New Mossbauer measurements of Fe~(3+)/ΣFe in chromites from the mantle section of the Oman ophiolite: evidence for the oxidation of the sub-oceanic mantle

摘要

Room temperature Mossbauer and electron-probe measurements of Fe~(3+)/ΣFe in chromite from the mantle section of the Oman ophiolite define two groups of samples: a low Fe~(3+)/ΣFe group (with Fe~(3+)/ΣFe = 0.21-0.36) have cr# = Cr/(Cr + Al) in the range 0.49-0.75, whereas a smaller more geographically localized high Fe~(3+)/ΣFe group (with Fe~(3+)/ΣFe = 0.71-0.78) have a more restricted range of cr# ratios of 0.72-0.75. The low Fe~(3+)/ΣFe chromitites have very variable Fe~(3+)/ΣFe ratios. Tliey are thought to have crystallized from melts that have interacted with depleted mantle and thereby acquired their variable Fe~(3+)/ΣFe ratio. The high Fe~(3+)/ΣFe chromitites are restricted to one small area of the mantle and their high oxidation state is thought to be post magmatic. They are either the product of later heating, related to melt flux or interaction with a later oxidising melt. A difference in oxygen fugacity between the MORB-depleted harzburgite host, which is at the quartz-fayalite-magnetite (QFM) buffer and the later chromite-bearing melts (QFM + 2) implies that there is a real difference in the oxidation state of the MORB and arc-magma sources.