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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >The source region and melting mineralogy of high-titanium and low-titanium lunar basalts deduced from Lu-Hf isotope data
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The source region and melting mineralogy of high-titanium and low-titanium lunar basalts deduced from Lu-Hf isotope data

机译：从Lu-Hf同位素数据推算的高钛和低钛月球玄武岩的源区和熔融矿物学

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摘要

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 ε_(Hf) and ε_(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 ε_(Hf) values, at a given ε_(Nd) 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 ε_(Hf) values and an assumed chondritic bulk moon initial ε_(Hf) 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.
机译：对来自阿波罗11号和17号登陆点的5个高钛玄武岩的ha同位素组成进行了分析。这些数据有助于更好地限制月球地幔的is同位素变化。低钛玄武岩和高钛玄武岩的初始ε_（Hf）和ε_（Nd）值的变化表明，这些月岩浆类型的源区矿物组合是不同的。在给定的ε_（Nd）值下，低钛玄武岩的初始ε_（Hf）值高于高钛玄武岩。低钛和高钛玄武岩的and和钕同位素组成的差异反映了一个事实，即低钛玄武岩源的[Lu / Hf] _n比约为其[Sm / Nd] _n比的四倍。 。相反，高Ti源区具有[Lu / Hf] _n和[Sm / Nd] _n之比。如果假设母马玄武岩是月球累积地幔的部分熔融，那么低钛和高钛玄武岩之间的差异只能用这些母岩岩浆类型来解释，这些母岩岩浆是由具有明显不同矿物组合的熔融源产生的。相对于Sm / Nd分离而言，低Ti玄武岩的大型Lu / Hf分离最好是由橄榄石和邻苯二甲醚与微量的clinopyroxene组合产生的，该组合在月球岩浆海洋（LMO）历史的早期就已结晶。高钛玄武岩的亚等分[Lu / Hf] _n和[Sm / Nd] _n分馏可以从多种含钛铁矿的矿物组合中产生。低钛和高钛玄武岩具有相似的Lu / Hf比，约为球粒陨石的0.6倍。从这些母岩浆测得的低Lu / Hf比值与根据初始ε_（Hf）值和假设的软骨体大月球初始ε_（Hf）值计算出的高Lu / Hf比值（大于软骨状）形成鲜明对比。熔岩的实测Lu / Hf与熔岩源的计算的Lu / Hf之间的差异表明，在部分融化过程中，相对于Hf，Lu优先保留在残余源中。为了解释测得的Lu / Hf比值的极端分馏，我们建议可以使用多角度熔融模型最好地解释母马玄武岩。石榴石承载源的最初熔化，然后在尖晶石稳定性场中继续熔化，可以产生所需的Lu / Hf分馏，并产生最后用橄榄石和邻二甲苯残渣平衡的液体。

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