Highly siderophile element depletion in the Moon

Abstract

70135) mare basalts, along with mare basalt meteorites La Paz icefield (LAP) 04841 and Miller Range (MIL) 05035. These mare basalts have consistently low HSE abundances, at ∼2 × 10 −5 to 2 × 10 −7 the chondritic abundance. The most magnesian samples have broadly chondrite-relative HSE abundances and chondritic measured and calculated initial 187 Os/ 188 Os. The lower abundances and fractionated HSE compositions of more evolved mare basalts can be reproduced by modeling crystalliquid fractionation using rock/melt bulk-partition coefficients of ∼2 for Os, Ir, Ru, Pt and Pd and ∼1.5 for Re. Lunar mare basalt bulk-partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions, leading to increased HSE compatibility. The chondritic-relative abundances and chondritic 187 Os/ 188 Os of the most primitive high-MgO mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface. Mare basalt compositions are best modeled as representing ∼5-11% partial melting of metal-free sources with low Os, Ir, Ru, Pd (∼0.1 ng g −1 ), Pt (∼0.2 ng g −1 ), Re (∼0.01 ng g −1 ) and S (∼75 μg g −1 ), with sulphide-melt partitioning between 1000 and 10,000. Apollo 12 olivine-, pigeonite-and ilmenite normative mare basalts define an imprecise 187 Re-187 Os age of 3.0 ± 0.9 Ga with an initial 187 Os/ 188 Os of 0.107 ± 0.010. This age is within uncertainty of 147 Sm-143 Nd ages for the samples. The initial Os isotopic composition of Apollo 12 samples indicates that the source of these rocks evolved with Re/Os within ∼10% of chondrite meteorites, from the time that the mantle source became a system closed to siderophile additions, to the time that the basalts erupted. Similarity in absolute HSE abundances between mare basalts from the Apollo 12, 15 and 17 sites, and from unknown regions of the Moon (La Paz mare basalts, MIL 05035), indicates relatively homogeneous and low HSE abundances within the lunar interior. Low absolute HSE abundances and chondritic Re/Os of mare basalts are consistent with a late accretion addition of ∼0.02 wt.% of the Moon's mass to the mantle, prior to the formation of the lunar crust. Late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation (>4.4 Ga), to enable efficient mixing and homogenization within the mantle. Low lunar HSE abundances are consistent with proportionally 40 times more late accretion to Earth than the Moon. Disproportional late accretion to the two bodies is consistent with the small 182 W excess (∼21-28 ppm) measured in lunar rocks, compared to the silicate Earth.

Highly siderophile element and 182 W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland

“…As such, the Isua data presented here provide strong support for previous interpretations that the lunar ε 182 W reflects that of the pre-late veneer BSE (Kruijer et al, 2015;Touboul et al, 2015), which in turn provides supportive evidence for the interpretation that the HSE inventory of lunar rocks indicates an almost negligible amount of lateaccreted mass added to the Moon (Day et al, 2007;Day and Walker, 2015). Collectively, these observations are best explained by disproportional late accretion to the Earth and Moon following the giant impact and the end of core formation on Earth (Day et al, 2007;Bottke et al, 2010;Day and Walker, 2015;Kruijer et al, 2015;Touboul et al, 2015).…”

The Moon: An Archive of Small Body Migration in the Solar System

“…For example, partial melts of the lunar interior such as mare basalts provide insights to the amount and isotopic composition of impactors delivered during the earliest part of the Moon's history in a period of late accretion (Day and Walker 2015;Barnes et al 2016). Samples of radiometrically dated impact melt breccias, which are linked to known crater or basin forming events, provide evidence of the highly siderophile element budget of ancient impactor species (e.g.…”

“…However, osmium isotopes and HSE signatures of mantle partial melts (i.e., mare basalt samples) suggest that *0.02 wt% of the Moon's mass could have been added in this interval from primitive (i.e. chondritic) asteroid sourced impactors (Day and Walker 2015). These impactors could also have contributed to the Moon's volatile and moderately volatile element budget and isotopic signature (McCubbin et al 2015;Tartèse 2016) (although degassing of the magma ocean may have contributed to some later depletion of volatile species Kato et al 2015).…”

“…The time interval has traditionally been defined as between lunar core segregation (determined by highly siderophile elements (HSE) partitioning into the lunar core; Dale et al 2012;Day et al 2016;Day and Walker 2015;Kruijer et al 2015) and the end of magma ocean solidification (which marks the point when the crust inhibited the addition of significant quantities of further exogenous material to the mantle). The time interval was dependent on the cooling rate of the Moon and is thought, on the basis of lunar sample ages, to have lasted until *200 Myr after lunar formation (Elkins-Tanton et al 2011).…”

“…It is generally agreed that after the Moon's formation at *4.5 Ga, collision rates were immediately high, in an interval when leftover protoplanetary disc material and planetary embryos were being depleted through collision and inwards migration towards the Sun. This early period of bombardment is often referred to as late accretion, where several large bodies may have collided with the Moon (Day and Walker 2015;Barnes et al 2016), adding their chemical constituents to the still hot ductile lunar magma ocean.…”