Young D. Reese

The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics

Samarium–neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial eNd143 value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87Sr/86Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite. Combining the eNd142-eNd143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513+0.033−0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios (87Rb/86Sr = 0.037–0.374; 147Sm/144Nd = 0.182–0.285; 176Lu/177Hf = 0.028–0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of fO2, suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.

Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516

Abstract Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial eNd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial eNd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143Nd/144Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental. Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.

Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta ∼4.50 Ga ago

We performed petrologic, radiometric (Ar-Ar, Sm-Nd, and Mn-Cr ages), and ion microprobe studies of the basaltic eucrite, EET90020. This is one of the few rare basaltic eucrites whose 39Ar-40Ar age has not been reset during impact bombardment on the HED parent body ≤4 Ga ago and, thus, should provide a unique opportunity to study the nature of the early thermal events on its parent body (presumably asteroid 4Vesta). Hand specimen inspection shows that the rock consists of a fine-grained and a coarse-grained lithology. Microscopy indicates that the fine-grained lithology has a granulitic texture, with a coarser-grained area and a large opaque assemblage embedded in the granulitic matrix. The coarse-grained lithology has an igneous, subophitic texture. The rock has pyroxenes similar to those in type 5 eucrites (type 5 pyroxene) and experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. However, minor mineral assemblages are unusual and suggest a complex thermal history. Tridymite occurs as large laths, irregular crystals ( 1–5 GPa after the reheating event. EET90020 seems to have experienced the following thermal history; (1) crystallization during rapid cooling near the surface; (2) some brecciation by impact; (3) thermal metamorphism that produced type 5 pyroxene; and (4) short reheating that caused partial melting and rapid cooling. 39Ar-40Ar measurements show a relatively flat pattern and an age of 4.49±0.01 Ga, which is consistent with rapid cooling from high temperature (event 4). Resetting of the Sm-Nd ages at 4.51 ± 0.04 Ga appears to be closely related to the remelting of Ca-phosphates. Rb-Sr data suggest Rb-loss from tridymite during partial melting. The resetting of the Mn-Cr age may have been related to the formation of Cr-ulvospinels (event 4). We suggest that all these ages were reset by partial melting (event 4). We further suggest that the partial melting event (event 4) that reset the ages ∼4.50 Ga ago was caused by an impact into EET90020 which was part of the hot crust of 4Vesta and resulted in an increase in the temperature from the ambient temperature of ∼ 870°C to above the subsolidus temperature of eucrites of ∼1060°C.