Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism, and compositional layering in the mantle

Masako Yoshikawa, Eizou Nakamura

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43 Citations (Scopus)

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. Al2O3, 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 (87Sr/86Sr = 0.7019 to 0.7066, εNd = + 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 143Nd/144Nd 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 lherzolite 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.

Original languageEnglish
Article number1999JB900344
Pages (from-to)2879-2901
Number of pages23
JournalJournal of Geophysical Research B: Solid Earth
Volume105
Issue numberB2
Publication statusPublished - Feb 10 2000

Fingerprint

peridotite
metasomatism
lherzolite
Earth mantle
melt
spinel
mantle
rare earth element
mid-ocean ridges
Rocks
Trace Elements
rocks
trace elements
Chemical analysis
signatures
trace element
rock
harzburgite
Garnets
mid-ocean ridge

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Atmospheric Science
  • Astronomy and Astrophysics
  • Oceanography

Cite this

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title = "Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism, and compositional layering in the mantle",
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. Al2O3, 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 (87Sr/86Sr = 0.7019 to 0.7066, εNd = + 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 143Nd/144Nd 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 lherzolite 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.",
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AB - 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. Al2O3, 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 (87Sr/86Sr = 0.7019 to 0.7066, εNd = + 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 143Nd/144Nd 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 lherzolite 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.

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