Clinopyroxene of spinel-peridotite mantle xenoliths from Lake Nji (Cameroon Volcanic Line, W Africa): crystal chemistry and petrological implications

2000 ◽  
Vol 139 (5) ◽  
pp. 503-508 ◽  
Author(s):  
Francesco Princivalle ◽  
Gabriella Salviulo ◽  
Andrea Marzoli ◽  
Enzo M. Piccirillo
Keyword(s):  
Crystal Chemistry ◽  
Mantle Xenoliths ◽  
Spinel Peridotite ◽  
Cameroon Volcanic Line ◽  
Peridotite Mantle

Textures in spinel peridotite mantle xenoliths using micro-CT scanning: Examples from Canary Islands and France

Lithos ◽  
2017 ◽  
Vol 276 ◽  
pp. 90-102 ◽  
Author(s):  
K.K. Bhanot ◽  
H. Downes ◽  
C.M. Petrone ◽  
E. Humphreys-Williams
Keyword(s):  
Canary Islands ◽  
Ct Scanning ◽  
Mantle Xenoliths ◽  
Spinel Peridotite ◽  
Micro Ct ◽  
Peridotite Mantle

scholarly journals Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Costanza Bonadiman ◽  
Valentina Brombin ◽  
Giovanni B. Andreozzi ◽  
Piera Benna ◽  
Massimo Coltorti ◽  
...  
Keyword(s):  
Oxygen Fugacity ◽  
Upper Mantle ◽  
Mantle Xenoliths ◽  
Spinel Peridotite ◽  
Simultaneous Formation ◽  
Empirical Potentials ◽  
Elastic Data ◽  
Mineralogical Study ◽  
Semi Empirical ◽  
T Locus

AbstractThe occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.

Is orthopyroxene really a reliable recorder of mantle water signatures? Insights from an experimental study

2020 ◽  
Author(s):  
Alexandra Demers-Roberge ◽  
Michael Jollands ◽  
Peter Tollan ◽  
Othmar Müntener
Keyword(s):  
Diffusion Coefficients ◽  
Mantle Xenoliths ◽  
Spinel Peridotite ◽  
Alkaline Magma ◽  
Silica Activity ◽  
Exponential Factors ◽  
Natural Olivine ◽  
Natural Settings ◽  
Water Contents

<p>Experiments have been conducted to assess the effects of temperature, oxygen fugacity, crystallographic orientation, silica activity and chemical composition on the diffusivity and substitution mechanisms of hydrogen in orthopyroxene (opx). Axially oriented ~cuboids of natural Tanzanian opx were dehydrated at 1 bar in a gas mixing furnace (H<sub>2</sub>-CO<sub>2</sub> mix) at three different oxygen fugacities (~QFM-1,~QFM+1, ~QFM-7), and two different silica activity buffers (olivine+pyroxene or pyroxene+quartz) between 700°C and 1000°C. Profiles of hydrogen content versus distance were extracted from experimental samples using Fourier-Transform Infrared (FTIR) spectroscopy, with diffusion coefficients extracted using relevant analytical solutions and numerical approximations of Fick’s second law. Diffusion is the fastest along [001] ( D<sub>[001]</sub>>D<sub>[010]</sub>>D<sub>[100]</sub>). Fitting the diffusion coefficients to the isobaric Arrhenius relationship (logD=logD<sub>0</sub>+(-Q/(2.303RT)) gives activation energies (Q) and pre-exponential factors (logD<sub>0</sub>) between 127 to 162 kJmol<sup>-1</sup> and –4.29 to -5.42  m<sup>2</sup>s<sup>-1</sup> , respectively, for ~QFM-1.</p><p>The extracted hydrogen diffusivities are faster than previously measured by 0.5 to 5 orders of magnitude at ~1000 °C and ~700°C, respectively (Carpenter (2003), Stalder and Skogby (2003), Stalder and Behrens (2006), Stalder and al. (2007)) and are slightly slower, but strikingly close, to those of the fastest experimentally-determined diffusivity of H in olivine (Kohlstedt and Mackwell, 1998), suggesting a mechanism akin to proton-polaron exchange. This presents a paradoxical decoupling between natural and experimental observations. In most cases for mantle xenoliths, natural olivine has low water contents (<35 ppm), or are dry, and show H diffusive loss of water, where natural opx contains between 10 and 460 ppm and rarely show H diffusive loss (Demouchy and Bolfan-Casanova (2016), suggesting opx is more capable of recording the mantle water signature. With hydrogen diffusivities of olivine and opx being quite similar, however, both minerals should suffer from the same rate of dehydration during ascent, thus show low or zero water content in natural settings, which is not the case. Therefore, the inference that pyroxenes are better recorder of water in the mantle (e.g. Warren et Hauri (2014), Peslier (2010)) cannot be a simple function of diffusivities. A case study on an opx crystal showing a dehydration profile from a spinel-peridotite xenolith, hosted in an alkaline magma, from Patagonia supports this. Using the H diffusion coefficients from this study, the calculated rates of ascent of the mantle xenolith in alkaline magma are comparable to those associated with kimberlite magmas. The two suggestions we present are the following: i) Changing the boundary conditions may modify the hydrogen diffusive flux through the xenolith history and ii) The measured diffusivities would be apparent diffusivities as there might be different pathways or mechanisms of diffusion.</p>

Ancient recycled mantle lithosphere in the Hawaiian plume: Osmium–Hafnium isotopic evidence from peridotite mantle xenoliths

2007 ◽  
Vol 257 (1-2) ◽  
pp. 259-273 ◽  
Author(s):  
Michael Bizimis ◽  
Melanie Griselin ◽  
John C. Lassiter ◽  
Vincent J.M. Salters ◽  
Gautam Sen
Keyword(s):  
Mantle Xenoliths ◽  
Mantle Lithosphere ◽  
Isotopic Evidence ◽  
Hawaiian Plume ◽  
Peridotite Mantle
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