The ferroan‐anorthositic suite, the extent of primordial lunar melting, and the bulk composition of the Moon

Abstract

The geochemical bimodality among compositionally pristine lunar rocks is strong evidence that a major component of the Moon's crust is fundamentally unrelated to the plagioclase flotation crust that accumulated over the primordial magma ocean. The ferroan anorthositic suite (FAS) has all the characteristics expected for a series of magma ocean plagioclase flotation cumulates, but we estimate, based on mass balance for Al2O3, that ∼ 30 wt% of the crust, and 20 wt% of the crust's total Al2O3 (or roughly 15% of the bulk‐Moon Al2O3 content, assumed to be 3–4 wt%), was emplaced by postmagma ocean, Mg‐suite “serial” magmatism. Remelting of plagioclase‐bearing magma ocean cumulates cannot plausibly account for the “late” emplacement of Al2O3, because the only plagioclase‐rich cumulates produced by a deep, ultramafic magma ocean would have mg ratios too low to engender the Mg‐suite. Conceivably, remelting of garnet‐rich magma ocean cumulates supplied the late Al2O3, but only if the bulk‐Moon Al2O3 content is substantially higher than generally assumed. High pressures in the magma ocean acted to enhance the py (pyroxene/(pyroxene+olivine)) ratio of the MO cumulates, and the Al2O3 content of whatever pyroxene formed. Remelting of pyroxene‐rich MO cumulates could conceivably have supplied the late Mg‐suite Al2O3 to the crust. The source region would have to encompass roughly 40 wt% of the mantle. Another possible source for late Al2O3 addition is a large portion of the deepest mantle that avoided more than a slight degree of partial melting, and thus remained Al‐rich, during formation of the magma ocean. The source region would have to encompass 20–30 wt% of the mantle. Ascent of diapiric masses from the lowermost mantle would likely coincide with (trigger, or be triggered by) a large‐scale overturn of the gravitationally unstable MO cumulate pile, with concomitant pressure‐release melting, in which case the Mg‐suite parental melts would be derived from a mix of deepest MO cumulates and the even deeper primitive mantle. The popular giant impact hypothesis of lunar origin implies an extremely hot beginning for the Moon, suggesting that the deep interior was either molten or depleted in Al2O3 (by differentiation in hot protomoons) as it formed. Origin of ∼ 30 wt% of the Moon's crust by postmagma ocean, serial magmatism is more consistent with coaccretion models that form the Moon without a giant, hot impact. In any scenario with a large fraction of the crust originating by partial remelting of deep magma ocean cumulates, mass balance for Al2O3 implies that the bulk‐Moon normative py ratio must be greater than roughly 0.5, i.e., significantly higher than generally estimated. In this respect, the Moon appears more compositionally similar to chondritic silicates than to the Earth's upper mantle.