Pore water chemistry of the Mariana serpentinite mud volcanoes: A window to the seismogenic zone

Abstract

[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.

Mariana Convergent Margin and South Chamorro Seamount

“…We mapped changes in the chemical composition of interstitial waters across the Mariana forearc in relation to subduction-related prograde diagenetic and metamorphic reactions that may trigger fluid and elemental releases (Figures F10, F11). Processes that occurred early (shallow) within the subduction system (i.e., Yinazao and Fantangisña Seamounts) were likely dominated by diagenesis and opal dehydration, whereas later processes (deeper) (i.e., Asùt Tesoru Seamount) included decarbonation and clay mineral decomposition Haggerty, 1991;Haggerty and Chaudhuri, 1992;Haggerty and Fisher, 1992;Mottl and Alt, 1992;Fryer et al, 1999;Mottl et al, 2003Mottl et al, , 2004Hulme et al, 2010;Wheat et al, 2008Wheat et al, , 2010. Higher DIC at greater depths-to-slab is likely linked to carbonate decomposition within the slab.…”

“…Slab devolatilization was identified through the compositions of interstitial water. Interstitial water data for Conical and South Chamorro Seamounts from Hulme et al (2010). For ease of comparison, all elemental concentrations are plotted in millimolar, but B was multiplied by 10 and Na was divided by 10 for ease of representation.…”

“…Depth-to-slab measurements for South Chamorro and Conical are extrapolation estimates (see text). Interstitial water measurements from Expedition 366, Mottl et al (2003, Mottl (2004), and Hulme et al (2010) summit; it was drilled during ODP Leg 195 ( Figure F1; Table T1). This seamount is primarily composed of unconsolidated flows of serpentinite mud with clasts consisting dominantly of serpentinized peridotite but also includes greenschist and blueschist rock fragments from the subducted Pacific plate (Shipboard Scientific Party, 2002).…”

“…Depth-to-slab was determined by seismic reflection profile for Yinazao, Fantangisña, and Asùt Tesoro Seamounts (Oakley et al, 2007(Oakley et al, , 2008Oakley, 2008), and by equilibrium mineral assemblages in metamafic clasts for South Chamorro and Conical Seamounts (Maekawa et al, 1993;Gharib, 2006). Distance to trench and temperature of slab from Hulme et al (2010). Depth-to-slab measurements for Yinazao, Fantangisna, and Asùt Tesoro from Oakley et al (2008) and Oakley (2008).…”

Effects of metal ions and pH on the formation and decomposition rates of di- and tri-peptides in aqueous solution

“…(Mottl et al, 2003;Tivey, 2007;Hulme et al, 2010). Although the concentrations of metal ions in the Lost City hydrothermal field are low, this system is enriched in Ca 2+ (Kelley et al, 2001(Kelley et al, , 2005.…”