Abstract

Two alternative petrogenetic models, plagioclase flotation and serial magmatism have been proposed to explain the origin of the lunar anorthositic crust, covering ~80% of the lunar highland. In this study, we re-examine the possible relict igneous texture present in an inferred lunar highland breccia clast (area ~1 mm²) in the Dhofar 081 meteorite. Our new petrographic and in-situ mineral microprobe chemical data on this clast show this coarse grained (average grain size ~0.5 mm) clast preserves relict igneous texture where subhedral, prismatic low-Ca pyroxene has intergrown with anhedral anorthitic plagioclase, suggesting its eutectic crystallization from its parent silicate magma. Absence of maskelynite and similarity of Na, K contents of plagioclase with the FAN assemblages negate the possibility of crystallization of the studied relict clast from an impact melt. The mineral-chemical data of Dhofar 081 suggest it is FAN (Ferroan anorthosite) in composition (after Warren in Annu. Rev. Earth Planet. Sci. 13:201–240, 1985). Hence, intergrown crystallization of minerals in the present relict igneous clasts and other reported FAN samples argues against a cumulate origin of the lunar anorthosite. The orthopyroxenes present in the unbrecciated portion of this meteoritic clast include bimodal low- and high-iron geochemical sub-groups. The application of orthopyroxene and plagioclase thermobarometry (after Gasparik in Contrib. Mineral. Petrol. 96:357–370, 1987) on our new microprobe data, and also two-pyroxene thermometry (after Lindsley in Am. Mineral. 68:477–493, 1983; Putirka in Rev. Mineral. Geochem. 69(1):61–120, 2008) on our new microprobe data and synthesis of literature data constrain the pressure and temperature of crystallization of lunar anorthosite parent magma close to 8 kbar and 1050°C, respectively. Application of Fo–An–Q experimental phase diagram at high pressure (up to 20 kbar) negates the possibility of generation of lunar anorthosite from a lherzolite source, the parent magma of these anorthosites probably lie on or close to Fo–An join of this phase diagram close to the spinel field.

Research Highlights

• Lunar anorthositic meteorite represents the global highland crust of Moon.

• Relict igneous clast of lunar anorthositic meteorite shows intergrown texture vis-a-vis eutectic crystallisation.

• Pressure and temperature of crystallisation of parent magma close to 8 kbar and 1050°C.

• Serial magmatism is consistent to explain the textural and mineral-chemical characters and vis-à-vis petrogenesis of lunar anorthosite.

中文翻译：

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月球长石陨石Dhofar 081：岩石化学对成岩作用的限制

摘要

已经提出了两种替代的岩石成因模型，斜长石浮选和连续岩浆作用来解释月球无脊椎动物壳的起源，覆盖了约80％的月球高地。在这项研究中，我们重新检查了在Dhofar 081陨石中推断出的高地角砾岩碎屑（面积约1 mm ²）中可能存在的遗迹火成岩质地。我们的新岩石学和原位该碎屑的矿物微探针化学数据显示，该粗粒（平均晶粒尺寸约为0.5 mm）碎屑保留了残质火成岩质地，其中亚面角形低钙辉石与无水无斜斜长石共生，表明它是由其母体硅酸盐岩浆形成的共晶结晶。缺少辉绿沸石，斜长石的Na，K含量与FAN组合相似，从而消除了研究熔渣碎屑从冲击熔体中结晶的可能性。Dhofar 081的矿物化学数据表明，其成分为FAN（铁铝钙硅铝石）（仅次于Warren，发表于Annu。Rev. Earth Planet。Sci 。13：201–240，1985年）。因此，目前的遗留火成岩碎屑中矿物的共生结晶和其他报道的FAN样品与月球无钙铁矿的累积成因相抵触。存在于该陨石层的未角砾化部分中的邻苯二甲include包括双峰低铁和高铁地球化学亚组。斜的应用和斜长石thermobarometry（Gasparik在后矿物汽油的Contrib。 96：357-370，1987）对我们的新探针数据，以及两个辉石测温（Lindsley在后上午矿物。 68：477-493 ，1983； Putirka，Rev.Mineral.Geochem。69（1）：61–120，2008）关于我们新的微探针数据和文献数据的综合约束，月球无钙铁矿母岩浆的结晶压力和结晶温度分别接近8 kbar和1050°C。在高压（最高20 kbar）下应用Fo–An–Q实验相图可消除从锂铁矿来源生成月球钙铁矿的可能性，这些钙铁矿的母岩浆可能位于或靠近Fo–An。相图接近尖晶石场。

研究重点

• 月球无定形陨石代表着月球的全球高地壳。

• 月球无定形陨石的残存火成岩岩屑相对于共晶结晶显示出相互融合的纹理。

• 母岩浆结晶的压力和温度接近8 kbar和1050°C。

• 连续岩浆作用可以解释月球钙长石的质地和矿物化学特征以及相对于成岩作用。