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doi:10.2204/iodp.proc.322.207.2013

Results and discussion

A total of 65 pore fluid samples were analyzed, and the data are listed in Table T1. Figure F2 shows the downcore ⁸⁷Sr/⁸⁶Sr depth profiles in the context of the corresponding lithology. At Site C0011, the shallowest sampled analyzed (Section 333-C0011D-1H-4; 24.98 mbsf) has an ⁸⁷Sr/⁸⁶Sr ratio of 0.708710, which is markedly lower than the seawater value of 0.70917, as shown by the red rectangle on the x-axis (Fig. F2). Sr isotopic ratios continue to decrease with depth to ~350 mbsf, which coincides with the boundary between the hemipelagic pyroclastic sediments of lithologic Unit I and the volcanic turbidites of Unit II and is denoted by a horizontal arrow marked “A” (Fig. F2). The ⁸⁷Sr/⁸⁶Sr ratio in pore fluids recovered from Unit II–IV sediments shows little variability, with values ranging between 7.06504 and 0.706916, except for the two deepest samples analyzed (850.44 and 858.34 mbsf), which have values of 0.706991 and 0.707298. A mixing diagram showing the ⁸⁷Sr/⁸⁶Sr ratio from Site C0011 versus the inverse of the Sr concentration measured onboard indicates a two-end-member system characterized by fluid mixing between seawater (⁸⁷Sr/⁸⁶Sr = 0.70917) and an evolved, ⁸⁷Sr-depleted fluid (⁸⁷Sr/⁸⁶Sr = 0.70650) (Fig. F3). The ⁸⁷Sr-depleted fluid likely results from alteration of the volcanic ash layers in lithologic Subunit IA (Fig. F2). It is worth noting that the shipboard sedimentologists from Expedition 333 reported that the ash recovered within Subunit IA shows little evidence for alteration, whereas the severity of ash alteration increases with depth below the transition from Subunit IA to IB at ~252 mbsf (Fig. F2) (Expedition 333 Scientists, 2012). The tuffaceous sandstones of Unit II (348–578 mbsf; Fig. F2) may also have contributed to the ⁸⁷Sr-depleted fluids at this site; however, smear slide observations revealed mostly unaltered glass within Unit II sediments (Expedition 333 Scientists, 2012). Unfortunately, because of technical drilling difficulties the hole was aborted before reaching the target basement depth. Because of this, the deepest sediments at this site, which may have shown a diffusional communication with a more evolved fluid in the upper crust, were not recovered.

The shallowest sample analyzed at Site C0012 (Section 322-C0012A-5R-2) is also highly depleted in ⁸⁷Sr relative to seawater, with an ⁸⁷Sr/⁸⁶Sr ratio of 0.707640. Similar to observations at Site C0011, the Sr isotopic ratio continues to decrease with depth toward the bottom of lithologic Unit I (denoted by the blue arrow “A” in Fig. F2), reaching a value of 0.706884 at ~144 mbsf. This lithologic unit is characterized by the presence of abundant ash layers. The potential contribution of ash to ⁸⁷Sr/⁸⁶Sr in the upper 150 mbsf at this site is also apparent in the mixing diagram in Figure F3. Consistent with observations at Site C0011, the Sr isotopic ratio of fluids recovered from lithologic Units II–IV remains relatively constant, with ⁸⁷Sr/⁸⁶Sr values ranging from 0.706884 to 0.706502 (Fig. F2). Deeper in the formation, the fluids recovered from Units V and VI show a marked decrease in the Sr isotope ratios, which reach a minimum ⁸⁷Sr/⁸⁶Sr value of 0.70436 at 529 mbsf, indicative of fluids modified by reaction with the underlying oceanic crust (⁸⁷Sr/⁸⁶Sr ~ 0.7030) (Fig. F3).

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