RESULTS

The calcium carbonate polymorphs aragonite and calcite have distinct trace element concentrations because their structures allow for preferential incorporation of different cations (Deer et al., 1992). The mineralogy of the Site 1256 carbonate veins is therefore determined from their Sr/Ca and Mg/Ca ratios, which reveal two distinct groups (Fig. F1; Table T1). The compact calcite structure can incorporate the small Mg cation, whereas aragonite has a larger cation site and favors accommodation of the larger Sr cation (Deer et al., 1992). Samples with Sr/Ca ratios less than ~0.2 mmol/mol are classified as calcite, and those with Mg/Ca ratios of less than ~2.5 mmol/mol are classified as aragonite (Table T1; Fig. F1). The remaining samples, with intermediate compositions (Sr/Ca >0.2 mmol/mol and Mg/Ca >2.5 mmol/mol), are interpreted to be aragonite-calcite mixtures, which were homogenized during hand-picking.

Fe and Mn are similar in diameter to Mg and are therefore also preferentially incorporated into calcite, with Fe/Ca ranging from ~3 to ~35 mmol/mol and Mn/Ca ratios up to ~80 mmol/mol (Fig. F1). The aragonite veins have lower Fe/Ca ratios (<2.5 mmol/mol), and their incorporation of Mn is extremely limited, with Mn/Ca ratios ranging from ~0.01 to ~2 mmol/mol.

The carbonate veins have ⁸⁷Sr/⁸⁶Sr ratios ranging from ~0.70804 to 0.70872. The majority of the aragonite veins have Sr isotopic compositions between ~0.7087 and ~0.7086, whereas the calcite veins precipitated from fluids with ⁸⁷Sr/⁸⁶Sr ranging from ~0.7086 to ~0.7080. The strontium isotopic composition of the oceans ~15 m.y. ago was ~0.70875 (Hodell et al., 1991), and it has increased progressively since then. All the carbonate veins precipitated from fluids with ⁸⁷Sr/⁸⁶Sr ratios lower than the seawater ⁸⁷Sr/⁸⁶Sr ratio has been since the eruption of the host basalt. The veins therefore all contain a discernible component of basalt-derived Sr (⁸⁷Sr/⁸⁶Sr ~0.7027). The carbonate ¹³C and ¹⁸O compositions range from –3 to 4_VPDB (Vienna Peede belemnite) and 20 to 32_VSMOW (Vienna standard mean ocean water), respectively, within the range previously established for hydrothermally altered mid-ocean-ridge basalt–hosted calcite breccia cements and veins (Stakes and O'Neil, 1982).