Fluid sources, fluid pathways and diagenetic reactions across an accretionary prism revealed by Sr and B geochemistry

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Abstract

ODP Leg 204, which drilled at Hydrate Ridge, provides unique insights into the fluid regime of an accretionary complex and delineates specific sub-seafloor pathways for fluid transport. Compaction and dewatering due to smectite–illite transition increase with distance from the toe of the accretionary prism and bring up fluids from deep within the accretionary complex to sampled depths (≤ 600 mbsf). These fluids have a distinctly non-radiogenic strontium isotope signature indicating reaction with the oceanic basement. Boron isotopes are also consistent with a deep fluid source that has been modified by desorption of heavy boron as clay minerals change from smectite to illite. One of three major horizons serves as conduit for the transport of mainly fluid. Our results enable us to evaluate fluid migration pathways that play important roles on massive gas hydrate accumulations and seepage of methane-rich fluids on southern Hydrate Ridge.

Introduction

Tectonic compaction in accretionary margins expels fluids that may have been altered by reaction with basement or by diagenetic processes at depth. Through these reactions fluids acquired characteristic geochemical and isotopic signals that can be used to trace their origins e.g. [1], [2], [3], [4]. These fluids migrate either as diffuse porous flow or as focussed advection through high permeability conduits such as fractures, faults or permeable horizons, and play important roles in the structural, thermal and chemical evolution of the margin.

Different tracers have been used to identify fluid sources and diagenetic reactions within the sediments. Among them, strontium (Sr) has proved to be a key tracer for solid–fluid reactions, establishing sources and fluid mixing patterns e.g. [5], [6], [7]. Potential source materials responsible for changes in Sr concentration and its isotopic composition in interstitial waters include: (1) continental detritus: 87Sr / 86Sr ∼0.7119–0.7133 (mean value [8], [9]); (2) biogenic calcite: 87Sr / 86Sr ∼0.7092–0.7075, and (3) oceanic crust: 87Sr / 86Sr ∼0.703 [10], [5].

The potential of boron (B) as a fluid tracer has recently become apparent and it develops from its high solubility in aqueous fluids, its large isotopic fractionation (ca. 60‰), its high abundance in the upper crust, its high mobilization at elevated temperatures and its affinity to sediments e.g. [11], [12], [4]. Sources of B include seawater (∼39.5‰), adsorbed B (∼15‰), lattice bound B (∼− 5‰ [13], [14]) and marine carbonates (∼20‰ [15], [16]). In shallow marine sediments the lighter tetrahedral B(OH)4 ion is adsorbed onto surfaces of detrital clay [14]. This process leads to pore fluids enriched in 11B and depleted in their total B concentration where B(OH)3 then becomes the dominant species at the in situ conditions. With increasing sediment burial the adsorbed B is released leading to an enrichment of 10B in the pore fluid [17]. The shallow release of light 10B has as well been attributed to boron release during organic matter decomposition [11], [2]. At greater depths, where high in situ temperatures and low porosities prevail, both the adsorbed and the lattice bound B can be released [17]. Because Sr and B have different sources, a combined study of both tracers can provide unique opportunities to investigate different geological questions and to gain a better understanding of the geochemical behavior of each element. In this paper, we combine our data on concentration and isotope data of strontium and boron with those from ODP Legs 146 [18] and 168 [19] to investigate the source of the sampled fluids and the factors influencing their migration patterns across the Cascadia accretionary prism. At depth, fluids are modified by reaction with oceanic crust, and by clay diagenesis. Three seismic reflectors (Horizons A, B and B′; Fig. 1D) are evaluated with respect to their role as gas and/or fluid pathways. Horizon A, which supplies the southern Hydrate Ridge summit with methane, seems to be a major pathway for gas transport [20] but not much fluid is advected through this path. In contrast, Horizon B appears to be a significant fluid pathway. The shallow sediments show evidence for significant boron release at the depth of bacterial organic matter degradation.

Section snippets

Geological setting

Hydrate Ridge is located offshore Oregon (U.S.A.) on the Cascadia margin. It is part of the seaward-verging thrust sequence of the accretionary prism which forms as the Juan de Fuca plate subducts beneath the North American plate.

The stratigraphic setting of Hydrate Ridge is characterized by having a core of highly deformed, underthrusted sediments of the accretionary complex (AC; Fig. 1D), which is thought to have high permeability [21]. This facies is overlain by dipping Pleistocene and

Methods

Pore water samples were obtained by squeezing whole-round sediment cores (5 to 20 cm in length) on board R/V JOIDES Resolution [22]. Boron and strontium concentration of all samples was measured onboard with a Jobin JY2000 ICP atomic emission spectrometer, using yttrium as an internal standard. Dilutions of IAPSO (International Association for the Physical Sciences of Oceans) standard seawater were used as calibration standards (for details see [22]). The reproducibility, expressed as 1σ

Results and discussion

The concentrations of dissolved Cl, Sr and B, and the isotopic composition of Sr and B for Sites 1244, 1245, 1248, 1250 and 1251 are listed in Table 1. Depth profiles are shown in Fig. 2. The complete data set of pore water data can be found in [22].

Measurements of the pore water B concentration and its isotopic composition may suffer from sampling artifacts because they were collected at a temperature and pressure that differ from the in situ conditions [17]. Therefore, all measurements were

Conclusions

  • 1.

    Non-radiogenic strontium isotopes in samples collected by drilling on the Cascadia accretionary prism indicate a deep fluid source that has reacted with oceanic basement. A combined transect encompassing data from Legs 168, 146 and 204 shows an increase in the deep fluid component with distance from the prism toe. The progressive influence of deep fluid away from the toe, likely reflects the effect of increased compaction and dewatering of the sediments. A larger component of deep sourced

Acknowledgments

We greatly appreciate the constructive comments of H. Elderfield, J. Gieskes and an anonymous reviewer who helped to improve this manuscript. We thank N. Gussone for helpful discussions on B geochemistry and A. Kolevica for laboratory support. We also express our gratitude to the captain and the crew of the JOIDES Resolution, and the ODP technical staff for their support at sea. This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S.

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