AQUEOUS DISSOLUTION AND ALTERATION OF OLIVINE IN LOW TEMPERATURE AND PRESSURE ENVIRONMENTS

Many olivine experiments at low temperature and pressure either focus on surface dissolution rate determination at ambient conditions (T = 25 °C, P = 0.1 MPa) or surface alteration characterization at conditions analogous to shallow crustal environments (e.g. mid-ocean ridge hydrothermal environments at T = 250–400 °C, P = 100–200 MPa). We conducted fluid-solid experiments with single crystal olivine under conditions that bridge this dissolution-alteration transition to investigate the physical, chemical and mineralogical changes to the mineral surface.

Experiments were performed over a range of temperature (25–300 °C), pressure (0.1­–200 MPa), and duration (1–90 d), with solutions that included deionized, distilled water (pH = 7), HCl (pH = 2, 0.01 M) and H₂SO₄ (pH = 2, 0.005 M). Starting material for experiments were single crystals of San Carlos olivine (Fo90) that were cut into plates ~4 mm in diameter and ~1 mm thick and polished to ensure a uniform surface layer.

To characterize surface topography, all plates were examined by profilometry before and after each experiment. These examinations indicated an increase in etching, pitting, and overall surface roughness resulting from experiment. To characterize chemical changes, each olivine was examined using SIMS and XPS depth profiling. XPS analysis of dissolution-dominated experiments showed surface depletion of Mg, with the depth of depletion varying with starting solution (e.g. ~100 nm Mg depletion zone for H₂O vs. >400 nm Mg depletion zone for HCl at 150 °C, 0.1 MPa, 28 d). SIMS depth profiles of ¹H, ²⁴Mg, ³⁰Si, and ⁵⁴Fe concentrations show protonation of surfaces in the lower T and P experiments, and formation of alteration rinds in the higher T and P experiments.

Surface dissolution rates for olivine vary with pH, with acidic systems having the fastest rates. Our experiments do not fix pH, but instead olivine buffers pH to values of 6–8. Increased pH results in slower dissolution rates and partly affects whether dissolution or alteration is the dominant process, since the formation of alteration phases (e.g. talc, antigorite, brucite) also depends on available cations. Our experiments show that temperature and pressure provide the initial kinetic control on dissolution, but the transition to alteration is a function of pH and concentration of Mg²⁺ in solution.