J. M. Karner

Comparative planetary mineralogy: Valence state partitioning of Cr, Fe, Ti, and V among crystallographic sites in olivine, pyroxene, and spinel from planetary basalts

Abstract This is a comparative planetary mineralogy study emphasizing the valence-state partitioning of Cr, Fe, Ti, and V over crystallographic sites in olivine, pyroxene, and spinel from planetary basalts. The sites that accommodate these cations are the M2 site (6 to 8-coordinated) and M1 site (6-coordinated) in pyroxene, the M2 site (6- to 8-coordinated) and M1 (6-coordinated site) in olivine, and the tetrahedral and octahedral sites in spinel. The samples we studied are basalts from Earth, Moon, and Mars, and range in oxygen fugacity conditions from IW-2 (Moon) to IW+6 (Earth), with Mars somewhere in between (IW to IW+2). The significant elemental valence-states at these fO₂ conditions are (from low to high fO₂): Ti4+, V3+, Fe2+, Cr2+, Cr3+, V4+, and Fe3+. V2+ and Ti3+ play a minor role in the phases considered for the Moon, and are found in very low concentrations. V5+ plays a minor role in these phases in oxidized terrestrial basalts because it is probably lower in abundance than V4+, and has an ionic radius that is so small (0.054 nm, 6-coordinated), that it is almost at the lower limit for octahedral coordination, and can even be tetrahedrally coordinated. The role of Cr2+ in the Moon is significant, as Cr2+ predominates in basaltic melts at fO₂ less than IW-1. Lunar olivine has been found to contain mostly Cr2+, whereas coexisting pyroxene contains mostly Cr3+. Fe3+ is very important in Earth, less so in Mars, and nonexistent in the Moon. The importance of the Fe2+ to Fe3+ transition cannot be overstated and, indeed, their crystal-chemical differences, in terms of behavior (based on size and charge), are similar to the differences between Mg and Al. We note that for pyroxene in six of the seven terrestrial suites we studied, Fe3+ (in the M1 site) coupled with Al (in the tetrahedral site) is one of the two most important charge-balance substitutions. This substitution is of lesser importance in Mars and does not exist in lunar basalts.

Determination of planetary basalt parentage: A simple technique using the electron microprobe

Abstract Many basaltic meteorites are being discovered in old and new meteorite suites including those from cold- deserts (e.g., Antarctica) and hot-desert environments. It is important to establish the specific planetary body source. Proven techniques for establishing planetary parentage include stableisotopic signatures (especially oxygen), certain elemental ratios in bulk samples, and certain elemental ratios in specific minerals. Some of these techniques are expensive, require considerable sample preparation, and are adversely affected by weathering processes on the parent body or on Earth. We have been seeking key major and minor elemental ratios (in pyroxene, olivine, and feldspar) that can be measured by the electron microprobe on standard thin sections. These ratios may be preserved in unweathered portions of mineral grains and thus “see through” weathering processes. In addition, if the sample is too small to provide a representative bulk composition, it may still have key information recorded in individual minerals. We have found that some of the most useful chemical parameters are Fe/Mn (atomic) in olivine or pyroxene and the percent anorthite (%An) in plagioclase solid solutions. A plot of Fe/Mn in pyroxene and/or olivine verses %An defines compositional fields that are significantly different for Earth, Mars, Moon, 4 Vesta, and the angrite parent body. This method may be especially powerful when combined with oxygen isotope data.

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.

Olivine from planetary basalts: Chemical signatures that indicate planetary parentage and those that record igneous setting and process

Abstract The systematics of Mn-Fe, Ni-Co, Ti, Cr, and V in olivine from 13 basalt suites from the Earth, Moon, and Mars were studied by electron and ion microprobe techniques. The results demonstrate that chemical signatures in olivine can be related to: (1) planetary parentage, where differences are the result of initial accretional ratios, source compositions, and oxygen fugacity; and also (2) igneous setting and process, where differences among basalt suites within a planet are a consequence of specific redox conditions in tectonic settings, differing melt compositions, and changes in element partitioning resulting from crystallization sequences and mineral modes. Manganese-Fe systematics indicate planetary parentage where the Mn/Fe ratio in olivine increases with increasing distance from the Sun (with the exception of the Moon, which can be explained). This sequence could be the result of initial Mn/Fe accretional ratios from the start of the solar system. Igneous processes such as differing melt compositions and crystallization sequences cause differences in the Mn/Fe ratios of olivine in basalt suites from the same planet. Nickel-Co and Ti systematics show that planetary signatures result from source-region differences among the three planets. For example, the lunar source regions are depleted in Ni and enriched in Ti, as compared with the Earth and Mars, and these characteristics are reflected in the olivine compositions. The differing partitioning behavior of Ni, Co, and Ti in planetary olivine suites is a result of crystallization sequences and initial melt compositions of the basalts during crystallization. Chromium concentrations in olivine result from differing oxygen fugacities and phase stabilities in the source regions of the three planets, whereas V concentrations in olivine are mostly a consequence of the different overall redox conditions on planetary bodies. Both Cr and V show igneous process signatures owing to different melt compositions, crystallization sequences, and modal mineralogy. Perhaps the most important conclusion from this study is that olivine in planetary basalts records information not only about igneous setting and process but planetary parentage as well, making the study of comparative planetary mineralogy an exciting way to gain new insights into basalt petrogenesis.

Application of a new vanadium valence oxybarometer to basaltic glasses from the Earth, Moon, and Mars

Abstract The redox states of volcanic and impact melts from the Earth, Moon, and Mars have been estimated from the valence state of V in basaltic glasses (Sutton et al. 2005). The V valence has been determined using synchrotron micro X-ray absorption near-edge structure spectroscopy (XANES) (Sutton et al. 2005), which allows for in situ measurements on samples with a micrometer spatial resolution and ~100 ppm elemental sensitivity. Here, we interpret those results for the natural samples and compare them to the literature. The results show that terrestrial melts are dominated by V4+, lunar samples by V3+, with Martian melts a mixture of both V3+ and V4+. The fO₂ estimates derived from the V valence are consistent with those determined by other proven methods, whereby terrestrial basalts experience fO₂ conditions within 1 or 2 log units of the QFM buffer, lunar basalts equilibrate at 1 to 2 log units below the IW buffer, and Martian basalts fall somewhere between the QFM and IW buffer. The results illustrate the usefulness of this technique; i.e., a robust oxybarometer covering over six orders of magnitude, applicable to samples that record fO₂ conditions from reduced extraterrestrial bodies to the oxidized Earth.

Valence state partitioning of Cr and V between pyroxene-melt: Estimates of oxygen fugacity for martian basalt QUE 94201

Abstract Based on the partitioning of Cr and V between pigeonite cores and bulk composition, we estimate that martian basalt QUE 94201 crystallized at an fO₂ between IW+0.2 and IW+0.9. These estimates are based on calibration curves for DCr, DV, and DCr/DV (pyroxene/melt) derived from experimental charges that were synthesized at fO₂ conditions of IW-1, IW, and IW+1. We believe our fO₂ estimate is robust because (1) the fO₂ is measured in the earliest crystallizing pyroxenes; (2) the calibration curves are based on the same bulk composition as the natural sample; and (3) that bulk composition represents a melt from the martian mantle, so an accurate DCr and DV are measured. Presently, the two best candidates for martian melts, Y 980459 and QUE 94201, indicate an fO₂ of IW to IW+1 for the upper martian mantle.

Comparative planetary mineralogy: V/(Cr + Al) systematics in chromite as an indicator of relative oxygen fugacity

Abstract We have been developing oxygen barometers based largely on the behavior of V, which can occur in four valence states (V2+, V3+, V4+, and V5+), and record at least 8 orders of magnitude of fO₂. Our first efforts in measuring these valence proportions were by XANES techniques in basaltic glasses from Earth, Moon, and Mars. We now address the behavior of V valence states in chromite in basalts from these bodies with a technique that uses the electron microprobe. Our first insights into this new technique resulted from running electron probe traverses across spinel grains from core to rim on grains that show zoning from chromite to ulvöspinel. The zoning profiles showed the normal trends of core to rim decreases of Cr, Al, and Mg, and increases of Fe, Ti, and Mn. However, the behavior of V was very different for Moon and Earth, with Mars in between. In terrestrial basalts V4+ > V3+, in lunar basalts V3+ > V4+, and in martian basalts V3+ and V4+ are both significant. The trends (core to rim) for the Moon show a strong positive correlation of V and Cr and negative correlation of V and Ti. For the Earth, the trends are just the opposite, with a strong negative correlation for V and Cr and a strong positive correlation of V and Ti. Chromite in martian basalts showed trends somewhere in between. We found that a convenient way to display these data for chromite is a plot showing the relative V/(Cr + Al) ratios. These ratios nicely reflect the oxygen fugacity ranges for Moon, Mars, and Earth.

Metastable eutectic, gas to solid, condensation in the FeO–Fe2O3–SiO2 system

Non-equilibrium gas to solid condensation produced three distinct groupings of ferrosilica solids with compositions ∽15, 30 and 87.5 wt.% FeO (or, ∽17, 33 and 97 wt.% Fe2O3). These solid compositions define metastable eutectic points in the FeO/Fe2O3–SiO2 binary system. The position of a silica-rich metastable eutectic is sensitive to the ratio FeO:Fe2O3 during gas-to-solid quenching. The Fe-rich metastable eutectic indicates a two-liquid field in this revised pseudo-binary equilibrium phase diagram.

Gas-to-solid condensation in a Mg–SiO–H2–O2 vapor: metastable eutectics in the MgO–SiO2 phase diagram

Kinetically controlled gas-to-solid condensation in Mg–SiO–H2–O2 vapors resulted in the formation of amorphous, chemically ordered, solid compositions at 26 ± 7, 46.5 ± 2 and 87 ± 3.5 wt.% MgO. The first two solid compositions match those of metastable eutectics in existing MgO–SiO2 phase diagrams. An explanation for the highest-MgO metastable solid compositions requires a revision of this phase diagram to include a eutectic at ∼95 wt.% MgO.

Pyroxene europium valence oxybarometer: Effects of pyroxene composition, melt composition, and crystallization kinetics

Abstract The behavior of multivalent elements such as Fe, Cr, V, and Eu in magmatic systems reflects the fO₂ of the environment. In particular, Eu behavior in pyroxene from basaltic systems has been demonstrated to be an effective measure of fO₂. We selected two nearly isochemical lunar pigeonite basalts (15058, 15499), a lunar high-Ti basalt (75035), and Pasamonte (representing asteroid 4 Vesta, an unequilibrated eucrite), to explore other potential variables that may affect this indicator of fO₂. All of these basalts crystallized at an fO₂ of approximately IW-1, yet they experienced different cooling and crystallization histories and their pyroxenes exhibit a wide range of compositional trajectories within the pyroxene quadrilateral. There are several variables that influence the Eu/Eu* recorded in pyroxene that may compromise the determination of fO₂. Previous experimental studies show that pyroxene composition influences the ability of pyroxene to accommodate REE and fractionate Eu2+ from Eu3+. We demonstrate that in addition to the influence of Ca in the M2 site, the Al content in the pyroxene and its influence on coupled substitutions will also influence the fractionation of Eu2+ from Eu3+. For example, the coupled substitution TSi4+ + M2R2+ → TAl3+ + M2REE3+ may accommodate REE3+ in preference to Eu2+, which is too large. Different pyroxene growth surfaces will incorporate Eu2+ and Eu3+ differently due to differences in growth rate, Al content, and site configuration. In consort with the pyroxene composition, fractionation of Eu2+ from Eu3+ will be aided by the Al content of the basaltic melt, which increases the activities of network-forming components such as CaAl2O4 and FeAl2O4 in the melt during pyroxene crystallization. The Al content will result in changing the partitioning behavior of Eu3+ while having very little effect on Eu2+. Melt composition, the appearance of plagioclase on the liquidus, and the kinetics of plagioclase crystallization are influenced by cooling rate. Data from the four basalts selected also suggest that Eu2+/Eu3+ in the melt remains buffered even with extreme differences in cooling rate and plagioclase crystallization kinetics. Unexpectedly, many of these same variables affect the substitution of multivalent V. If the fO₂ determined from Eu behavior in pyroxene is not placed within a petrologic and crystal-chemical context, errors of 1 to 2 log units may result. The influence of these variables may be reduced by using multiple, co-crystallizing phases (i.e., plagioclase and pyroxene) and ratioing DEu to adjacent REE (DSm, Gd).