40 research outputs found
Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 75 (2011): 5247-5268, doi:10.1016/j.gca.2011.06.026.We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from
the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic)
meteorites. The accuracy of our new Mg isotope ratio measurement protocol is
substantiated by a combination of standard addition experiments, the absence of mass
independent effects in terrestrial samples and our obtaining identical values for rock
standards using 2 different separation chemistries and 3 different mass-spectrometric
introduction systems. Carbonaceous, ordinary and enstatite chondrites have
irresolvable mean stable Mg isotopic compositions (δ25Mg = -0.14 ± 0.06; δ26Mg = -
0.27 ± 0.12‰, 2sd), but our enstatite chondrite samples have lighter δ7Li (by up to
~3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2sd),
possibly as a result of spallation in the early solar system. Measurements of
equilibrated, fertile peridotites give mean values of δ7Li = 3.5 ± 0.5‰, δ25Mg = -0.10
± 0.03‰ and δ26Mg = -0.21 ± 0.07‰. We believe these values provide a useful
estimate of the primitive mantle and they are within error of our average of bulk
carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic
samples, covering a variety of geological histories, show a broad positive correlation
between bulk δ7Li and δ26Mg, which vary from -3.7 to +14.5‰, and -0.36 to +0.06‰,
respectively. Values of δ7Li and δ26Mg lower than our estimate of primitive mantle
are strongly linked to kinetic isotope fractionation, occurring during transport of the
mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models
suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities
of Li and Mg in olivine. The isotopically lightest samples require ~5-10 years of
diffusive ingress, which we interpret as a time since volatile loss in the host magma.
Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope
ratios, likely as a result of little prior degassing in these explosive events. High δ7Li,
coupled with high [Li], in rapidly cooled arc peridotites may indicate that these
samples represent fragments of mantle wedge that has been metasomatised by heavy,
slab-derived fluids. If such material is typically stirred back into the convecting
mantle, it may account for the heavy δ7Li seen in some oceanic basalts.PPvS was supported
by NERC grant NER/C510983/
Petrology of Mantle Wedge Lithosphere: New data on Supra-Subduction Zone Peridotite Xenoliths from the Andesitic Avacha Volcano, Kamchatka
International audienc
Melt- and Fluid-Rock Interaction in Supra-Subduction Lithospheric Mantle: Evidence from Andesite-hostedVeined Peridotite Xenoliths
We report petrographic, major and trace element data for xenoliths from the andesitic Avacha volcano (Kamchatka), which host orthopyroxene (opx)-rich veins of mantle origin formed either by rapid crystallization of intruded melts or by their interaction
Tracing partial melting and subduction-related metasomatism in the Kamchatkan mantle wedge using noble gas compositions
International audienc
Lithium abundances and isotopic compositions in mantle xenoliths from subduction and intra-plate settings: Mantle sources vs. eruption histories
International audienc
Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia
International audienc
Melt evolution in subarc mantle: evidence from heating experiments on spinel-hosted melt inclusions in peridotite xenoliths from the andesitic Avacha volcano (Kamchatka, Russia)
International audienc
Composition of the Lithospheric Mantle in the Siberian Craton: New Constraints from Fresh Peridotites in the Udachnaya-East Kimberlite
International audienc
Effects of Oxygen Fugacity on Hydroxyl Incorporation in Garnet at 1-3 GPa and 800-1000°C and Implications for Water Storage in the Mantle
International audienceGarnet, an important mineral component in Earth's interior, usually contains minor water as structural OH, but experimental data on OH incorporation in the mineral as a function of chemical and thermodynamic parameters remain rare. In particular, the role of oxygen fugacity (fO2) that is variable in the Earth has not been well assessed. We examine the effects of fO2 on the incorporation of OH at 1-3 GPa and 800-1000°C, by H-annealing gem-quality spessartine-rich, grossular-rich, and pyrope-rich garnets. For each type of annealed garnet, the OH peak positions and spectral shapes are the same, suggesting the same OH incorporation mechanism(s). Under identical conditions, spessartine-rich and grossular-rich garnets are rich in OH, with solubility orders of magnitudes greater than for pyrope-rich garnets. Over the studied conditions, the solubility of OH increases with pressure and temperature but decreases with fO2. The negative correlation of fO2 and OH solubility is probably due to the Fe3+ to Fe2+ reduction at more reduced conditions and the coupled OH incorporation in the structure. The effects of fO2 on OH solubility are more significant in Fe-rich garnets. The ascertained small OH solubility in peridotite garnets, together with comparisons with relevant studies, suggest that mantle garnet is unlikely to be a major water carrier. In combination with literature-reported positive correlation of fO2 and OH solubility in olivine, the partition coefficient of OH between garnet and olivine in the upper mantle is expected to be redox-dependent