The redox state of subduction zones: insights from arc-peridotites

Spinel peridotites from a variety of island arcs have been utilised to calc ulate the redox state of the mantle wedge above subduction zones. Oxygen fu gacities (f(o2) values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J ., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of I herzolite spinels by Fe-57 Mossbauer spectroscopy and resultant oxygen fuga cities. Geochim. Cosmochim. Acta, 53, 1277-1291.], yield values which range from 0.3 to 2.0 above the fayalite-magnetite-quartz (FMQ) buffer. These da ta provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation be tween f(o2) values and the presence of hydrous phases and, in fact, the mos t oxidised samples contain no hydrous phases. Within individual suites ther e is no correlation between f(o2) and degree of depletion as indicated by s pinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation betwee n f(o2) and spinel Cr# suggesting that partial melting processes may influe nce the redox state of the mantle wedge. We suggest that the ultimate sourc e of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous flui d or a water-bearing silicate melt. However, our data does indicate that si licate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primit ive subduction zone magmas indicates that the source region must contain 0. 6-1.0 wt.% Fe2O3. This amount of Fe2O3 in a fertile spinel peridotite yield s an oxygen fugacity of 0.5-1.7 log units above FMQ in the IAB source. If w ater is the sole oxidising agent in the mantle wedge then 0.030-0.075 wt.% H2O is required which is considerably less than the 0.25% H2O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in t he petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 29 3-325.], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fl uid. This model would produce spinel, orthopyroxene or amphibole in the wed ge with only a slight increase in f(o2) of the source region. Although it i s unclear which model is correct the maximum f(o2) of the fertile mantle we dge is unlikely to be above FMQ + 2 and therefore some decompression meltin g in the mantle wedge is required to explain the higher f(o2) values of pri mitive are lavas than arc-peridotites. (C) 1999 Elsevier Science B.V. All r ights reserved.