Larry Taylor

CONSTRAINTS ON MARTIAN DIFFERENTIATION PROCESSES FROM RB-SR AND SM-ND ISOTOPIC ANALYSES OF THE BASALTIC SHERGOTTITE QUE 94201

Isotopic analyses of mineral, leachate, and whole rock fractions from the Martian shergottite meteorite QUE 94201 yield Rbue5f8Sr and Smue5f8Nd crystallization ages of 327 ± 12 and 327 ± 19 Ma, respectively. These ages are concordant, although the isochrons are defined by different fractions within the meteorite. Comparison of isotope dilution Sm and Nd data for the various QUE 94201 fractions with in situ ion microprobe data for QUE 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite. This component is likely to have been added to QUE 94201 by secondary alteration processes on Mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions. Initial 87Sr/86Sr of 0.701298 ± 14, ϵNd143 of +47.6 ± 1.7, and whole rock ϵNd142 of +0.92 ± 0.11 indicate that QUE 94201 was derived from a source that was strongly depleted in 87Rb/86Sr and enriched in 147Sm/144Nd early in its history. Modeling demonstrates that the Smue5f8Nd isotopic compositions of QUE 94201 can be produced by either four episodes of melting at 327 Ma of cumulates crystallized from a magma ocean at 4.525 Ga or five episodes of melting of an initially solid Mars at 4.525 Ga and 327 Ma. The neodymium isotopic systematics of QUE 94201 are not consistent with significant melting between 4.525 Ga and 327 Ma. The estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the QUE 94201 parental magma. Rb-Sr-based partial melting models are unable to reproduce the composition of QUE 94201 using the same model parameters employed in the Smue5f8Nd-based models, implying a decoupling of Rbue5f8Sr and Smue5f8Nd isotopic systems. The initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate Rb from Sr compared to Sm from Nd. The fact that all Martian meteorites analyzed so far define a Rbue5f8Sr whole rock isochron age of 4.5 Ga suggests that virtually all Rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time. Thus, the Martian mantle cumulates and restites are not expected to evolve past 87Sr86Sr of 0.700 and could not have been significantly enriched in incompatible elements by crustal recycling processes. All Martian meteorites have initial 87Sr86Sr values that are higher than ∼0.700 and are, therefore, likely to be produced by mixing between evolved crustal-like and depleted mantle reservoirs. The absence of crustal recycling processes on Mars may preserve the geochemical evidence for decoupling of the Rbue5f8Sr and Smue5f8Nd isotopic systems, underscoring one of the fundamental differences between geologic processes on Mars and the Earth.

Isotopic studies of ferroan anorthosite 62236: a young lunar crustal rock from a light rare-earth-element-depleted source

Isotopic analyses of mineral fractions and whole rocks from the ferroan anorthosite 62236 yield a Sm-Nd isochron with an age of 4.29 ± 0.06 Ga and an initial eNd143 value of +3.1 ± 0.9. We have also measured eNd142 anomalies of +0.25 on two fractions of 62236. These values are higher than the value of −0.1 predicted if 62236 was derived from a chondritic source at 4.29 Ga, but are consistent with the positive initial eNd143 value. The Sm-Nd isotopic composition of 62236 has been modified by the capture of thermal neutrons such that the 147Sm/144Nd, 143Nd/144Nd, and 142Nd/144Nd ratios measured on the mineral fractions and whole rocks must be corrected. The corrections do not significantly alter the Sm-Nd isotopic results determined on 62236. Despite the fact that the Ar-Ar and Rb-Sr isotopic systematics of 62236 have been reset by impact metamorphism at 3.93 ± 0.04 Ga, the Sm-Nd systematics appear to have been unaffected. The Sm-Nd isotopic systematics of 62236 provide several constrains on models of lunar crustal differentiation provided they have not been reset since crystallization. First, the relatively young age of 62236, as well as the old ages determined on several crustal plutonic rocks of the Mg-suite, require multiple sources of magmas on the Moon very early in its history. Second, positive eNd143 values determined on all analyzed ferroan anorthosites suggest that they were derived from sources depleted in light rare earth elements. And third, models based on initial eNd143 and present-day eNd142 values suggest that the source of 62236 was depleted in light rare earth elements at ∼4.46 Ga. In order to reconcile these observations with the lunar magma ocean model (1) the magma ocean must have existed for a very short period of time, and may have had a sub-chondritic Nd/Sm ratio, and (2) the youngest ferroan anorthosites, such as 62236, cannot be cumulates from the magma ocean, but must form by other processes.

Lunar mare deposits associated with the Orientale impact basin: New insights into mineralogy, history, mode of emplacement, and relation to Orientale Basin evolution from Moon Mineralogy Mapper (M3) data from Chandrayaan‐1

[1]xa0Moon Mineralogy Mapper (M3) image and spectral reflectance data are combined to analyze mare basalt units in and adjacent to the Orientale multiring impact basin. Models are assessed for the relationships between basin formation and mare basalt emplacement. Mare basalt emplacement on the western nearside limb began prior to the Orientale event as evidenced by the presence of cryptomaria. The earliest post-Orientale-event mare basalt emplacement occurred in the center of the basin (Mare Orientale) and postdated the formation of the Orientale Basin by about 60–100 Ma. Over the next several hundred million years, basalt patches were emplaced first along the base of the Outer Rook ring (Lacus Veris) and then along the base of the Cordillera ring (Lacus Autumni), with some overlap in ages. The latest basalt patches are as young as some of the youngest basalt deposits on the lunar nearside. M3 data show several previously undetected mare patches on the southwestern margins of the basin interior. Regardless, the previously documented increase in mare abundance from the southwest toward the northeast is still prominent. We attribute this to crustal and lithospheric trends moving from the farside to the nearside, with correspondingly shallower density and thermal barriers to basaltic magma ascent and eruption toward the nearside. The wide range of model ages for Orientale mare deposits (3.70–1.66 Ga) mirrors the range of nearside mare ages, indicating that the small amount of mare fill in Orientale is not due to early cessation of mare emplacement but rather to limited volumes of extrusion for each phase during the entire period of nearside mare basalt volcanism. This suggests that nearside and farside source regions may be similar but that other factors, such as thermal and crustal thickness barriers to magma ascent and eruption, may be determining the abundance of surface deposits on the limbs and farside. The sequence, timing, and elevation of mare basalt deposits suggest that regional basin-related stresses exerted control on their distribution. Our analysis clearly shows that Orientale serves as an excellent example of the early stages of the filling of impact basins with mare basalt.