Northwest Africa 773 Clan Olivine Cumulate Gabbros: Crystallization Trends Compared with a Gabbroic Sill from Murotomisaki, Japan
Northwest Africa 773 Clan Olivine Cumulate Gabbros: Crystallization Trends Compared with a Gabbroic Sill from Murotomisaki, Japan
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发表时间：
2017-07
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通讯作者：
T. Fagan;H. Nagaoka
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作者：
T. Fagan;H. Nagaoka
导言：橄榄石堆积辉长岩(OC)是一种独特的碎屑类型，在西北非洲773(西北非洲773)族月球陨石中常见且丰富[1-4]。有机碳的岩石学意义源于，除其他原因外，将有机碳解释为同岩浆序列结晶过程中的早期岩性，现在被西北地区773族中的几个碎屑保存下来[5]。这种解释取决于辉石中Ti#与Fe#分带的空间变化(图1a)。然而，一个问题是，OC内的分带显示在固定的Fe#处有很大范围的Ti#，而向晚期岩浆碎屑演化需要增加Ti#和Fe#(图1a)。推断有机碳代表岩浆演化的早期阶段是否合理，即使有机碳辉石内的分带并没有显示产生晚期岩性所需的铁#的增加？在这个项目中，我们比较了NWA773族OC的辉石分带趋势与日本室本辉长岩的趋势[6]。室本辉长岩(MGS)的样品已知为同源岩浆，其结晶序列可由场约束得到。方法：Muroto Misaki地质公园的MGS样本[7]于2014年8月采集。从结晶早期到后期，本研究的三个样品是Muro-7、Muro-4和Muro-14(图1C)。制备了抛光薄片，并用岩相显微镜进行检查。使用早稻田大学的JEOL JXA-8900电子探针显微分析仪收集了矿物的图像和定量分析。结果和讨论：在Muro-7和Muro-14(图1B)中，确定了辉石中以恒定的Fe#增加Ti#的早期分带趋势。目前分析的Muro-4辉石在拐点具有恒定的Ti#和Fe#，其中Muro-14的分带由增加的Ti#转变为增加的Fe#(图1B)。Muro-14中最新阶段的辉石显示，在固定的Ti#下，Fe#增加到Ti#和Fe#的高值(图1B)。因此，来自MGS的早期辉石在恒定的Fe#时增加了Ti#，类似于NWA773族OC中的分带(图1a，b；见[5]的“钛-克里普-富集”趋势)。来自MGS的晚期辉石具有较高的Ti#和Fe#值，类似于NWA773晚期碎屑岩，推测为与OC同岩浆[5]。我们推测，MGS和NWA773OC岩浆系统均在早期向高Ti#演化，后期向高Fe#演化，但从早期到晚期的演化路径不同。在镁合金中，在保持钛#不变的情况下，钛#增加之后是铁#增加(图1B)。在NWA773系统中，在OC的捕获口袋中，Ti#增加，液体主体通过同时增加Ti#和Fe#而形成(图1A)。
Introduction: Olivine cumulate gabbro (OC) is a distinctive clast type that is common and abundant in the Northwest Africa 773 (NWA 773) clan of lunar meteorites [1-4]. The petrologic significance of the OC stems from, among other reasons, the interpretation of the OC as an early stage lithology during crystallization of a co-magmatic sequence, now preserved by several clasts in the NWA 773 clan [5]. This interpretation depends on spatial variations in Ti# vs. Fe# zoning in pyroxene (Fig. 1a). One problem, however, is that zoning within the OC shows a wide range of Ti# at fixed Fe#, whereas an evolution toward late-stage magmatic clasts requires increases in both Ti# and Fe# (Fig. 1a). Is it reasonable to infer that the OC represents an early stage of magmatic evolution, even though zoning within the OC pyroxene does not show the increase in Fe# needed to produce late-stage lithologies? In this project, we compare pyroxene zoning trends of the NWA 773 clan OC with those of a gabbroic sill from Murotomisaki, Japan [6]. Samples from the Murotomisaki gabbroic sill (MGS) are known to be co-magmatic and the sequence of crystallization can be derived from field constraints. Methods: Samples of the MGS, which is in the Muroto Misaki Geopark [7], were collected in August 2014. From early to later stages of crystallization, the three samples of this study are Muro-7, Muro-4 and Muro-14 (Fig. 1c). Polished thin sections were prepared and examined using petrographic microscopes. Images and quantitative analyses of minerals were collected using a JEOL JXA-8900 electron probe micro-analyzer at Waseda University. Results and Discussion: An early-stage zoning trend of increasing Ti# at constant Fe# in pyroxene was identified in Muro-7 and Muro-14 (Fig. 1b). The Muro-4 pyroxene analyzed at present has constant Ti# and Fe# at an inflection point, where zoning in Muro-14 changes from increasing Ti# to increasing Fe# (Fig. 1b). The latest stage pyroxene in Muro-14 exhibits increasing Fe# at constant Ti# to values that are high in both Ti# and Fe# (Fig. 1b). Thus, early stage pyroxene from the MGS has increasing Ti# at constant Fe#, similar to zoning in NWA 773 clan OC (Figs. 1a,b; see "titanian-KREEP-enrichment" trend of [5]). Late stage pyroxene from the MGS has high values for both Ti# and Fe#, similar to late-stage NWA 773 clasts inferred to be co-magmatic with the OC [5]. We infer that the MGS and NWA 773 OC magmatic system both evolved toward higher Ti# at early stages and higher Fe# later; however, the paths from early to latest stages of the two magmatic systems differed. In the MGS, Ti# increase was followed by Fe# increase at ~ constant Ti# (Fig. 1b). In the NWA 773 system, Ti# increased in trapped pockets of the OC, and the main body of liquid evolved by simultaneous increases in both Ti# and Fe# (Fig. 1a).