Gary Fowler

Diogenites as asteroidal cumulates: insights from orthopyroxene trace element chemistry

Eucrite, howardite, and diogenite members of the achondrite meteorites are considered by many to be genetically related. Therefore, each provides a piece of the puzzle for reconstructing magmatic processes on the eucrite parent body (EPB). The interpretation of the magmatic history of the diogenites (orthopyroxenites) is compromised to a great extent because the magmatic major element signature of orthopyroxene has been reset and some minor elements such as Al have been compromised by coupled substitution mechanisms. As a further test of the models for the origin of diogenites, we have analyzed a suite of twenty-one diogenites (≈160 individual analyses) for minor and trace elements using ion microprobe techniques. The concentrations of incompatible elements are low in the orthopyroxenes analyzed, while their variability in the orthopyroxenes is both extensive and consistent. The range of averages in Yb varies by a factor of 16 from Ellemeet to LEW 88679. Over this suite of diogenites, Zr varies by a factor of 117 and Y varies by a factor of 151. This variability exceeds the range noted by previous INAA studies of orthopyroxene separates. These incompatible trace elements exhibit a strong positive correlation with Ti. The consistent incompatible element variability among diogenites, limited textural evidence for subsolidus exsolution modification, and the expected slower diffusion rates of the REE, Ti, and Y relative to Fe-Mg indicate that the trace elements in the diogenitic orthopyroxene may reliably preserve the magmatic history of the diogenites. Based on the incompatible trace element systematics of Y and Yb, over 90% crystallization is necessary to explain the variation in concentrations from Peckelsheim (most depleted) to LEW 88679 (most enriched) assuming constant Ds. Over 70% crystallization of orthopyroxene is required if DY and DYb increase by a factor of three over the same suite of diogenites. Based on terrestrial analogs, it appears highly unlikely that a single basaltic magma will produce such a mono-mineralic orthopyroxene cumulate horizon with 70–90% crystallization of the parental melt. Two models that potentially explain this extensive incompatible element variability are: (1) the melts from which the diogenites formed are normative orthopyroxene enriched and normative plagioclase depleted or; (2) the suite of diogenites represent multiple basaltic melts with distinctly different incompatible element enrichments. Melt compositions that were back-calculated from the orthopyroxene data indicate that the diogenites crystallized from melts that had a wider range in incompatible elements than that exhibited by the main group eucrites. If the assumptions made in the calculation of these melts compositions are correct, this may be interpreted to mean that either many of the diogenites are not fractional crystallization products of eucritic melts or that the eucritic melts that were parental to the incompatible element enriched diogenites have not yet been sampled.

Ion microprobe investigation of plagioclase and orthopyroxene from lunar Mg-suite norites: Implications for calculating parental melt REE concentrations and for assessing postcrystallization REE redistribution

The lunar Mg-suite, which includes dunites, troctolites, and norites, makes up to 20–30% of the Moons crust down to a depth of ∼ 60 km. The remainder is largely anorthosite. This report focuses on norites (which consist mostly of orthopyroxene and plagioclase) because we have found that both phases are effective recorders of their parental melt compositions. In an earlier report, we analyzed orthopyroxene from twelve samples (three from Apollo 14, two from A-15, and seven from A-17) by SIMS for eight REE (La, Ce, Nd, Sm, Eu, Dy, Er, Yb). Inversion of these data to estimated melt compositions yielded extremely high REE concentrations similar to KREEP. In this study, we report SIMS REE data for plagioclase from these same twelve samples. The major objective of this study is to estimate parental REE concentrations from both orthopyroxene and plagioclase data to see if both data inversions produce concordant melt compositions and thus better constrain the composition of melts parental to Mg-suite norites. The estimated REE concentrations from both phases show some evidence of slight postcrystallization REE redistribution. Comparison of the observed ratio of REE for pyroxene/ plagioclase to the ratio of the Ds for pyroxene/plagioclase is consistent with REE redistribution which involves LREE diffusing from pyroxene into plagioclase and HREE diffusing from plagioclase into pyroxene. However, apparently these postcrystallization exchanges have not seriously affected our ability to estimate melt REE concentrations. The estimated melt REE concentrations using both plagioclase and orthopyroxene, are similar to high-K KREEP especially for melts parental to A-15 norites. This study provides support for mineral trace element inversion estimates for parental melts. Even though these lunar crustal lithologies experienced high temperature annealing, apparently the cores of plagioclase and pyroxene continue to carry a record of their igneous crystallization history.

Diogenites as asteroidal cumulates: Insights from orthopyroxene major and minor element chemistry

Abstract Diogenites appear to be cumulates formed from one or more igneous reservoirs in the interior of asteroid 4 Vesta. Magmatism in this parent body gave rise to a series of related lithologies designated howardites, eucrites, and diogenites or “HED.” Eucrites are pigeonite/plagioclase basalts, and diogenites are orthopyroxenites. Howardites are brecciated mixtures of eucrites and diogenites. The major objective of this paper is to characterize the major and minor element chemistry of orthopyroxene in diogenites and to interpret these data in terms of a petrogenetic model. This study, which involves interpretation of ~ 1,200 high-quality electron microprobe analyses, demonstrates that the systematics of Ca, Fe, and Mg in orthopyroxene were largely reset by reaction with melt and by subsolidus exchange. However, the systematics of the minor elements Al, Cr, and Ti still effectively record their igneous history. Aluminum and Ti are highly correlated with the incompatible trace elements (e.g. Yb, Y) and thus are useful fractionation indicators. As many as twenty of the twenty-three diogenites studied may have formed from one igneous system based on Al, Cr, and Ti systematics in orthopyroxene. The Al content of the orthopyroxene in the twenty diogenites forms the basis for a fractionation sequence with Peckelsheim the least fractionated and LEW 88008 the most fractionated. Roda and Manegaon, and possibly ALHA77256, fall off the main compositional trends and may belong to different melt systems. The coupled substitutions that incorporate Al, Cr, and Ti into the orthopyroxene crystal structure are VICr3+-IVAl3+, VIAl3+-VIAl3+, and VITi4+-2IVAl3+. The dominant substitutional couple in the early stages of crystallization is VICr3+-IVAl3+ while VIAl3+-IVAl3+ is dominant in the late stages. These findings demonstrate the importance of understanding the coupled substitutions for minor elements when assessing the appropriate mineral/melt partition coefficients (D values) or when discussing whether an element is compatible or incompatible. For example, in diogenitic orthopyroxene, Al has three different compatibilities (D values) with VICr3+-IVAl3+ more compatible than VIAl3+-IVAl3+ which is more compatible than VITi4+-2IVAl3+. Based on the assumption that D Al increases during crystallization from 0.05 to 0.1 corresponding to the crystallization interval represented by Peckelsheim to LEW 88008, 60% crystallization is required. The melt parental to LEW 88008 is estimated to contain ~12 wt% Al2O3, very near plagioclase saturation.

Evolution of the lunar crust: SIMS study of plagioclase from ferroan anorthosites

Abstract The lunar crust, down to a depth of ∼65 km, is composed of older (>4.5 Ga) ferroan anorthosites and younger (4.43-4.17 Ga) Mg-suite lithologies which include dunites, troctolites, and norites. The anorthosites are generally inferred to represent floating cumulates in a lunar magma ocean (possible depth 800 km, moons radius ∼ 1,738 km). The cumulates that are inferred to be located near the base of the magma ocean are dominantly olivine and pyroxene. The last dregs of the magma ocean are enriched in incompatible elements and have been named KREEP (K, rare earth elements, P). KREEP, formed in this manner, is probably concentrated near the crust/mantle boundary at ∼70 km depth. We are attempting to characterize melts parental to ferroan anorthosites and Mg-suite norites by analyzing REEs (La, Ce, Nd, Sm, Eu, Dy, Er, Yb) and Ba, Sr, and Y in their cumulus plagioclase. If the cumulus grains have not been compromised by postcrystallization effects and if we know the relevant mineral/melt partition coefficients (Ds) we can invert the trace element data for plagioclase to parental melt compositions. Melts parental to ferroan anorthosites are estimated to contain REE at concentrations ten to fifty times chondrites. Melts parental to the earlier crystallizing anorthosites (lower REE) have virtually no Eu anomaly, while melts parental to later crystallizing anorthosites (higher REE) have small negative Eu anomalies. This is qualitatively consistent with the fractionation of Eu relative to other REE by crystallization of plagioclase with large positive Eu anomalies. Melts parental to the Mg-suite have much higher total REE and very large negative Eu anomalies. The characteristics of Mg-suite norite parental magmas may have been derived by the assimilation of KREEP (near the crust/mantle boundary) by Mg-rich basaltic melts formed deeper in the cumulate pile or near the contact between the lower cumulate horizons of the magma ocean and primitive, unprocessed lunar mantle (∼800 km).

Systematics of Ni and Co in olivine from planetary melt systems; lunar mare basalts

Abstract The systematics of Co and Ni in olivine from six Apollo 12 olivine basalts were studied by SIMS techniques and correlated with major and minor element data obtained with the electron microprobe. Our results, together with previous studies, demonstrate that one of these basalts (12009) was extruded, as a liquid, onto the lunar surface and was parental to five other cumulates (12075, 12020, 12018, 12040, and 12035). The concentrations of Ni in zoned olivine phenocrysts behave as expected for a highly compatible element with initial estimated DNi = 9.9. On the other hand, Co concentrations in olivine vary hardly at all and show flat patterns across crystals that retain normal zoning trends for Mg, Fe, Mn, and Ni. The explanation for this behavior is not that DCo ≈ 1 (our estimated DCo = 4) or that Co zoning has been erased by rapid diffusion of Co compared to the other elements (e.g., Mg and Fe) that still show normal zoning. The explanation for the behavior, as originally suggested by Kohn et al. (1989), is that the increase in DCo with crystallization exactly balances the depletion of Co in the melt. This decoupling of the behavior of Ni and Co in olivine results in significant increases in Co/Ni with crystallization.

The Lodran primitive achondrite: petrogenetic insights from electron and ion microprobe analysis of olivine and orthopyroxene

The Lodran primitive achondrite is thought to represent some of the earliest events in the differentiation of chondritic asteroids. Most researchers who studied Lodran believe that it is a restite from which a melt, enriched in the incompatible trace and minor elements, was extracted. This process is reflected by a lack of feldspar in the Lodran mineral assemblage. Presumably, after or during melting, a reduction event took place resulting in an increase in Mg/Fe near the rims of both olivine and orthopyroxene. The reduction process is far more advanced in olivine than in pyroxene because of the more rapid FeMg diffusion rates in olivine. Narrow reaction rims (<10 μm) around orthopyroxene grains show a depletion in Ca, Al, Cr, Ti, REEs, and Y and an increase in Mg (reverse zoning). These systematics are largely the result of melting and to a lesser extent of reduction. Significant reaction of olivine has taken place by reduction and/or sulfidatuon and to a lesser extent by melting. This is reflected by a decrease in Fe, Co, and Ni and an increase in Mg towards the rims of olivine. The reactions by which reduction and/or sulfidation of olivine took place are not confirmed. However, the following represent two possible reactions:

SIMS studies of planetary cumulates; orthopyroxene from the Stillwater Complex, Montana

Abstract Igneous cumulate rocks provide an important record of planetary magmatism, but there are pitfalls in their interpretation. The rocks are composed of cumulus minerals plus assemblages that crystallized from trapped melt. Cumulus minerals may react with the trapped melt and other cumulus phases during subsolidus reactions, thus losing a direct record of their igneous history. One of the best approaches for estimating the melt compositions parental to the cumulates is to analyze the cores of cumulus phases for elements with slow diffusion rates (e.g., REE) because these most reliably retain a record of the mineral-melt partitioning. Many of the cumulus orthopyroxene grains from the Stillwater Complex analyzed in this study have augite lamellae in their interiors but lamellae-free rims. Secondary ion mass spectrometer (SIMS) analysis shows that these orthopyroxene rims have lower Sr, Y, Zr, and Ce concentrations relative to the cores. These systematics were apparently caused by migration of augite exsolution lamellae, which preferentially incorporate these trace elements, out of the rims of the orthopyroxene grains. SIMS analyses for REE in orthopyroxene cores show lower LREE concentrations, (Ce/ Yb)n and (Dy/Yb)n than the isotope dilution data on mineral separates (Lambert and Simmons, 1987). We believe that these differences result from one or more contaminating phases (e.g., augite and plagioclase) in the orthopyroxene mineral separates. The SIMS data yield calculated parental melts similar to sills found below the Stillwater Complex, which have previously been suggested as possible parental melts (Helz, 1985). A major focus of this study was to determine whether the onset of plagioclase crystallization at the bronzitite zone-norite zone contact was the result of fractionation until plagioclase saturation of the magmas parental to the bronzitites, or if mixing of a second magma with higher Al activity was the cause of plagioclase crystallization. The data presented in this paper, coupled with existing data, support the magma-mixing model.