Reconciling hydrological models with geological constraints in the upper oceanic crust at ODP Holes 504B and 896A

It has been proposed that ridge flank hydrothermal circulation by outcrop-to-outcrop (lateral) flow may be the dominant mode of advective heat extraction from the oceanic crust globally[1]. In this model the upper crust is an aquifer overlain by low permeability sediments, and aquifer-ocean fluid exchange occurs through basement outcrops. Thermally-induced pressure gradients drive fluid laterally from recharge outcrops to discharge outcrops. Drilling at ODP Holes 504B and 896A (in 6.9 Myr crust), along with hydrological monitoring and geophysical surveys of the area, provide a wealth of data to test the model of outcrop-to-outcrop flow in a young, rapidly sedimented area. Outcrop-to-outcrop flow in this region has been shown to be consistent with heat flow data and measured upper crustal temperatures[2], but has not been tested against geological constraints. With a numerical model of sedimentation, and constraints from geophysical data, the time-dependant distribution of outcrops near Holes 504B and 896A is estimated. From this, temperature histories and fluid fluxes in the upper crust are estimated for the drilling locations with a quantitative model of outcrop-to-outcrop fluid flow and heat exchange. Temperatures predicted by the outcrop-to-outcrop flow model for both holes are <20°C for most of the crust's history, reaching 30-55°C at Hole 504B and 30-45°C at Hole 896A only in the last 2 Myrs. Calculated δ18O-derived precipitation temperatures of secondary carbonate in the upper 100 m of crust at Holes 504B and 896A range from ~30-70°C[3-5]. The high δ18O-derived carbonate temperatures at 896A are difficult to reconcile with the regional sediment distribution and a model of outcrop-to-outcrop flow. We consider whether δ18O-derived carbonate temperatures may be overestimated due to uncertainty in the hydrothermal fluid composition. Although there is a 1-2% enrichment in the δ18Orock of Hole 896A lavas[3], the modeled water-rock ratio of ~50 requires a relatively small corresponding decrease in δ18Ofluid relative to seawater, insufficient to lower carbonate temperatures to within range of the outcrop-to-outcrop flow model. These high temperature carbonates could be explained if vertical heat advection is important. The solubility of calcium carbonate is determined for temperatures predicted for outcrop-to-outcrop flow. Carbonate solubility and modeled water/rock ratios are used to estimate the timing and abundance of carbonate precipitation. Outcrop-to-outcrop flow requires that most upper crustal carbonates in both holes precipitated within the last ~2 Myrs. The 2-3x enrichment of bulk rock CO2 at Hole 896A[6] is consistent with greater fluid fluxes at Hole 896A relative to Hole 504B and/or a closer proximity of Hole 896A to a focused recharge site. Ongoing modeling of carbonate precipitation in off-axis hydrothermal systems near other drill holes will also be presented. [1] Fisher and Becker, Nature 2000, v403, p71. [2] Davis et al., EPSL 2004, v222, p863. [3] Teagle et al., Proc. ODP, Sci. Res. v148, p119. [4] Coggon et al., Science 2010, v327, p1114. [5] Alt et al., Proc. ODP, Sci. Res. 1996, v148, p435. [6] Alt and Teagle, Geochim. Cosmochim. Acta 1999, v63, p1527.