SERPENTINITE MUD GEOCHEMISTRY

Chemical interaction between the serpentinite muds and seawater was a major concern for several key elements in this study (e.g., Na, B, Sr, and K). However, the geochemistry of the pore waters indicates that seawater penetration is restricted to depths <5 mbsf, ruling out the likelihood of seawater contamination of the muds.

Our Leg 195 samples closely resemble those from Conical Seamount (Fryer and Mottl, 1992; Lagabrielle et al., 1992), where mixing was observed between the seawater and only the uppermost 5 m of the recovered core samples. At South Chamorro Seamount, the pore waters below depths of ~5 mbsf show stable and very high pH (always >12), high alkalinity (40–120 mmol/kg), Na/Cl ratios between 1.15 and 1.2, and high B (>3000 µmol/mg) and K (>18 mmol/kg) (for comparison, the seawater values for B and K are 420 µmol/kg and 10.2 mmol/kg, respectively) (Mottl, 1992). Mottl et al. (2003) also observed that the South Chamorro pore waters are highly depleted relative to seawater values in Mg and Sr and have very low Li abundances (with an order of magnitude lower than the seawater). They also have an extremely fractionated Li isotope signature (~12 heavier than seawater) (Savov et al., 2002).

As shown by Savov et al. (2002, 2004), the serpentinized peridotites from Site 1200 have ⁸⁷Sr/⁸⁶Sr = 0.704–0.705, with the highest ⁸⁷Sr/⁸⁶Sr near the top of the section and stable and consistently low ⁸⁷Sr/⁸⁶Sr ratios (~0.7045) for the deep samples, significantly lower than the seawater ratio (⁸⁷Sr/⁸⁶Sr = 0.709). These values are similar to the ⁸⁷Sr/⁸⁶Sr ratio of serpentinized peridotites and corresponding pore waters from Leg 125, Conical Seamount (Haggerty and Chaudhuri, 1992; Savov et al., 2002, 2004).

Near the seafloor, high-alkalinity/low-Ca deep fluids interact with the low-alkalinity/high-Ca seawater. The result is the well-documented precipitation of needlelike (centimeter sized) aragonite crystals within the serpentinite muds. This process is best recorded in the CaCO₃ content of the serpentine muds. At core depths of ~2 mbsf, CaCO₃ concentrations are elevated and may reach as high as 36 wt%, whereas below ~5 mbsf, CaCO₃ concentrations drop to consistent values <1 wt% (Salisbury, Shinohara, Richter, et al., 2002). The presence or absence and relative abundance of aragonite crystals was also used to estimate approximately how deeply the seawater has penetrated below the surface of the seamount. The low abundances of Ca, Na, Sr, and Mn in the serpentinite muds (Tables T1, T2), along with visual inspections, confirms that aragonite crystals are absent below depths of ~5 mbsf.

In Tables T1 and T2 we report the major and minor element abundances of the serpentinite muds. We observe several small but important differences between the compositions of the average serpentinite muds and the average serpentinized peridotites from the Mariana forearc sites (Fig. F3). Serpentinite muds have higher average Al₂O₃ (0.9 wt%), CaO (1.4 wt%), and TiO₂ (0.1 wt%) (Tables T1, T2). Figure F4A shows that all of the studied serpentine muds have average loss on ignition (LOI) values similar to serpentinized peridotite clasts (ranging between 14.27 and 15.56 wt%) but higher SiO₂/MgO ratios. The Leg 195 muds and clasts have comparable SiO₂ contents (~43 wt%) but are higher in MgO relative to the muds from Leg 125 (Lagabrielle et al., 1992; Benton, 1997) (Fig. F4B, F4C).

Using Figure F4A, F4B, and F4C, we investigate relationships between LOI, MgO, and SiO₂ contents of the serpentinite muds. When compared to the Leg 125 serpentinite muds of Lagabrielle et al. (1992) and Benton (1997), the Leg 195 muds contain as much as 10 wt% higher average MgO (Fig. F4B) and 4 wt% higher average SiO₂ (Fig. F4C). The Leg 195 muds also have elevated Ni concentrations (as much as 1500 ppm higher), lower Rb, and higher Cr compared to the data of Lagabrielle et al. (1992) and higher average Sr (as much as 50 ppm higher) compared to the data set of Benton (1997). When combined, the entire serpentinite mud data set (Legs 125 and 195) shows positive correlations between SiO₂/MgO ratios and TiO₂ and Al₂O₃ contents (Fig. F5A, F5B). No such correlation was observed between the SiO₂/MgO ratios and Fe₂O₃ and CaO contents (Fig. F5C, F5D). The serpentinite muds always have TiO₂ > 0.01 wt%, and in almost all cases this parameter discriminates them well from the serpentinized peridotite clasts, which contain average TiO₂ ~ 0.01 wt%.

When compared to the hydrated ultramafic rocks (serpentinized peridotite clasts) from Legs 125 and 195, the muds generally show lower MgO, Fe₂O₃, Ni, and Cr (Figs. F4C, F5D, F6) and elevated TiO₂, Al₂O₃, CaO, and Sr (Figs. F5A, F5B, F5C, F7A, F7B, F7C) or trends toward the compositions of altered seafloor basalts (Kelley et al., 2003) or metabasic clasts reported by Johnson (1992) and in this study.

We report here the first measurements of rare earth elements (as well as Nb, Ta, Hf, Th, U, Pb, Zr, Cs, As, and Sb) in the muds from serpentinite seamounts in the Mariana forearc (Table T3). The analyzed serpentinite muds (total of 14 samples) were recovered from Leg 195 Holes 1200A, 1200D, and 1200E. Chondrite-normalized plots of REE for the serpentinite muds (Fig. F8A, F8B) show relatively flat patterns with average [La/Sm]_N = 2.3 and average [Sm/Yb]_N = 2. In Holes 1200D, 1200E, and 1200A, the [La/Sm]_N range is 0.13–1.9, 0.2–4.2, and 1.16–1.32, respectively. In Hole 1200D, the serpentinite muds exhibit slight negative Eu anomalies (Fig. F8A). In Holes 1200D, 1200E, and 1200A, the [Sm/Yb]_N range is 0.1–0.5, 0.35–1.2, and 0.3–0.4, respectively.

These patterns are drastically different from the relative abundances and chondrite-normalized REE patterns of the serpentinized peridotites from Legs 125 or 195. As seen in Figure F8A and F8B, the serpentinized peridotite clasts from Legs 125 and 195 have a noticeable positive Eu anomaly and they also never reach more than 0.1 x chondrite (CI) level (i.e., their REE abundances are a factor of 1 or 2 lower than the serpentinite muds) (see Table T3).