ABUNDANCE AND ISOTOPIC VARIATIONS OF B, BE, AND LI IN TERRESTRIAL PLANETS AND METEORITES: INSIGHTS INTO DIFFERENTIATION "FUNDAMENTALS"

Li, Be, and B have become key tools in geochemical studies of subduction and the evolution of the Earth's mantle, especially as regards the recycling of surficial materials (sediments, water) into the planet's interior. Li-Be-B systematics of other terrestrial bodies (the Moon, the eucrite parent planet, Mars) indicate different fractionation processes are fundamental in the development of these worlds: Meteorites: Chondrite B/Be ratios >20, higher than the Earth's mantle (<3). d¹¹B are variable, though CI-CM chondrties are -3 to -6 ‰. d⁷Li is much less variable (~+3 ‰, w/ E chondrites-aubrites ~0 ‰) EPB: High Li and Be contents point to low F melting. B contents in eucrites are lower than in ocean ridge/intraplate basalts (MORBs/OIBs), suggesting B losses during accretion, or accretion of low B materials. Moon: Elevated Li, B, and Be in highland, regolith and some mare suites point to the important role of KREEP in the geochemical development of lunar lavas. Mare basalts and highlands rocks are elevated in B compared to MORBs/OIBs and terrestrial cumulates, though d¹¹B in for both lunar rocks and eucrites, at -4 to -6 ‰, are intermediate between MORBs and OIBs. Lunar picritic glasses have lower B than mare whole rocks. Mars: SNC magmatic contents inferred from shergottite/nahklite pyroxenes are higher in Li, much higher in B, and lower in Be than terrestrial or lunar basalts, indicating the accretion of materials poorer in refractories and richer in volatiles than the Earth/Moon; and the absence of subduction as a major differentiation process in the formation of the Martian crust. Declines in B and Li from core to rim in shergottite pyroxenes indicate that the release of H₂O-rich phases during differentiation may play a role in forming Mars surface rocks. Examining possible hydrothermal phases in SNC's (carbonates, evaporates) may afford a test of this hypothesis.