Petrographic and geochemical analysis of spinel from 35 lower crustal dunites, harzburgites, wehr... more Petrographic and geochemical analysis of spinel from 35 lower crustal dunites, harzburgites, wehrlites and gabbros recovered from the inner trench slope of the Bonin Ridge (BR) reveals 2 groups of samples which reacted with distinct melt compositions. The first group (Group M) consists of peridotites (cpx-harzburgite), wehrlites, and gabbroic rocks with medium Cr# (100 x Cr / Cr + Al) spinels ranging from 45 to 60 and high TiO2 and Al2O3 spanning ~0.1-2.25 and ~12-30 wt. % respectively. The second group (Group B) consists of only dunites and cpx-free peridotites with high Cr# spinels ranging from 65 to 94 and low TiO2 and Al2O3 spanning ~0-0.12 and ~3-21 wt. % respectively. Clinopyroxene is present in samples from group M but not group B. Clinopyroxene major element compositions range from 98 to 86 in Mg# with low TiO2 (0 - 0.11 wt. %) and heavily depleted REE compositions similar to depleted MORB mantle peridotite clinopyroxenes. The group M and group B samples are the result of melt-rock reaction with a mid-ocean ridge basalt (MORB)-like melt and a more depleted boninitic melt respectively. MORB-like forearc basalts (~50-52 Ma) and boninites (~44-48 Ma) recovered from the BR have been interpreted to represent a change from decompression melting at subduction initiation to flux melting and boninitic volcanism. The group M and group B samples are a record of the change from MORB-like melts created by decompression melting of already depleted mantle at or soon after subduction initiation to arc-type flux melting and boninite volcanism. Further, the presence of melt-hybridized peridotites and gabbroic rocks with spinels belonging to group M and not group B suggests that the lower crust and the mantle transition zone of the BR may be dominated by gabbroic rocks and material related to the FABs. This would imply that a large portion of the lower crust in the fore-arc was formed during or shortly after subduction initiation and is similar in composition to MOR lower crust.
Most of the recent discussions on the number and kinds of components which can be distinguished i... more Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with
Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymiu... more Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.
The oldest rocks-3.85 billion years old-from southwest Greenland have coupled neodymium-142 exces... more The oldest rocks-3.85 billion years old-from southwest Greenland have coupled neodymium-142 excesses (from decay of now-extinct samarium-146; half-life, 103 million years) and neodymium-143 excesses (from decay of samarium-147; half-life, 106 billion years), relative to chondritic meteorites, that directly date the formation of chemically distinct silicate reservoirs in the first 30 million to 75 million years of Earth history. The differences in 142Nd signatures of coeval rocks from the two most extensive crustal relicts more than 3.6 billion years old, in Western Australia and southwest Greenland, reveal early-formed large-scale chemical heterogeneities in Earth's mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of very-early-formed mantle chemical domains.
Coupled 186 Os and 187 Os evidence for core-mantle interaction. AD BRANDON, RJ WALKER, JW MORGAN,... more Coupled 186 Os and 187 Os evidence for core-mantle interaction. AD BRANDON, RJ WALKER, JW MORGAN, MD NORMAN, HM PRICHARD ...
ABSTRACT Re-Os systematics of quenched- and slowly-cooled angrites suggests that Re behaved as an... more ABSTRACT Re-Os systematics of quenched- and slowly-cooled angrites suggests that Re behaved as an incompatible and compatible element during the genesis of each of these magma-groups. This difference potentially relates to variations in oxidation state.
Petrographic and geochemical analysis of spinel from 35 lower crustal dunites, harzburgites, wehr... more Petrographic and geochemical analysis of spinel from 35 lower crustal dunites, harzburgites, wehrlites and gabbros recovered from the inner trench slope of the Bonin Ridge (BR) reveals 2 groups of samples which reacted with distinct melt compositions. The first group (Group M) consists of peridotites (cpx-harzburgite), wehrlites, and gabbroic rocks with medium Cr# (100 x Cr / Cr + Al) spinels ranging from 45 to 60 and high TiO2 and Al2O3 spanning ~0.1-2.25 and ~12-30 wt. % respectively. The second group (Group B) consists of only dunites and cpx-free peridotites with high Cr# spinels ranging from 65 to 94 and low TiO2 and Al2O3 spanning ~0-0.12 and ~3-21 wt. % respectively. Clinopyroxene is present in samples from group M but not group B. Clinopyroxene major element compositions range from 98 to 86 in Mg# with low TiO2 (0 - 0.11 wt. %) and heavily depleted REE compositions similar to depleted MORB mantle peridotite clinopyroxenes. The group M and group B samples are the result of melt-rock reaction with a mid-ocean ridge basalt (MORB)-like melt and a more depleted boninitic melt respectively. MORB-like forearc basalts (~50-52 Ma) and boninites (~44-48 Ma) recovered from the BR have been interpreted to represent a change from decompression melting at subduction initiation to flux melting and boninitic volcanism. The group M and group B samples are a record of the change from MORB-like melts created by decompression melting of already depleted mantle at or soon after subduction initiation to arc-type flux melting and boninite volcanism. Further, the presence of melt-hybridized peridotites and gabbroic rocks with spinels belonging to group M and not group B suggests that the lower crust and the mantle transition zone of the BR may be dominated by gabbroic rocks and material related to the FABs. This would imply that a large portion of the lower crust in the fore-arc was formed during or shortly after subduction initiation and is similar in composition to MOR lower crust.
Most of the recent discussions on the number and kinds of components which can be distinguished i... more Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with
Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymiu... more Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.
The oldest rocks-3.85 billion years old-from southwest Greenland have coupled neodymium-142 exces... more The oldest rocks-3.85 billion years old-from southwest Greenland have coupled neodymium-142 excesses (from decay of now-extinct samarium-146; half-life, 103 million years) and neodymium-143 excesses (from decay of samarium-147; half-life, 106 billion years), relative to chondritic meteorites, that directly date the formation of chemically distinct silicate reservoirs in the first 30 million to 75 million years of Earth history. The differences in 142Nd signatures of coeval rocks from the two most extensive crustal relicts more than 3.6 billion years old, in Western Australia and southwest Greenland, reveal early-formed large-scale chemical heterogeneities in Earth's mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of very-early-formed mantle chemical domains.
Coupled 186 Os and 187 Os evidence for core-mantle interaction. AD BRANDON, RJ WALKER, JW MORGAN,... more Coupled 186 Os and 187 Os evidence for core-mantle interaction. AD BRANDON, RJ WALKER, JW MORGAN, MD NORMAN, HM PRICHARD ...
ABSTRACT Re-Os systematics of quenched- and slowly-cooled angrites suggests that Re behaved as an... more ABSTRACT Re-Os systematics of quenched- and slowly-cooled angrites suggests that Re behaved as an incompatible and compatible element during the genesis of each of these magma-groups. This difference potentially relates to variations in oxidation state.
Abstract–Northwest Africa (NWA) 5298 is an evolved basaltic shergottite that has bulk characteris... more Abstract–Northwest Africa (NWA) 5298 is an evolved basaltic shergottite that has bulk characteristics and mineral compositions consistent with derivation from an oxidized reservoir in Mars. Chemically zoned clinopyroxene (64.5%, augite and pigeonite), with ...
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