Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v14b..02p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V14B-02
Other
[1026] Geochemistry / Composition Of The Moon, [1041] Geochemistry / Stable Isotope Geochemistry, [1060] Geochemistry / Planetary Geochemistry, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
It is difficult to estimate the bulk chemical and isotopic properties of planets, especially for the Moon for which our sampling is far more limited than for the Earth. As a result, there is currently a debate on the bulk Fe isotope composition of the Moon: Whereas in a first study we proposed that the bulk lunar Fe isotope composition (δ57Fe ~0.2‰) was twice as heavy as that of the Earth (δ57Fe ~0.1‰) relative to chondrites (δ57Fe ~0‰), normalized to IRMM-14, others proposed that there is no difference between Earth and Moon. This question is of crucial importance because the first case may track the Moon-forming giant interplanetary impact, whereas the alternative situation may also result from a very high pressure metal-silicate fractionation during the Earth’s core formation, assuming that the Moon subsequently inherited the Earth Fe isotope composition. To reassess this question, we analyzed a suite of 18 mare basalts (both high- and low-Ti) by MC-ICP-MS using the nickel doping technique developed at LMTG. Combined with our previous measurements of lunar rocks, we obtained a mean δ57Fe of 0.138±0.035‰ (2SE, n=11) for low-Ti basalts, 0.269±0.026‰ (2SE, n=16) for high-Ti ones and 0.177±0.036‰ (2SE, n=6) for highland rocks. T-tests confirm that averages of low- and high-Ti basalts are significantly different at the 95% confidence level. Similarly, t-tests indicate that highland rocks are significantly different from high-Ti basalts, but not from low-Ti ones. These new data therefore confirm suggestion from previous groups that low- and high-Ti basalts contain distinct Fe isotope signatures. This shows that on the Moon, high temperature processes can significantly change the Fe isotope composition of bulk mafic rocks at the planetary scale. This cannot result from simple equilibrium magmatic fractionation or assimilation of ilmenite given its Fe isotope fractionation factor, however. We conclude that another process, yet to be identified, is responsible for this peculiar isotopic effect observed in the high-Ti basalts that are unknown on Earth. It might be linked to armalcolite, a Fe- and Ti-oxide specific to the Moon. Estimating the bulk Moon Fe isotope composition remains difficult. The now clarified Fe isotope difference between high- and low-Ti basalts shows that a high temperature process generates a planetary-scale Fe isotope fractionation. Hence, these basalts cannot be easily taken as the direct proxies of the Fe isotope composition of the deep Moon. On the other hand, highland rocks, both anorthosites and rocks from the Mg-suite that are older than the mare basalts, display the same Fe isotope composition whatever their petrology. They may well yield a more pristine iron isotope composition of the bulk Moon. The value obtained here (δ57Fe = 0.177±0.036‰) is undistinguishable from our previous estimate for the Moon (0.206±0.029‰). It must be recognized, however, that this conclusion is based on a limited number of samples (n = 6) and more highland bulk rock data are required to assess whether the Moon is isotopically similar to or different from the Earth.
Magna Tomáš
Neal Chris R.
Poitrasson Franck
Zambardi T.
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