Osmium isotope and highly siderophile element geochemistry of mantle xenoliths from NW Turkey: implications for melt depletion and metasomatic history of the sub-continental lithospheric mantle


Aldanmaz E.

INTERNATIONAL GEOLOGY REVIEW, cilt.54, ss.799-815, 2012 (SCI İndekslerine Giren Dergi) identifier identifier

  • Cilt numarası: 54 Konu: 7
  • Basım Tarihi: 2012
  • Doi Numarası: 10.1080/00206814.2011.581799
  • Dergi Adı: INTERNATIONAL GEOLOGY REVIEW
  • Sayfa Sayısı: ss.799-815

Özet

Ultramafic xenoliths entrained in the late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate spinel-lherzolites. Whole-rock major and trace element characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (similar to 4-similar to 15%), but some have geochemical signatures reflecting the processes of melt/rock interaction. Mantle-normalized trace element patterns for the peridotites vary from LREE-depleted to strongly LREE-enriched, reflecting multistage mantle processes from simple melt extraction to metasomatic enrichment. Rhenium and platinum group element (PGE) abundances and Os-187/Os-188 systematics of peridotites were examined in order to identify the nature of the mantle source and the processes effective during variable stages of melt extraction within the sub-continental lithospheric mantle (SCLM). The peridotites are characterized by chondritic Os/Ir and Pt/Ir ratios and slightly supra-chondritic Pd/Ir and Rh/Ir ratios, representing a mantle region similar in composition to the primitive mantle (PM). Moderate enrichment in PPGE (Pd-Pt-Rh)/IPGE (Ir-Os-Ru) ratios with respect to the PM composition in the metasomatized samples, however, reflects compositional modification by sulphide addition during possible post-melting processes. The Os-187/Os-188 ratios of the peridotites range from 0.11801 to 0.12657. Highly unradiogenic Os isotope compositions (gamma Os at 10 Ma from -7.0 to -3.2) in the chemically undisturbed mantle residues are accompanied by depletion in Re/Os ratios, suggesting long-term differentiation of SCLM by continuous melt extraction. For the metasomatized peridotites, however, systematic enrichments in PPGE and Re abundances, and the observed positive covariance between Re-187/Os-188 and gamma Os can most likely be explained by interaction of solid residues with basaltic melts produced by melting of relatively more radiogenic components in the mantle. Significantly, the wide range of Os-187/Os-188 ratios characterizing the entire xenolith suite seems to be consistent with multistage evolution of SCLM and suggests that parts of the lithospheric mantle contain materials that have experienced ancient melt removal (similar to 1.3 Ga) which created time-integrated depletion in Re/Os ratios; in contrast, some other parts display evidence indicative of recent perturbation in the Re-Os system by sulphide addition during interaction with metasomatizing melts.