Lu-Hf sysematics and the early silicate differentiation of the Moon
Final Report Abstract
We have shown that the Hf isotope compositions of lunar samples are strongly affected by capture of secondary neutrons produced during cosmic ray exposure on the Moon. In conjunction with Sm isotope data and models developed during this study, neutron capture effects can be quantified and pristine radiogenic isotope compositions for the Lu-Hf, the Sm-Nd, and the Hf- W system that are unbiased by neutron capture effects can be obtained. With bias induced by neutron capture removed, the Lu-Hf and Sm-Nd systematics of major reservoirs in the lunar mantle are most consistent with their primary formation during inside-out crystallization of a possibly global lunar magma ocean whose Lu-Hf and Sm-Nd systematics are equal to those of chondrites and subsequent mixing of the crystallization products during a global mantle overturn. These findings are in perfect agreement with the Lu-Hf signatures and U-Pb age data of lunar and terrestrial zircon grains, whose compositions converge on condritic Lu-Hf compositions towards the age of the solar system. Our study on the young, ca. 3 billion year-old KREEP-gabbro NWA 6950 produced a greatly extended evolution line of lunar basalts and plutonic equivalents whose mantle source was a residual liquid-component of the crystallizing lunar magma ocean (KREEP). This evolution line further substantiates that the bulk lunar silicate portion has chondritic Lu-Hf properties in that it departs from a chondritic evolution within the first 50 milion years of the solar system. Applying the knowledge gained from the newly developed Hf-Sm neutron dosimeter for lunar rocks to their 182Hf-182W systematics revealed the Moon to exceed Earth in the abundance of radiogenic 182W after removal of a neutroncapture-induced 182W component. Our study of the extended HFSE-Th-U-W concentration systematics in a wide selection of lunar mantle-derived rocks, aided by results of our companion studies on the partition behaviour of these elements during melting of the lunar mantle, revealed that the distinct concentrations of these elements and their relative abundance in different suites of lunar rocks are best explained by the same magma-ocean crystallization models that reproduced the Sm-Nd-Lu-Hf systematics of mantlederived lunar rocks. Further, or data and these models require higher-thanterrestrial Hf/W of the silicate Moon, in striking agreement with the lunar excess in 182W stemming from the formation of the Moon during the first ca. 60 million years of the solar system, i.e. when 182Hf was still extant. Further, experiments to further substantiate our neutron-capture-correction schemes were initiated. Preliminary data strongly support the developed correction scheme thus substantiating our findings in lunar rocks.
Publications
- (2013): Isotopic evidence for chondritic Lu/Hf and Sm/Nd of the Moon. Earth and Planetary Science Letters 380: 77-87
Sprung, P., Kleine, T., Scherer, E.E.
(See online at https://doi.org/10.1016/j.epsl.2013.08.018) - (2014): Evidence for a Common Initial 176Hf/177Hf of the Earth, Moon, and Chondrites. 45th Lunar and Planetary Science Conference, abstract # 2821
Sprung, P., Kleine, T., Scherer, E.E.
- (2014): Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the W/U ratio of the lunar mantle. Earth and Planetary Science Letters 404, 1 – 13
Fonseca, R.O.C, Mallmann, G., Sprung, P., Sommer, J.E., Heuser, A.,Speelmanns, I.M., Blanchard, H.
(See online at https://doi.org/10.1016/j.epsl.2014.07.015) - (2015): Lunar tungsten isotopic evidence for the late veneer. Nature 520, 534 – 537
Kruijer, T.S., Kleine, T., Fischer-Gödde, M., Sprung, P.
(See online at https://doi.org/10.1038/nature14360) - (2016) Extended HFSE systematics of Apollo samples – wrenching further Secrets from the Lunar Mantle. AGU Fall meeting 2016
Thiemens, M.M., Sprung, P., Münker, C.
- (2016): The effect of titanium on the partitioning behavior of high-field strength elements between silicates, oxides and lunar basaltic melts with applications to the origin of mare basalts. Chem. Geol. 440, 219 – 238
Leitzke, F.P., Fonseca, R.O.C., Michely, L.T., Sprung, P., Münker, C., Heuser, A., Blanchard, H.
(See online at https://doi.org/10.1016/j.chemgeo.2016.07.011) - (2018) Isotope Planetology, Diss., Mathematisch-Naturwissenschaftliche Fakultät der Universität zu Köln
Thiemens, M.M.
