Petrogenesis of picritic mare magmas: Constraints on the extent of early lunar differentiation

Publication Type  Journal Article
Year of Publication  2006
Authors  Longhi, J.
Journal Title  Geochimica Et Cosmochimica Acta
Volume  70
Issue  24
Pages  5919-5934
Journal Date  Dec 15
ISBN Number  0016-7037
Accession Number  ISI:000243166200003
Key Words  evolution; moon; basalts; spectrometer; volcanism; pressure; glasses; surface; mantle; model
Abstract  

Calculations of isobaric batch, polybaric batch, and polybaric fractional melting have been carried out on a variety of proposed lunar and terrestrial source region compositions. Results show that magmas with a generally tholeiitic character-plagioclase and high-Ca pyroxene crystallize before low-Ca pyroxene reflecting relatively high Al2O3 concentrations (> 12 wt%)-are the inevitable consequence of anhydrous partial melting of source regions composed primarily of olivine and two pyroxenes with an aluminous phase on the solidus. Low-Al2O3 magmas (< 10 wt%), as typified by the green picritic glasses in the lunar maria require deep (700-1000 km), low-Al2O3 source regions without an aluminous phase. The difference between primitive and depleted mantle beneath mid-ocean ridges amounts to less than 0.1 wt% Al2O3, whereas formation of the green glass source region requires a net loss of between 1.5 and 2.5 wt% Al2O3. Basalt extraction cannot account for fractionations of this magnitude. Accumulation of olivine and pyroxene at the base of a crystallizing magma ocean is, however, an effective method for producing the necessary Al2O3 depletions. Both olivine-rich and pyroxene-rich source regions can produce the picritic magmas, but mixing calculations show that both types of source region are likely to be hybrids consisting of an early- to intermediate-stage cumulate (olivine plus enstatite) and a later stage cumulate assemblage. Mass balance calculations show that refractory element-enriched bulk Moon compositions contain too much Al2O3 to allow for the deep low-Al2O3 source regions even after extraction of an Al2O3-rich (26-30 wt%) crust between 50 and 70 km thick. (c) 2006 Elsevier Inc. All rights reserved.

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URL  <Go to ISI>://000243166200003
DOI  DOI 10.1016/j.gca.2006.09.023