Publications (2)0 Total impact
Article: The Source Region and Melting Mineralogy of High-Titanium and Low-Titanium Lunar Basalts Deduced from Lu-Hf Isotope Data[show abstract] [hide abstract]
ABSTRACT: Five high-Ti basalts from the Apollo 11 and 17 landing sites have been analyzed for their hafnium isotope composition. These data serve to better constrain the hafnium isotope variation of the Moon’s mantle. Variations in initial ϵHf and ϵNd 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 ϵHf values, at a given ϵNd value, than high-Ti basalts. The differences in the hafnium and neodymium isotopic composition of low- and high-Ti basalts reflect the fact that the source of low-Ti basalts 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 are partial melts of the Moon’s cumulate mantle, the differences between low- and high-Ti basalts can only be explained by these mare magma types being generated from melting sources with distinctly different mineral assemblages. The large Lu/Hf fractionations, relative to Sm/Nd fractionations, of low-Ti basalts can best be produced by an assemblage of olivine and orthopyroxene with trace amount of clinopyroxene that crystallized early in the history of the Lunar Magma Ocean (LMO). The subequal [Lu/Hf]n and [Sm/Nd]n fractionations of high-Ti basalts can be produced from a variety of ilmenite-bearing mineral assemblages.Low- and high-Ti basalts have similar Lu/Hf ratios, approximately 0.6 times chondrite. The low Lu/Hf ratios measured for these mare magmas contrast sharply with the high Lu/Hf ratios (greater than chondritic) calculated for their sources from initial ϵHf values and an assumed chondritic bulk moon initial ϵHf value. The difference between the measured Lu/Hf of a lava, vs. the calculated Lu/Hf of its source, implies that during partial melting, Lu was preferentially retained in the residual source, relative to Hf. In order to explain the extreme fractionation of measured Lu/Hf ratios we suggest mare basalts can best be explained using a polybaric melting model. Initial melting of a garnet bearing source followed by continued melting in the spinel stability field can produce the required Lu/Hf fractionations and produce a liquid that last equilibrated with a residuum of olivine and orthopyroxene.Geochimica et Cosmochimica Acta.
[show abstract] [hide abstract]
ABSTRACT: Owing to the association with diamonds, eclogite xenoliths have received disproportionate attention given their low abundance in kimberlites. Several hypotheses have been advanced for the origin of eclogite xenoliths, from the subduction and high-pressure melting of oceanic crust, to cumulates and liquids derived from the upper mantle. We have amassed a comprehensive data set, including major- and trace-element mineral chemistry, carbon isotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os, and oxygen isotopes in ultrapure mineral and whole-rock splits from eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia. Furthermore, eclogites from two other Yakutian kimberlite pipes, Mir and Obnazhennaya, have been studied in detail and offer contrasting images of eclogite protoliths. Relative to eclogites from southern Africa and other Yakutian localities, Udachnaya eclogites are notable in the absence of chemical zoning in mineral grains, as well as the degree of light rare earth element (LREE) depletion and unradiogenic Sr; lack of significant oxygen, sulfur, and carbon isotopic variation relative to the mantle; and intermineral radiogenic isotopic equilibration. Several of these eclogites could be derived from ancient, recycled, oceanic crust, but many others exhibit no evidence for an oceanic crustal protolith. The apparent lack of stable-isotope variation in the Udachnaya eclogites could be due to the antiquity of the samples and consequent lack of deep oceanic and biogenically diverse environments at that time. Those eclogites that are interpreted to be non-recycled have compositions characteristic of Group A eclogites from other localities that also have been interpreted as being directly from the mantle. At least two separate and diverse isotopic reservoirs are suggested by Nd isotopic whole-rock reconstructions. Most samples were derived from typical depleted mantle. However, two groups of three samples each indicate both enriched mantle and possible ultra-depleted mantle present beneath Yakutia during the late Archean and early Proterozoic. The vast majority of eclogites studied from the Obnazhennaya pipe also exhibit characteristics of Group A eclogites and are probably derived directly from the mantle. However, the eclogites from the Mir kimberlite are more typical of other eclogites world-wide and show convincing evidence of a recycled, oceanic crustal affinity. We concur with the late Ted Ringwood that eclogites can be formed in a variety of ways, both within the mantle and from oceanic crustal residues.