The Os isotopic composition of Proterozoic upper mantle: evidence for chondritic upper mantle from the Outokumpu ophiolite, Finland

Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary Os-rich PGM

“…back-arc basins [38], or from the lithosphere formed at a mid-ocean ridge and later experienced arc magmatism prior to obduction [39,40,27]. The careful determination of the in-situ Re-Os isotope systematics of encapsulated PGM in chromian spinel, combined with other petrological characteristics, could constrain the tectonic setting of ophiolite formation because the low Re/Os ratio of residual mantle samples may provide a relatively precise initial Os isotopic composition [7].…”

“…Bulk-rock Re-Os isotope systematics have been examined for two early Proterozoic ophiolites, the Outokumpu and Jormua [7,46]. The Os-isotope ratios are more or less chondritic, but are significantly heterogeneous in these early Proterozoic ophiolites [7].…”

“…The Os-isotope ratios are more or less chondritic, but are significantly heterogeneous in these early Proterozoic ophiolites [7]. The authors concluded that the least radiogenic Os is representative of the primary magmatic mantle source, whilst the more radiogenic materials reflect contamination either by sea water or by continental crust [7].…”

“…The isotopic heterogeneity of mantle rocks was attributed to at least two factors; (1) addition or subtraction of Re, and (2) addition of Os with different isotope ratios to the depleted mantle source [7]. The least radiogenic values are most probably characteristic of the primary magmatic mantle system [7], whereas the more radiogenic ones are attributed to the assimilation of crustal material or to the interaction with sea water [49,50,29].…”

“…Although discrete PGM inclusions within spinel in chromitite can provide constraints on the initial Os isotopic composition of the melt from which they formed, Os isotope data on such minerals in ophiolites are very limited [6][7][8][9]. This paper presents 187 Os/ 188 Os ratios of insitu Os-rich PGM included within chromian spinel from two ophiolitic chromitites: the Proterozoic ophiolite of the Eastern Desert, Egypt, and the Phanerozoic ophiolite of northern Oman ( Fig.…”