G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 11, Number 1 20 January 2010 Q01X09, doi: ISSN: Click Here for Full Article Pore water chemistry of the Mariana serpentinite mud volcanoes: A window to the seismogenic zone Samuel M. Hulme SOEST, University of Hawai i at Mānoa, 1680 East-West Road, POST 504, Honolulu, Hawaii 96822, USA (hulme@hawaii.edu) C. Geoffery Wheat Global Undersea Research Unit, PO Box 475, Moss Landing, California 95039, USA Patricia Fryer and Michael J. Mottl SOEST, University of Hawai i at Mānoa, 1680 East-West Road, POST 504, Honolulu, Hawaii 96822, USA [1] In 2003, we conducted a survey of 11 serpentinite mud volcanoes in the Mariana fore arc. Here we report sediment pore water data from navigated gravity and piston cores and from push cores collected by the ROV system Jason2-Medea. Systematic variations in profiles of pore water chemical compositions from these mud volcanoes are consistent with models that include active upflow of pore water relative to the surrounding serpentinite matrix. The speed of upwelling, based on fits of an advection-diffusion model to observed data (K, Na, Rb, and Cs), reaches a maximum of 36 cm/yr at Big Blue Seamount. Results from these simulations constrain the pore water composition at depth and the degree of additional alteration as the pore water ascends through the sampled section. For example, the transition metals (e.g., Mn, Fe, Co, Ni, Cu, and Mo) are mobilized under conditions of low upwelling speeds and microbial activity. Similarly, the rare earth elements (REE) show evidence of near-surface alteration. In addition to these surficial reactions, distinctive pore water compositional patterns exist as a function of the distance from the trench axis, which is a proxy for the depths of water generation from the downgoing plate below each seamount. Systematic trends in the chemical composition of these slab-sourced fluids are consistent with increasing temperature and pressure at depth west of the trench. These trends include an increase in K, sulfate, carbonate alkalinity, Na/Cl, B, Mn, Fe, Co, Rb, Cs, Gd/Tb, Eu, and light REE (LREE) and a decrease in Ca, Sr, and Y with increasing distance from the trench. Mg and U are universally depleted in the upwelling water. We constrain the thermal conditions along the décollement using concentrations of fluid mobile elements (K, B, Cs, and Rb) and the mobilization of LREE relative to heavy REE (HREE). The 80 C isotherm is estimated at a depth of 15 km between Blue Moon Seamount and Cerulean Springs. At slab depths of 17 to 24 km, pore waters lack significant Rb and K enrichments relative to seawater, suggesting an upper bound near 150 C. There is an observed enrichment in LREE relative to HREE at Big Blue Seamount (slab depth 25 km) indicating that the décollement at this site is 200 C. The relative mobilization of Cs outpaced that of Rb at all seamounts sampled in this survey. On the basis of laboratory experiments, this observation sets an upper limit of 350 C at a depth of 30 km below the seafloor. Components: 16,276 words, 15 figures, 5 tables. Keywords: serpentine; subduction; REE; Mariana; trace element. Index Terms: 1050 Geochemistry: Marine geochemistry (4835, 4845, 4850); 1065 Geochemistry: Major and trace element geochemistry. Copyright 2010 by the American Geophysical Union 1 of 29

2 Received 10 June 2009; Revised 29 September 2009; Accepted 29 October 2009; Published 20 January Hulme, S. M., C. G. Wheat, P. Fryer, and M. J. Mottl (2010), Pore water chemistry of the Mariana serpentinite mud volcanoes: A window to the seismogenic zone, Geochem. Geophys. Geosyst., 11, Q01X09, doi:. Theme: Izu-Bonin-Mariana Subduction System: A Comprehensive Overview Guest Editors: S. Kodaira, S. Pozgay, and J. Ryan 1. Introduction [2] Within subduction zones the lithospheric slab is subject to increases in temperature and pressure as it sinks, remobilizing elements from its crustal sections and transferring them upward either along the décollement or into the overlying plate [Fryer et al., 1985, 1999, 2000, 2006; Silver, 2000]. Water plays a critical role in the transfer of elements by several distinct processes: dissolution of material in the sediment [Benton et al., 2001]; serpentinization of the overlying mantle [Fryer and Fryer, 1987]; and metasomatism during serpentinization and arc magma generation [Peacock, 1990; Girardeau and Lagabrielle, 1992]. Examining the flux of elements through subduction zones is integral to models of elemental recycling [e.g., Stern et al., 1991; Mottl, 1992; Peacock, 1993; Domanik and Holloway, 1996; You et al., 1996; Bebout, 1995; Fryer et al., 1999; Chan and Kastner, 2000; Spivak, 2002]. Because processes that govern elemental recycling occur at depths unreachable within the present limits of the Integrated Ocean Drilling Program (IODP), direct observation of P-T conditions, mineral phase changes, and physical properties at the décollement are not possible. To understand these processes researchers rely upon seismic and tomographic imaging, geochemical balances of subduction inputs and outputs, and the examination of exposed paleosubduction zones when constructing models of elemental cycling and subduction processes. [3] Within the outermost fore arc of an accretionary margin, pore water is released at low temperatures through sediment compaction, which can influence the geometry of the accreting wedge [Moore and Vrolijk, 1992]. As the slab continues to subduct, dewatering of clays and mafic minerals within the oceanic crust contribute water to the fore arc and subarc mantle through prograde metamorphism at depths approaching km below the seafloor [Domanik and Holloway, 1996]. During dehydration, removal and fractionation of mobile elements occurs (e.g., B, N, As, Be, Cs, Li, Pb, Rb, and the rare earth elements (REEs)) [You et al.,1996;bebout et al., 1999]. However, varying pressure and temperature (P-T) conditions along the broad source zone and the exchange of elements along the path of fluid migration are coupled with potential hydration-dehydration or recrystallization reactions, thus obscuring many potential geochemical tracers. [4] The focus of this study is to explore processes within the nonaccreting Mariana fore arc that distill components from the subducting slab through a succession of dehydration/decarbonation reactions well before reaching the zone of arc magmatism. Within the active Mariana fore arc, deep-seated materials ascend from the décollement and underlying mantle, forming mud volcanoes. These volcanoes provide the only means to sample directly the subducting slab as it evolves while migrating into the mantle. These conditions allow us to collect pore waters and mineral parageneses to elucidate in situ P-T conditions at the source region. This manuscript builds upon previous research in the Mariana fore arc [Mottl, 1992; Fryer et al., 1999;Benton et al., 2001;Mottl et al., 2004; Alt and Shanks, 2006; Fryer et al., 2006] by presenting new and more inclusive chemical data from navigated sediment cores within areas of active pore water upwelling on serpentinite mud volcanoes. These data are used to constrain the pore water composition at depth, which differs systematically as a function of the distance to the trench. On the basis of this systematic difference in composition, we constrain the extent of element mobilization, possible reaction mechanisms, and temperature along the décollement. These data, coupled with mineralogical data from deep ocean drilling, form the foundation for future modeling efforts to further constrain physical and chemical mechanisms and conditions 2of29

3 along the décollement in the Mariana fore arc and subduction zones globally. 2. A Geological Description of the Mariana Fore Arc [5] The Mariana fore arc (Figure 1) supports serpentinite mud volcanism that exposes suprasubduction zone mantle-sourced serpentinite mud, greenschist and blueschist facies metabasites, and slab-sourced water at the seafloor during the formation of serpentinite seamounts [Bloomer, 1982; Fryer et al., 1985; Fryer and Fryer, 1987; Fryer and Mottl, 1992; Fryer et al., 1999, 2000; Mottl et al., 2003]. Little or no accretionary prism, combined with extensive faulting of the overriding fore-arc plate, facilitates serpentinite mud volcanism in the Mariana fore arc [Fryer et al., 1985, 1990]. Here, serpentinite mud volcanoes erupt along high-angle tectonic faults that extend far below the seafloor to the décollement, possibly reaching depths up to 30 km [Fryer et al., 1999, 2000, 2006]. Tectonic movement along these faults mylonizes the serpentinite [Fryer, 2002], which, in combination with water released from the downgoing plate, forms mud with densities of g/cm 3 [Phipps and Ballotti, 1992; Salisbury et al., 2002]. This is 0.5 to 1.3 g/cm 3 less than the density of mafic oceanic crust and lithospheric mantle rocks ( g/cm 3 ). The erupting mud often carries larger serpentinite clasts or small greenschist to blueschist facies clasts to the seafloor in a series of episodic flows [Fryer, 1992; Fryer and Mottl, 1997; Fryer et al., 2000, 2006]. [6] Pore waters that upwell through serpentinite seamounts do so within a mud matrix that consists of (1) medium blue-green to dark blue serpentine; (2) veins or precipitates of chlorite, brucite, magnetite and Cr-spinel; (3) moderately to completely serpentinized ultramafic harzburgite and dunite clasts containing olivine and orthopyroxene with lesser amounts of clinopyroxene; and (4) calcite and aragonite carbonates [Salisbury et al., 2002]. Results from chemical analyses of pore waters and the serpentinite host have concluded that the pore waters are not significantly altered on their migration to the surface through serpentinized peridotite conduits, and that their composition represents conditions deep within the subduction zone [Mottl, 1992; Fryer et al., 1999; Benton et al., 2001; Mottl et al., 2003, 2004; Savov et al., 2005; Fryer et al., 2006]. Figure 1. Locations of serpentinite seamounts that were sampled during the 2003 Mariana fore-arc expedition. Also shown are contours of the distance from the trench axis produced in ArcGIS. Background map data from combining EM-300, Hydrosweep, MR1, and ETOPO 1 data sets and gridding at 10 arc sec resolution. Contours every 250 m. [7] Previous direct seafloor explorations by submersibles, ROV, and gravity coring discovered active pore water venting at the summit regions of three mud volcanoes. The submersible Alvin 3of29

4 surveyed active mud volcanism on Conical Seamount, prior to drilling ODP Leg 125, and discovered carbonate chimneys [Fryer et al., 1990; Haggerty, 1991]. The submersible Shinkai 6500 was deployed in 1995 to investigate S. Chamorro Seamount near ODP Site 1200 and discovered the only known benthic megafaunal community associated with serpentinite mud volcanism [Fryer and Mottl, 1997]. Pacman Seamount was surveyed with the Shinkai 6500 submersible and with the ROV Jason-Medea. Active seeps along the SE flank of Pacman Seamount, the Cerulean Springs, produce thin brucite chimneys almost 1 m in length [Fryer et al., 1999]. 3. Methods [8] A multidisciplinary survey of the Mariana fore arc, from 23 March through 4 May 2003, utilized acoustic swath-mapping surveys and sediment gravity/piston coring operations to locate active pore water seeps on 11 of the Mariana fore-arc serpentinite mud volcanoes (Figure 1). [9] Sediment from gravity and piston cores was immediately split and sampled. Sediment was scooped into acid-washed polycarbonate centrifuge tubes using Teflon-coated stainless steel spatulas. Sediment within 0.5 cm of the PVC liner and on the bisected surfaces was discarded to avoid sample smearing and contamination from the core wall. Sediment-filled centrifuge tubes were cooled to 1 4 C, placed in a cooled rotor to eliminate warming during centrifuging, and spun in a centrifuge for 5 min at 10,000 rpm. Pore water was siphoned off the top of the sediment using acid-washed highdensity polyethylene (HDPE) syringes and filtered (0.45 micron) into acid-leached HDPE bottles and glass ampules. Some aliquots were immediately acidified using subboiled (6 N) HCl to lower the ph below 2. [10] Select push cores collected during ROV operations were immediately transported to a walk-in refrigerator (4 C) and placed in a nitrogen-filled glove bag. These push cores were sectioned in the glove bag into 3 cm intervals, depending on the water content and the presence of rocks or precipitates. Because of the high volume of cores recovered on some dives, only the highest-priority cores were sampled within the freezer. The remaining cores were sectioned at ambient temperatures and atmospheric conditions. Only the centers of the extruded core sections were collected to avoid sample smearing and contamination artifacts. Sediment samples were placed in acid-washed polycarbonate centrifuge tubes and capped within the nitrogen-filled glove bag. These samples were subsequently processed in the same method as the gravity and piston core samples. [11] Several chemical analyses were conducted immediately at sea. Chlorinity was measured by titration with silver nitrate [e.g., Knudsen et al., 1902] using an automated electrochemical endpoint. For those fluids with high dissolved sulfide content, samples were dried, rehydrated, and analyzed. Pore water ph was measured on 2 ml aliquots that were then titrated with 0.1 N hydrochloric acid for a measure of the alkalinity by the Gran function method [Stumm and Morgan, 1981]. Concentrations of hydrogen sulfide were measured by the photocolorimetric method developed by Cline [1969]. [12] The major elements (Ca, Mg, Na, K, S, Sr, and Li) and minor elements (Sr, Li, Ba, B, Mn, Fe, and Si) were measured using standard inductively coupled plasma atomic emission spectrometer (ICP-AES) techniques. High-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) was used to measure a selection of diluted (1% in 0.5 N QHNO 3 ) samples for Rb, Mo, Cs, Ba, and U. A standard addition technique was used to analyze V, Cr, Co, Ni, Cu, and Zn with 10% sample dilutions. For this analysis, a metal-free seawater solution was produced by pumping bottom seawater through an 8-hydroxyquinoline (8-HQ) column to provide a matrix-matched standard addition procedural blank. Because of their minimal concentrations and susceptibility to interferences from a seawater matrix, the REEs, Y and Cd were extracted from pore water samples using a combination of pumps, valves, and 8-hydroxyquinoline (8-HQ) columns (for more detail see Hulme [2005] or Hulme et al. [2008]). H and O stable isotopic measurements were made using a mass spectrometer at the University of California, Berkeley [DePaolo et al., 2004]. A total of 42 chemical species were analyzed within the highest-priority samples. 4. Results [13] Eleven geologic features related to serpentinite mud volcanism were sampled with 15 gravity cores, 14 piston cores, and 51 push cores (Unless otherwise noted, data that are discussed are from this 2003 expedition.). Pore water chemical profiles of gravity, piston and ROV push cores from 4of29

5 Figure 2. Major and minor element pore water chemical profiles recovered on the 2003 expedition. Profiles shown are from representative cores that were calculated to have the highest pore water upwelling speeds at each seamount. Pore water profiles from S. Chamorro collected in 2003 are not indicative of active fluid upwelling. Concentrations shown for S. Chamorro are from water samples taken from a venting borehole and a background bottom seawater sample [Wheat et al., 2008]. the 2003 expedition with the highest speed of pore water upward flow are presented in Figures 2 and 3. Pore water gradients at all of the active sites display decreasing (approaching 0 mmol/kg) Mg concentrations and increasing ph (up to 12.3 at Big Blue) down-core. Ca and Sr concentrations exhibit increases with depth relative to seawater at seamounts near the trench (e.g., E. Quaker; maximum of 78 mmol/kg Ca and 914 mmol/kg Sr) and decreases with depth at the more distal seamounts (e.g., Big Blue; minimum 0.1 mmol/kg Ca and 10 mmol/kg Sr). K and B concentrations generally 5of29

6 Figure 3. Results of minor and trace elemental pore water composition from cores recovered on the 2003 expedition. The same samples from Figure 2 are presented here. behave inversely to Ca and Sr across the Mariana fore arc with maxima of 14 mmol/kg K at Conical and 1800 mmol/kg B at Big Blue and minima of 2.2 mmol/kg K and 15 mmol/kg B at Cerulean Springs. [14] Concentrations of Rb show internally consistent trends down-core at each site (Figure 3). Pore waters that seep from three seamounts, Blue Moon, Cerulean Springs and NE Quaker, are depleted in Rb relative to seawater. For example, measured Rb values of 0.45 mmol/kg at Cerulean Springs are less than half the seawater concentration of 1.37 mmol/kg. In contrast, the highest pore water concentration of Rb is 6.65 mmol/kg at Big Blue. Unlike Rb, Cs concentrations within the deepest-sampled intervals are greater than those of the surface pore water concentration at all of the seamounts, with the exception of Blue Moon (Figure 3). [15] Other trace elements measured in pore water, Mo and U, show a marked degree of mobilization and irregular variation in the uppermost sediment but approach asymptotic concentration profiles deeper in the cores (Figure 3). For example, U 6of29

7 Figure 4. Depth profiles of redox-sensitive elements and K in pore water recovered during the 2003 expedition at Big Blue Seamount. Cores labeled J36 are from ROV Jason-Medea sampling. The other cores are from piston (PC5) and gravity (GC4) coring. Modeled flow speeds (cm/yr) based on the K data are shown. concentrations within the upper tens of centimeters of the sediment column exceed 40 nmol/kg in a push core from the summit of Big Blue and are significantly elevated at E. Quaker and Blue Moon. Despite the surface enrichments, down-section pore water concentrations approach zero at all of the sampled sites with active pore water upwelling, similar to Mg profiles. [16] Two other elements, Ba and Li, are apparently affected during ascent from depths below the deepest sampled interval, but this can only be confirmed at a single mud volcano for each element (E. Quaker for Ba and Quaker for Li). For example, Ba concentrations are elevated in borehole samples from S. Chamorro [Wheat et al., 2008], but the lack of a pore water profile precludes determining if the mobilization occurs at depth or as a result of microbial redox reactions. Mobilization of Ba at Cerulean Springs does fit an asymptotic profile, so the results are inconclusive there as well. Regardless of these uncertainties, concentrations of Ba and Li in deep-sourced pore waters do not follow any trend with increasing distance from the trench axis. [17] The complex behavior of the fourth period transition metals is attributed to differing redox potentials within the sediment column (Figures 4 7of29

8 Figure 5. Depth profiles of redox-sensitive elements and K in pore water recovered during the 2003 expedition at Cerulean Springs. Cores labeled J31 are from ROV Jason-Medea sampling. PC11 is a piston core. Modeled flow speeds (cm/yr) based on the K data are shown. and 5). The most abundant transition metals in pore water, Fe and Mn, behave nearly identical to one another. Often there are two distinct zones of enrichment as well as depletion. Other transition metals show similar zones of enrichment. For example, the Co enrichment is up to 22 nmol/kg in pore waters from Big Blue. These upwelling pore waters have high concentrations of sulfate and high alkalinities. In general, faster upward pore water flow compresses the vertical zone of transition metal reactivity, in contrast to slower upwelling seeps where mobilization of these elements occurs over a greater depth range. Cr, however, is mobilized in pore waters from the upper sediment section at Big Blue independent of flow speed, and high flow speeds correlate with the greatest degree of V enrichment. [18] The entire suite of rare earth elements (REE) and Y were measured in pore waters from 5 of the Mariana fore-arc seamounts and water sampled from the venting borehole at S. Chamorro [Wheat et al., 2008]. Given the minimal volumes extracted from the sediment during centrifuging, it was not possible to measure pore water concentrations of REE from all of the sampled cores. Concentrations of the REE at Big Blue were studied in greater detail and show a progressive depletion with faster pore water upwelling speeds (Figure 6). Examination of the REE concentrations from the deepest pore water sample from the core with the highest 8of29

9 +16%). This anomalous sample also has a significant decrease 18 O( 1.3%) compared with the bracketing samples. The lowest values ( 3%) are from Blue Moon. The range of typical surface pore water values from all of the seamounts is 0.9 to 18 O and 2 to 5. Discussion Figure 6. REE-Y results of HR-ICP-MS analysis of pore waters from Big Blue. Calculated pore water upwelling speeds are as follows: J36-4, 36 cm/yr; GC-19, 1.4 cm/yr; J36-2, 0.7 cm/yr. Sample depths are 0 3 cm (J36-2-1), cm (J36-2-9), cm (J36-4-1), cm (J36-4-6), and cm (GC19-7). Values are normalized to CI Carbonaceous chondrites [McDonough and Sun, 1995] and presented on a log scale. Bottom seawater data are shown for comparison. The dashed line indicates the standard deviation of the procedural blank normalized to CI Estimating Depth to the Subducting Slab [20] Previous results from the Mariana serpentinite volcanoes show a trend in the chemical composition of the upwelling pore water as a function of distance from the Mariana Trench axis [Fryer et al., 1999; Mottl et al., 2004]. This distance was then used as a proxy for the depth to the décollement from which a temperature at depth was estimated. We refine these previous estimates by first quantifying the distance from the trench for each sample location using a vectorized trace of the Mariana Trench axis and the commercial GIS software pack- seepage speed reveals the elemental fingerprint for each seamount (Figure 7). Some observed trends across the fore arc include: Cerulean Springs and Blue Moon have the highest abundances of REEs, distributed in a slightly LREE enriched pattern; E. Quaker has the lowest measured concentrations of La and the only positive Ce anomaly; Big Blue, E. Quaker, and Baby Blue have LREE-enriched pore waters and a positive Gd anomaly; S. Chamorro haslree-enriched pore waters and a positive Eu anomaly. [19] Selected pore water profiles of the stable 18 O (SMOW) (Figure 8) show considerable variation and no clear relationship with distance from the trench. The highest measured value 18 O (+1.8%) occurs at Cerulean Springs and the lowest measured value ( 1.5%) occurs at Blue Moon. The highest within these samples is from 1 m depth at Big Blue (+27%), but values below this horizon converge with values from nearby cores (at around +12 to Figure 7. REE-Y results of HR-ICP-MS analysis. Values are normalized to CI Carbonaceous chondrites [McDonough and Sun, 1995] and presented on a log scale. Bottom seawater data are shown for comparison. Shaded regions represent the range of values for seamounts near the trench (blue) and farther from the trench (red). The dashed line indicates the standard deviation of the procedural blank normalized to CI. 9of29

10 Figure 8. Oxygen and hydrogen stable isotopic ratios in pore waters from selected piston and gravity cores. Values are in % relative to SMOW. age ArcView8 (trace shown in Figure 1). After combining the subducting slab slope with distances from the trench, we applied a correction to account for bathymetric variations up to 3 km to for each sample location (Table 1). [21] The slab-depth estimate beneath Celestial produced by the trench distance-bathymetry model (19 km) fell within the range of depths (16 20 km) predicted from seismic imaging of the subducting slab by Oakley et al. [2007]. Depth estimate errors include an additional ±2 km resulting from the presence of subducting seamounts that alter the shape of the trench axis and reduce the depth to the décollement. An additional source of uncertainty is the differing slab dip angles across the Mariana Trench [Fryer et al., 2003; Miller et al., 2004], although this effect is minimal at shallow depths near the trench. These estimates are used in section 5.4 to discuss geochemical and thermal conditions within the subducting slab and upper mantle Estimates of Pore Water Upwelling Speeds [22] Quantifying the speed that pore water upwells at each site provides a benchmark for establishing the degree to which the pore water composition is altered during ascent. Slower upwelling speeds result in longer residence times, allowing chemical exchange with the surrounding matrix and possible alteration by microbial populations [e.g., Wheat Table 1. Results of the Distance-From-Trench Calculations and Estimates of the Depth to the Subducting Slab Under the Serpentinite Seamounts a Distance to Mariana Trench Axis (km) Depth to the Subducting Slab (km) Blue Moon Cerulean Springs E Quaker Celestial NE Quaker Pacman Summit Baby Blue Big Blue Quaker S. Chamorro Conical a The estimates are based on combining the distance from the trench axis with estimates of the slab dip angle under the fore arc [Fryer et al., 1985]. Corrections based on the bathymetric depth of each seamount were made to account for up to 3 km variations in seamount summit heights. 10 of 29

11 and Mottl, 2000]. In addition, the downward diffusion of seawater in such areas can result in the formation of secondary minerals such as carbonates. The degree of pore water sediment/rock alteration becomes apparent by quantifying the upward speed of flow at multiple locations within a seamount and assessing systematic variations in pore water chemical profiles. Cores that sample the highest speed of pore water upwelling are the primary basis for estimates of the composition of formation water at depth, presumably at or near the décollement. [23] The general equation for pore water flow is described by Berner [1980] and simplified for this application. In the simplified model the concentration [C] of an element within the sediment column is C ¼ b e vz=ds 1 þ C 0 where; v is the velocity of pore water upwelling, z is the depth within the sediment, Ds is the sediment diffusion coefficient of the modeled ion, C 0 is the concentration of the ion in bottom seawater, and b is a function of the composition (C) at a particular depth (z f ), b ¼ C f C o = e vzf=ds ð1þ ð2þ The variable Ds is a function of the conditions in the sediment, 6=f p Ds ¼ Dw v 10 ð3þ where Dw is the molecular diffusion coefficient of the element in seawater at 4 C [Li and Gregory, 1974], v is the viscosity of the water (0.95 at 0 C [Li and Gregory, 1974]), f is the sediment formation factor, and p is the porosity. Values for the formation factor and porosity are 3.5 and 0.55, respectively, based on previous measurements of serpentinite mud [Shipboard Scientific Party, 2002]. We chose an end-member depth of 60 m for these models to ensure the theoretical profiles were far below the sampled depth. We quantify upwelling speeds using this model by iteratively calculating the sum of the residuals between the observed and theoretical concentrations. [24] We apply this model to all of the sampled sites across the Mariana fore arc. K, Rb, and Cs concentration profiles are modeled because the source of these elements is the subducting slab, not the overlying mantle, suggesting that these elements are the most conservative elements during pore water ascent in this system [e.g., Mottl et al., 2004]. We rely more on the Rb results, because Rb appears to be the most conservative element in these systems and there is a large quantifiable difference between pore water and seawater concentrations. However, for three of the seamounts (Pacman Summit, Quaker, and Baby Blue) Rb concentrations in the upwelling pore waters are nearly indistinguishable from bottom seawater concentrations and thus, we rely mostly on the Cs concentration profiles to model flow, as pore water concentrations of Cs are readily distinguishable from seawater. Pore waters from cores with the highest calculated upwelling speeds from each seamount are used to assess the composition of the upwelling pore water from depth (Tables 2 and 3) Pore Water/Matrix Reactions [25] We address the role of pore water/matrix reactions by assuming the deep-seated composition of upwelling pore water is based on cores from areas with the fastest pore water upwelling speeds. By examining a wide variety of chemical species and their systematic differences in cores from areas with different pore water upwelling speeds, it is possible to estimate the degree of alteration as water upwells and to determine which elements are conservative during ascent. Such characterizations are important because they provide a measure of confidence in our estimates for the fluid composition at depth, which is thought to reflect slab dehydration reactions that are constrained by P-T conditions along the décollement [Mottl, 1992; Fryer et al., 1999; Mottl et al., 2004]. [26] In a natural seafloor setting, however, mobilization of some elements is not limited to abiotic reactions, microbially mediated redox reactions with readily available oxidants, such as oxygen and nitrate from bottom seawater, affect chemical compositions [e.g., Berner, 1980]. Other microbial mediated diagenetic processes are present in sediment pore waters (e.g., reduction of metallic oxyhydroxides and sulfur species [Van Cappellen and Wang, 1996; Thullner et al., 2005]). These abiotic and microbially mediated reactions affect chemical compositions of pore waters within surface sediments of the Mariana fore-arc serpentinite mud volcanoes [Peacock, 1990; Mottl et al., 2003; Takai et al., 2005]. Such reactions can mask the composition of upwelling deep-sourced water, if chemical fluxes from reaction are a significant portion of the advective transport portion of the chemical flux. 11 of 29

12 Table 2. Major Element Composition of Samples Most Representative of Deep-Sourced Fluids That Were Sampled in the 2003 Expedition a Maximum Flow (cm/yr) Minimum Mg (mmol/kg) Ca (mmol/kg) K (mmol/kg) Na (mmol/kg) Cl (mmol/kg) Na/Cl (mol/mol) ph Alk (meq/kg) Sulfate (mmol/kg) Maximum Sulfide (mmol/kg) Bottom seawater Blue Moon b >54.2 <5.19 <398 <469 <0.85 >8.2 <0.3 < Cerulean Spr E. Quaker NE Quaker b >33 <7.64 <420 <487 <0.86 >8.3 <1.3 <23 0 Pacman Summit b < < >8.9 <1.0 <25 2 Quaker Baby Blue b < >489 <527 >0.93 >9.0 <1.0 <22 0 Big Blue S. Chamorro c Conical b <3.58 >14.3 <452 <427 >1.05 >9.2 >1.4 >29 0 Conical c a The maximum flow rates at each site as well as the minimum Mg are given to show the proximity to an end-member composition (i.e., high flow, low Mg). Where the end-member compositions were not reached, the minimum or maximum values are shown based on the down-core asymptotic trends in composition. Because upwelling pore water was not sampled at S. Chamorro during the 2003 cruise, values from ODP Leg 195 are presented [Shipboard Scientific Party, 2002; Mottl et al., 2004]. The Conical Seamount 2003 pore water composition of the most altered interval is shown along with measured endmember values from ODP Leg 125 drilling results [Mottl, 1992]. b Asymptote not reached. c ODP drilling results. 12 of 29

13 Table 3. Minor, Trace Element, and Stable Isotopic Composition of Samples Most Representative of Deep-Sourced Fluids Sampled During the 2003 Expedition a B (mmol/kg) Li (mmol/kg) Si (mmol/kg) Sr (mmol/kg) Rb (mmol/kg) Cs (nmol/kg) Ba (nmol/kg) U (nmol/kg) d 18 O dd Bottom seawater Blue Moon b <7.8 >645 <0.72 < < Cerulean Spr Nip Blip b <185 < >563 <1.23 >6.1 >277 <2.4 Pacman Summit b <387 > <56 >1.4 >9.1 <118 <0.28 Quaker Baby Blue b >31 <61 >1.69 > < Big Blue < S. Chamorro 3000 c 0.4 c 84.6 c 10 c 10 c 53.5 c 400 c <0.1 d 2.5 c 12 c Conical b >890 <6.4 <41 <78 >5.3 >61.6 < Conical c a Where the end-member compositions were not reached, the minimum or maximum values are shown based on the down-core asymptotic trends in composition. Because upwelling pore water was not sampled at S. Chamorro during the 2003 cruise, values from ODP Leg 195 are presented [Shipboard Scientific Party, 2002; Mottl et al., 2004]. Because U was not measured in the ODP Leg 195 samples, the concentration of U in water venting from the Site 1200 borehole is presented (Mg = 26.3 mmol/kg [Wheat et al., 2008[). Conical Seamount estimates are shown along with measured end-member values from ODP Leg 125 drilling results [Mottl, 1992]. b Asymptote not reached. c ODP drilling results. d Borehole fluid sample. Thus, we need to recognize these reactions and their affects to elucidate the chemical composition of the deep-seated fluid from which we constrain the P-T conditions within the downgoing slab Major Elements [27] Upwelling pore water is depleted in Mg relative to seawater at each seamount, approaching sub-mm concentrations in the deepest cores. The depleted mantle beneath the Mariana fore arc is primarily harzburgite and dunite containing up to 99% olivine [Fryer, 1996; Saboda et al., 1992; Parkinson et al., 1992; Parkinson and Pearce, 1998]. Serpentinization of these protoliths produces secondary mineral phases such as brucite or magnesite, which can incorporate dissolved Mg from the pore water. [28] Brucite formation is favored under high ph and olivine-rich conditions [Bach et al., 2004] and has been observed at many of the seamounts [Fryer and Mottl, 1992; Lagabrielle et al., 1992; Fryer et al., 1999, 2000, 2006]. [29] Typically, in hydrothermal systems, Mg depletion of the end-member fluid is the basis for estimating the degree of mixing with seawater and is used for constraining the end-member composition; conservative elements when plotted together produce a line joining the two end-member values. However, for the slower pore water upwelling speeds observed in the Mariana cores, Mg is reactive in the sediment column, presumably being removed in brucite. We highlight Mg-Ca systematics at two seamounts of differing pore water compositions, indicating significant nonconservative behavior in the cored sediment (Figure 9a). Ca in pore waters also is reactive. For example, pore water from Cerulean Springs has high concentrations of Ca and low alkalinities, whereas pore water from Big Blue is depleted in Ca but contains appreciable levels of alkalinity (>68 meq/kg). Near the surface of Big Blue, Ca from seawater diffuses into the sediment and reacts with this high-alkalinity pore water, forming carbonates [Mottl, 1992; Fryer et al., 1999; Mottl et al., 2003]. [30] Concentrations of K in the ascending pore waters vary dramatically across the fore arc, being depleted relative to bottom seawater near the Mariana Trench (e.g., Cerulean Springs) and enriched farther from the trench (e.g., Big Blue). This trend is consistent with increasing temperatures in the subducting slab and is not related to serpentinization reactions [Mottl, 1992; Janecky and Seyfried, 1986]. By plotting the conservative component K against the molar ratio of Mg/Ca (Figure 9b), reactions in the surface sediment noted above become more evident. For example, the loss of Ca to carbonate formation at Big Blue correlates with a dramatic rise in Mg/Ca molar ratio. At Cerulean Springs, which has low-alkalinity pore water, the shallow slope of K versus the Mg/Ca molar ratio in the near surface is the result of rapid 13 of 29

14 [32] One possible explanation is that dissolved chloride is removed in, or added to, the sampled serpentinite mud as a product of precipitation or dissolution of chloride-bearing minerals such as iowaite (Mg 3 Fe 3+ (OH) 8 Cl2H 2 O) [Sharp and Barnes, 2004]. However, the presence of iowaite has yet to be documented. Another, less likely, possibility is that Na is substituted for Mg and/or Ca in surficial sedimentary serpentinite, thus maintaining charge balance [e.g., Mottl et al., 2004]. Magnesium is universally depleted in the upwelling pore water and is not the likely cation involved with Na exchange. In contrast, Ca-Na exchange has been observed in marine sediment pore water [e.g., Wheat et al., 2000], but the composition of sedimentary serpentinite vastly differs from that of typical pelagic sediment. Even though Ca varies dramatically with increasing distance from the Mariana Trench axis, the Ca concentration is too low at Big Blue for the required molar equivalent Figure 9. Plots of Mg, Ca, and K in the pore water from Big Blue and Cerulean Springs show the effects of reaction within the surficial sediment. (a) A plot of Mg versus Ca at the two seamounts highlights the similar low-mg composition and the opposite Ca trends. (b) Plotting the more conservative element K against Mg/Ca highlights various reactions; Ca is removed by carbonate precipitation at Big Blue, whereas Mg is removed by brucite formation at Cerulean Springs. Mg loss because K versus Ca are conservative (linear mixing relationship) at this site. The uptake of Mg in the upwelling pore water is probably results from brucite precipitation. [31] Experiments by Janecky and Seyfried [1986] showed that while Na and Cl can vary during serpentinization reactions, the Na/Cl molar ratio is conserved. A plot of Na versus Cl at Big Blue and Cerulean Springs shows two contrasting trends (Figure 10). There is a high linear correlation (0.9968) between Na and chlorinity at Cerulean Springs. This trend is consistent with these two species being conservative in the upwelling pore water. In contrast, the behavior of these two components at Big Blue is nonlinear (Figure 10), indicating that at least one of these elements is not conservative in the sampled section. Figure 10. Plots illustrating the relationship between Na and Cl in pore water from Big Blue and Cerulean Springs. (a) Na and Cl have a linear relationship at Cerulean Springs, but the low correlation at Big Blue illustrates the effect of alteration reactions. (b) Plotting K versus Na/Cl produces linear trends for both Big Blue and Cerulean Springs, illustrating the covariation of Na and Cl in the surface sediment, even during alteration. 14 of 29

15 Figure 11. Differences in Rb and Cs concentrations in pore waters from Big Blue and Cerulean Springs. Linear trends are indicative of conservative mixing. exchange with Na. Furthermore, the lack of any significant deviation in linear trends between Na and carbonate alkalinity versus K reinforces the possibility of pore water Cl concentrations being altered by Iowaite formation in the sampled section. Rb does not suffer from the same susceptibility to near-surface reactions as Cs. Therefore nonlinear element/rb relationships provide a measure of how reactive an element is in this regional setting, and in the sampled sections. [35] Uranium is the only element other than Mg that uniformly approaches sub-nm (<0.1 nmol U/kg) concentrations in upwelling pore water throughout the Mariana fore arc, but unlike Mg, pore water U enrichments are observed near the seafloor. A plot of U versus Rb at Big Blue and Cerulean Springs clearly illustrates this behavior (Figure 12a). Uranium in surface pore water from Big Blue is enriched by a factor of 3.5 over bottom seawater concentrations. In contrast, no significant mobilization of U occurs at Cerulean Springs where carbonate alkalinity is low. These distributions are related to microbially mediated dissolution of metallic oxyhydroxides present in the shallow sediments [Morford et al., 2005, 2007, 2009] Minor and Trace Elements [33] In general, pore water concentrations of Rb, Cs, and K are conservative in the uppermost sediment section, as concentrations are governed by reactions with potassic minerals in the subducting slab [Sadofsky and Bebout, 2003; Barr et al., 2002]. The products of differing P-T conditions at depth result in differing Rb, Cs, and K concentrations in the upwelling fluid. Rubidium concentrations at Big Blue and Cerulean Springs differ in a manner similar to K. In contrast, Cs is enriched in the upwelling pore water at both localities (Figure 11). The linear relationship between Rb and Cs reaffirms our assumption that these elements are conservative within the upper serpentinite mud. The only observation for the mobilization of Cs in the sediment matrix is in the uppermost (<10 cm) of serpentinite mud/pelagic sediment at Big Blue. [34] Because K, Rb, and Cs are generally conservative in the pore waters from our cores, especially those from below the upper tens of centimeters, we can use any of these elements to ascertain the behavior of other elements. We used K in the example above to establish a relationship between Mg and Ca alteration reactions at Big Blue and Cerulean Springs. We chose Rb to constrain the degree of alteration of pore water constituents across the entire Mariana fore arc because of its large dynamic range in concentration, and because Figure 12. Element-element plots of trace elements in pore water sampled at Big Blue and Cerulean Springs. (a) Uranium concentrations at both seamounts approach sub-nm concentrations at depth. Complexing with carbonate ions present in Big Blue pore water permits increased levels of U to concentrate within the upper sediment. (b) Each seamount exhibits distinctive patterns of Mo versus Ba as a result of differing sulfur species. Low sulfate at Cerulean Springs allows Ba to remain mobile in the pore water. 15 of 29

16 Although U is released during Mn and/or Fe oxidation at both sites, it is kept in solution at Big Blue by the formation of uranyl-carbonate complexes. [36] Other trace elements show differing degrees of mobilization and are likely linked to major dissolved ions present in the pore water. Sulfate and sulfide concentrations at Big Blue and Cerulean Springs have a marked affect on the degree of Ba and Mo mobilization, respectively (Figure 12b). At Cerulean Springs, depleted sulfate concentrations (8.5 mmol/kg) allow Ba mobilization that is normally inhibited by the formation of barite (BaSO 4 ). Elevated concentrations of Ca (49.5 mmol/kg) and Sr (0.32 mmol/kg) saturate the solution with respect to their sulfate minerals gypsum (CaSO 4 2(H 2 O)) and celestite (SrSO 4 ), and the precipitation of these minerals increases Ba mobilization [You et al., 1996]. Mottl et al. [2004] also suggested gypsum saturation, but these minerals have not been found, in spite of a search for them. In contrast, low Ba concentrations occur at all depths in cores recovered from Big Blue, which has elevated concentrations of sulfate relative to seawater (30 versus 28 mmol/kg). [37] Molybdenum mobilization occurs dramatically (up to 6,420 nmol/kg) at Big Blue associated with intervals that have high concentrations of dissolved Mn. Previous studies described the removal mechanism of Mo from solution by adsorption onto Mn oxyhydroxides [e.g., Bertine and Turekian, 1973; Emerson and Huested, 1991].Microbiallymediated dissolution of Mn oxyhydroxides was determined as a significant source of Mo mobilization in near-surface pore water by Morford et al. [2005] and is likely responsible for Mo mobilization at Big Blue. Minimal, or much less than that observed at Big Blue, Mo enrichment is apparent at Cerulean Springs even though the surface interval includes Mn mobilization. Concentrations of Ba and Mo in the deepest core sections at Big Blue converge to concentrations of 47 and 220 nmol/kg, respectively. In contrast, concentrations of Ba and Mo at Cerulean Springs converge to concentrations of 297 and 20 nmol/kg, respectively (Figures 3 and 12b) Transition Metals [38] Mobilization patterns in pore water profiles for Mn and Fe show a large degree of symmetry. Many of the remaining transition metals (Co, V, Cr and Cu) have profiles that appear to behave opposite to or independent of Fe and Mn (Figures 4 and 5). The underlying reason for the observed profiles in the shallow serpentinite mud is related to microbially mediated redox states that, in part, result in the consumption of Fe-Mn oxyhydroxides and the precipitation of metal sulfides [Poulton et al., 2004; Morford et al., 2009]. The reduction of oxyhydroxides results in the mobilization of Fe and Mn, which can then react with dissolved sulfide to precipitate pyrite and metal sulfides. The presence of high concentrations of carbonate and hydroxide ions further complicates the shape of the profiles by increasing metal carbonate and metal hydroxide complexing, and possibly precipitation [Zachara et al., 2001]. [39] Reactions among Fe, Mn, carbonates, oxyhydroxides and sulfur species impact the degree of mobility for a variety of other transition metals. For example, at Big Blue, Co enrichment occurs within the zone of Fe and Mn depletion (Figure 4). Experiments by Zachara et al. [2001] demonstrated the preferred dissolution of Co(II) over Fe(II) from oxyhydroxides through bacterial metabolism. Nickel shows the same behavior as Co in this zone, and shows significant mobilization at Big Blue. In contrast, pore waters from Cerulean Springs do not show the same magnitude of Co enrichment observed at Big Blue, and samples that are enriched in Co do not exhibit significant Ni mobilization. Instead, a correlation between Co and Cu exists at Cerulean Springs (Figure 5). This suggests the possibility that chemical species incorporated into serpentinite at depth under reducing conditions can be mobilized at the seafloor. Serpentinization at depth has a significant effect on the transition metals because the water is simultaneously reducing and hyperalkaline. Water reacting with basaltic crust will become acidic causing metals to mobilize, but serpentinization consumes metals by sequestering them into native metal and sulfur species depending on water-rock ratios [Palandri and Reed, 2004]. High water-rock ratios release additional sulfur into the system, which is reduced, leading to the formation of metallic sulfides. At low water-rock ratios, serpentinizing fluids also are reducing, but, in the absence of available sulfur, dissolved metals will precipitate as native metals and/or oxyhydroxide species [Palandri and Reed, 2004] Rare Earth Elements [40] Concentrations of REEs in pore water from Mariana fore-arc mud volcanoes are uniformly depleted relative to bottom seawater, with the exception of slightly elevated Ce at S. Chamorro 16 of 29

17 Figure 13. End-member estimates 18 O in the upwelling pore water across the Mariana fore arc. Pacman Summit, Baby Blue, and Quaker seamounts show little variation from values in surface pore water samples. Values for Conical and S. Chamorro are from ODP drilling Legs 125 and 195, respectively [Mottl, 1992; Mottl et al., 2003, 2004]. (Figure 7). One possibility is that carbonates coprecipitate REEs from solution (e.g., Ancylite-(La): Sr(La,Ce)(CO3)2(OH).(H2O)). Evidence for carbonate coprecipitation is clear in push core J36 4 (Figure 6), which was taken from a region of fast pore water upwelling at the summit of Big Blue with the highest alkalinity (68 mmol/kg). Significant levels of LREE depletion are present in the uppermost pore waters of the serpentinite mud where the carbonate-rich pore water reacts with Ca-rich seawater to form carbonates (including hydrated hydroxycarbonates). Scavenging by Fe oxyhydroxides has been documented as producing positive La, Gd, Lu and Y anomalies [Bau, 1999], and must not be overlooked in this system. Another possibility is that REEs are not mobilized in such settings, consistent with bulk chemical analyses of land-based blueschist formations [Bebout et al., 1999], and require much warmer temperatures for mobilization to occur at the high pressures expected along the décollement [e.g., Klinkhammer et al., 1994]. The origin of REE signatures are discussed in more detail later Stable Isotopic Effects [41] The O and H stable isotopic composition of pore waters indicates considerable alteration of the initial deep-sourced fluid composition. On the based of previous studies of Mariana fore-arc serpentinites, pore 18 O values should respond to variations in temperature along the décollement with increasing temperature resulting in 18 O[Alt and Shanks, 2006]. This trend is violated in the case of Cerulean Springs, which has a relatively 18 O value (+1.76%). Yet, the other chemical data do not support such a high projected temperature (e.g., Cs, Rb, and K data). If serpentinization in the near surface was responsible for altering 18 O values, then one would expect a shift in the opposite direction toward 18 O because of cooler temperatures at the seafloor. The cause of this anomalous shift toward 18 O may be attributed to isotopic fractionation during brucite formation, which is more abundant at Cerulean Springs than at the other seamounts. [42] Similar anomalous behavior is observed in pore water data, which differ inconsistently with projected depth to the subducting slab. The highest values from the 2003 survey were measured at E. Quaker and Big Blue, similar to results reported from ODP operations at Conical and S. Chamorro [Benton, 1997; Mottl et al., 2003] (Figure 13). There is a possibility that our values in gravity cores from Conical, compared to ODP drilling data, are the result of serpentinization reactions at shallow depths, but this does not account for the at E. Quaker. It is more likely that there is a significant isotopic effect related 17 of 29