月球冷海高铝玄武岩遥感识别及其地质特征分析

Study on remote sensing identification and geological characteristics analysis of high alumina mare basalt in Mare Frigoris

高铝玄武岩是美国Apollo和前苏联Luna任务采集的月球样品中极其特殊且具有重要意义的一类样品,对月幔的不均匀性分异和月球早期热演化活动具有重要的指示性意义.本文以地形和成因极其独特的月球冷海中小型撞击坑坑壁或者近端溅射物中新鲜裸露的岩石(岩石丰富>0.5%)为突破口,利用中心和临近像元之间的Al_(2)O_(3)含量和镁指数(Mg#)间的线性关系消除了月海-高地混合物影响,将实验室岩性分类标准应用于Lunar Prospector(LP)钍(Th)和Lunar Reconnaissance Orbiter(LRO)Diviner主要元素氧化物反演产品中,获得月球表面新鲜裸露的高铝玄武岩质(Al_(2)O_(3)>11 wt%)岩石位置信息.以此为基础,通过目视解译的方法识别了埋藏在冷海表面风化层以下的高铝玄武岩斑块(10个)和单元(1个).在LRO宽角(WAC)相机和数字地形图影像(LOLA GLD100)中对高铝玄武岩形态学地质特征统计表明,冷海中的高铝玄武岩总面积和体积为3694 km^(2)和1160 km^(3),岩层的最大埋藏深度和厚度为331 m和207 m.研究结果表明,冷海高铝玄武岩岩层一般具有不一致且不连续的厚度和较大的成分变化范围,并且不是广泛地分布于冷海之中,只是零散地分布于中西部,以及集中于东部的两个表层月海玄武岩地质单元之下.这些分布特征意味着月球冷海早期的高铝型火山活动可能以随机的小型(面积小于500 km^(2)或者体积小于100 km^(3))喷发为主,偶尔会出现大型的喷发.

High alumina(HA)mare basalts are a crucial group of the United States Apollo and former Soviet Union Luna sample collection missions and have important indicative significance for understanding lunar mantle heterogeneity and early thermal evolution.In this paper,the fresh exposed rocks(>0.5%rock abundance)on the small-middle impact crater walls and proximal ejecta surfaces in Mare Frigoris were selected as a window for HA basalts remote sensing identification.After eliminating the mare-highland mixtures influences using the linear relationships between Al_(2)O_(3) abundance and Mg#among the central and adjacent pixels,these exposed rocks lithologies were identified following the laboratory lithologic classification criteria based on Lunar Prospector Th and LRO Diviner major element oxide images.Then,>11 wt%Al_(2)O_(3) compositional constraint was applied to mare basaltic rock pixels and the geographic location information of fresh exposed HA basalt rocks was obtained.Using visual interpretation approach,10 HA basalt patches and one unit in Mare Frigoris were identified.The morphologic features estimated from LRO WAC and LOLA GLD100 images indicate that the total area and volume of Frigoris HA basalts are 3694 km^(2) and 1160 km^(3),the maximum depth and thickness are 331 m and 207 m respectively.The results of this study indicate that HA basalts in Mare Frigoris have discontinuous and variable thicknesses and a wide compositional range.They are not widely distributed in Mare Frigoris,but scattered in the central and eastern Frigoris and mainly concentrated in two western mare basalt units.This distribution of Frigoris high-alumina basalts indicates that the early aluminous volcanic activity may have been dominated by small(less than 500 km^(2) in area or less than 100 km^(3) in volume)random eruptions,with occasional large eruptions.