Origin of the eclogitic clasts with graphite-bearing and graphite-free lithologies in the Northwest Africa 801 (CR2) chondrite: Possible origin from a Moon-sized planetary body inferred from chemistry, oxygen isotopes and REE abundances
Origin of the eclogitic clasts with graphite-bearing and graphite-free lithologies in the Northwest Africa 801 (CR2) chondrite: Possible origin from a Moon-sized planetary body inferred from chemistry, oxygen isotopes and REE abundances
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西北非洲含石墨和无石墨岩性的榴辉岩碎屑的起源 801 (CR2) 球粒陨石：根据化学、氧同位素和稀土元素丰度推断，可能起源于月球大小的行星体
DOI：
10.1016/j.gca.2016.04.030
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发表时间：
2016
影响因子：
5
通讯作者：
Y. Takehana
中科院分区：
文献类型：
--
作者：
H. Hiyagon;N. Sugiura;N.T. Kita;M. Kimura;Y. Morishita;Y. Takehana
为了阐明在NWA 801（CR2）球粒陨石中发现的榴辉碎屑的起源（Kimura等人，2013），特别是在高温高压条件下（~ 3GPa和~ 1000 °C），对碎屑进行了氧同位素和稀土元素丰度的离子探针分析。含石墨岩性（GBL）和不含石墨岩性（GFL）的氧同位素组成在O三同位素图解中显示出斜率为0.6的相关性，略低于CR-CH-CB辉长岩场，前者的变化较大，后者的组成几乎是均匀的。两种岩性的稀土元素平均丰度几乎呈未分馏模式。基于这些新获得的数据，以及矿物学观察，体化学，并考虑各种元素的扩散时标，我们详细讨论了碎屑的形成历史。Kimura等人（2013年）使用的地质温压计的一致性表明，不同矿物对之间的各种元素是平衡的，这对高P-T条件的持续时间提供了强有力的限制。我们认为高PT状态持续了102- 103年。这显然排除了冲击高压（HP）模型，因此，强烈支持静态HP模型。一个静态的HP模型需要一个月球大小的行星体，半径为1500公里。此外，它意味着两个连续的剧烈碰撞，首先是在大行星体的形成时，当碎屑被放置在其内部深处时，第二，在大行星体的破裂时，当碎屑被驱逐出母体，后来被运送到CR陨石的吸积区域。我们还讨论了GBL中O同位素变化的可能来源，以及GBL/GFL中石墨的存在/不存在，分别与形成这两种岩性的火成岩过程中可能发生的熔炼有关。最后，我们提出了一个可能的榴辉岩碎屑的形成方案。
In order to clarify the origin of the eclogitic clasts found in the NWA801 (CR2) chondrite (Kimura et al., 2013), especially, that of the high pressure and temperature (P–T) condition (∼3 GPa and ∼1000 °C), we conducted ion microprobe analyses of oxygen isotopes and rare earth element (REE) abundances in the clasts. Oxygen isotopic compositions of the graphite-bearing lithology (GBL) and graphite-free lithology (GFL) show a slope ∼0.6 correlation slightly below the CR-CH-CB chondrites field in the O three-isotope-diagram, with a large variation for the former and almost homogeneous composition for the latter. The average REE abundances of the two lithologies show almost unfractionated patterns. Based on these newly obtained data, as well as mineralogical observations, bulk chemistry, and considerations about diffusion timescales for various elements, we discuss in detail the formation history of the clasts. Consistency of the geothermobarometers used by Kimura et al. (2013), suggesting equilibration of various elements among different mineral pairs, provides a strong constraint for the duration of the highP–Tcondition. We suggest that the highP–Tcondition lasted 102–103years. This clearly precludes a shock high pressure (HP) model, and hence, strongly supports a static HP model. A static HP model requires a Moon-sized planetary body of ∼1500 km in radius. Furthermore, it implies two successive violent collisions, first at the formation of the large planetary body, when the clasts were placed its deep interior, and second, at the disruption of the large planetary body, when the clasts were expelled out of the parent body and later on transported to the accretion region of the CR chondrites. We also discuss possible origin of O isotopic variations in GBL, and presence/absence of graphite in GBL/GFL, respectively, in relation to smelting possibly occurred during the igneous process(es) which formed the two lithologies. Finally we present a possible formation scenario of the eclogitic clasts.