Geochemical constraints on the modes of carbonate precipitation in peridotites from the Logatchev Hydrothermal Vent Field and Gakkel Ridge
Introduction
Seawater circulation within the ocean crust and associated water–rock interactions has profound consequences for the chemical and isotopic compositions of both the ageing ocean crust and seawater (e.g., Elderfield and Schultz, 1996). During the transformation of seawater into a hydrothermal fluid, calcium (Ca) concentration increases while magnesium (Mg) and sulfate concentrations decrease (e.g., von Damm, 1990). These changes are due to seawater–rock Ca–Mg exchange reactions and to precipitation of carbonate, anhydrite, and hydrous magnesium-silicates (Mottl, 1983, Seyfried, 1987, Teagle et al., 1998). High-temperature vent fluids discharge at or near mid-ocean ridge axes is manifested by the formation of black smokers and massive sulfide deposits (Von Damm, 1990, Tivey, 2007). In contrast, low-temperature alteration follows a different reaction path, characterized by the formation of palagonite, clay minerals, and carbonate minerals (e.g., Staudigel et al., 1981, Alt, 1995). Even at low temperatures (< 60 °C), seawater loses Mg while it leaches Ca from the rock (Elderfield et al., 1999, Wheat et al., 2000). The increase in Ca in seawater is matched by the decrease in Mg, so that the overall reaction does not significantly change the pH of the circulating seawater. Calcite and aragonite precipitate during low-temperature seawater–rock interactions and represent useful indicators for the determination of water–rock reactions and fluid mixing processes within the ocean crust (Hart et al., 1994, Coggon et al., 2004, Amini et al., 2008). Several previous studies of ultramafic-hosted hydrothermal environments along the Mid-Atlantic Ridge (MAR) report that aragonite has exclusively formed at low temperatures from virtually unmodified seawater (e.g., Bonatti et al., 1980, Blusztajn and Hart, 1996, Ribeiro da Costa et al., 2008). Despite the apparent dominance of carbonates formed at ambient seafloor conditions, there is also evidence for aragonite and calcite precipitating at high temperatures from mixtures of ambient seawater and discharging hydrothermal fluids (Früh-Green et al., 2003, Pichler and Veizer, 2004, Ludwig et al., 2006, Amini et al., 2008).
The main subjects of this study are carbonate minerals from the Logatchev Hydrothermal Field (LHF) at the MAR at 14°45′N. Aragonite hosted in serpentinites from the Gakkel Ridge (GR) in the Arctic Ocean was also included in the study. This paper investigates the chemical aspects of precipitation of carbonate minerals at both sampling sites using Sr isotopes (87Sr/86Sr) and minor and trace element concentrations of aragonite, calcite and dolomite formed during water–rock interaction. Our previous study (Eickmann et al., 2009) revealed that δ18O and δ13C values agree with aragonite precipitation at low temperatures, similar to other seafloor weathering environments (Bonatti et al., 1980) and moderate-temperature hydrothermal sites (Kelley et al., 2005). In contrast, low δ18O and δ13C values of serpentinites-hosted calcite from the LHF indicate precipitation from hydrothermal solutions dominated by magmatic CO2 (Eickmann et al., 2009). The study of LHF carbonate minerals consequently provides a good opportunity to investigate both calcite precipitation at high temperatures and aragonite precipitation at low temperatures within the same hydrothermal field. Here, we demonstrate that calcite was precipitated from conductively cooled hydrothermal fluids, whereas dolomite has been precipitated from conductively heated seawater. Although aragonite precipitated from near end-member seawater, REE reveal a minor hydrothermal influence.
Section snippets
Geological background
Both the slow-spreading MAR and the ultraslow-spreading GR (Fig. 1) are characterized by the exposure of upper mantle rocks along detachment faults (Cannat, 1993, Tucholke and Lin, 1994, Dick et al., 2003). The area south of the Fifteen-Twenty Fracture Zone is typified by extensive exposure of peridotite and gabbroic rocks on both flanks along the spreading axis of the MAR (Cannat et al., 1997, Fujiwara et al., 2003). Widespread fluid–rock interaction in the sub-seafloor is evident from
Sampling
Samples were collected during the METEOR cruise M60/3 in 2003 (LHF) and the POLARSTERN cruise AMORE 1 in 2001 (GR; Table 1). Serpentinite-hosted aragonite and calcite, sulfide-hosted aragonite, and dolomite hosted in talus breccia from the LHF have been studied. The sulfide-hosted aragonite samples and the dolomite were sampled in close proximity to active vents in the LHF, whereas serpentinite-hosted aragonite was sampled south of the active LHF sites (Fig. 2). Serpentinite-hosted calcite has
Sr isotope–O isotope relations
Strontium isotope ratios of the carbonates reveal precipitation from variably evolved fluids, with 87Sr/86Sr values ranging from 0.70460 to 0.70917 (Table 2). O isotope compositions of aliquots of the same powders are indicated in Table 2 (cf., Eickmann et al., 2009) as well as the corresponding calculated precipitation temperatures. The lowest 87Sr/86Sr values (0.70460 to 0.70499) were measured for the LHF serpentinite-hosted calcite. In contrast, aragonite hosted in serpentinites and sulfides
Serpentinite-hosted and sulfide-hosted aragonite
87Sr/86Sr ratios for aragonite hosted in serpentinites from the LHF indicate that it precipitated from unmodified seawater, just like the serpentinite-hosted aragonite from the GR (Fig. 4B). The Sr isotope data (Table 2) hence corroborate the conclusion derived from δ18O and δ13C values that the aragonites precipitated from seawater at ambient seafloor temperatures (Eickmann et al., 2009). These aragonite veins closely resemble those commonly found in serpentinites that underwent oxidative
Conclusions
The chemical composition and 87Sr/86Sr ratios of carbonate minerals from the Logatchev Hydrothermal Field (LHF) on the Mid-Atlantic Ridge (MAR) and from the ultraslow-spreading Arctic Gakkel Ridge (GR) show different modes of formation. 87Sr/86Sr ratios and trace element (Mg, Sr, REE) compositions of the LHF and GR serpentinite-hosted aragonite indicate precipitation from seawater, which has undergone very limited chemical and isotopic exchange with the host rock. The REE patterns for these
Acknowledgments
We thank D. Birgel and E. Hellebrand for providing samples from the LHF and the GR and F. Böhm for helpful discussions. The help and assistance of H. Anders and A. Klügel with LA-ICP-MS analyses is gratefully acknowledged. We thank D.A.H Teagle, I. Ribeiro da Costa and an anonymous reviewer for their constructive and insightful reviews. This study was financed by grants from the Special Priority Program 1144 “From Mantle to Ocean: Energy-, Material- and Life-cycles at Spreading Axes” and the
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Present address: BAM Bundesanstalt für Materialforschung und-prüfung, Unter den Eichen 87, D-12205 Berlin, Germany.