Graham D. Layne

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.

Sulfur isotopic systematics in alteration assemblages in martian meteorite Allan Hills 84001

Abstract ALH84001 is a coarse-grained, clastic orthopyroxenite meteorite related to the SNC meteorite group ( s hergottites, n akhlites, C hassigny). Superimposed upon the orthopyroxene-dominant igneous mineral assemblage is a hydrothermal signature. This hydrothermal overprint consists of carbonate assemblages occurring in spheroidal aggregates and fine-grained carbonate-sulfide vug-filling. The sulfide in this assemblage has been identified as pyrite, an unusual sulfide in meteorites. Previously, Burgess et al. (1989) reported a bulk δ 34 S for a SNC group meteorite (Shergotty) of −0.5 ± 1.5‰. Here, we report the first martian δ 34 S values from individual sulfide grains. Using newly developed ion microprobe techniques, we were able to determine δ 34 S of the pyrite in ALH84001 with a 1 a precision of better than ±0.5‰. The δ 34 S values for the pyrite range from +4.8 to +7.8‰. Within the stated uncertainties, the pyrite from ALH84001 exhibits a real variability in δ 34 S in this alteration assemblage. In addition, these sulfides are demonstrably enriched in 34 S relative to Canon Diablo troilite and sulfides from most other meteorites. This signature implies that the planetary body represented by ALH84001 experienced processes capable of fractionating sulphur isotopes and that hydrothermal conditions changed during pyrite precipitation (T, pH, fluid composition, etc.). The fractionated signature of the sulphur in the pyrite is most likely attributed to either conditions of pyrite precipitation (low temperature, reduced (low f o 2 ) and moderately alkaline (pH > 8) environment) or enrichment of fluids in 34 S by surface processes (weathering or impact processes) prior to precipitation. These new data are not consistent with the pyrite recording either biogenic activity or atmospheric fractionation of sulphur through nonthermal escape mechanisms or oxidation processes. This study also demonstrates the usefulness of ion microprobe measurements of sulphur isotopes in constraining conditions on other planetary bodies.

SIMS analyses of minor and trace element distributions in fracture calcite from Yucca Mountain, Nevada, USA

Abstract Fracture-lining calcite samples from Yucca Mountain, Nevada, obtained as part of the extensive vertical sampling in studies of this site as a potential high-level waste repository, have been characterized according to microbeam-scale (25–30 μm) trace and minor element chemistry, and cathodoluminescent zonation patterns. As bulk chemical analyses are limited in spatial resolution and are subject to contamination by intergrown phases, a technique for analysis by secondary ion mass spectrometry (SIMS) of minor (Mn, Fe, Sr) and trace (REE) elements in calcite was developed and applied to eighteen calcite samples from four boreholes and one trench. SIMS analyses of REE in calcite and dolomite have been shown to be quantitative to abundances Bulk chemical signatures noted by Vaniman (1994) allowed correlation of minor and trace element signatures in Yucca Mountain calcite with location of calcite precipitation (saturated vs. unsaturated zone). For example, upper unsaturated zone calcite exhibits pronounced negative Ce and Eu anomalies not observed in calcite collected below in the deep unsaturated zone. These chemical distinctions served as fingerprints which were applied to growth zones in order to examine temporal changes in calcite crystallization histories; analyses of such fine-scale zonal variations are unattainable using bulk analytical techniques. In addition, LREE (particularly Ce) scavenging of calcite-precipitating solutions by manganese oxide phases is discussed as the mechanism for Ce-depletion in unsaturated zone calcite.

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:

DECIPHERING BASALTIC MAGMATISM ON THE MOON FROM THE COMPOSITIONAL VARIATIONS IN THE APOLLO 15 VERY LOW-TI PICRITIC MAGMAS

Picritic glass beads of volcanic origin have been interpreted as representing extremely primitive lunar magmas that approach primary magma compositions. Based on this, they have been used for developing models for lunar magmatism, estimating the mineralogical and chemical characteristics of their mantle sources, constraining the dynamics and structure of the lunar mantle, developing models for lunar core formation, reconstructing the bulk composition of the Moon, and evaluating the genetic relationships between the Earth and the Moon. Yet, we still do not truly understand many aspects of the compositional diversity among individual glass types or even within a single glass type. Using high precision ion microprobe techniques, we have analyzed a total of forty very low-Ti glass beads from the Apollo 15 (A15) site and twenty additional picritic glass beads from other sampling sites for a suite of highly compatible (Ni, Co), weakly compatible to weakly incompatible (Mn, Cr, V), and highly incompatible (Ce, Ti, Zr, Nb) elements. Within the A15 very low-Ti glass population, Ni and Co concentrations are positively correlated, exhibit limited fractionation (of Ni/Co), and are distinctive for individual glass groups as defined by major elements. Manganese is also colinear with both Ni and Co. The highly incompatible elements also appear to be generally colinear with Ni, Co, and Mn, although there are distinguishable compositional “offsets” in the glass populations. The Ni/Co ratio does not differ dramatically among the very low-Ti glasses from other sampling sites, although incompatible element concentrations may differ significantly. These trace element data eliminate fractional crystallization and partial melting of a homogeneous source (under extremely low fOstaggered2 conditions) as possible models for the origin of the compositional variations observed in the A15 very low-Ti glass population. These data are consistent with the generation of these magmas through partial melting of a spatially restricted, heterogeneous, lunar magma ocean (LMO) cumulate sequence. The compositional variation in these magmas can be produced by at least two different melting processes: (1) Partial melting of a cumulate sequence consisting of relatively early cumulates with a higher proportion of an “intercumulus melt component” (Green glass A, D, E) and later cumulates with lower proportions of an “intercumulus melt component” (Green glass C). (2) Different degrees of partial melting of a cumulate sequence. Small degrees of partial melting (less than 5%) of a more primitive cumulate mantle source will generate one group of magmas (Green A, D, E), whereas a group of other picritic magmas (Green C) were produced by larger degrees of partial melting (10 to 20%) of a mineralogically similar, but a more evolved cumulate mantle source. In both models, the Green B glass group is formed by either cumulate commingling or magma mixing processes. These magmas cannot be rapidly transported from the source region to the lunar surface in a single eruptive event without extensive fractional crystallization. Yet, the colinear relationship among Ni, Co, and Mn is not compatible with extensive and variable fractional crystallization. Therefore, a polybaric melting model as suggested by Longhi (1992b) appears to be the only alternative to producing relatively unfractionated, Apollo 15 very low-Ti picritic magmas with high-pressure signatures. The trace element data provides additional constrains for this type of model.

The role of ilmenite in the source region for mare basalts: Evidence from niobium, zirconium, and cerium in picritic glasses

Abstract To investigate models for the generation of lunar high Ti-basalts, we have analyzed lunar picritic glasses for Zr, Nb, Ce, and Ti at high precision using ion microprobe techniques. The picritic magmas represented by these glasses have experienced minor crystallization, which has allowed us to partially eliminate the effects of post-melting processes commonly experienced by crystalline high-Ti mare basalts. The Nb Zr for these glasses ranges from .05-.11. The high-Ti glasses generally tend to have higher values of Nb Zr (.072-.109) than the very low-Ti glasses (.048-.085). The crystalline mare basalts tend to have slightly higher Nb Zr than glasses with similar Ti from the same site. For example, the Apollo 17 (A17) high-Ti basalts have Nb Zr of approximately .09. whereas, the A17 high-Ti glasses have Nb Zr of .07. KREEP has Nb Zr of approximately .06. Thus Zr is fractionated from Nb to different degrees in the various picritic magmas. The concentrations of Zr, Nb, and Ce increase from the very low-Ti glasses to the high-Ti glasses, and along the trajectory the Nb Ce and Zr Ce increase. Nb Ce (.25-1.6) and Zr Ce (4–15) for the picritic glasses overlap with KREEP ( Nb Ce = .36 and Zr Ce =5 ). Zr Ti and Nb Ti show a wide range of variation in these glasses. Both Zr Ti and Nb Ti in the glasses range from approximately .0014 to slightly less than .0003. The Zr Ti and Nb Ti for these glasses overlap with that of the crystalline mare basalts. Generally, with increasing Ti, Nb, and Zr, Zr Ti and Nb Ti decrease. The exceptions to this are the Apollo 14 (A14) glasses that exhibit an increase in Nb Ti and Zr Ti . Based on these data for the picritic glasses and experimentally determined partition coefficients for Nb, Zr, and Ce, the mantle sources for these picritic magmas are slightly to moderately fractionated from C1 chondrite and previous estimates of the bulk silicate Moon. Our best fit model for our data and this observation is that both the very low-Ti and high-Ti picritic magmas were derived through small to moderate degrees of nonmodal melting of lunar mantle sources consisting of a mixture of late-stage LMO cumulates (derived after >95% crystallization of the LMO) and early to intermediate LMO cumulates (derived prior to 80% crystallization of the LMO). The early LMO cumulates had Nb Zr , Zr Ce , and Nb Ce ratios near C1 chondrite, whereas these ratios were fractionated in the late-stage LMO cumulates. This hybridization of mantle sources occurred during large scale overturning of the LMO cumulate pile. The source for the low-Ti picritic magmas had very minor amounts of ilmenite, whereas the source for the high-Ti picritic magmas probably contained less than 6% ilmenite. For all the picritic magmas, ilmenite was exhausted from the residua during melting. Models suggesting that the high-Ti magmas are derived through the assimilation of an ilmenite-bearing cumulate layer or preferential assimilation of ilmenite by low-Ti primary magmas are not consistent with the Nb, Zr, Ce, and Ti data magmas (Hubbard and Minear, 1975; Wagner and Grove, 1993, 1995). In particular, the preferential assimilation by very low-Ti picritic magmas of ilmenite with expected Nb Ce (20,000–22,000) and Nb Zr (55) signatures would displace the resulting high-Ti magma too far from our observed data. Large scale overturning of the LMO cumulate pile also accounts for the trace element signatures found in the A14 picritic glasses. The evolved signature found in these primitive very low-Ti picritic glasses is most likely a product of KREEP incorporation into the LMO cumulate source rather than either contamination by evolved ilmenite-bearing cumulates or incorporation of higher proportions of locally derived intercumulus melt.

The systematics of light lithophile elements (Li, Be and B) in lunar picritic glasses: Implications for basaltic magmatism on the Moon and the origin of the Moon

Lunar picrites, represented by high-Mg volcanic glasses, are thought to be products of either partial melting of the deep lunar mantle followed by rapid ascent or polybaric partial melting initiated in the deep lunar mantle. The near primary compositions of these volcanic glasses provide us with a unique perspective for evaluating basaltic magmatism, the characteristics and evolution of the lunar mantle, and the origin of the Moon. The light lithophile elements (LLE = Li, Be, B) in planetary materials have been used to estimate planetary compositions and evaluate magmatic processes. Ion microprobe analyses of these glasses for LLEs were conducted using a Cameca 4f ion microprobe. This suite of glass beads ranged in TiO2 from 0.3 to 17 wt%. Seventy-one individual glass beads were analyzed for the LLEs. In addition, core-rim analyses of individual glass beads were made. The LLEs show a wide range of variability with Li ranging from 1.2 to 23.8 ppm, Be ranging from 0.06 to 3.09 ppm and B ranging from 0.11 to 3.87 ppm. B/Be ranges from 0.40 to 4.6. Li/Be ranges from 2.7 to 41.7, although 90% of the Li/Be values range from 14 to 30. Both B/Be and Li/Be values for the picritic glasses are less than chondrite. Be/Nd for the glasses ranges from .04 to .06 and are similar to chondrite (.058). Traverses across individual beads indicate that they are generally homogeneous with regards to LLEs regardless of TiO2 content. The individual glass groups show limited variations in LLE characteristics. The exceptions to this observation are the A17 VLT and the A15 yellow glasses. At individual Apollo sampling sites, the LLE content is generally correlated to TiO2. The high-Ti glasses are displaced toward higher Li at similar B and Be relative to the very low-Ti glasses. LLE concentrations also parallel the enrichments of other lithophile elements such as Ba, Zr, Sr and REEs. As noted for other trace element characteristics, glasses from each sampling site have similar LLE signatures. For example, the Apollo 14 glasses generally have higher LLE concentrations relative to glasses of similar TiO2 content from other sites. The LLE data support mantle inhomogeneity and Lunar Magma Ocean (LMO) cumulate overturn models suggested by previous studies. A KREEP component had been incorporated into some of these picritic glasses. This is consistent with other trace elements and probably reflects the recycling of KREEP and/or other late stage LMO cumulates into the deep lunar mantle. The picritic glasses are compositionally distinct from the crystalline mare basalts in LLEs. They are not related by either fractional crystallization or partial melting processes. This suggests that they were derived from distinctively different mantle sources. Estimates of the bulk compositions of the Earth and the Moon have previously been made based on the assumption that the ratio of Li to Be is a direct measure of the ratio of the high temperature condensates (HTC, refractory components) to the Mg-silicates (less refractory components) in a planet. We assert that if Li/Be is to be used to estimate bulk Moon composition, the picritic glasses provide fewer pitfalls and a better estimate than the crystalline mare basalts. Differences in partition coefficients (D) for Li and Be indicate that fractional crystallization and partial melting will modify the LiBe ratio. Estimates based on the picritic glasses infer a higher Li/Be for the bulk Moon than estimated from the mare basalts. This would indicate that the bulk Moon is less refractory than previously calculated by Li/Be and approaches the bulk composition of the Earth.

Subsolidus REE partitioning between pyroxene and plagioclase in cumulate eucrites: An ion microprobe investigation

Abstract In an attempt to elucidate the relationship between the cumulate eucrites and the noncumulate (main series) eucrites, we have examined the trace-element systematics of plagioclase grains that coexist with inverted pigeonites of the cumulate eucrites. Analyses were done by secondary ion mass spectrometry (SIMS). Plagioclase compositions are uniform in trace-element abundances within grains from each cumulate eucrite, but are variable between the four analyzed: Moama, Moore County, Serra de Mage, and Binda. Shapes of CI-normalized trace-element abundance patterns of the plagioclase grains of the cumulates are typical of plagioclase, showing a LREE-enriched pattern with positive Eu anomalies reflecting the site preference of Eu 2+ in the feldspar structure. Using trace-element concentrations of the pyroxenes (orthopyroxene, pigeonite, augite; Pun and Papike, 1995) and plagioclase (this study), we calculate the subsolidus partition coefficients (Ds) for the REEs and Y between plagioclase and pyroxenes in the examined cumulate eucrites. Trace-element systematics suggest that igneous trace-element signatures have been altered by subsolidus exchange for both the plagioclase and pyroxene. Calculated hypothetical parental-melt patterns determined from pyroxene and plagioclase are different and can differ by a factor of 4–5 for individual REEs.

The behavior of light lithophile and halogen elements in felsic magma: geochemistry of the post-caldera Valles Rhyolites, Jemez Mountains Volcanic Field, New Mexico

Abstract Post-collapse rhyolite lava domes, lava flows and pyroclastic rocks from Valles caldera (1140 ka), erupted from 1133 ka to approximately 520-60 ka, have been sampled to study variations of light lithophile (Li, Be, B) and halogen (F, Cl) elements. Our principal objectives were: (1) to examine the mobility of these elements during post-eruptive devitrification and hydration; and (2) to study their behavior during magma differentiation. Compared to fresh glassy samples, devitrified rocks from the same dome are depleted in B, Li, F and Cl, but not in Be. During devitrification, Be was immobile while the other elements were progressively more mobile in the order B