Ophiolites mantle

We shall hereafter follow the current terminology. It is convenient to distinguish three main types of mantle occurrences and to examine them in the following sequence (i) orogenic peridotite massifs, (ii) ophiolitic mantle rocks, and (iii) oceanic peridotites. 

Moores E., Kellogg L. H., and Dilek Y. (2000) Tethyan ophiolites, mantle convection, and tectonic historical contingency a resolution of the ophiolite conundrum . In Ophiolites and Oceanic Crust New Insights from Field Studies and the Ocean Drilling Program, Special Paper (eds. Y. Dilek, E. M. Moores, D. Elthon, and A. Nicolas). Geological Society of America, Boulder, CO, USA, vol. 349, pp. 3-12. 

Ophiolitic mantle therefore provides a great deal of insight into the uppermost part of the oceanic mantle and in particular into the processes of melt accumulation and melt migration beneath a midocean ridge. In recent years new insights have been gained into the process of melt reaction whereby melts derived at depth are modified in composition through reaction with shallow mantle. 

FIGURE 3.6 A summary diagram showing the relative contributions of ophiolites, mantle xenoliths, diamond inclusions, and basaltic melts to our knowledge of the nature of the upper and lower mantle. 

Seafloor spreading eventually pushes oceanic crust into subduction zones where the hydrothermal sediments and rock are recycled back into the mantle. A small fraction of these deposits is uplifted, or obducted, onto land. These rescued deposits are termed ophiolites. Because of their metal enrichments, they serve as major ore bodies and have been mined for various precious metals, such as copper, for thousands of years. 

Fig. 1. Primitive mantle (PM) normalized whole rock PGE profiles for stratiform chromitites from the Stillwater Complex and for podiform chromitites from the mantle section of Thetford Mines Ophiolite (TMO). Note the similarity between IPGE contents of chromitites from these different tectonic settings. D.L.= detection limit.

We analyzed chromites from 1) podiform chromitite deposits from the mantle section of the Thetford Mines Ophiolite (TMO), southern Quebec Appalachians (Canada) 2) stratiform chromitite seams found within the Stillwater Complex, Montana (USA). In addition, we also analyzed chromite in lavas from different tectonic settings in order to 1) verify whether the IPGE solubility in chromite could be related to geological setting and... 

Lundstrom CC, Chaussidon M, Hsui AT, Keleman P, Zimmermann M (2005) Observations of Li isotope variations in the Trinity ophiolite evidence for isotope fractionation by diffusion during mantle melting, Geochim Cosmochim Acta 69 735-751 Luz B, Barkan E (2000) Assessment of oceanic productivity with the triple-isotope composition of dissolved oxygen. Science 288 2028-2031... 

Ozawa, K. and Shimizu, N., 1995. Open-system melting in the upper mantle Constraints from the Hayachine-Miyamori ophiolite, northeastern Japan. J. Geophys. Res., 100 22315-22335. 

Crystal field theory predicts that Ni2+, because of its high CFSE in octahedral coordination in minerals, should remain behind in refractory phases resulting from partial fusion of the oceanic lithosphere and become enriched in the Upper Mantle. It is possible, however, that the nickel reappears in alpine peri-dotites or obducted ophiolite suites. 

Osmium isotopes currently provide the strongest case for mineral-to-mineral disequilibrium, and for mineral-melt disequilibrium available from observations on natural rocks. Thus, both osmium alloys and sulfides from ophiolites and mantle xenoliths have yielded strongly heterogeneous osmium isotope ratios (Alard et al., 2002 Meibom et al., 2002). The most remarkable aspect of these results is that these ophiolites were emplaced in Phanerozoic times, yet they contain osmiumbearing phases that have retained model ages in excess of 2 Ga in some cases. The melts that were extracted from these ophiolitic peridotites contained almost certainly much more radiogenic osmium and could, in any case, not have been in osmium-isotopic equilibrium with all of these isotopically diverse residual phases. 

These two end-members provide a simple frame for the classification of the ophiolites in two main groups (e.g., Boudier and Nicolas, 1985 Ishiwatari, 1985 Nicolas and Boudier, in press). They are briefly examined in the following, together with the rare subarc mantle sections exposed at the base of obducted island arcs (e.g., Burg et al., 1998). 

The ophiolite consists of several large massifs. Each of them exposes a more or less complete ophiolitic sequence, comprising a thick mantle section (up to 12 km), a gabbro sequence (0.5-6 km), a sheeted dike complex (1-1.5 km), a thick extrusive sequence (0.5-2 km), and interbedded oceanic sediments (e.g., Lippard et al., 1986). 

Most of the ophiolites worldwide display similar zoning of their mantle lithologies, i.e., from the top to the base... 

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