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Project B1: Origin of fractionations of highly siderophile and siderophile volatile elements in lunar rocks and in the Earth
The abundances of siderophile elements in the silicate fraction of differentiated rocky planets and asteroids reflect the conditions of metal–silicate and sulfide-silicate segregation processes during core formation, but possibly also the composition of the building materials of the planets, volatile loss resulting from large impacts, and late additions of material. Ancient lunar impact rocks retain a direct compositional and chronological record of late accretion. They were produced by single or multiple impacts into a heterogeneous lunar surface during the basin forming period >3.8 Ga ago. Highly siderophile elements and siderophile volatile elements determined in impactites and pristine samples of the lunar crust provide insights into the composition of impactors as well as small- and large-scale processes during lunar differentiation. By comparison with Earth and differentiated meteorite groups which are attributed to planets like Mars and asteroids like Vesta we seek to understand the inventory budged of late accreted material onto terrestrial planets.
Current work: Origin of fractionations of highly siderophile and siderophile volatile elements in lunar granulitic rocks
In recent years highly siderophile elements (HSE) have been studied in ancient lunar impactites by reliably precise and accurate methods. The HSE ratios of many ancient lunar impactites are non-chondritic whereas others match those of chondritic meteorites. A broad linear trend defined by the impactites in diagrams of HSE ratios was interpreted to reflect a mixture of chondrite–like material and a minor proportion of fragments of planetesimal or embryo cores. Alternatively, the differences in HSE ratios between impactites from different landing sites could represent unique compositions that reflect different processing conditions of dust precursors of primitive meteorites, which later impacted the lunar surface. Since KREEP-rich mafic impactites predominantly show differentiated iron meteorite-like HSE ratios, a strong bias by a small number of impacts into the PKT is likely. In contrast, available data for KREEP-poor feldspatic breccias and granulites display less fractionated HSE, but the number of analysis is still too small to generalize this observation. Another important question is whether or not late accreted materials were predominantly rich in volatiles or not. If the differentiated iron-meteorite impactor hypothesis holds, this question cannot answered by HSE alone. We will further evaluate this question by study of ratios of some key siderophile volatile elements (SVE) like Se/Te, Cu/Ag, Cd/In in different lunar impactites.
Granulitic breccias and granulites are important components of the lunar crust which are often characterized by complex histories, including recrystallization during impact induced prolonged heating at temperatures >1000°C. Since all granulites seem to be contaminated with meteoritic siderophile elements, these signatures may pre-date impact-related recrystallization. Most granulites have been derived from KREEP-poor precursor rocks, which implies a different target rock composition than more common KREEP-rich mafic impactites. This is interpreted as resembling very early lunar crust, which may be formed before formation of PKT, or KREEP-free material potentially derived from outside the PKT and the Apollo landing sites. This is further supported by Ar-Ar ages >4 Ga, interpreted as major metamorphic events which formed the granulites, thus postdating the primary formation of the impactites. Our new HSE and SVE data together with new age constraints from TRR170 subproject A1 this will provide new insight into early lunar crust formation and impact processes.
Fig. 1. Preparation of granulitic breccia 67955 in the Lunar Sample Laboratory Facility at NASA Johnson Space Center, Houston.
Fig. 2. Petrographic characterization of subsample 67955,108 before analysis at Geochemistry Laboratory, Freie Universität Berlin.
Gleißner, P., 2019: The Earth–Moon late-accretion conundrum. Nature Geoscience, Vol. 12, pp. 683–684. 10.1038/s41561-019-0445-0
Gleißner, P. and H. Becker, 2019: Origin of lunar fragmental matrix breccias - Highly siderophile element constraints. Meteoritics & Planetary Science, Vol. 1, pp. 21. 10.1111/maps.13363
Gleißner, P. and H. Becker, 2017: Formation of Apollo 16 impactites and the composition of late accreted material: constraints from Os isotopes, highly siderophile elements and sulfur abundances, 200, 1-24, Geochimica et Cosmochimica Acta. 10.1016/j.gca.2016.12.017
Gleißner, P. and Becker, H. (2019): The composition of basin forming impactors and large-scale impact gardening in the lunar highlands. 50th Lunar and Planetary Science Conference
Gleißner, P. and Becker, H. (2018): Siderophile elements in lunar granulitic impactites: constraints on the composition of pre 4 Ga late accretion. American Geophysical Union, Fall Meeting Washington, D.C.
Gleißner, P. and Becker, H. (2018): The composition of lunar basin forming impactors: Constraints from siderophile elements in ancient impactites. 49th Lunar and Planetary Science Conference
Gleißner, P. and Becker, H. (2017): Origin of non-chondritic highly siderophile element patterns in lunar fragmental matrix breccia. Paneth Kolloquium
Gleißner, P. and Becker, H. (2017): Combining S-Se-Te and highly siderophile element abundances in ancient lunar impactites – New constraints on impactor composition, mixing and lunar differentiation. Interdisciplinary workshop "Accretion and Early Differentiation of the Earth and Terrestrial Planets"
Gleißner, P. and Becker, H. (2017): Late accreted material on the lunar surface: Constraints from highly siderophile and chalcophile elements in ancient lunar impactites. New Views of the Moon 2 — Europe
Gleißner, P. and Becker, H. (2017): Combining S-Se-Te and highly siderophile element abundances in ancient lunar impactites – New constraints on impactor composition, mixing and lunar differentiation. 48th Lunar and Planetary Science Conference
Gleißner, P. and Becker, H. (2016): Highly siderophile and chalcophile elements in lunar impact rocks; Constraints on the composition of late accreted material. 79th Annual Meeting of the Meteoritical Society
Gleißner, P. and Becker, H. (2016): Highly siderophile element fractionations in Apollo 16 impact melt rocks: Large-scale fractionation processes. 47th Lunar and Planetary Science Conference
Gleißner, P. and Becker, H. (2016): Highly siderophile element fractionations in Apollo 16 impact melt rocks: Effects of small-scale processes. 47th Lunar and Planetary Science Conference