Aluminum-dependent trace element partitioning in clinopyroxene |
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| Authors: | Don Francis William Minarik |
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| Institution: | (1) Earth and Planetary Sciences, McGill University, GEOTOP UQAM-McGill, Montreal, QC, Canada, H3A 2A7 |
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| Abstract: | As technical advances have dramatically increased our ability to analyze trace elements, the need for more reliable data on
the compositional dependence of trace element partitioning between minerals and melt has become increasingly important. The
late-Cretaceous Carmacks Group of south central Yukon comprises a succession of primitive high-Mg ankaramitic lavas characterized
by shoshonitic chemical affinities and containing large complexly zoned clinopyroxene phenocrysts. The compositional zonation
of the clinopyroxene phenocrysts is characterized by relatively Fe-rich (Mg# = Mg/(Mg + Fe) = 0.85), but mottled, cores surrounded by mantles of cyclically-zoned clinopyroxene whose Mg# varies repeatedly between 0.9 and 0.80. These cyclically zoned clinopyroxene mantles appear to record the repeated influx
and mixing of batches of primitive with more evolved magma in a deep sub-crustal (∼1.2 GPa) magma chamber(s). Laser ablation
ICP-MS was used to analyze the trace element variation in these zoned clinopyroxenes. The results indicate more than a threefold
variation in the absolute concentrations of Th, Zr, rare earth elements (REE), and Y within individual clinopyroxene phenocrysts,
with no apparent change in the degree of REE or high field strength element (HFSE) fractionation. The variation in absolute
abundances of trace elements correlates closely with the major element composition of the clinopyroxene, with the most enriched
clinopyroxene having the lowest Mg# and highest Al contents. The problem is that the amount of crystal fractionation required to explain the major element variation
(∼20%) in these clinopyroxene phenocrysts cannot explain the increase in the abundance of the incompatible trace elements,
which would require more than 70% crystal fractionation, if constant partition coefficients are assumed. The anomalous increase
in incompatible trace elements appears to reflect an increase in their partition coefficients with increasing AlIV in the clinopyroxene; with an increase in Al2O3 from 1.5 to 4.0 wt.% during ∼20% crystal fractionation over a temperature decrease of ∼100°C being associated with more that
a threefold increase in the partition coefficients of Th, Zr, REE, and Y. The magnitude of these increases may indicate that
the substitution of these trace elements into clinopyroxene is better modeled in some natural systems by a local charge balance
model, rather than the distributed charge model that better replicates the results of annealed experiments. These findings
indicate that the effect of Al on the partition coefficients of incompatible trace elements in clinopyroxene may be under
appreciated in natural magmatic systems and that the application of experimentally determined clinopyroxene partition coefficients
to natural systems must be done with caution. |
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| Keywords: | Clinopyroxene Trace element partitioning Aluminum Ankaramite Carmacks Phenocryst |
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