Showing papers by "Eizo Nakamura published in 2000"
TL;DR: In this article, major and trace element compositions and isotopic ratios of Sr and Nd were determined for bulk rocks and their constituent clinopyroxenes from the Horoman peridotite complex, Japan.
Abstract: Major and trace element compositions and isotopic ratios of Sr and Nd were determined for bulk rocks and their constituent clinopyroxenes from the Horoman peridotite complex, Japan. Al 2 O 3 , CaO, and heavy rare earth elements (HREE) contents of peridotites generally decrease from plagioclase lherzolite through spinel lherzolite to spinel harzburgite, indicating simple melt extraction from a single source. However, the extremely large variations in isotopic ( 87 Sr/ 86 Sr = 0.7019 to 0.7066, e Nd = + 110 to -10) and trace element compositions ([Ce/Yb] N = 0.006 to 4.0) cannot be explained by a simple melt extraction mechanism. The samples can be divided into two groups: one suite has depleted isotopic and light REE (LREE) characteristics (DP), while the other suite shows enriched isotopic and LREE signatures (EP). Sm-Nd isotope systematics of whole-rock DP samples yield an isochron age of 833 ± 78 Ma with an initial 143 Nd/ 144 Nd ratio of 0.5119 ± 2, which is identical to the isotopic composition of mid-ocean ridge basalt (MORB) source mantle at that time. The relationship between MgO and Yb abundances of whole rocks shows that melt extraction was initiated at pressures near the garnet and spinel Iherzolite transition. Peridotites that formed at different depths presently occur in close proximity to each other, sometimes within tens of meters. The chemical and isotopic signatures of the EP samples can be explained by mixing between mantle residue and an isotopically and more incompatible element enriched fluid derived from a subducted slab. These observations suggest that the small-scale compositional layering observed in the complex may have formed in a wedge mantle by water-enhanced thinning and folding of metasomatized peridotites which had previously developed large-scale simple stratification as a result of melt extraction beneath a mid-ocean ridge.
55 citations
TL;DR: In this paper, the authors reported that two-pyroxene granulite is a relict left after formation of the garnet hornblendite, and that the formation of a garnet-clinopyroxene pyramid was coeval with the tectonic accretion of the Kohistan arc to the Asian continent.
Abstract: Abstract Mafic to ultramafic granulites in the northeastern part of the Jijal complex include two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite. Field and textural relations indicate that two-pyroxene granulite is a relict left after formation of the garnet-clinopyroxene granulite and garnet hornblendite was an originally intrusive rock which dissected the protoliths of mafic granulites. Sm-Nd mineral isochron ages of 118 ± 12 Ma, 94.0 ± 4.7 Ma and 83 ± 10 Ma were determined for two-pyroxene granulite, garnet-clinopyroxene granulite and garnet hornblendite respectively. These ages, together with previously reported chronological data, led to the following tectonic implications: (1) crystallization of the granulite protoliths predates, or is coeval with, the tectonic accretion of the Kohistan arc to the Asian continent; (2) crustal thickening related to the accretion was probably responsible for the high-pressure granulite-facies metamorphism in the Jijal complex; (3) formation of the garnet hornblendite assemblage was probably after crystallization of garnet-clinopyroxene granulite.
41 citations
TL;DR: In this paper, a mixing of the HIMU endmember and the MORB source was used to explain the vertical nature of the mixing trend, which favors a mixing model between extensively dehydrated oceanic crust and sediment.
Abstract: were analyzed for isotopic composition. Cpx samples from HIMU islands show quite uniform 87 Sr/ 86 Sr ratios (~0.70274), while leached and unleached whole rock samples show variable and higher 87 Sr/ 86 Sr than those of cpx samples. These results suggest that even leached whole rock samples have been affected by secondary contaminations of sea water. On the other hand, cpx preserves a pristine isotopic signature with minimal secondary effects. Using only the cpx analyses, HIMU form a vertical linear trend in the Sr-Nd isotope diagram with small variation in e Nd (+3.3~+5.5) and constant 87 Sr/ 86 Sr. This trend is explained by a mixing of the HIMU endmember and the MORB source. Since e Nd of the HIMU end-member is constrained to be less than +3.3, the HIMU source should include former sediment added to oceanic crust. To explain the vertical nature of the mixing trend, the HIMU end-member should have similar Rb/Sr to the MORB source, or much lower Sr/Nd ratio than the MORB source, which favors a mixing model between extensively dehydrated oceanic crust and sediment as the HIMU source. The correlation between e Nd and trace element ratios such as Pb/Ta also supports the model.
30 citations
TL;DR: In this paper, high-pressure melting experiments on primitive mantle composition material (PM1) and thoreiitic basalt (JB2) were performed from 3 to 15 GPa, and garnet (majorite)/silicate melt partition coefficients, D, were measured for major elements.
11 citations