[PPS10-16] Petrology of ferroan diogenites and cumulate eucrites and their genetic relationships
キーワード：分化隕石、ユークライト、ダイオジェナイト
Achondrites record the early differentiation of the planetesimals and protoplanets. The HED (howardites, eucrites, diogenites) meteorites are the largest group of achondrites, possibly derived from a differentiated asteroid, 4 Vesta. Eucrites are basalts or gabbros, and diogenites are mainly orthopyroxenites. The genetic relationship between eucrites and diogenites is a subject of debate. Classically, diogenites are believed to be cumulates crystallized from an early magma ocean, whereas eucrites are residual liquids after extensive fractional crystallization (e.g., Takeda and Mori 1986). However, the wide range of the trace element abundances indicates that most diogenites are not genetically related (Mittlefehldt 1994) and crystallized from melts produced by remelting of early magma ocean cumulates (Barrat 2008). In this study, we focused on ferroan diogenites, whose FeO/MgO values are similar to those of cumulate eucrites and attempted to examine their petrogenetic relationships.
We examined ferroan diogenites, Y-75032 type (Y-75032, Y-791199) and A-881839, and cumulate eucrites, Y-791195, Y 980433, Moore County, and Moama by an optical and an FE-SEM-EDS and an EPMA. We determined bulk major and trace element compositions of pyroxene and plagioclase by an EPMA with ~30 µm beam diameter and averages of 10-35 analyses and LA-ICP-MS (Thermo Element XR equipped with LSX-213 Laser) at NIPR with line mode (a few mm long) and a beam 50-100 µm in beam diameter.
On the pyroxene quadrilateral, pyroxene compositions in the ferroan diogenites are plotted between cumulate eucrites and typical diogenites. This is consistent with the presence of three types of pyroxenes (one type of orthopyroxene, and two types of inverted clinopyroxenes) (Mori and Takeda 1985). A diogenite, A-881839 is relatively ferroan consistent with the presence of Binda type pyroxene. In contrast, the situation is quite different in the case of minor and trace elements. For example, pyroxenes in the Y-75032, Y-791199, and A-881389 are richer in TiO2, compared to the two other ferroan diogenites (e.g., Dho 700), and other diogenites. The trace element compositions of pyroxenes in Y-75032 types are rather similar to those of cumulate eucrites.
We estimated the REE compositions of parental melts by the same approach made by Barrat (2004). The presence of plagioclase indicates that Y-75032, Y-791199, and A-881399 contain significant amounts of trapped liquid. Using the REE data of pyroxenes and assuming 5-10% of trapped melts, the compositions of the melts are estimated to be roughly similar to those of basaltic eucrites. This may indicate that Y-75032 type is pigeonite cumulate eucrites rather than diogenite from a petrogenetic point of view. It seems likely that there are two types of diogenites from the petrogenetic point of view, digenites crystallized from secondary melts from early-formed cumulates (normal diogenites) (Barrat et al. 2008) and diogenites crystallized from eucritic magma (magma ocean).
References: Barrat J.A. et al. (2004) Meteor. Planet. Sci. 39, 1767-1779. Barrat J.A. et al. (2008) Meteor. Planet. Sci. 43, 1759-1775. Mittlefehldt D.W. (1994) Geochim. Cosmochim. Acta 58, 1537-1552. Takeda H. and Mori H. (1985) Proc. 15th LPSC, JGR 90, suppl., C636-C648.
We examined ferroan diogenites, Y-75032 type (Y-75032, Y-791199) and A-881839, and cumulate eucrites, Y-791195, Y 980433, Moore County, and Moama by an optical and an FE-SEM-EDS and an EPMA. We determined bulk major and trace element compositions of pyroxene and plagioclase by an EPMA with ~30 µm beam diameter and averages of 10-35 analyses and LA-ICP-MS (Thermo Element XR equipped with LSX-213 Laser) at NIPR with line mode (a few mm long) and a beam 50-100 µm in beam diameter.
On the pyroxene quadrilateral, pyroxene compositions in the ferroan diogenites are plotted between cumulate eucrites and typical diogenites. This is consistent with the presence of three types of pyroxenes (one type of orthopyroxene, and two types of inverted clinopyroxenes) (Mori and Takeda 1985). A diogenite, A-881839 is relatively ferroan consistent with the presence of Binda type pyroxene. In contrast, the situation is quite different in the case of minor and trace elements. For example, pyroxenes in the Y-75032, Y-791199, and A-881389 are richer in TiO2, compared to the two other ferroan diogenites (e.g., Dho 700), and other diogenites. The trace element compositions of pyroxenes in Y-75032 types are rather similar to those of cumulate eucrites.
We estimated the REE compositions of parental melts by the same approach made by Barrat (2004). The presence of plagioclase indicates that Y-75032, Y-791199, and A-881399 contain significant amounts of trapped liquid. Using the REE data of pyroxenes and assuming 5-10% of trapped melts, the compositions of the melts are estimated to be roughly similar to those of basaltic eucrites. This may indicate that Y-75032 type is pigeonite cumulate eucrites rather than diogenite from a petrogenetic point of view. It seems likely that there are two types of diogenites from the petrogenetic point of view, digenites crystallized from secondary melts from early-formed cumulates (normal diogenites) (Barrat et al. 2008) and diogenites crystallized from eucritic magma (magma ocean).
References: Barrat J.A. et al. (2004) Meteor. Planet. Sci. 39, 1767-1779. Barrat J.A. et al. (2008) Meteor. Planet. Sci. 43, 1759-1775. Mittlefehldt D.W. (1994) Geochim. Cosmochim. Acta 58, 1537-1552. Takeda H. and Mori H. (1985) Proc. 15th LPSC, JGR 90, suppl., C636-C648.