Mare basalt petrogenesis in a dynamic moon |
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| Authors: | SE Kesson AE Ringwood |
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| Institution: | Research School of Earth Sciences, Australian National University, Canberra A.C.T. ,Australia |
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| Abstract: | Previous hypotheses for mare basalt petrogenesis involving either the remelting or assimilation of 4.6-4.4-b.y. cumulates cannot satisfactorily account for certain key geochemical similarities between high-Ti and low-Ti primitive basalts, e.g. Mg/(Mg + Fe) ratios, Cr2O3 contents, eruption temperatures, and high-pressure liquidus phase relations. In addition, many thermal problems remain unanswered. A new petrogenetic hypothesis which appears to satisfy these constraints, is outlined below.The 4.6-4.4-b.y. melting event affected only the outer few hundred kilometers of the moon, leaving the undifferentiated primordial interior with its full complement of radiogenic elements. The differentiated lithosphere consisted of the crust and an underlying sequence of Mg-rich olivine + pyroxene cumulates and refractory residua. The late-stage differentiates were sandwiched between the crust and the cumulate sequence in large (perhaps 5–20 km diameter) segregations. The plagioclase-depleted late-stage residual system itself differentiated to form dense, Fe, Ti-rich, pyroxene + ilmenite±olivine cumulates, overlain by a complementary, solidified, residual liquid component, rich in incompatible elements. The dynamical consequence of this gravitationally unstable situation was the sinking of the dense (about 3.8 g/cm3) ilmenite-bearing cumulate pods into the lunar interior (density about 3.35 g/cm3). Mean-while the primordial mantle was approaching solidus temperatures due to radiogenic heating. The arrival of the sinking Fe, Ti-rich pods into this region initiated partial melting and complex assimilative interactions. High-Ti hybrid liquids were produced under equilibrium conditions, with olivine-pyroxenite remaining as the refractory residuum. Incompatible elements including Ti, U, and REE (depleted in Eu) derived from the sinking cumulates were strongly partitioned into the liquid, thereby accounting for the high TiO2 contents, rare earth element characteristics, and two-stage isotopic record displayed by high-Ti mare basalts. Equilibrium between high-Ti hybrid liquids and the olivine-pyroxenite residuum was also responsible for controlling and buffering Mg/(Mg + Fe) and Cr distributions, thereby producing the relatively high Mg and Cr contents of primitive high-Ti magmas.By about 3.3 b.y., the primordial mantle immediately beneath the differentiated lithosphere had experienced partial melting due to radiogenic heating. Smaller and less-efficiently differentiated pods of 4.6-4.4-b.y. late-stage assemblages, sinking more slowly, contaminated the partially molten region and produced low-Ti hybrid liquids in equilibrium with residual olivine-pyroxenite. These hybrid magmas, although predominantly primordial in character, acquired incompatible-element characteristics from the sinking cumulates, and thereby obtained the Eu anomaly in their REE patterns, and their two-stage isotope record.The above model, invoking hybridization at depth accompanied by equilibrium between the hybrid liquids and the local olivine-pyroxenite residuum, accounts for the similar Mg/(Mg + Fe) ratios and Cr2O3 contents observed in primitive high-Ti and low-Ti basalts. |
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