REE in Mantle Xenolith and Mantle Fluids
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Abstract:
Study on REE in mantle fluids has significance for understanding of regional geochemical difference, mantle enrichment and depletion. Recently, REE study on mantle fluids is indirectly achieved through the comparison of CO2rich and CO2poor fluid inclusions in mantle minerals. REE in fluidmelt inclusions of mantle xenolith from Changbaishan are measured directly by ICPMS. The primary study shows that fluidmelt inclusions enrich REE, particularly with higher LREE. The REE distribution pattern curve is decline to the right with a slight positive Eu anomaly and is similar to that of the mantle xenolith host basalt. It indicates that the source of mantle xenolith may be experienced a metasomatic process.Keywords:
Xenolith
Metasomatism
Hotspot (geology)
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Metasomatism
Peridotite
Xenolith
Isotope Geochemistry
Primitive mantle
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Peridotites that sample Archean mantle roots are frequently incompatible trace element enriched despite their refractory major element compositions. To constrain the trace element budget of the lithosphere beneath the Canadian craton, trace element and rare earth element (REE) abundances were determined for a suite of garnet peridotites and garnet pyroxenites from the Nikos kimberlite pipe on Somerset Island, Canadian Arctic, their constituent garnet and clinopyroxene, and the host kimberlite. These refractory mantle xenoliths are depleted in fusible major elements, but enriched in incompatible trace elements, such as large ion lithophile elements (LILE), Th, U and light rare earth elements (LREE). Mass balance calculations based on modal abundances of clinopyroxene and garnet and their respective REE contents yield discrepancies between calculated and analyzed REE contents for the Nikos bulk rocks that amount to LREE deficiencies of 70–99%, suggesting the presence of small amounts of interstitial kimberlite liquid (0·4–2 wt %) to account for the excess LREE abundances. These results indicate that the peridotites had in fact depleted or flat LREE patterns before contamination by their host kimberlite. LREE and Sr enrichment in clinopyroxene and low Zr and Sr abundances in garnet in low-temperature peridotites (800–1100°C) compared with high-temperature peridotites (1200–1400°C) suggest that the shallow lithosphere is geochemically distinct from the deep lithosphere beneath the northern margin of the Canadian craton. The Somerset mantle root appears to be characterized by a depth zonation that may date from the time of its stabilization in the Archean.
Lile
Trace element
Xenolith
Rare-earth element
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Xenolith
Metasomatism
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Metasomatism
Xenolith
Amphibole
Trace element
Incompatible element
Fractional crystallization (geology)
Petrogenesis
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Citations (157)
Peridotite
Xenolith
Rare-earth element
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Citations (9)
Metasomatism
Phlogopite
Amphibole
Xenolith
Peridotite
Ultramafic rock
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Carbonatite
Metasomatism
Fractional crystallization (geology)
Trace element
Incompatible element
Petrogenesis
Isotope Geochemistry
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Spinel lherzolite occurs at Salt Lake Crater, Hawaii, intimately associated in single xenoliths with garnet pyroxenite. Investigations of the rare earth element (REE) distributions in these two-assemblage rocks show that (1) pyroxenite mineral REE patterns sum very nearly to their whole-rock pattern; (2) chondrite-normalized REE patterns for pyroxenites are gently curved and convex upward (they have maximums in the range Pr to Gd, with total REE contents 4 to 10 times the REE content of chondrites); (3) lherzolites are depleted in REE relative to associated pyroxenites, but show a greater light REE enrichment; and (4) lherzolite chrome diopsides and associated pyroxenite clinopyroxenes have very similar REE distributions. The data suggest the following interpretations: (1) Xenoliths are not contaminated by the host basalt. (2) Solid-liquid distribution coefficients applied to pyroxenite clinopyroxenes give liquids with REE patterns unlike those of any Hawaiian basalt. If the pyroxenites crystallized from some liquid, this liquid was chemically dissimilar to Hawaiian lavas, and pyroxenite formation was probably unrelated to the Hawaiian volcanism. Pyroxenites have REE patterns whose shapes are very similar to the shape of Hawaiian tholeiites and have REE contents about half the REE contents of the tholeiites. The pyroxenites, therefore, may be parental to tholeiites. (3) The similarity in REE patterns of the lherzolite and the pyroxenite clinopyroxenes from a single xenolith is inconsistent with the usual genetic interpretation of these bimodal xenoliths, i.e., that the pyroxenite was a liquid that intruded upper mantle lherzolite (peridotite) and crystallized at depth. The REE data are consistent with textural observations that suggest that the lherzolite has formed as residue when basaltic components were removed from the original pyroxenite by a permeating melt. Relative to Iherzolite, garnet pyroxenite is the more primitive upper-mantle material.
Xenolith
Peridotite
Rare-earth element
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Citations (50)
Carbonatite
Phenocryst
Metasomatism
Trace element
Phlogopite
Xenolith
Rare-earth element
Peridotite
Melt inclusions
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Citations (112)
Radiogenic nuclide
Peridotite
Xenolith
Metasomatism
Massif
Mantle plume
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Citations (88)