CLAY MINERAL FORMATION ON EARLY MARS VIA OPEN-SYSTEM ACID ALTERATION OF BASALTIC GLASS

Abstract:

Smectite on Mars has been detected by orbital and landed instrumentation, but the detection of associated carbonate deposits is limited. Early Mars may have experienced mildly acidic conditions that allowed smectite formation but prevented widespread carbonate formation. Our previous studies showed formation of smectite from basaltic glass in an acidic closed hydrologic system, but it remains unknown if flow regime could affect smectite formation. The objective of this work was to evaluate smectite formation from basaltic glass reacted with H₂SO₄ (pH_in 2, 3 and 4 input solutions) in a flow-through reactor mimicking an open hydrologic system. The experiments were performed at 190°C for 7d at 0.25 and 0.01 mL min^-1 flow rate. X-ray diffraction analysis of the final solids revealed formation of serpentine at 0.25 mL min^-1 flow rate at pH_in 3 and 4, but no phyllosilicates at pH_in 2. Flow rate of 0.01 mL min^-1 permitted smectite and kaolinite formation at pH_in 2 and serpentine formation at pH_in 3 and 4. The presence of kaolinite with smectite at pH_in 2 suggests that these phases formed simultaneously, and future work will investigate possible interstratification. Analysis of solution chemistry showed higher Si activity at slower flow rate and lower pHs. Equilibrium modeling (Fig. 1) indicated that alteration mineralogy was controlled by solution pH and Si and Mg activity. The higher Si activities (~9 mM) achieved at pH_in 2 permit kaolinite formation at both flow rates, although kaolinite dissolves by the end of the experiment at 0.25 mL min^-1 (Fig. 1). Modeling with bulk solution chemistry did not capture smectite formation, indicating that smectite may form in microenvironments in these experiments. Lower Si activities (~1 mM) at pH_in 3 and 4 allowed only serpentine formation (data not shown). These results demonstrate that low leaching rates may be required to achieve Si and Mg activities high enough for smectite formation under acidic conditions. Smectite formation from basaltic glass on early Mars might occur under low leaching rates and mildly acidic conditions. Our modeling results indicate that carbonate is undersaturated under such conditions, which may have prevented widespread carbonate formation on early Mars.

Fig. 1. Equilibrium model with pH_in 2 experiments at 0.25 mL/min (solid) and 0.01 mL/min (open) flow rates.