The elevated oxygen fugacity recorded by subduction-related lavas and peridotites, relative to their mid-ocean ridge counterparts, fundamentally influences the petrogenesis of arc magmas. However, the timing, process, and spatial extent of oxidizing mass transfer at subduction zones remain unknown. Forearc peridotites, which are sometimes exposed on the trench wall of the overriding plate, record chemical fingerprints of the melting and melt-rock interaction processes that occur during and following subduction initiation, and thus provide insight into the spatial and temporal evolution of this oxidized signature. In this study, we present new major element, trace element, and oxygen fugacity data for a suite of forearc peridotites recovered from the Tonga Trench, in addition to a new assessment of literature data for previously studied forearc peridotites. For Tonga samples and literature data for forearc, ridge, and subduction-zone peridotites, we calculate oxygen fugacity (fO2) using an updated method. In contrast to previous studies, we find that spinel Cr#, a proxy for extent of melt extraction, does not correlate with oxygen fugacity, such that many forearc peridotites with high spinel Cr# do not record oxygen fugacity higher than the mid-ocean ridge peridotite array. Combining these observations with trace element modeling, we conclude that forearc peridotites are less pervasively influenced by oxidation owing to subduction processes than previously reported. The oxygen fugacity recorded by Tonga forearc peridotites is heterogeneous between dredges and homogeneous within dredges. To explore these variations, we grouped the dredges into two categories. Group I peridotites have high spinel Cr#, extremely depleted trace element compositions and oxygen fugacity values consistent with the mid-ocean ridge peridotite array. We interpret these to be the residues of large degrees of fractional melting, with little influence from arc-like melts or fluids, formed during the first stages of subduction initiation. Group II peridotites have lower spinel Cr#, enriched light rare earth elements, and oxygen fugacity elevated by ≥1 log unit above the mid-ocean peridotite array. We interpret these peridotites to be the residues of flux melting, initiated once corner flow is established in the young subduction zone. We conclude that the forearc mantle is not pervasively oxidized relative to midocean ridge mantle, and that the asthenospheric mantle in the proto-subduction zone region is not oxidized prior to subduction initiation. As the oxidized signature in Group II peridotites accompanies geochemical evidence of interaction with subduction-related fluids and melts, this suggests that the sub-arc mantle is oxidized concurrently with addition of subduction fluids to the mantle wedge.
|Original language||English (US)|
|Number of pages||26|
|Journal||Journal of Petrology|
|State||Published - Sep 1 2017|
Bibliographical noteFunding Information:
We thank Sherman Bloomer, Chris MacLeod, and Henry Dick for access to the samples, Peter Clift for providing spreadsheets of the BMRG08 dredge logs, and Robert Stern for his encouragement in working on Tonga. We thank Yan Liang for providing the disequilibrium melting code, Megan D'Errico for providing a starting point for the melt addition code, Gerry Salinas for help with SolidWorks, Tim Gooding and Tim Rose for laboratory support at the Smithsonian Institution, and Katie McGoldrick and Phil Robinson for laboratory support at the University of Tasmania. We thank Peter Tollan and an anonymous reviewer for thoughtful and thorough reviews, and Simon Turner for editorial handling that greatly improved the paper. This work was supported by the National Science Foundation (OCE-1433212 to E.C. and F.D., OCE-1433182 to K.K., OCE-1434199 to J.W.). S.B. was supported by a Stanford Graduate Fellowship. T.F. acknowledges funding from the Australian Research Council
© The Author 2017.
- Oxygen fugacity
- Upper mantle