UUnniivveerrssiittyy ooff RRhhooddee IIssllaanndd DDiiggiittaallCCoommmmoonnss@@UURRII Graduate School of Oceanography Faculty Graduate School of Oceanography Publications 12-2014 33++ VVaarriiaattiioonnss iinn FFee //∑FFee ooff MMaarriiaannaa AArrcc BBaassaallttss aanndd MMaannttllee WWeeddggee fOO 22 M. N. Brounce K. A. Kelley University of Rhode Island, [email protected] E. Cottrell Follow this and additional works at: https://digitalcommons.uri.edu/gsofacpubs TThhee UUnniivveerrssiittyy ooff RRhhooddee IIssllaanndd FFaaccuullttyy hhaavvee mmaaddee tthhiiss aarrttiiccllee ooppeennllyy aavvaaiillaabbllee.. PPlleeaassee lleett uuss kknnooww hhooww OOppeenn AAcccceessss ttoo tthhiiss rreesseeaarrcchh bbeenneefifittss yyoouu.. This is a pre-publication author manuscript of the final, published article. Terms of Use This article is made available under the terms and conditions applicable towards Open Access Policy Articles, as set forth in our Terms of Use. CCiittaattiioonn//PPuubblliisshheerr AAttttrriibbuuttiioonn M. N. Brounce, K. A. Kelley and E. Cottrell. (2014). "Variations in Fe3+/∑Fe of Mariana Arc Basalts and Mantle Wedge fO2." Journal of Petrology. 55(12): 2513-2536. Available at: https://doi.org/10.1093/petrology/egu065 This Article is brought to you for free and open access by the Graduate School of Oceanography at DigitalCommons@URI. It has been accepted for inclusion in Graduate School of Oceanography Faculty Publications by an authorized administrator of DigitalCommons@URI. For more information, please contact [email protected]. Variations in Fe3+/∑Fe of Mariana Arc basalts and mantle wedge fO 2 M.N. Brounce1,2*, K.A. Kelley1 and E. Cottrell2 1Graduate School of Oceanography, University of Rhode Island, Narragansett Bay Campus, Narragansett, RI 02882, USA 2Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA *Corresponding author: Telephone (401) 674-6010. Fax: (401) 874-6811. E-mail: [email protected] ABSTRACT Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation processes in the Earth’s crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with Fe3+/ΣFe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions of the Mariana trough. For individual eruptions, Fe3+/ΣFe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine + clinopyroxene + plagioclase fractionation, CO ± H O degassing) or 2 2 reducing (olivine + clinopyroxene + plagioclase ± magnetite fractionation, CO + H O + S 2 2 degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic H O and magmatic oxygen fugacity, where wetter, higher oxygen 2 fugacity magmas display greater affinity for calc-alkaline differentiation. We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe3+/ΣFe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized over mid-ocean ridge basalts (~QFM), ranging from QFM+1.0 – QFM+1.6 for Mariana arc basalts, while back-arc related samples record primary oxygen fugacities that range from QFM+0.1 – QFM+0.5. This Mariana arc sample suite comprises a diversity of subduction influences, from lesser influence of a homogeneous H O-rich component 2 in the back-arc, to sediment melt- and fluid-dominated influences along the arc. Primary melt oxygen fugacity does not correlate significantly with sediment melt contributions (e.g., Th/La), nor can it be attributed to previous melt extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g., Ba/La) from the Mariana trough through the Mariana arc, increasing by 1.5 orders of magnitude as Ba/La increases by a factor of 10 over mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized. Key words: differentiation; Mariana arc; melt inclusions; oxygen fugacity; redox; subduction INTRODUCTION Oxygen fugacity (fO ) is a fundamental thermodynamic property that governs reduction- 2 oxidation (redox) equilibria in solid Earth systems. It controls material transfer from the interior to the exterior of the Earth by setting the speciation of multi-valent elements (e.g., Fe, S, V, C), which in turn controls their crystal/melt partitioning behaviors (e.g., Canil, 2002), their physical state and mobility in the mantle (e.g., Rohrbach & Schmidt, 2011), and their solubility in silicate melts (e.g., Jugo et al., 2010). Despite its power in dictating chemical exchange in the Earth however, the fO of the upper mantle and whether it varies through space and geologic time is 2 widely debated (e.g., Ballhaus, 1993, Bezos & Humler, 2005, Bryndzia & Wood, 1990, Carmichael, 1991, Christie et al., 1986, Cottrell & Kelley, 2011, Cottrell & Kelley, 2013, Kelley & Cottrell, 2009, Kelley & Cottrell, 2012, Lee et al., 2005, Lee et al., 2012, Lee et al., 2010, Parkinson & Arculus, 1999, Rowe et al., 2009, Trail et al., 2011, Wood et al., 1990). Oceanic crust ages and oxidizes as it moves from spreading centers to subduction zones, where it is recycled into the mantle, and material from the down-going slab contributes chemically to the mantle source of arc magmas (e.g., Alt & Teagle, 2003, Elliott et al., 1997, Lecuyer & Ricard, 1999, Plank & Langmuir, 1993). Arc basalts have a higher proportion of oxidized (Fe3+) relative to reduced (Fe2+) iron, expressed as the Fe3+/ΣFe ratio (i.e., Fe3+/[Fe2++Fe3+]), than do MORB (Carmichael, 1991). There is disagreement as to whether this arises due to differentiation processes (e.g., crystal fractionation, crustal assimilation, degassing) in the arc crust or to differences in the fO of the mantle source. Experimentally calibrated trace 2 element proxies for mantle fO , which are potentially more immune to differentiation processes 2 in the arc crust, suggest that the fO of arc mantle is similar to the MORB primary magmas (Lee 2 et al., 2005, Lee et al., 2012, Lee et al., 2010). Magmatic oxidation may perhaps be influenced by later stage crustal processes, such as the extensive fractionation of Fe2+-bearing minerals (e.g., olivine) or by the assimilation of oxidized crustal material, although such relationships have not yet been observed or quantitatively modeled. Yet, a global study of basaltic glasses shows that those magmas most heavily influenced by subduction have higher Fe3+/ΣFe ratios than MORB (Kelley & Cottrell, 2009). Moreover, olivine-hosted melt inclusions from a single eruptive event from Agrigan volcano in the Marianas show that the least differentiated melts have the highest Fe3+/ΣFe ratios, and the Fe3+/ΣFe ratios of reconstructed primary melts correspond to a source mantle that is oxidized 1 – 1.6 orders of magnitude over the MORB source (Kelley & Cottrell, 2012). In addition, a paired study of whole rock Fe3+/ΣFe ratios determined by wet chemical methods and fO calculated 2 from magnetite-ilmenite mineral pairs demonstrates that andesites from the Mexican volcanic belt experienced no net change in bulk Fe3+/ΣFe ratios despite significant changes in volatile content and extent of crystal fractionation (Crabtree & Lange, 2011). These observations suggest that low-pressure crystallization and degassing do not significantly oxidize arc magmas and instead indicate that high Fe3+/ΣFe ratios recorded by arc magmas reflect a mantle source that has higher fO than MORB source mantle. 2 Outside of mid-ocean ridge settings, Fe redox studies that specifically address the effects of differentiation on Fe speciation have thus far been limited. For example, elevated magmatic water contents, derived from the subducting plate, may suppress plagioclase saturation and decrease the temperature difference between the appearance of silicates and magnetite on the liquidus (Sisson & Grove, 1993), potentially influencing whether a basaltic magma follows a calc-alkaline (Fe-depleted) or tholeiitic (Fe-enriched) differentiation path (e.g., Zimmer et al., 2010). Yet, magmatic H O and Fe3+/ΣFe ratios are strongly correlated (Kelley & Cottrell, 2009), 2 and high magmatic fO also enhances the appearance of oxides relative to silicates on the basalt 2 liquidus (Botcharnikov et al., 2008, Osborn, 1959). The effects of fO and H O on magmatic 2 2 differentiation may thus be difficult to segregate. Magnetite fractionation in a system closed to oxygen is also expected to reduce magmatic Fe3+/ΣFe ratios, but this phenomenon has not been observed directly in the natural rock record. If source mantle fO at convergent margins is 2 elevated over MORB, the cause of this oxidation and the extent to which it varies are central to developing models for the structure and growth of arc crust, and of the oxygen evolution of Earth through time. Does primary fO change as subduction influence varies or diminishes? What 2 effect do variable extents of fluid or sediment melt infiltration have on primary fO ? 2 To answer these questions, we examine the relationships between crystal fractionation, degassing, mantle source composition, subduction influence, and magmatic or mantle fO along 2 the entire Mariana subduction zone. With this work, we investigate a variety of crystal fractionation and degassing processes recorded by arc and back-arc basaltic glasses and examine the relationships between these processes and magmatic Fe redox. We present new major, trace, and volatile element concentrations as well as Fe3+/ΣFe ratios in olivine-hosted melt inclusions from single eruptive events at five sub-aerial volcanic centers along the Mariana arc (Sarigan, Guguan, Alamagan, Pagan, and Agrigan), in addition to submarine glasses from NW Rota-1 and Pagan volcanoes (Tamura et al., 2013, Tamura et al., 2011) and the Mariana trough back arc spreading center (Fig. 1). After assessing the effects of differentiation on magmatic redox, we use major element trends defined by the data to reconstruct primary melt compositions and mantle source fO conditions. We then pair these with key trace element ratios (Ba/La, Th/La, 2 and Zr/Y) to assess the extent to which different slab derived materials may influence the fO of 2 the mantle wedge. GEOLOGIC SETTING The Mariana subduction system is a well-studied ocean-ocean convergent margin with an active sub-aerial and submarine arc made up of ~40 volcanic centers and the Mariana trough, an actively extending back-arc basin (Fig. 1; Bloomer et al., 1989, Fryer, 1996, Hickey-Vargas & Reagan, 1987, Stern, 1979, Tollstrup & Gill, 2005, Woodhead, 1989). The arc is split into three distinct segments, the Northern Seamount Province, the Central Island Province, and the Southern Seamount Province. The Central Island and Southern Seamount Provinces are both built on oceanic lithosphere previously rifted by the opening of the Mariana trough and the Parece-Vela basin (Fryer, 1996). The composition of erupted products along these arc volcanic centers are well studied and are primarily basaltic in composition (Bloomer et al., 1989, Kelley et al., 2010, Martindale et al., 2013, Meijer & Reagan, 1981, Pearce et al., 2005, Shaw et al., 2008, Wade et al., 2005). The northern to central Mariana trough, here termed collectively the northern Mariana trough, is opening asymmetrically in an east-west direction and generally mimics the arcuate shape of the volcanic front (Fryer, 1996). The volcanic arc follows the strike of the Mariana trench north of ~13ºN. South of this latitude, the trench curves sharply to an east- west orientation. In this area, both arc and back-arc volcanism approach the trench and the subducting Pacific plate is shallower beneath this magmatically active area (Ribeiro et al., 2013, Syracuse & Abers, 2006). Taken together, the oceanic upper plate, mafic magmatism, and the presence of a mature back-arc spreading center make the Mariana arc an ideal setting for studying the competing effects of source fO and shallow crustal processes on the Fe3+/ΣFe ratios 2 of arc and back-arc basalts. SAMPLES AND METHODS Mariana arc tephra samples Olivine hosted melt inclusions were targeted for this study for several reasons. First, suites of melt inclusions from a single eruptive event at a volcano potentially display a range of variable, pre-eruptive magmatic compositions that correspond to the changing compositions of a differentiating magma. Olivine is an early fractionating phase in the evolution of basaltic magma, such that melt inclusions hosted in olivine often record early stages of differentiation compared to plagioclase- or clinopyroxene-hosted inclusions, and so their compositions may be closer to the composition of parental magmas than the final erupted lavas. Finally, melt inclusions have also been shown to preserve less degassed volatile concentrations than erupted lavas, allowing the study of the effects of volcanic degassing along with crystal fractionation on Fe redox in subduction zone magmas. The glass inclusions analyzed in this study were picked from nine Mariana arc tephra samples originating from five volcanoes from the Central Island province of the Mariana arc (numbers indicate disparate eruptions): Sarigan (Sari15-04), Guguan (Gug11 and Gug23-02), Alamagan (Ala02 and Ala03; Shaw et al., 2008), Pagan (Paga8), and Agrigan (Agri07, Agri05 and Agri04, Fig. 1). These samples were collected by a MARGINS-NSF field expedition to the Mariana arc in 2004 and donated to this study by T. Plank (http://sio.ucsd.edu/marianas; Figure 1). Each tephra sample was washed in de-ionized water and sieved, taking care to avoid any samples with clasts larger than two centimeters to ensure that all material had a short cooling history upon eruption (e.g., Lloyd et al., 2012). Olivine crystals were either hand-picked from sieved size fractions or separated using lithium poly-tungstate heavy liquid separation, using modified techniques from Luhr (2001). Large (0.5 -1 mm), euhedral olivines or olivine fragments were immersed in mineral oil to identify glass inclusions, which were selected for analysis if they were >50 µm in diameter, completely glassy, without daughter or co-entrapped minerals, fully contained by the host olivine, and contained no more than one vapor bubble. Representative photomicrographs are shown in Figure 2. Photomicrographs of every inclusion are shown in electronic appendix K. Submarine glass samples Glassy pillow lavas from the southernmost Mariana trough (Malaguana-Gadao ridge) were dredged from the seafloor between 12.5° - 13.2°N, during expedition TN273 of the R.V Thomas G. Thompson in 2011-2012 (Southern Mariana trough, Fig. 1). Glassy pillow lavas from submarine volcanic exposures at Pagan and NW Rota-1 volcanoes were provided by Yoshi Tamura (Tamura et al., 2013, Tamura et al., 2011). Glass chips were chiseled and hand picked from the freshest pillow lavas in each dredge and washed in de-ionized water prior to preparation for analysis. We also incorporate previously published data for submarine glass samples from the northern Mariana trough (18.1° - 20.9°N; Kelley & Cottrell, 2009, Newman et al., 2000, Pearce et al., 2005, Stolper & Newman, 1994; Fig. 1) Analytical methods Electron Microprobe Analysis Submarine glass chips and glass inclusions were exposed on a single side and polished for electron microprobe analyzer (EMPA) analysis on a JEOL-8900 5 spectrometer microprobe at the Smithsonian Institution. During major element analysis, the beam was operated at 10nA, an accelerating voltage of 15 kV and 10 µm beam diameter. Sodium and potassium were measured first with 20 second peak count times to minimize alkali loss. Subsequently, Si, Ti, Al, Fe*, Mn, Ca and P were measured with 30-40 second peak count times. All data were subject to ZAF correction procedures. Primary calibration standards include VG-2 glass, Kakanui hornblende, anorthite, microcline, ilmenite, and apatite (Jarosewich et al., 1980). The VG-2 and VG-A99 glasses were monitored as secondary standards during each run (Jarosewich et al., 1980). Sulfur and chlorine were measured separately using a beam operated at 80 nA, an accelerating voltage of 15 kV and 10 µm beam diameter. Scapolite was used as the primary calibration standard (0.529 wt% S, 1.49 wt% Cl). The VG-2 (1320 ppm S, 300 ppm Cl) and NIST 620 (1121 ppm S) glasses were used as secondary standards in each run (Jarosewich et al., 1980, Carroll & Rutherford, 1988, Wallace & Carmichael, 1991). The major element compositions of the olivine hosts were measured adjacent to the glass inclusions as well as at the rims of the olivines to eliminate zoned hosts that reflect potentially complex magmatic histories. A focused electron beam was operated at 10 nA and an accelerating voltage of 15 kV. San Carlos olivine and fayalite were used as primary calibration standards, San Carlos olivine and Springwater olivine were used as secondary standards during each run (Jarosewich et al., 1980). Significant olivine zoning was not observed for any samples in this study and the olivine compositions reported in electronic appendix E are average values of all three to six analysis spots on each olivine. FTIR Analysis After EMPA analysis of melt inclusions, all sample pits were polished away, being careful to account for possible electronic damage within the activation volume of each EMPA spot. Melt inclusions were then polished from the opposite side until doubly exposed, and submarine glasses were wafered to a nominal thickness of 80 µm (though some were as thin as 20 µm) to create wafers with analyzable pools of optically clear glass. All wafered samples were washed gently with acetone to remove all epoxy residues. Dissolved H O and CO 2 2 concentrations in glasses and glass inclusions were analyzed by Fourier-transform infrared (FTIR) spectroscopy at the Smithsonian Institution. All samples were analyzed using either a Bio-Rad MA-500 microscope attached to a Bio-Rad Excalibur FTS 3000 FTIR spectrometer or a Continuum microscope coupled with a Thermo-Nicolet 6700 FTIR spectrometer. Spectra for all samples were collected between 1000-6000 cm-1 using a tungsten-halogen source, KBr beamsplitter and a liquid-nitrogen cooled MCT-A detector. The bench, microscope, and samples