HR: 1340h
AN: V23A-1225 [Abstracts]
TI: Experimental Investigation of the Dissolution and Alteration of Olivine at Low Temperatures and Pressures
AU: * DeAngelis, M T
EM: mdeangel@utk.edu
AF: Department of Earth and Planetary Sciences - University of Tennessee, 306 Earth and
Planetary Sciences Bldg., Knoxville, TN 37995, United States
AU: Labotka, T C
EM: tlabotka@utk.edu
AF: Department of Earth and Planetary Sciences - University of Tennessee, 306 Earth and
Planetary Sciences Bldg., Knoxville, TN 37995, United States
AU: Cole, D R
EM: coledr@ornl.gov
AF: Chemical Sciences Division - Oak Ridge National Laboratory, 1 Bethel Valley Rd., Oak
Ridge, TN 37831, United States
AB:
We have determined the surface characteristics of single crystals of olivine that have been altered under a variety
of experimental conditions relevant to processes occurring on the Martian surface and in shallow crustal
environments (i.e. mid-ocean ridge systems and hydrothermal environments). Most previous experiments have
focused on dissolution rates of mineral powders at near-surface conditions of temperature (25 °C),
pressure (1 atm), and pH (2--7) (see Brantley, 2003, for summary). The intent of our study is to explore the
surface physical, chemical and mineralogical features attendant with reactions of single crystals of olivine.
Surface analytical methods such as SEM, XRD, TEM, SIMS, IR and XPS are being used to interrogate the reacted
mineral surfaces.
Our experiments were performed over a range of T=25--150 °C, t=1--30 d, and pH=2--7. The starting
material for these experiments consisted of single grains of San Carlos olivine (Fo90) that were cut to a size of
~4 mm in diameter and ~1 mm in thickness. The larger, flat surfaces of these olivine ‘slices' were
polished to ensure a uniform surface layer. Initial starting solutions included doubly-distilled water (pH=7), HCl
(pH=2) and H2SO4 (pH=2). Two olivine crystals per experiment were loaded into a pyrex tube, and the
accompanying solution was added with a fluid to rock ratio of either 5:1 or 15:1. Solutions were frozen and tubes
were evacuated prior to sealing, ensuring that any reaction or dissolution occurred at an internal pressure
corresponding to the temperature of interest along the liquid-vapor curve for the solution.
Preliminary results indicate that all olivine samples, regardless of experiment duration, temperature, or pH
conditions, have some measurable amount of weight loss (0.1--0.3%). SEM analysis shows the development of
dissolution pits in most of the experimental samples, and the presence of secondary phases on the surface of
olivine samples reacted with acidic starting solutions (pH=2). BSE and SE images indicate that these secondary
phases (as yet not quantitatively determined) are compositionally distinct from starting olivine. A characteristic of
these experiments is that the reaction rapidly increases the pH of the solution, even in experiments with a fluid to
rock ratio of 15:1. At a fluid to rock ratio of 5:1, the short duration experiments (t=1d, T=150 °C) exhibited a
change in pH from 2 to 6.17 for HCl (aq) starting solutions, and a similar change was also observed for
H2SO4 (aq) starting solutions. These results have an important bearing on the interpretation of olivine
reactivity in acidic solutions relevant to numerous planetary crustal settings.
DE: 3617 Alteration and weathering processes (1039)
DE: 3630 Experimental mineralogy and petrology
DE: 5415 Erosion and weathering
DE: 5470 Surface materials and properties
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting