2010 Annual Research Report

 

Tosca's research involves understanding the controls on the chemical composition of sedimentary rocks and Earth and Mars and using this information to reconstruct ancient climates. He approaches these problems using mineralogical and geochemical analysis and experimentation. Ongoing projects include:

 

Clay-bearing assemblages from synthetic Martian basalt: Funded through the MFR program in 2009, this project involves the experimental generation of alteration assemblages under controlled environmental conditions using synthetic martian basalt. The goal of the project is to identify the initial stages of alteration of martian basalt with a focus on clay-bearing assemblages. Attention is paid to the response of the entire alteration assemblage to changes in: (1) atmospheric composition, (2) initial pH, (3) water/rock ratio, (4) temperature. Low temperature alteration experiments are conducted in the laboratory at low temperature in a controlled atmosphere glovebox and high temperature hydrothermal experiments are conducted at JPL in collaboration with Joel Hurowitz using sealed Parr hydrothermal vessels. Since the project was funded, the project has focused on the laboratory synthesis of martian basalt. The goal is to produce a homogeneous, well characterized pool of starting material for all alteration experiments. We are currently in the process of building pools of glassy basalt and crystalline basalt beginning with one composition representative of pristine Noachian basalt.

 

Physico-chemical properties of concentrated martian surface waters: Understanding the processes controlling chemical sedimentation is an important step in deciphering paleoclimatic conditions from the rock records preserved on both Earth and Mars. Clear evidence for subaqueous sedimentation at Meridiani Planum, widespread saline mineral deposits in the Valles Marineris region, and the possible role of saline waters in forming recent geomorphologic features all underscore the need to understand the physical properties of highly concentrated solutions on Mars in addition to, and as a function of, their distinct chemistry. Using thermodynamic models predicting saline mineral solubility we have recently completed a project devoted to understanding sedimentation in highly concentrated surface waters on the martian surface. We generate likely brine compositions ranging from bicarbonate-dominated to sulfate-dominated and predict their saline mineralogy. For each brine composition, we then estimate a number of thermal, transport and colligative properties using established models that have been developed for highly concentrated multi-component electrolyte solutions. The available experimental data and theoretical models that allow estimation of these physico-chemical properties encompass, for the most part, much of the anticipated variation in chemistry for likely martian brines. These estimates allow significant progress in building a detailed analysis of physical sedimentation at the ancient martian surface is possible, in addition to more accurate predictions of thermal behavior and the diffusive transport of matter through chemically distinct solutions under comparatively non-standard conditions.

 

 

Early diagenetic clay mineralogy in Proterozoic carbonate successions: Funded through the Royal Society Research Grants Program (UK) in 2009/2010, this project involves an experimental approach toward understanding the formation of early diagenetic silicates in the Proterozoic. As an example, we have recently completed a project based on mineralogical, petrographic and sedimentological observations that document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~800-700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25^oC with low-SO₄ synthetic seawater solutions at varying pH, Mg²⁺ and SiO₂(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg²⁺ and/or SiO₂(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~8.6-8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg²⁺, elevated SiO₂(aq), and pH > ~8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration.

 

Papers:

 

Hurowitz, J.A., Fischer, W.W., Tosca, N.J., Milliken, R.E. (2010) Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars. Nature Geoscience, 3, 5, 323-326.