Abstract
The recent discoveries of hydrogen (H) bearing species on the lunar surface and in samples derived from the lunar interior have necessitated a paradigm shift in our understanding of the water inventory of the Moon, which was previously considered to be a ‘bone-dry’ planetary body. Most sample-based studies have focused on assessing the water contents of the younger mare basalts and pyroclastic glasses, which are partial-melting products of the lunar mantle. In contrast, little attention has been paid to the inventory and source(s) of water in the lunar highlands rocks which are some of the oldest and most pristine materials available for laboratory investigations, and that have the potential to reveal the original history of water in the Earth–Moon system. Here, we report in-situ measurements of hydroxyl (OH) content and H isotopic composition of the mineral apatite from four lunar highlands samples (two norites, a troctolite, and a granite clast) collected during the Apollo missions. Apart from troctolite in which the measured OH contents in apatite are close to our analytical detection limit and its H isotopic composition appears to be severely compromised by secondary processes, we have measured up to ∼2200 ppm OH in the granite clast with a weighted average δD of ∼−105±130‰, and up to ∼3400 ppm OH in the two norites (77215 and 78235) with weighted average δD values of −281±49‰ and −27±98‰, respectively. The apatites in the granite clast and the norites are characterised by higher OH contents than have been reported so far for highlands samples, and have H isotopic compositions similar to those of terrestrial materials and some carbonaceous chondrites, providing one of the strongest pieces of evidence yet for a common origin for water in the Earth–Moon system. In addition, the presence of water, of terrestrial affinity, in some samples of the earliest-formed lunar crust suggests that either primordial terrestrial water survived the aftermath of the putative impact-origin of the Moon or water was added to the Earth–Moon system by a common source immediately after the accretion of the Moon.
Highlights
The OH contents and H isotopic compositions of apatite grains measured in this study considerably expand the existing OH–δD database for lunar highlands samples
The final total uncertainties for both δD and OH were calculated by propagating the uncertainties in H and D production rates and taking into account the uncertainties associated with both the cosmic ray exposure (CRE) ages and the analytical error on the measured OH–δD values
The OH contents of apatites in the granite clast 14303 display a range from ∼130 to 2240 ppm with associated δD values ranging from 110 ± 295h to −321 ± 390h, the weighted average δD value being −105 ± 130h (95% confidence level) (Fig. 2)
Summary
The recent findings of ‘water’ in lunar materials have called into question mechanisms related to the retention of volatiles during accretion and have highlighted the possibility of late delivery of volatiles to the Moon (Barnes et al, 2013a; Boyce et al, 2010; Füri et al, 2014; Greenwood et al, 2011; Hauri et al, 2011; Hui et al, 2013; Liu et al, 2012; McCubbin et al, 2010a, 2010b, 2011; Saal et al, 2008, 2013; Tartèse et al, 2013). Hui et al (2013) measured the H2O content of nominally anhydrous minerals (NAMs) from two ferroan anorthosites (FAN) and a Mg-suite troctolite Their results predict a relatively wet lunar mantle, with the late-stage residual melt, termed urKREEP (K – potassium, REE – rare earth elements, P – phosphorous), of the lunar magma ocean (LMO) containing up to 1.4 wt.% H2O. We have measured the OH contents and H isotopic compositions of apatite grains from four lunar highlands samples belonging to the alkali-suite and Mg-suite group of rocks, namely, granite 14303,205, troctolite 76535,51, norite 77215,202, and norite 78235,43 Based on their geochemical characteristics and crystallisation ages (∼4.44 to 4.29 Ga), these rocks are considered to represent mixtures of some of the most primitive magmas that formed from the partial-melting of early LMO cumulates, which assimilated urKREEP and crustal components (Shearer and Papike, 2005; Elardo et al, 2011). The results presented in this study provide key insights into the history of water in the primitive Moon and help place constraints on the source of this water in the context of the evolution of the Earth–Moon system
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