TY - JOUR
T1 - The variety of lithologies in the Yamato-86032 lunar meteorite
T2 - Implications for formation processes of the lunar crust
AU - Yamaguchi, A.
AU - Karouji, Y.
AU - Takeda, H.
AU - Nyquist, L.
AU - Bogard, D.
AU - Ebihara, M.
AU - Shih, C. Y.
AU - Reese, Y.
AU - Garrison, D.
AU - Park, J.
AU - McKay, G.
N1 - Funding Information:
We thank Drs. Kojima H., and Arai T. for technical assistance and discussion, and R. Korotev, P.H. Warren, G.J. Taylor, and T. Mikouchi for reviews of earlier version of the paper. This work is partly supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Science, and Technology, Japan No. 19540511 (A.Y.) and 19340169 (M.E.) and by NIPR Research Project Funds, P-8 (Evolution of the early Solar System materials). This work is further supported in part by funds from the cooperative program (No. 005, 2007) provided by Ocean Research Institute, the University of Tokyo (H.T.) and by the NASA Cosmochemistry Program (L.N., D.B., C.S., Y.R., D.G., G.M.).
PY - 2010/8
Y1 - 2010/8
N2 - We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an " An97 anorthosite" that has An contents similar to those of nearside FANs. Mg' (= molar Mg/(Mg+Fe)×100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg' gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2×3.6cm2) mainly consists of fragments of anorthosites (" An93 anorthosite" ) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg' variation indicates that the " An97 anorthosite" is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg' in mafic minerals. The highest Mg' values in mafic minerals for the " An97 anorthosite" and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and " An93 anorthosite" , and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.
AB - We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an " An97 anorthosite" that has An contents similar to those of nearside FANs. Mg' (= molar Mg/(Mg+Fe)×100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg' gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2×3.6cm2) mainly consists of fragments of anorthosites (" An93 anorthosite" ) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg' variation indicates that the " An97 anorthosite" is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg' in mafic minerals. The highest Mg' values in mafic minerals for the " An97 anorthosite" and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and " An93 anorthosite" , and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.
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U2 - 10.1016/j.gca.2010.04.015
DO - 10.1016/j.gca.2010.04.015
M3 - Article
AN - SCOPUS:77954029058
VL - 74
SP - 4507
EP - 4530
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
IS - 15
ER -