Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere
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Date
2014
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Abstract
The chalcophile elements, as associated with sulfides, are believed mainly from the study of
ophiolites to be generally enriched in the upper mantle, but depleted by magmatic processes
in the lower and upper ocean crust. However, studies of some orogenic lherzolites suggest a
copper depletion of peridotites in relation to the primitive mantle, suggesting that a portion of
the sulfides is melted during decompression and incorporated into the ascending magmas.
The rarity of abyssal peridotites and the high degree of their alteration have not allowed these
results to be verified in situ in the oceans.
Here, we present the first complete study of chalcophile elements based on a suite of rocks
from an oceanic core complex (OCC), the Kane Megamullion at 22°30’N at the MidAtlantic
Ridge. OCCs provide large exposures of mantle and lower crustal rocks on the seafloor on
detachment fault footwalls at slow and ultraslow spreading ridges. The Kane Megamullion is
one of the best sampled OCCs in the world, with 1342 rocks from 28 dredge sites and 14
dives. We have made XRF, TDMS
and INAA analyses of 129 representative peridotites,
gabbroic rocks, diabases and basalts.
Our results suggest a depletion of some peridotites in relation to the primitive mantle (28 ppm
Cu). Dunites, troctolites and olivine gabbros are relatively enriched in chalcophile elements.
The amount of sulfides decreases gradually with progressive differentiation, reaching a
minimum in gabbronorites and diabases. The highest bulk abundance of chalcophile
elements in our sample suite was observed in dunites (up to ~ 300 ppm Cu in several
samples) and a contact zone between residual peridotite and a mafic vein (294 ppm Cu).
Plagioclasebearing
harzburgites, generally formed by latestage
melt impregnation in the
mantle, are typically more enriched in Cu than unimpregnated residual peridotites. For these
reasons, our initial results indicate sulfide melting during mantle melting, and their local
precipitation in the mantle lithosphere due to late-stage
melt impregnation.
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Keywords
Chalcophile elements, Mantle, Lower crust, Kane Megamullion
Citation
Ciazela, J., Dick, H., Koepke, J., Kuhn, T., Muszynski, A., & Kubiak., M., 2014. Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere. Abstract V31B-4756 presented at AGU Fall Meeting, San Francisco, Calif., 15-19 Dec.