JOURNAL OF PETROLOGY VOLUME 55 NUMBER12 PAGES 2513^2536 2014 doi:10.1093/petrology/egu065 P Variations in Fe3þ/ Fe of Mariana Arc Basalts and Mantle Wedge fO2 M. N. BROUNCE1,2,3,*, K. A. KELLEY1 AND E. COTTRELL2 1GRADUATE SCHOOL OF OCEANOGRAPHY, UNIVERSITY OF RHODE ISLAND, NARRAGANSETT BAY CAMPUS, NARRAGANSETT, RI 02882, USA 2 DEPARTMENT OF MINERAL SCIENCES, SMITHSONIAN INSTITUTION, NATIONAL MUSEUM OF NATURAL HISTORY, Downloaded from WASHINGTON,DC20560,USA 3DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA CA 91125, USA http://petrology.oxfordjournals.org/ RECEIVED JANUARY 22, 2014; ACCEPTED NOVEMBER 6, 2014 Arc basalts are more oxidized than mid-ocean ridge basalts, but it is homogeneous H2O-rich component in the back-arc, to sediment unclear whether this difference is due to differentiation processes in melt- and fluid-dominated influences along the arc. Primary melt the Earth’s crust or to a fundamental difference in the oxygen fuga- oxygen fugacity does not correlate significantly with sediment melt city of their mantle sources. Distinguishing between these two hypoth- contributions (e.g.Th/La),nor can it be attributed to previous melt eses is important for understanding redox-sensitive processes related extraction in the back-arc. Primary melt oxygen fugacity correlates at University of Rhode Island on January 14, 2015 to arc magmatism, and thus more broadly how Earth materials strongly with indices of slab fluids (e.g. Ba/La) from the Mariana cycle globally.Wepresent major, volatile,P and trace element concentra- Trough through the Mariana arc, increasing by 1Á5 orders of magni- 3þ tions in combination with Fe / Fe ratios determined in olivine- tude as Ba/La increases by a factor of 10 relative to mid-ocean hosted glass inclusions and submarine glasses from five Mariana ridge basalts. These results suggest that contributions from the slab arc volcanoes andP two regions of the Mariana Trough. For single to the mantle wedge may be responsible for the elevated oxygen fuga- 3þ eruptions, Fe / Fe ratios vary along liquid lines of descent that city recorded by Mariana arc basalts and that slab fluids are poten- are either slightly oxidizing (olivine þ clinopyroxene þ plagioclase tially very oxidized. fractionation, CO2 Æ H2O degassing) or reducing (olivine þ clino- pyroxene þ plagioclase Æ magnetite fractionation, CO2 þ H2O þS degassing). Mariana samples are consistent with a global relation- KEY WORDS: differentiation; Mariana arc; melt inclusions; oxygen fu- ship between calc-alkaline affinity and both magmatic H2O and gacity; redox; subduction magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiationP cannot explain INTRODUCTION 3þ the majority of variations observed in Fe / Fe ratios for Oxygen fugacity (fO2) is a fundamental thermodynamic Mariana arc basalts, requiring primary differences in magmatic property that governs reduction^oxidation (redox) equili- oxygen fugacity. Calculated oxygen fugacities of primary mantle bria in solid Earth systems. It controls material transfer melts at the pressures and temperatures of melt segregation are sig- from the interior to the exterior of the Earth by setting nificantly oxidized relative to mid-ocean ridge basalts (QFM, the speciation of multivalent elements (e.g. Fe, S, V, C), where QFM is quartz^fayalite^magnetite buffer), ranging from which in turn controls their crystal^melt partitioning be- QFM þ1Á0toQFMþ1Á6 for Mariana arc basalts, whereas back- haviors (e.g. Canil, 2002), their physical state and mobility arc related samples record primary oxygen fugacities that range from in the mantle (e.g. Rohrbach & Schmidt, 2011), and their QFM þ 0Á1toQFMþ 0Á5.This Mariana arc sample suite includes solubility in silicate melts (e.g. Jugo et al., 2010). Despite its a diversity of subduction influences, from lesser influence of a power in dictating chemical exchange in the Earth, ß The Author 2014. Published by Oxford University Press. All *Corresponding author: Telephone: 626-395-6409. 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For Permissions, please e-mail: journals.permissions@ [email protected] oup.com JOURNAL OF PETROLOGY VOLUME 55 NUMBER 12 DECEMBER 2014 however, the fO2 of the upper mantle and whether it varies magmatic water contents, derived from the subducting spatially and over geological time is widely debated (e.g. plate, may suppress plagioclase saturation and decrease Christie et al., 1986; Bryndzia & Wood, 1990; Wood et al., the temperature difference between the appearance of sili- 1990; Carmichael, 1991; Ballhaus, 1993; Parkinson & cates and magnetite on the liquidus (Sisson & Grove, Arculus, 1999; Bezos & Humler, 2005; Lee et al., 2005, 1993), potentially influencing whether a basaltic magma 2010, 2012; Kelley & Cottrell, 2009, 2012; Rowe et al., 2009; follows a calc-alkaline (Fe-depleted) or tholeiitic (Fe-en- Cottrell & Kelley, 2011, 2013; Trail et al., 2011). riched) differentiation path (e.g. ZimmerP et al., 2010). 3þ Oceanic crust ages and oxidizes as it moves from spread- However, magmatic H2O and Fe / Fe ratios are ing centers to subduction zones, where it is recycled into strongly correlated (Kelley & Cottrell, 2009), and high the mantle, and material from the downgoing slab contrib- magmatic fO2 also enhances the appearance of oxides rela- utes chemically to the mantle source of arc magmas (e.g. tive to silicates on the basalt liquidus (Osborn, 1959; Plank & Langmuir, 1993; Elliott et al., 1997; Lecuyer & Botcharnikov et al., 2008). The effects of fO2 and H2Oon Ricard, 1999; Alt & Teagle, 2003). Arc basalts have a magmatic differentiation may thus be difficult to segregate. Downloaded from higher proportion of oxidized (Fe3þ) relative to reduced Magnetite fractionation in a system closedP to oxygen is P 3þ (Fe2þ) iron, expressed as the Fe3þ/ Fe rat i o [ i. e. Fe 3þ/ also expected to reduce magmatic Fe / Fe rat i o s, but (Fe2þ þ Fe 3þ)], than do mid-ocean ridge basalts this phenomenon has not been observed directly in the nat- (MORBs) (Carmichael, 1991). There is disagreement as to ural rock record. If source mantle fO2 at convergent mar- whether this arises owing to differentiation processes (e.g. gins is elevated over MORB, the cause of this oxidation crystal fractionation, crustal assimilation, degassing) in and the extent to which it varies are central to developing http://petrology.oxfordjournals.org/ models for the structure and growth of arc crust, and of the arc crust or to differences in the fO2 of the mantle source. Experimentally calibrated trace element proxies the oxygen evolution of Earth through time. Does primary fO2 change as subduction influence varies or diminishes? for mantle fO2, which are potentially more immune to dif- What effect do variable extents of fluid or sediment melt in- ferentiation processes in the arc crust, suggest that the fO2 of arc mantle is similar to that of MORB primary filtration have on primary fO2? magmas (Lee et al., 2005, 2010, 2012). Magmatic oxidation Toanswer these questions, we examine the relationships between crystal fractionation, degassing, mantle source may perhaps be influenced by later stage crustal processes, þ composition, subduction influence, and magmatic or such as the extensive fractionation of Fe2 -bearing min- mantle fO2 along the entire Mariana subduction zone. erals (e.g. olivine) or by the assimilation of oxidized crus- at University of Rhode Island on January 14, 2015 With this work, we investigate a variety of crystal fraction- tal material, although such relationships have not yet ation and degassing processes recorded by arc and back- been observed or quantitatively modeled. arc basaltic glasses and examine the relationships between A global study of basaltic glasses shows that those these processes and magmatic Fe redox. We present new magmas most heavily influenced by subduction have P major, trace, and volatile element concentrations as well higher Fe3þ/ Fe ratios than MORB (Kelley & Cottrell, P as Fe3þ/ Fe ratios in olivine-hosted melt inclusions from 2009). Moreover, olivine-hosted melt inclusions from a single eruptive events at five subaerial volcanic centers single eruptive event from Agrigan volcano in the along the Mariana arc (Sarigan, Guguan, Alamagan, Marianas showP that the least differentiatedP melts have the 3þ 3þ Pagan, and Agrigan), in addition to submarine glasses highest Fe / Fe ratios, and the Fe / Fe ratios of recon- from NW Rota-1 and Pagan volcanoes (Tamura et al., structed primary melts correspond to a source mantle 2011, 2014) and the Mariana Trough back-arc spreading Á that is oxidized 1^1 6 orders of magnitude relative to the center (Fig. 1). After assessing the effects of differentiation MORB source (Kelley & Cottrell,P 2012). In addition, a 3þ on magmatic redox, we use major element trends defined paired study of whole-rock Fe / Fe ratios determined by the data to reconstruct primary melt compositions and by wet chemical methods and fO2 calculated from mag- mantle source fO2 conditions. We then pair these with key netite^ilmenite mineral pairs demonstrates that andesites trace element ratios (Ba/La, Th/La, and Zr/Y) to assess from the MexicanP volcanic belt experienced no net the extent to which different slab-derived materials may in- change in bulk Fe3þ/ Fe ratios despite significant changes fluence the fO2 of the mantle wedge. in volatile content and extent of crystal fractionation (Crabtree & Lange, 2011). These observations suggest that low-pressure crystallization and degassing do not signifi- GEOLOGICAL SETTING cantlyP oxidize arc magmas and instead indicate that high The Mariana subduction system is a well-studied ocean^ Fe 3þ/ Fe ratios recorded by arc magmas reflect a mantle ocean convergent margin with an active subaerial and sub- source that has higher fO2 than MORB-source mantle. marine arc made up of 40 volcanic centers and the Outside mid-ocean ridge settings, Fe redox studies that Mariana Trough, an actively extending back-arc basin specifically address the effects of differentiation on Fe speci- (Fig.