TY - JOUR
T1 - Behaviour of boron, beryllium, and lithium during melting and crystallization
T2 - constraints from mineral-melt partitioning experiments
AU - Brenan, J. M.
AU - Neroda, E.
AU - Lundstrom, C. C.
AU - Shaw, H. F.
AU - Rverson, F. J.
AU - Phinney, D. L.
N1 - Funding Information:
Ian Hutcheon and Adam Kent are thanked for their advice and assistance concerning SIMS analysis. Reviews by Jeff Ryan and Tom Latourrette helped to improve the clarity of the manuscript, especially Ryan’s comments regarding Li-V systematics. Support for this research was provided by the Geosciences Research Program of the U.S. Department of Energy’s Office of Basic Energy Research and by LLNLs branch of the Institute for Geophysics and Planetary Physics. E. Neroda’s research at LLNL was made possible by the USDOEs Science and Engineering Research Semester administered by LLNL. Work at LLNL was performed under the auspices of the U.S. Department of Energy under contract No. W-7405-Eng-48. J. Brenan also acknowledges funding from the Natural Sciences and Engineering Research Council of Canada under operating grant OGP 0194228.
PY - 1998/6
Y1 - 1998/6
N2 - In order to provide a more substantial foundation for interpreting the behaviour of B, Be, and Li during the production and early crystallization of primitive igneous rocks, we have measured olivine-, clinopyroxene-, and orthopyroxene-, and amphibole-melt partition coefficients for these elements involving broadly basaltic-andesitic melt compositions. Experiments were conducted at both one atmosphere and 1.0-1.5 GPa and employed a time-temperature history that yielded large crystals with minimal compositional zoning. Experiment temperatures ranged from 1000 to 1350°C and were selected to minimize the total crystal fraction in a given experiment. Partition coefficients for olivine and clinopyroxene were found to be independent of run duratiion or total concentration of B, Be, or Li suggesting that crystal-liquid equilibrium was closely approached. Olivine-, orthopyroxene-, and clinopyroxene-melt partition coefficients decrease in the order: Li (0.1-0.2) ≫ Be ~ B (0.002-0.03), whereas amphibole-melt partition coefficients for Be and Li are similar (~0.2) and larger than those for B (~0.02). Comparison of partition coefficients measured in this study with previous determinations yields good agreement, with the exception of some of our mineral-melt values for B, which are uniformly lower (up to 10 times) than values determined at similar conditions of pressure and temperature. The latter discrepancy could be due to mineral or melt compositional effects, but this hypothesis is currently untestable owing to the absence of reported mineral compositions in previous studies. Partition coefficients for olivine and clinopyroxene have been found to vary as a function of mineral and melt composition, and with the exception of B partitioning into clinopyroxene, this variation can be modeled using simple exchange reactions involving the trace element and a substituent element, such as Na, Mg, or Al. Partition coefficients measured in this study were combined with simple models of melting and crystallization to evaluate how accurately element ratios such as B/Be, B/K, B/Nb, Be/Nd, Li/V, and Li/Yb in primitive magmas reflect that of their source. These models further confirm that the source regions of IAB magmas are enriched in B/Be, B/Nb, and Li/Yb relative to the MORB source, thus lending further support to the notion of metasomatic enrichment of the IAB source by slab-derived fluids. Moreover, our modeling also indicates that the low B/Be and B/Nb in primitive OIB magmas is indicative of similarly low values in OIB sources, which is consistent with the hypothesis that OIB sources contain a B-depleted component, such as subducted dehydrated oceanic crust. Partial melting models have also been constructed to explore the possibility of using the Li/V ratio in MORB and IAB as a monitor of redox conditions in their source-regions. Models indicate that this ratio does not uniquely constrain source fO2 without a priori knowledge of the degree of melting. However, the small amount of dispersion in MORB Li/V is consistent with (1) the small variation in source-region fO2 inferred for MORB by independent means and (2) degrees of melting close to clinopyroxene exhaustion. The very large dispersion in Li/V ratios in the IAB suite can be reconciled by melt generation under more oxidising conditions than that for MORB, in addition to variation in source composition resulting from metasomatism involving a Li-rich component.
AB - In order to provide a more substantial foundation for interpreting the behaviour of B, Be, and Li during the production and early crystallization of primitive igneous rocks, we have measured olivine-, clinopyroxene-, and orthopyroxene-, and amphibole-melt partition coefficients for these elements involving broadly basaltic-andesitic melt compositions. Experiments were conducted at both one atmosphere and 1.0-1.5 GPa and employed a time-temperature history that yielded large crystals with minimal compositional zoning. Experiment temperatures ranged from 1000 to 1350°C and were selected to minimize the total crystal fraction in a given experiment. Partition coefficients for olivine and clinopyroxene were found to be independent of run duratiion or total concentration of B, Be, or Li suggesting that crystal-liquid equilibrium was closely approached. Olivine-, orthopyroxene-, and clinopyroxene-melt partition coefficients decrease in the order: Li (0.1-0.2) ≫ Be ~ B (0.002-0.03), whereas amphibole-melt partition coefficients for Be and Li are similar (~0.2) and larger than those for B (~0.02). Comparison of partition coefficients measured in this study with previous determinations yields good agreement, with the exception of some of our mineral-melt values for B, which are uniformly lower (up to 10 times) than values determined at similar conditions of pressure and temperature. The latter discrepancy could be due to mineral or melt compositional effects, but this hypothesis is currently untestable owing to the absence of reported mineral compositions in previous studies. Partition coefficients for olivine and clinopyroxene have been found to vary as a function of mineral and melt composition, and with the exception of B partitioning into clinopyroxene, this variation can be modeled using simple exchange reactions involving the trace element and a substituent element, such as Na, Mg, or Al. Partition coefficients measured in this study were combined with simple models of melting and crystallization to evaluate how accurately element ratios such as B/Be, B/K, B/Nb, Be/Nd, Li/V, and Li/Yb in primitive magmas reflect that of their source. These models further confirm that the source regions of IAB magmas are enriched in B/Be, B/Nb, and Li/Yb relative to the MORB source, thus lending further support to the notion of metasomatic enrichment of the IAB source by slab-derived fluids. Moreover, our modeling also indicates that the low B/Be and B/Nb in primitive OIB magmas is indicative of similarly low values in OIB sources, which is consistent with the hypothesis that OIB sources contain a B-depleted component, such as subducted dehydrated oceanic crust. Partial melting models have also been constructed to explore the possibility of using the Li/V ratio in MORB and IAB as a monitor of redox conditions in their source-regions. Models indicate that this ratio does not uniquely constrain source fO2 without a priori knowledge of the degree of melting. However, the small amount of dispersion in MORB Li/V is consistent with (1) the small variation in source-region fO2 inferred for MORB by independent means and (2) degrees of melting close to clinopyroxene exhaustion. The very large dispersion in Li/V ratios in the IAB suite can be reconciled by melt generation under more oxidising conditions than that for MORB, in addition to variation in source composition resulting from metasomatism involving a Li-rich component.
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U2 - 10.1016/S0016-7037(98)00131-8
DO - 10.1016/S0016-7037(98)00131-8
M3 - Article
AN - SCOPUS:0032100968
SN - 0016-7037
VL - 62
SP - 2129
EP - 2141
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 12
ER -