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

Abstract

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.

Olivine dissolution rates: A critical review

“…Similarly Pokrovsky and Schott (2000b) and Rosso and Rimstidt (2000) reported that low concentrations of aqueous Mg had little effect on forsterite dissolution rates. This conclusion was confirmed and expanded upon by Olsen et al (2015) who reported that there was little effect of aqueous Mg concentration on forsterite dissolution rates at concentrations as high as 4 mol/kg.…”

“…In contrast to the role of Mg and Si, a significant effect on forsterite dissolution rates was found when varying the activity of H2O. In a regression of results from highly saline aqueous solutions of MgSO4, Na2SO4, Mg(NO3)2 and KNO3, Olsen et al (2015) reported that forsterite dissolution rates at 25 o C and pH ranging from 2 to 4 are proportional to the activity of H2O to the 3.26±0.65 power. The role of water in the dissolution mechanism has been attributed to its involvement in releasing silica tetrahedra from the structure during the rate-limiting step of dissolution (Liu et al, 2006).…”

“…Further recent interest in olivine dissolution rates stems from its potential role in the weathering of the Martian surface (e.g. Stopar et al, 2006;Olsen and Rimstidt, 2007;Hausrath et al, 2008;Hausrath and Brantley, 2010;Dehouck et al, 2014;Velbel, 2014;Olsen et al, 2015;Niles et al, 2017). Such interest has led to a large number of studies aimed at characterizing forsterite dissolution behavior and rates at various fluid compositions and temperatures (e.g.…”

Assessing hydrodynamic effects on jarosite dissolution rates, reaction products, and preservation on Mars

“…This suggests either that no sodium sulfate minerals were formed in the experiments due to their high solubility or that the reaction products are X‐ray amorphous and/or in quantities below the detection limit. If sodium sulfates are undersaturated in the system or fluid flow inhibits sodium sulfate precipitation, the chloride‐attack mechanism described for the calcium chloride experiments may not occur, thus resulting in slower dissolution rates due to the lower activity of water and/or a lower activity of free chloride ions in the system [ Pritchett et al ., ; Olsen et al ., ].…”

“…Basalt dissolution slows significantly in high ionic strength NaCl and CaCl 2 brines [ Hausrath and Brantley , ]. Olivine dissolution rates have also been shown to slow as activity of water decreases in sulfate‐rich brines [ Olsen et al ., ]. However, in the case of jarosite dissolution, rates accelerate over time in saturated NaCl ( a H 2 O = 0.75) and CaCl 2 ( a H 2 O = 0.35) solutions likely due to chloride complexation with Fe 3+ following precipitation of sulfate reaction products in closed system experiments [ Pritchett et al ., ].…”