Characterising modal metasomatic processes in young continental lithospheric mantle: a microsampling isotopic and trace element study on xenoliths from the Middle Atlas Mountains,Morocco |
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| Authors: | Jacqueline Malarkey Nadine Wittig D Graham Pearson Jon P Davidson |
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| Institution: | (1) Northern Centre for Elemental and Isotopic Tracing, Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK;(2) Friedrich-Alexander Universitaet Erlangen, GeoZentrum Nordbayern, Lithosphere Dynamics, Schlossgarten, 5, 91054 Erlangen, Germany;(3) Department of Earth, Ocean, and Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA |
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| Abstract: | Clinopyroxene is a major host for lithophile elements in the mantle lithosphere, and therefore it is critical whether we are
to understand the constraints that this mineral puts on mantle evolution and melt generation. This study presents a detailed
in situ trace element and Sr isotope study of clinopyroxene, amphibole and melt from two spinel lherzolites from the Middle
Atlas Mountains, Morocco. The results show that there is limited, but discernable, Sr isotopic variation between clinopyroxene
crystals within these xenoliths 87Sr/86Sr ranging from 0.703416 (±11 2SE) to 0.703681 (±12 2SE)]. Trace element patterns show similar interelement fractionation
with LREE enrichment, but there is a considerable range in terms of elemental concentration (e.g. over 100 ppm in Sr concentrations).
Observed modal clinopyroxene is far more abundant than that predicted from estimates of melt depletion. This along with isotope
and trace element variability found in these xenoliths supports a multistage metasomatic process in which clinopyroxene and
amphibole are recent secondary additions to the lithospheric mantle. Elemental systematics indicate that the metasomatic mineral
assemblage has most recently equilibrated with a carbonatitic melt prior to inclusion in the host basalt. The clinopyroxene
from this study is typical of global off-craton clinopyroxene in terms of Sr isotope composition, suggesting that the majority
of clinopyroxene in off-craton settings may have a recent metasomatic origin. These findings indicate that caution is required
when using peridotite xenoliths to estimate the degree of elemental enrichment in the subcontinental lithosphere. |
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