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Experimental trace element partition coefficients for upper mantle pyroxenites coexisting with H2O with CO2, NaCl and NH3 fluids
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DOI：10.1007/s00410-026-02349-2.png)
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Abstract
En 中文
Experimental determinations of trace element distribution between rocks/minerals and fluids at mantle pressures and temperatures have concentrated on eclogite and on fluids containing H2O with or without the addition of NaCl. Here we present data for websterite (pyroxenite) coexisting with five different fluids: H2O, H2O+CO2, H2O+NaCl, H2O+CO2+NaCl and H2O+NH3. All experiments were conducted at 1.5 GPa and 800˚C using glassy carbon fluid traps that were kept in a frozen state following experiments until trace elements in the fluid traps were analysed by laser cryoablation-ICP-MS. Results show that Rb, Ba, Sr, Pb and Li are fluid-mobile, with lower rock/fluid partition coefficients (DRock/Fluid) for all mixed fluids relative to pure H2O. Lowest values (0.0004–0.123) are found for H2O+NH3 fluids. Almost all for the REE are < 1, contrasting with results at higher pressures for eclogite, and show very little fractionation of LREE from HREE. The REE are least mobile in H2O+CO2+NaCl and H2O+NH3 fluids, as are the HFSE, which also show lower DRock/Fluid than for eclogites, with or without NaCl. Re and Pt are fluid-mobile, with DRe < DPt and lowest values for both in H2O+CO2 fluids. The first row transition elements show D-values ranging from 0.0004 to 2.88 with only 4 of 35 data points > 1. DRock/Fluid increase in the order Cu < Ni ≈ Zn < Co < Cr, V, Sc. Rock/fluid partitioning of trace elements for pyroxenite resembles peridotite more than eclogite due to the abundance of minerals with low DRock/Fluid,particularly low-Al clinpyroxenes.
Keywords:
Fluids
Mantle
Partition coefficients
Trace elements
Cryocell
High-pressure experiments
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