17:15 〜 18:45
[PPS07-P08] Varieties of Gabbro in Lunar Meteorite NWA 2727: Implications for Crystallization Processes in a Lunar Magmatic System
キーワード：月隕石、火成岩の結晶化、火成岩の岩石学、月
Introduction: The NWA 773 clan consists of a group of petrogenetically related lunar meteorites with several lithologies from a common magmatic system on the Moon [1,2]. The olivine cumulate gabbro (OCG) is a characteristic rock-type of the NWA 773 clan, with main minerals consisting of olivine, low-Ca pyroxene, high-Ca pyroxene, plagioclase feldspar. The OCG represents an early stage of crystallization in the NWA 773 clan magmatic system. Previous studies have described the OCG as a homogeneous lithology [1-4]. In this study, we identify compositionally and texturally diverse varieties of the OCG that shed light on crystallization processes in the NWA 773 magmatic system.
Method: Electron probe micro-analysis (EPMA) was conducted for both elemental mapping and quantitative analyses using the JEOL JXA-8900 EPMA at Waseda University. Elemental maps of were collected from one thin section of NWA 2727 and one thin section of NWA 6950 [5]. Quantitative analyses were collected from olivine, pyroxene, and plagioclase feldspar in three clasts of OCG from NWA 2727: clasts-1, -4a and -5.
Results: NWA 2727 clast-1 is more magnesian (less ferroan) than clasts-4a and 5. Olivine analyses from clast-1 have an average of Fa28(range, Fa27-29), whereas analyses from clast-4a and clast-5 have averages of Fa38 (range, Fa37-39) and Fa39 (range, Fa37-42), respectively. In contrast, plagioclase analyses for all three clasts overlap in composition, with averages of An90-91, and a narrow range of An87-94 for analyses from all three clasts.
Analyses of pyroxenes from clast-1 and clast-4a are consistent with the olivine compositions, indicating that clast-1 is more magnesian than clast-4a. Pyroxene analyses from clast-1 yield an average Fe# = 25 (range, Fe# 20-26; Fe# = molar FeO/[MgO+FeO]x100), whereas pyroxene from clast-4a has average Fe# = 33 (range, Fe# 33-34). In contrast, pyroxene analyses from clast-5 have two clusters of compositions: one magnesian (mean Fe# = 25, range Fe# 22-26, n = 9), similar to clast-1; and the other more ferroan (mean Fe# = 33, range Fe# 29-34), similar to clast-4a.
After the two clusters of pyroxene compositions (Fe# = 25 and Fe# = 33) were detected, the textures of clast-5 were examined in more detail and two varieties of OCG were identified. “Islands” of pyroxene-rich OCG (pyx-OCG) are surrounded by feldspar-rich OCG (fsp-OCG) (Fig. 1). Of 9 pyroxene analyses in the pyx-OCG, 8 are magnesian (Fe# ~ 25); and 9 of 10 analyses in the fsp-OCG are ferroan (Fe# ~ 33). Elemental maps show that K-P-rich incompatible element-rich pockets are concentrated in the fsp-OCG. Similar results were obtained from pyroxene-rich and feldspar-rich varieties of OCG in NWA 6950 (Fig. 2). Based on analyses from a previous study [5], average compositions of olivine are Fa30 for the pyx-OCG and Fa31 for the fsp-OCG in NWA 6950.
Discussion: The NWA 773 clan OCG can be divided into at least four varieties based on compositions, mineral abundances and textures. Magnesian OCGs have olivine with Fo-contents near Fo30. Ferroan OCGs have olivine with Fo-contents near Fo40. Within the magnesian and ferroan OCGs, pyroxene-rich varieties (pyx-OCG) occur as islands surrounded by feldspar-rich varieties (fsp-OCG) (Figs. 1,2).
We infer that the pyx-OCG formed by crystal growth and pieces were entrained in later magma which crystallized to form the fsp-OCG by a combination of crystal growth and settling. Incompatible element-rich pockets of liquid were trapped during crystal settling in the fsp-OCG. Pyx-OCG and fsp-OCG both formed during magnesian and ferroan stages of crystallization of the NWA 773 magmatic system on the Moon.
References: [1] Fagan T. J. (2014) GCA, 133, 97-127. [2] Valencia et al. (2019) MaPS, 54, 2083-2115. [3] Jolliff B.L. (2003) GCA, 67, 4857-4879. [4] Korotev R. L. and Irving A.J. (2021) MaPS, 56, 206-240. [5] Hori Y. (2015) Waseda Univ. undergraduate thesis, 18 pages plus figures & tables.
Method: Electron probe micro-analysis (EPMA) was conducted for both elemental mapping and quantitative analyses using the JEOL JXA-8900 EPMA at Waseda University. Elemental maps of were collected from one thin section of NWA 2727 and one thin section of NWA 6950 [5]. Quantitative analyses were collected from olivine, pyroxene, and plagioclase feldspar in three clasts of OCG from NWA 2727: clasts-1, -4a and -5.
Results: NWA 2727 clast-1 is more magnesian (less ferroan) than clasts-4a and 5. Olivine analyses from clast-1 have an average of Fa28(range, Fa27-29), whereas analyses from clast-4a and clast-5 have averages of Fa38 (range, Fa37-39) and Fa39 (range, Fa37-42), respectively. In contrast, plagioclase analyses for all three clasts overlap in composition, with averages of An90-91, and a narrow range of An87-94 for analyses from all three clasts.
Analyses of pyroxenes from clast-1 and clast-4a are consistent with the olivine compositions, indicating that clast-1 is more magnesian than clast-4a. Pyroxene analyses from clast-1 yield an average Fe# = 25 (range, Fe# 20-26; Fe# = molar FeO/[MgO+FeO]x100), whereas pyroxene from clast-4a has average Fe# = 33 (range, Fe# 33-34). In contrast, pyroxene analyses from clast-5 have two clusters of compositions: one magnesian (mean Fe# = 25, range Fe# 22-26, n = 9), similar to clast-1; and the other more ferroan (mean Fe# = 33, range Fe# 29-34), similar to clast-4a.
After the two clusters of pyroxene compositions (Fe# = 25 and Fe# = 33) were detected, the textures of clast-5 were examined in more detail and two varieties of OCG were identified. “Islands” of pyroxene-rich OCG (pyx-OCG) are surrounded by feldspar-rich OCG (fsp-OCG) (Fig. 1). Of 9 pyroxene analyses in the pyx-OCG, 8 are magnesian (Fe# ~ 25); and 9 of 10 analyses in the fsp-OCG are ferroan (Fe# ~ 33). Elemental maps show that K-P-rich incompatible element-rich pockets are concentrated in the fsp-OCG. Similar results were obtained from pyroxene-rich and feldspar-rich varieties of OCG in NWA 6950 (Fig. 2). Based on analyses from a previous study [5], average compositions of olivine are Fa30 for the pyx-OCG and Fa31 for the fsp-OCG in NWA 6950.
Discussion: The NWA 773 clan OCG can be divided into at least four varieties based on compositions, mineral abundances and textures. Magnesian OCGs have olivine with Fo-contents near Fo30. Ferroan OCGs have olivine with Fo-contents near Fo40. Within the magnesian and ferroan OCGs, pyroxene-rich varieties (pyx-OCG) occur as islands surrounded by feldspar-rich varieties (fsp-OCG) (Figs. 1,2).
We infer that the pyx-OCG formed by crystal growth and pieces were entrained in later magma which crystallized to form the fsp-OCG by a combination of crystal growth and settling. Incompatible element-rich pockets of liquid were trapped during crystal settling in the fsp-OCG. Pyx-OCG and fsp-OCG both formed during magnesian and ferroan stages of crystallization of the NWA 773 magmatic system on the Moon.
References: [1] Fagan T. J. (2014) GCA, 133, 97-127. [2] Valencia et al. (2019) MaPS, 54, 2083-2115. [3] Jolliff B.L. (2003) GCA, 67, 4857-4879. [4] Korotev R. L. and Irving A.J. (2021) MaPS, 56, 206-240. [5] Hori Y. (2015) Waseda Univ. undergraduate thesis, 18 pages plus figures & tables.