Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

Abstract

[1] Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Srisotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter-scale hydrothermal veins, show a range of 87 Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr-isotope exchange is similar in all of these locations. Models that assume that fluid-rock reaction occurs during one-dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid-rock exchange. In this scenario the fluid at the base of the crust is little modified in 87 Sr/ 86 Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh-rock 87 Sr/ 86 Sr, but this is not observed. Alternatively, models in which fluid-rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87 Sr/ 86 Sr with depth in the crust. Minimum time-integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 Â 10 6 kg m À2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the crust. Because this leaching lowers the fluid 87 Sr/ 86 Sr without changing the rock 87 Sr/ 86 Sr, these mass balance models must underestimate the time-integrated fluid flux. Additionally, these values do not account for fluid flow that is channelized within the crust.

Crustal faults exposed in the Pito Deep Rift: Conduits for hydrothermal fluids on the southeast Pacific Rise

“…The distribution and composition of hydrothermal mineral phases result from the reaction progress as seawater-derived hydrothermal fluids pass through the ocean crust at variable pressure, temperature, pH, and redox state of the fluids [Mottl, 1983;Seyfried et al, 1991;Alt et al, 1996]. Such reaction progress is also tracked by stable isotope compositions of altered rocks, which are reported elsewhere for PDR samples [Barker et al, 2007[Barker et al, , 2008. Here, we present bulk rock geochemical data from the fault zones (Tables 1 and 2) and interpret the data in the context of current models for hydrothermal systems in the following discussion.…”

“…The fact that magnesium was actually added to both types of fault rocks at greenschist conditions suggests that relatively fresh seawater was able to flow into the fault zones and reach temperatures of >400°C [Saccocia and Seyfried, 1994]. Other, independent evidence for seawater infiltration into the fault rocks also comes from 87/86 Sr compositions reported elsewhere that require some amount fault and fracture-focused flow of mixed seawater and hydrothermal fluids [Barker et al, 2007[Barker et al, , 2008A. Barker, personal communication, 2007].…”

“…[52] One of the key observations from altered ocean crust (and ophiolites) is that 87/86 Sr values from altered basalt become more rock-buffered with depth [Bickle and Teagle, 1992;Barker et al, 2008]. Another apparently universal property of the ocean crust is a metamorphic gradient with depth wherein low-temperature clay minerals dominate the alteration mineralogy in the lavas, with an abrupt jump to ''greenschist'' through ''amphibolite'' metamorphism in the sheeted dikes [Alt et al, 1996;Gillis et al, 2001].…”

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“…No systematic variations are apparent in alteration assemblages or peak metamorphic temperatures with depth, similar to other sheeted dike sections (ODP Holes 504B [Alt et al, 1996] and 1256D [Alt et al, 2010]; Pito Deep [Heft et al, 2008]). The average minimum fluid/ rock ratio and fluid flux for the sheeted dike complex, calculated by mass balance using whole-rock Sr isotope compositions, are 0.7 × 10 6 and 1.5 × 10 6 kg/m 2 , respectively (Gillis et al, 2005), within the range for sheeted dike complexes from other fast-and intermediate-spreading ridges (Barker et al, 2008;Teagle et al, 2003).…”

Deep drilling of intact ocean crust: harnessing past lessons to inform future endeavors

“…An important recent advance comes from the recognition that the sheeted dike complexes of all intermediate to fast-spread systems studied (DSDP Hole 504B and ODP Hole 1256D and seafloor samples from Hess Deep and Pito Deep tectonic windows) provide relatively consistent estimates of axial hightemperature fluid fluxes (e.g., Teagle et al, 1998aTeagle et al, , 2003Gillis et al, 2005;Barker et al, 2008;Harris et al, 2008;Harris, 2011;Coggon, 2006;Nielsen et al, 2006;Chan et al, 2002). These estimates are all much lower than hydrothermal fluxes estimated from global seawater budgets, hydrothermal vent observations (e.g., Elderfield and Schultz, 1996), or studies of ophiolites (Bickle and Teagle, 1992), but their consistency with thermal calculations gives confidence in their validity.…”