Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels
Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels
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DOI：
10.1111/j.1945-5100.1996.tb02121.x
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发表时间：
1996-11
影响因子：
2.2
通讯作者：
T. Arai;P. Warren;H. Takeda
中科院分区：
文献类型：
--
作者：
T. Arai;P. Warren;H. Takeda
- 本文研究了来自月海地区的四块南极陨石中辉石和尖晶石的结晶趋势：Y-793169和A-881757（YA陨石）为未角砾化的火成岩玄武岩，EET 87521为碎屑角砾岩，Y-793274为风化角砾岩。所有的陨石都有相对较低的块状岩石二氧化钛含量，YA陨石是罕见的古老。我们的电子探针微区分析（EPMA）数据表明，YA陨石和占主导地位的月海成分的Y-793274和EET 87521符合钛贫（低钛和极低钛）月海玄武岩的一般趋势。其辉石在Fe/（Fe + Mg）（Fe#）和Ti/（Ti + Cr）（Ti#）之间表现出很强的相关性，这两个比值通常从核到边缘增加。这些趋势可能反映了间隙熔体的局部结晶分异。以前的研究（M。Drake及其同事）提出，这种Fe#与Ti#趋势的详细配置可能反映了母岩浆（玄武岩）的大量TiO 2含量。作为一个更系统的方法来解决这个问题，我们绘制散装岩石TiO 2作为函数的Fe# = 0.50截距的每一块岩石的辉石Fe#与Ti#的趋势。我们称这个截距为Fe#归一化Ti#。基于我们的数据EET 87521，YA陨石，阿波罗12号玄武岩12031和12064，再加上其他几个钛贫月海玄武岩的文献数据，我们发现Fe#-归一化Ti#和大量的TiO 2含量的母玄武岩之间有很强的相关性。这种相关性证实，破碎角砾岩EET 87521几乎是纯极低钛（VLT）玄武岩和YA陨石，其中大块岩石TiO 2的结果分散由于异常粗粒度（A-881757）或可用的样品（Y-793169）的稀缺性，是一块不常见的钛贫，但不完全VLT，各种低钛月海玄武岩。从这种相关性推断，风化角砾岩Y-793274的主要组成部分可能是VLT亲和力。除了Fe#和Ti#之间强相关性的正常月海辉石趋势外，Y-793274还包括两个额外的辉石成分趋势，尽管Fe#相对恒定（并且较低，按月海标准），但两者都显示出较宽的Ti#范围。这些趋势中最富镁的是一个单一的碎屑，模式为斜方辉石+富含MgO的钛铁矿。这两种趋势的起源并不确定。可能是一个或两个代表高地成分的风化角砾岩，虽然，不像大多数高地辉石，这些似乎相对不受影响角砾岩和变质作用。粗粒A-881757中尖晶石的组成显示出非凡的分布：铬铁矿和ulvospinel组分在颗粒之间变化，但在颗粒内几乎是恒定的。尽管它年代久远，粒度异常粗，但矿物学证据（即，辉石和尖晶石的不均匀性;典型的辉石出溶规模非常粗的月海标准，但超过了辉石的EET 87521和Y-793274）表明，A-881757只是略慢于典型的月海玄武岩冷却，并可能已形成一个不寻常的厚熔岩流的中心附近。这两个VLT玄武岩月球陨石角砾岩，EET 87521和Y-793274，主要是由辉石与出溶粗比正常的月海玄武岩。可能VLT玄武岩流往往是系统性的厚，因此更缓慢地冷却，比更多的富钛流。
— We studied crystallization trends of pyroxene and spinel in four Antarctic meteorites known to be derived from mare regions of the Moon: Y-793169 and A-881757 (YA meteorites) are unbrecciated igneous basalts, EET 87521 is a fragmental breccia, and Y-793274 is a regolith breccia. All have relatively low bulkrock TiO2 content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data indicate that the YA meteorites and the dominant mare components of Y-793274 and EET 87521 conform to a general trend for Ti-poor (low-Ti and very low-Ti) mare basalts. Their pyroxenes show a strong correlation between Fe/(Fe + Mg) (Fe#) and Ti/(Ti + Cr) (Ti#), both ratios typically increasing from core to rim. These trends presumably reflect local crystallization differentiation of interstitial melt. Previous studies (M. J. Drake and coworkers) have suggested that the detailed configurations of such Fe# vs. Ti# trends may reflect the bulk TiO2 contents of the parent magmas (basalts). As a more systematic approach to this problem, we plot bulk-rock TiO2 as a function of the Fe# = 0.50 intercept of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept the Fe#-normalized Ti#. Based on our data for EET 87521, the YA meteorites, and Apollo 12 basalts 12031 and 12064, plus literature data for several other Ti-poor mare basalts, we find a strong correlation between Fe#-normalized Ti# and the bulk TiO2 content of the parent basalt. This correlation confirms that fragmental breccia EET 87521 is nearly pure very low-Ti (VLT) basalt and that the YA meteorites, for which bulk-rock TiO2 results scatter due to unusually coarse grain size (A-881757) or scarcity of available sample (Y-793169), are pieces of an uncommonly Ti-poor, but not quite VLT, variety of low-Ti mare basalt. Extrapolating from this correlation, the dominant mare component of regolith breccia Y-793274 is probably of VLT affinity. Besides the normal mare pyroxene trend of strong correlation between Fe# and Ti#, Y-793274 includes two additional pyroxene compositional trends, both showing a wide range of Ti# despite relatively constant (and low, by mare standards) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO-rich ilmenite. These two trends are of uncertain origin. Possibly one or both represents the highland component of this regolith breccia, although, unlike most highland pyroxenes, these appear relatively unaltered by impact brecciation and metamorphism. Compositions of spinels in the coarse-grained A-881757 show an extraordinary distribution: chromite and ulvospinel components vary among grains but are nearly constant within grains. Despite its old age and unusually coarse grain sizes, mineralogical evidence (i.e., heterogeneity within both pyroxene and spinel; typical pyroxene exsolution scale very coarse by mare standards but exceeded by the pyroxenes of EET 87521 and Y-793274) indicates that A-881757 was cooled only slightly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic lunar meteorite breccias, EET 87521 and Y-793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Possibly VLT basalt flows tend to be systematically thicker, and thus more slowly cooled, than more Ti-rich flows.