Amanda A. Olsen

Forsterite dissolution rates in Mg‐sulfate‐rich Mars‐analog brines and implications of the aqueous history of Mars

High salinity brines, although rare on Earths surface, may have been important in the geologic history of Mars. Increasing evidence suggests the importance of liquid brines in multiple locations on Mars. In order to interpret the effect of high ionic strength brines on olivine dissolution, which is widely present on Mars, 47 new batch reactor experiments combined with 35 results from a previous study conducted at 25°C from 1 < pH < 4 in magnesium sulfate, sodium sulfate, magnesium nitrate, and potassium nitrate solutions with ionic strengths as high as 12 m show that very high ionic strength brines have an inhibitory effect of forsterite dissolution rates. Multiple linear regression analysis of the data suggests that the inhibition in dissolution rates is due to decreased water activity at high ionic strengths. Regression models also show that mMg up to 4 m and mSO4 up to 3 m have no effect on forsterite dissolution rates. The effect of decreasing dissolution rates with decreasing aH2O is consistent with the idea that water acts as a ligand that participates in the dissolution process. Less available water to participate in the dissolution reaction results in a slower dissolution rate. Multiple linear regression analysis of the data produces the rate equation logr=−6.81−0.52pH+3.26logaH2O. Forsterite in dilute solutions with a water activity of one dissolves twice as fast as those in brines with a water activity of 0.8.

Quantum mechanical modeling of hydrolysis and H2O-exchange in Mg-, Ca-, and Ni- silicate clusters: Implications for dissolution mechanisms of olivine minerals

Abstract Barrier heights (BHs) for hydrolysis and H2O exchange reactions at M-O-Si (M = Ni2+, Mg2+, and Ca2+) linkages on olivine (M2SiO4) mineral surfaces were determined via DFT calculations. BHs for hydrolysis of protonated Ni-O-Si, Mg-O-Si, and Ca-O-Si sites are 76, 54, and 27 kJ/mol, respectively, and are 69 and 24 kJ/mol for H2O exchange reactions of protonated Mg-O-Si and Ca-O-Si sites, respectively. Rate constants were calculated via classical transition state theory (TST) using these BHs. For protonated Ni-O-Si, Mg-O-Si, and Ca-O-Si sites, these are 7.2 × 10-1, 4.7 × 104, and 1.5 × 109 s-1 [pseudo-first-order where (H2O) is assumed to be constant], respectively, and for H2O exchange at protonated Mg-O-Si and Ca-O-Si sites are 2.6 × 101 and 3.7 × 109 s-1 [pseudo-first-order where (H2O) is assumed to be constant], respectively. Approach of an H2O molecule from the second hydration sphere toward a protonated Ni-O-Si site leads to breakage of the Ni-O bond and subsequent release of Ni2+ to solution. For protonated Mg-O-Si sites, however, H2O exchange does not lead to rupture of the Mg-O bond and would not be a step toward dissolution of the mineral. Potential energy surface (PES) scans of H2O exchange indicated formation of a hepta-coordinated Ca2+, so neither H2O exchange nor hydrolysis of the Ca-O-Si linkage occurred in this case. Calculated rate constants are consistent with experimental data for end-member composition olivine minerals where observed rates of dissolution increase in the order Ni2+ < Mg2+ < Ca2+.