A 200-year 210Pb record from Greenland

A continuous profile of 210Pb activity extending back to 1766 has been developed for a firn/ice core collected at Site D in central Greenland in 1984. Unexpectedly high activities of 210Pb were found at the base of this core (0.032 pCi kg−1 in samples more than 200 years old), calling into question the common assumption that supported 210Pb can be neglected when constructing chronologies in glacial snow and ice. It is problematic to assert that all of the 210Pb measured at depth should be attributed to the supported fraction, given previous estimates of dust loading in Greenland ice cores. However, even if an estimated constant value of 0.032 pCi supported210Pb kg−1 is subtracted from the measured values to estimate excess 210Pb, the 210Pb chronology for Site D yields ages that are significantly younger (mean accumulation rate too high) than an independent depth-age scale based on annual layer counting. It is apparent that the flux of excess and/or supported 210Pb to this site must have decreased over the past 2 centuries, with decreasing trends in both fractions most likely. Previously published 210Pb profiles for cores from Summit and Dye 3, Greenland, show similar trends, which had been interpreted as decreasing fluxes of excess 210Pb only. For all three sites, it is not possible to separate variations in the fluxes of the excess and supported fractions of 210Pb, but variations in the total 210Pb flux will impact 210Pb-based chronologies generally if these variations have not been restricted to the Greenland ice sheet

Quantitative Ion Microprobe Analysis of the Rare Earth Elements in Minerals

The lanthanides or rare earth elements (REE) which are present in trace concentrations in rocks are most useful for investigating the origin of these objects. Of the microbeam techniques presently used to measure the REE concentrations of individual crystals, secondary ion mass spectrometry (SIMS) is the only method sensitive enough to allow the determination of REE abundances in most natural minerals. Usually, energy filtering is applied to remove all complex molecular interferences. All the REE, down to a level of ≤ 100 ppb, can be measured in spots 5 to 20 m in diameter. A growing number of studies involving both terrestrial and extraterrestrial materials have been undertaken

Trace element distributions in the Yamato 000593000749,NWA 817 and NWA 998 nakhlites: Implications for their petrogenesis and mantle source on Mars

We report here results of ion microprobe analyses of rare earth element abundances in various phases in the new Antarctic (paired) nakhlites Yamato 000593 and Yamato 000749, as well as in two additional nakhlites recovered from the Saharan desert, NWA 817 and NWA 998. Although these nakhlites are all composed predominantly of augite and some olivine, they differ from each other, and from the three previously known nakhlites, in the abundance and degree of crystallinity of the interstitial mesostasis. Trace element abundances in various phases in these new nakhlites indicate that they are petrogenetically related to (and comagmatic with) each other and the previously known nakhlites. The calculated parent melt compositions (in equilibrium with augite core compositions) are LREE-enriched and have REE patterns parallel to those of their whole rocks. This suggests that subsequent to accumulation of the olivine and augite, the intercumulus trapped melt evolved in a closed system. The similarity in the estimated parent melt compositions and trace element zonation in the augites of the various nakhlites indicates that these rocks are likely to have formed within a single lithologic unit on Mars. In this scenario, the differences among these nakhlites may be explained in terms of differences in the depth of crystallization within the cumulus pile, represented by different horizons within the same lithologic unit. Based on the partitioning of Eu in their augite cores, the magmatic redox conditions for the nakhlites are estimated to be relatively oxidizing (QFM), implying an oxidized source reservoir in the martian mantle. Late metasomatism of their mantle source by LREE-enriched, oxidizing fluids is suggested to be responsible for the LREE-enrichment and oxidation condition of the nakhlite parent melts

Lunar surface dynamics: Some general conclusions and new results from Apollo 16 and 17

Exposure ages of Apollo 17 rocks as measured by tracks and the Kr-Kr rare gas method are reported. Concordant ages of 22 - or + 1 million year (my) are obtained for the station 6 boulder sample 76315. This value is interpreted as the time when the station 6 boulder was emplaced in its present position. Reasonable agreement is also obtained by the two methods for another station 6 boulder, sample 76015. Discordant ages (respectively 5 and 28 my by the track and rare gas methods) are obtained for the station 7 boulder sample, 77135, indicating that the boulder was emplaced at least 5 my ago. The 72 my exposure age of 75035, in general agreement with previous measurements of approximately 85 my for another Camelot boulder, may well date the formation of Camelot. Rock 76015 was split and one surface exposed to the sky through a very small solid angle

Trace element distributions in Yamato-793605, a chip off the "martian lherzolite" block

In situ ion microprobe analyses of various phases in Yamato-793605 (Y79) confirm that it is very similar to the other two lherzolitic shergottites, ALHA77005 and LEW88516. Differences in absolute REE abundances between bulk samples of these meteorites can be largely accounted for by sample heterogeneity. The three lherzolites were formed by essentially identical processes and they may even have originated from the same lithological unit on Mars. Preservation of major element zonation in olivines of Y79 indicates that it is less equilibrated than the other lherzolitic shergottites, and may have crystallized at shallower depth. The parent magmas of lherzolitic shergottites, like those of other shergottites, were derived by partial melting of a partly depleted martian mantle

Trace element constraints on the origins of highly metamorphosed Antarctic eucrites

We have compared the trace element distributions of four highly metamorphosed Antarctic eucrites with those of previously studied noncumulate eucrites. All eucrites studied here exhibit some evidence for reequilibration of the REE and/or other trace elements. LREE-enrichments are observed in the silicate phases, and are attributed to remobilization of the REE from Ca-phosphates. Similarly, elevated Zr and Ti abundances in some pigeonites may be the result of redistribution from oxide phases such as ilmenite. Thus, trace element distributions cannot provide petrogenetic information about the origin of these eucrites. However, combined with petrographic information, the data do provide information about the nature and degree of heating and metamorphism experienced by these rocks. For example, Y-86763 appears to have experienced a rapid secondary reheating, in addition to an extended period of thermal metamorphism. Two granulitic eucrites appear to have been less extensively affected by metamorphism, despite the fact that they are strongly recrystallized. Although plagioclase REE compositions are LREE-enriched compared to those of noncumulate eucrites, pigeonites have LREE/HREE ratios that fall close to the non-cumulate line. Furthermore, they seem to have retained their original Ti, Zr and Y abundances. Finally, some trace elements, such as Na, K, Sr and Ba in plagioclase, do not appear to have been redistributed between mineral phases in these eucrites, and provide links to less equilibrated non-cumulate eucrites

Nuclear interaction tracks in minerals and their implications for extraterrestrial materials

Geometrical characteristics and thermal stability of nuclear interaction tracks produced by protons and alpha particles in mic

Multiple carriers of Q noble gases in primitive meteorites

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. Indirect techniques have revealed that phase Q might be composed of two subphases, one of them associated with sulfide. Here we provide experimental evidence that noble gases trapped within meteoritic sulfides present chemically- and thermally-driven behavior patterns that are similar to Q-gases. We therefore suggest that phase Q is likely composed of two subcomponents: carbonaceous phases and sulfides. In situ decay of iodine at concentrations levels consistent with those reported for meteoritic sulfides can reproduce the 129Xe excess observed for Q-gases relative to fractionated Solar Wind. We suggest that the Q-bearing sulfides formed at high temperature and could have recorded the conditions that prevailed in the chondrule-forming region(s)

Lunar meteorite regolith breccias: an in situ study of impact melt composition using LA-ICP-MS with implications for the composition of the lunar crust

Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104/88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104/88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA-ICP-MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg#10), and have low incompatible trace element (ITE) concentrations (i.e., typically 10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc/Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP-poor and Sc-rich, and represent mixing between feldspathic lithologies and low-Ti or very low-Ti (VLT) basalts. As PCA 02007 and MAC 88104/05 were likely sourced from the Outer-Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low-Ti basalt contribution