Seawater-peridotite interaction at 300°C and 500 bars: implications for the origin of oceanic serpentinites |
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| Authors: | WE Seyfried WE Dibble |
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| Institution: | Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, U.S.A.;Department of Geology, Stanford University, Stanford, CA 94305, U.S.A. |
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| Abstract: | An experiment has been performed reacting seawater with fresh peridotite (80% olivine, Fo90 and ～- 15% enstatitic orthopyroxene En95 and minor clinopyroxene and spinel) at 300°C, 500 bars and water/rock mass ratio of 20. The duration of the experiment was approximately 1500 hr.Seawater chemistry was appreciably modified during the experiment. Mg, Ca, Sr, SO4 and H2O were removed, while H2S(aq), Fe, Mn and Zn were added. H2S(aq) resulted from the inorganic reduction of seawater SO4. pH was initially acid (2.8), but then rose slowly to a value of 5.2. The aqueous concentrations of Na, K, Cl and boron (B) changed little from that in seawater prior to reaction. However, as the solution was cooled to room temperature at the end of the experiment, the B concentration decreased. This suggests that the B content of oceanic serpentinites may be the result of retrograde reactions between a previously serpentinized body and ‘cold’ seawater.The primary minerals in the peridotite were replaced to varying degrees by serpentine (lizardite), magnetite. Mg-hydroxysulfate, anhydrite and possibly pyrite and sphalerite. Mg-hydroxysulfate and much anhydrite dissolved on quench.The alteration mineral assemblage generated during this experiment is consistent with that predicted from equilibrium phase relations and is similar in chemical composition, mineralogy and paragenesis to that reported for oceanic serpentinites. |
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