THE SOURCE REGION AND MELTING MINERALOGY OF HIGH-TITANIUM AND LOW-TITANIUM LUNAR BASALTS DEDUCED FROM LU-HF ISOTOPE DATA

Five high-Ti basalts from the Apollo 11 and 17 landing sites have been analyzed for their hafnium isotope composition. These data serve to b etter constrain the hafnium isotope variation of the Moon's mantle. Va riations in initial epsilon(Hf) and epsilon(Nd) values of low-and high -Ti basalts imply that the source region mineral assemblages of these lunar magma types are distinct. Low-Ti basalts have higher initial eps ilon(Hf) values, at a given epsilon(Nd) value, than high-Ti basalts. T he differences in the hafnium and neodymium isotopic composition of lo w- and high-Ti basalts reflect the fact that the source of low-Ti basa lts had a [Lu/Hf](n) ratio approximately four times greater than its [ Sm/Nd](n) ratio. In contrast, the high-Ti source region had subequal [ Lu/Hf](n) and [Sm/Nd](n) ratios. If it is assumed that mare basalts ar e partial melts of the Moon's cumulate mantle, the differences between low-and high-Ti basalts can only be explained by these mare magma typ es being generated from melting sources with distinctly different mine ral assemblages. The large Lu/Hf fractionations, relative to Sm/Nd fra ctionations, of low-Ti basalts can best be produced by an assemblage o f olivine and orthopyroxene with trace amount of clinopyroxene that cr ystallized early in the history of the Lunar Magma Ocean (LMO). The su bequal [Lu/Hf](n) and [Sm/Nd](n) fractionations of high-Ti basalts can be produced from a variety of ilmenite-bearing mineral assemblages. L ow- and high-Ti basalts have similar Lu/Hf ratios, approximately 0.6 t imes chondrite. The low Lu/Hf ratios measured for these mare magmas co ntrast sharply with the high Lu/Hf ratios (greater than chondritic) ca lculated for their sources from initial epsilon(Hf) values and an assu med chondritic bulk moon initial epsilon(Hf) value. The difference bet ween the measured Lu/Hf of a lava, vs. the calculated Lu/Hf of its sou rce, implies that during partial melting, Lu was preferentially retain ed in the residual source, relative to Hf. Zn order to explain the ext reme fractionation of measured Lu/Hf ratios we suggest mare basalts ca n best be explained using a polybaric melting model. Initial melting o f a garnet bearing source followed by continued melting in the spinel stability field can produce the required Lu/Hf fractionations and prod uce a liquid that last equilibrated with a residuum of olivine and ort hopyroxene. Copyright (C) 1998 Elsevier Science Ltd.