Our results suggest the possibility that melting experiments may not have been carried out under conditions appropriate for crystallization of the shergottites. We consider two possible explanations: crystallization of pyroxene cores at depth or under different redox conditions. Prior arguments for polybaric crystallization of shergottites, based on pyroxene intergrowths and kaersutite inclusions, are inconclusive. Using the MELTS program, we have explored crystallization at higher pressure. A greater proportion of pyroxene crystallizes at higher pressures, rather than a smaller amount as necessary in this model. Even in calculations at 1 bar, pigeonite crystallizes before augite, and the temperature interval between the appearance of pigeonite and augite expands with pressure. Both pigeonite and augite appear to be in equilibrium with the same liquid composition and thus are thought to have co-crystallized in Shergotty, so it seems likely that MELTS may not correctly predict pyroxene crystallization behavior in this Fe-rich magma. Fe,Ti-oxide compositions in Shergotty were previously used as evidence that the magma crystallized at redox conditions near QFM. However, Ghosal et al. (1998) recently suggested that the oxides record subsolidus, reducing conditions of QFM-3 at 602°C, and our analyses support that conclusion, if a new solution model for oxides is used. They also proposed that Fe3+ in Shergotty pyroxenes indicates magmatic crystallization at approximately QFM-3, and our estimates of Fe3+ calculated from pyroxene stoichiometry in microprobe analyses match their reported values. However, we doubt that stoichiometric calculations provide accurate Fe3+ determinations. Other indicators suggest that magmatic conditions were oxidizing. The FeO∗ content of Shergotty maskelynite is greater and the negative Eu anomaly in pyroxene cores is shallower than in another basaltic shergottite, QUE94201, which is thought to have crystallized under reducing conditions near QFM-4. Also, the presence of silica, fayalite, and magnetite in mesostasis indicates oxiding conditions, at least at a late magmatic stage. Thus we argue that Shergotty crystallized under redox conditions near QFM.

Rare Earth Element Partition Coefficients from Enstatite/Melt Synthesis Experiments

Enstatite (En80Fs19Wo01) was synthesized from a hypersthene normative basaltic melt doped at the same time with La, Ce, Nd, Sm, Eu, Dy, Er, Yb, and Lu. The rare earth element concentrations were measured in both the basaltic glass by EMPA and the enstatite by SIMS. Rare earth element concentrations in the glass were determined by electron microprobe analysis with uncertainties less than two percent relative. Rare earth element concentrations in enstatite were determined by secondary ion mass spectrometry with uncertainties less than five percent relative. The resulting rare earth element partition signature for enstatite is similar to previous calculated and composite low-Ca pigeonite signatures, but is better defined and differs in several details. First, the enstatite LREE signature is steeper than for pigeonite. Second, the signature has a negative Eu anomaly which relates to oxygen fugacity and third, the dependence of the partition coefficients on alumina content is linked to crystal structural constraints.

Valence state partitioning of vanadium between olivine-liquid: Estimates of the oxygen fugacity of Y980459 and application to other olivine-phyric martian basalts

Abstract The valence state of vanadium (V2+, V3+, V4+, and V5+) is highly sensitive to variations in redox conditions of basaltic magmas. Differences in valence state will influence its partitioning behavior between minerals and basaltic liquid. Using partitioning behavior of V between olivine and basaltic liquid precisely calibrated for martian basalts, we determined the oxidation state of a primitive (olivine- rich, high Mg no.) martian basalt (Y980459) near its liquidus. The behavior of V in the olivine from other martian olivine-phyric basalts (SaU005, DaG476, and NWA1110) was documented. The combination of oxidation state and incompatible-element characteristics determined from early olivine indicates that correlations among geochemical characteristics such as fO₂, LREE/HREE, initial 87Sr/86Sr, and initial εNd observed in many martian basalts is also a fundamental characteristic of these primitive magmas. These observations are interpreted as indicating that the mantle sources for these magmas have a limited variation in fO₂ from IW to IW+1 and are incompatible-element depleted. Moreover, these mantle-derived magmas assimilated a more oxidizing (>IW+3), incompatible-element enriched, lower-crustal component as they ponded at the base of the martian crust.