Late-stage magmatic outgassing from a volatile-depleted Moon

Abstract

The abundance of volatile elements and compounds, such as zinc, potassium, chlorine, and water, provide key evidence for how Earth and the Moon formed and evolved. Currently, evidence exists for a Moon depleted in volatile elements, as well as reservoirs within the Moon with volatile abundances like Earth's depleted upper mantle. Volatile depletion is consistent with catastrophic formation, such as a giant impact, whereas a Moon with Earth-like volatile abundances suggests preservation of these volatiles, or addition through late accretion. We show, using the "Rusty Rock" impact melt breccia, 66095, that volatile enrichment on the lunar surface occurred through vapor condensation. Isotopically light Zn (δZn = -13.7‰), heavy Cl (δCl = +15‰), and high U/Pb supports the origin of condensates from a volatile-poor internal source formed during thermomagmatic evolution of the Moon, with long-term depletion in incompatible Cl and Pb, and lesser depletion of more-compatible Zn. Leaching experiments on mare basalt 14053 demonstrate that isotopically light Zn condensates also occur on some mare basalts after their crystallization, confirming a volatile-depleted lunar interior source with homogeneous δZn ≈ +1.4‰. Our results show that much of the lunar interior must be significantly depleted in volatile elements and compounds and that volatile-rich rocks on the lunar surface formed through vapor condensation. Volatiles detected by remote sensing on the surface of the Moon likely have a partially condensate origin from its interior.

Chlorine isotopic compositions of apatite in Apollo 14 rocks: Evidence for widespread vapor-phase metasomatism on the lunar nearside ∼4 billion years ago

“…This process has also been identified in 'rusty-rock' 66095 and is similar to the mechanism proposed to explain crystallization of large apatite with elevated δ 37 Cl values in granulite 79215 (Treiman et al, 2014). Vapor condensation has also been proposed as a mechanism to explain volatile contents in lunar glass beads (74220, 15426) and 66095 (Day et al, 2017). The two samples, 66095 and 79215, have yielded dates around 3.9 Ga (Fischer-Gödde and Becker, 2012;Norman et al, 2006;Snape et al, 2017), similar to the Apollo 14 samples studied here (Table 1).…”

Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation

“…Compared with Earth, the Moon is poor in volatile compounds (e.g., H 2 O, CO 2 ) and elements (e.g., K, Na, Cl, Zn, Rb, Sn). Comparison of volatile element data for these planetary bodies shows that the Moon has lower ratios of element pairs that behave similarly during igneous processes but have a more volatile numerator over denominator, including K/U and Rb/Sr (e.g., Herzog et al, 2009;Sharp et al, 2010;Paniello et al, 2012a;Kato et al, 2015;Boyce et al, 2015;Wang and Jacobsen, 2016;Day et al, 2017aDay et al, , 2019Kato and Moynier, 2017;Pringle and Moynier, 2017;Wang et al, 2019).…”

Metal grains in lunar rocks as indicators of igneous and impact processes

“…2014; Day et al. 2017, 2019). The interior of the Moon is considered to be strongly reducing with lunar volcanic gases dominated by CO, and highly reduced conditions (IW to IW‐2.5; Fogel and Rutherford 1995; Nicholis and Rutherford 2009).…”