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We present a diagram that shows the effect of pH, temperature, grain size, composition, hydrodynamics, and the laboratory/fi eld discrepancy on the lifetime of olivine grains in weathering environments. Because the persistence of olivine grains on Mars can be used to constrain the duration of liquid water, we can use this diagram to predict a range of possible maximum contact times for olivine grains with liquid water before they dissolve away completely. Depending upon the physicochemical conditions, this contact time could range between a few thousand and several million years.

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High salinity brines, although rare on Earth's 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 m Mg up to 4 m and m SO4 up to 3 m have no effect on forsterite dissolution rates. The effect of decreasing dissolution rates with decreasing a H2O 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 log r ¼ À6:81 À 0:52pH þ 3:26log a H2O. 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.