Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11i..04s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P11I-04
Other
[1026] Geochemistry / Composition Of The Moon, [5480] Planetary Sciences: Solid Surface Planets / Volcanism, [6250] Planetary Sciences: Solar System Objects / Moon, [8430] Volcanology / Volcanic Gases
Scientific paper
Separate accretion and capture is a physically plausible alternative to the giant Earth impact hypothesis for the origin of the Moon. Like the capture hypothesis, giant impact relies on gravitational interaction of the Earth and a smaller planetesimal under very specific orbital encounter conditions. In addition to questions about the physics of debris re-aggregation in Earth-orbit, the giant impact hypothesis, as currently formulated by computer models, fails a critical reality check; namely, the Moon contains reservoirs of volatiles that would have been dispersed at ejection temperatures predicted by the models... Researchers have previously noted that concentrations of many volatile elements in both the Apollo 17 and Apollo 15 pyroclastic glass samples indicate the existence of volatile reservoirs at depth. Both the orange and green glasses are enriched over mare basalts by factors >100 in Cl, F, Br, Zn, Ge, Dc, Tl, and Ag and by factors >10 in Pb, Ga, Sb, Bi, In, Au, Ni, Se, Te, and Cu. These elements exist almost entirely in the non-glass components of the pyroclastic samples. The recent identification of significant water within the Apollo 17 orange pyroclastic glasses further emphasizes the existence of volatile reservoirs in the lunar mantle... No evidence exists that the volatiles in the vesicles of mare basalts, derived by partial melting of a differentiated and solidified lunar magma ocean (upper mantle), were comparable to those in the pyroclastic glasses. Accretionary thermal effects that produced the magma ocean, combined with low lunar gravity, to would have depleted primordial volatiles. Any remaining magma ocean water would be converted to hydrogen and FeO by migration of early-formed, broadly disseminated, immiscible FexNiySz liquid. The reservoirs for the pyroclastic volatiles, therefore, would be below about 500km (lower mantle), that is, below the base of the original magma ocean. Relatively inert hydrogen and carbon monoxide probably made up the vesicle phase in the mare basalt... Release and upward migration of lower mantle volatiles, near the end of the Basaltic Maria Stage of lunar evolution, would have stimulated additional partial melting of remaining relatively low temperature components in the still hot upper mantle. Associations of regional pyroclastic deposits with large extensional faulting systems in young large basins suggest that release of lithostatic pressure on source regions played an important role in the generation of large volatile-rich volcanic eruptions... The presence of a relatively cool lower lunar mantle with a ~1200km radius, the relic chondritic protocore of the Moon, sets a rough limit on the amount of accretion that could take place before impact generated magma oceans begin to form on growing planetesimals. As some planetesimals grew larger than others, they would reach a gravitational state when volatile elements and compounds would not be lost from a growing magma ocean. Lead's primitive isotopic signature in the pyroclastic volatiles supports the chondritic protocore hypothesis.
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