Kinetics of Aqueous Alteration Why worry about kinetics? General principles of kinetics Examples: Jarosite dissolution Olivine dissolution rates Weathering rinds Jeff Taylor Aqueous Alteration on Mars

Importance of Kinetics of Aqueous Alteration On Earth (a small sampling): Weathering is an important part of the rock cycle Important geochemical cycles dependent on rates of weathering, e.g., Sr and P budgets in oceans, carbon cycle Rates give information about how rapidly continents erode Rates give information about alteration of basalts on sea floor Allow study of climate history Rates of weathering change with time for a given system Jeff Taylor Aqueous Alteration on Mars

Importance of Kinetics of Aqueous Alteration On Mars: Duration of aqueous events (floods, oceans, lakes, rain, hydrothermal systems, etc.) Interpretation of surface mineralogy Nature of sediments and sedimentary rocks Climate history Geochemical cycling within crust Effects on distribution and nature of organic compounds Jeff Taylor Aqueous Alteration on Mars

General Principles of Kinetics Basic rate law is simply: Where R is the weathering rate of a mineral (mol/m2/s) Q is the number of moles reacted S is the surface area t is time Rate is actually dependent on pH, temperature, and how saturated the solution is. For olivine (Fo100): Jeff Taylor Aqueous Alteration on Mars

General Principles of Kinetics From classic Arrhenius relation k = koe-E/RT log k = log ko-(E/2.303R)(1/T) Allow experiments at different temperatures to determine E, so we can extrapolate to other temperatures Olivine follows this type of relation Jeff Taylor Aqueous Alteration on Mars

General Principles of Kinetics Olivine particularly useful: Dissolves relatively rapidly, so its presence implies small amount of weathering Dissolves congruently (releases its constituents into solution in same proportions as in its stoichiometry) Does not necessarily allow for much reprecipitation of constituents, though Fe-oxides might be an exception Typical reaction (hydrolysis): Mg2SiO4 + 4H2CO32Mg+ + 4HCO3- + H4SiO4 (Hydrolysis produces an excess of H+ or OH- ions in solution) Jeff Taylor Aqueous Alteration on Mars

Weathering Rates and Time Comparison of experiments and field conditions indicates that weathering rates are not constant Increases uncertainty in extrapolating experimental results White, A. F. and Brantley, S. L. (2003) The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field? Chem. Geol. 202, 479-506. White and Brantley (2003) White and Brantley (2003) Jeff Taylor Aqueous Alteration on Mars

Weathering Rates and Time Weathering rate decreases 4-5 orders of magnitude with duration of weathering Consistent with data from other feldspars, biotite, and hornblende Average silicate weathering described by a power function: R = 3.1 x 10-13 t(-0.61) White and Brantley (2003) Jeff Taylor Aqueous Alteration on Mars

Why Weathering Rate Varies with Time Intrinsic properties (applies to minerals) Surface area increases with weathering (which is inversely proportional to weathering rate) Easy to reach defects are rapidly weathered Secondary mineral precipitation blocks access to surface by water Extrinsic properties (the environment) Composition of solutions, especially as they approach thermodynamic saturation Hydrologic heterogeneity—variations in rate of water migration through different parts of a rock, caused partly by formation of secondary phases that decrease permeability Jeff Taylor Aqueous Alteration on Mars

Jarosite Dissolution as a Time Constraint This is from a study by Megan Elwood Madden et al. (2009). From the abstract: Jarosite dissolution rates can be used to determine the duration of water at Meridiani Planum and other environments containing this ephemeral, metastable ferric sulfate salt. The maximum duration of aqueous fluids in the region is calculated based on the dissolution rate of jarosite under varying temperature and ionic strength conditions. Jarosite lifetimes predicted in this study range from 1.5 a in warm dilute environments to 100 ka in NaCl-saturated brines at 250 K. Elwood Madden, M. E. (2009) How long was Meridiani Planum wet? Applying a jarosite stopwatch to determine the duration of aqueous diagenesis. Geology 37, 635–638. Jarosite lifetime only 1.5 to 100,000 years Jeff Taylor Aqueous Alteration on Mars

Olivine Dissolution and Geologic Setting Upper dashed line corresponds to grain size of 0.1 cm; lower one to 0.01 cm Stopar, J. S., G. J. Taylor, V. E. Hamilton, L. Browning (2006) Kinetic Model of Olivine Dissolution and Extent of Aqueous Alteration on Mars. Geochem. Cosmochim. Acta 70, 6136-6152. Stolpar et al. (2006) Jeff Taylor Aqueous Alteration on Mars

Aqueous Alteration on Mars Rinds on Rocks Costa Rica, Earth Gusev Crater, Mars Weathering rind formation can be quantified in a number of ways, including by complicated reaction transport modeling. The Costa Rica case is described along with two other locations by Sak et al. (2004). The formation of the rind on Mars is described by Hausrath et al. (2008). The image on the right is of Humphrey, a rock at the Gusev landing site. The area shown that appears smooth has been ground down with the rock abrasion tool. The image appeared in McSween et al. (2004). Hausrath, E. et al. (2008) Basalt weathering rates on Earth and the duration of liquid water on the plains of Gusev Crater, Mars. Geology 36, 67-70. Sak, P. B., D. M. Fisher, T. W. Gardner, K. Murphy, and S. L. Brantley (2004), Rates of weathering rind formation on Costa Rican basalt, Geochimica et Cosmochimica Acta, 68, 1453-1472. McSween, H.Y., and 34 others, et al., 2004, Basaltic rocks analyzed by the Spirit rover in Gusev Crater : Science, v. 305, p. 842–845, doi: 10.1126/science.3050842. McSween et al. (2004) Sak et al. (2004) Detailed aqueous reaction transport modeling by Hausrath et al. (2008) indicates a total weathering time of 22,000 years to make the rind on Mars. Jeff Taylor Aqueous Alteration on Mars

Clay Mineral Distribution Elhmann, B.L. et al. (2011) Subsurface water and clay mineral formation during the early history of Mars. Nature 479, 53-60. This is Fig 1a, whose caption reads: CRISMtargeted images surveyed for the presence of clay minerals, grouped by geological setting and superimposed on a shaded relief map. Open symbols mark sites where no clays were found. Ehlmann et al. (2011) Jeff Taylor Aqueous Alteration

Clay Mineral Geological Settings Crustal: Ancient terrains deeply degraded by impacts. Sedimentary: fan-shaped and layered deposits within basins—clearly depositional “In stratigraphies”: multiple units in which clays and other alteration products are in contact with each other—ultimate origin (depositional, impact modified, etc.) cannot be determined Diagrams on right are from Ehlmann et al. (2011), Fig 1 b,c,d. Caption reads: Percentage frequency of detection of alteration phase(s), grouped by geological setting; n is the total number of images within which clay minerals were detected (a total of 365 of 639 images included in this meta-analysis). The percentage given is not areal coverage but rather the number of detections of a given phase divided by the number of detections of all alteration phases within the geological setting. Ehlmann et al. (2011) Aqueous Alteration Jeff Taylor

Deep Phyllosilicates (crustal) Mawrth Vallis Ehlmann (2010) Jeff Taylor Aqueous Alteration

'Deep phyllosilicates' as exposed in a 600m tall scarp beneath a mafic LCP-bearing cap rock (figures after Mustard et al., 2008; 2009). Arrows indicate that Fe/Mg smectites exposed beneath the younger Syrtis Major lavas. Olivine is present here in as mobile sands. The two CRISM images are overlain on and use to colorize a grayscale, high resolution Context Imager (CTX) image. Uncolored parts of the image do not have CRISM coverage. From Ehlmann (2010), Geochemical Newsletter (online) Ehlmann (2010)

Phyllosilicates in Intracrater Fans Red to magenta areas contain a mixture of Fe/Mg smectite and carbonate Fig. 4 from Murchie et al (2011) (JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, E00D06, doi:10.1029/2009JE003342) Phyllosilicates and carbonate in the layered bright materials comprising the western delta of Jezero crater. (a) Spectral parameter image of CRISM observation HRL000040FF at 18.5N, 77.4E related to hydrated materials (red, D2300; green, not applicable; blue, D1900). Red to magenta areas contain a mixture of Fe/Mg smectite and carbonate [Ehlmann et al., 2009], and grayish areas have a low content of altered phases. The white box shows the location of Figure 4b. (b) Part of HiRISE image PSP_002387_ 1985_RED showing the eroded light-toned materials comprising the delta which have the strongest hydrated mineral signature. The white box shows the location of Figure 4c. (c) Zoom of Figure 4b showing horizontally layered materials at the base of the fan. Murchie et al (2009) Jeff Taylor Aqueous Alteration

Carbonates Circumferential to the Isidis Impact Basin Example of the partially altered ultramafic unit and phyllosilicate/carbonate stratigraphy. CRISM spectra at the upper right correspond to colored units in this CRISM false color plus grayscale HiRISE (23cm/pixel) composite image. Arrows point to raised fractures (white) and breccia blocks (black) in the Fe/Mg smectite-bearing unit (figure from Ehlmann et al., 2009). Ehlmann (2010) Jeff Taylor Aqueous Alteration

Ehlmann et al (2011) Aqueous Alteration Jeff Taylor Fig 3 of Ehlmann et al (2011) The caption reads: Published regional stratigraphies from different locations on the planet—Nili Fossae/ northeastern Syrtis41,49,57, Argyre39, Valles Marineris11; Mawrth Vallis and Arabia Terra46,50,68–70; Terra Meridiani53; and sedimentary clays at Gale crater47 (latitudes and longitudes as shown)—have been compiled and correlated. Columns, except those for Valles Marineris and Gale, are drawn on the same vertical scale. Section thickness ranges from,250 m, atMawrth Vallis, to 8 km, at Valles Marineris. Characteristic minerals or compositions that characterize each unit in near-infrared spectral data are shown. Hatching indicates the interbedding of two minerals in the same large-scale unit, and colour gradation indicates that the corresponding minerals occupy the same unit, grading from one to the other at various points. Upper units have been dated to the Noachian/Hesperian boundary and can be correlated globally. Correlations between lower stratigraphic units can sometimes be inferred on a regional basis39,50. Unit labels correspond to inferred age (EH, early Hesperian; LN, late Noachian;MN, middle Noachian; A,Amazonian) or assigned global geological map units (for example Npl1) identified in the section by the authors of the original published stratigraphy, where the first letter indicates the age of the rock unit. The geological setting of clay formation is given in italics, and, where consensus does not exist in the literature, a question mark is appended. ‘In situ’ means in-place formation of the stratigraphic unit by in situ alteration; sed., sedimentary. HCP, high-calcium pyroxene; LCP, low-calcium pyroxene. Ehlmann et al (2011) Aqueous Alteration Jeff Taylor

Jeff Taylor Aqueous Alteration

Ehlmann et al (2011) Aqueous Alteration Jeff Taylor From Ehlmann et al (2011): Figure 4 | Timeline ofmajor processes in Mars history. a–c, Major geological processes influencing water availability, including the presence of a magnetic field95 (a), impact cratering96,97 (b) and volcanism86,98 (c). d, Schematic depicting the changing nature of environments hosting liquid water, as implied by the geological evidence discussed herein. e, f, Evidence of liquid water: timing of valley network and outflow channel activity15,16 ages of key minerals formed by aqueous alteration (e) and important regional units with alteration minerals (f). Relative timing is determined using relative crater densities and stratigraphic relationships. Absolute ages of period boundaries5 have uncertainties of several hundred million years, inherent to extrapolation from cratering statistics99. NF, Nili Fossae; MV, Mawrth Vallis; VM, Valles Marineris; Carb., carbonates; Chl., chlorites; Sulph., sulphates. Figure 4 | Timeline ofmajor processes in Mars history. a–c, Major geological processes influencing water availability, including the presence of a magnetic field95 (a), impact cratering96,97 (b) and volcanism86,98 (c). d, Schematic depicting the changing nature of environments hosting liquid water, as implied by the geological evidence discussed herein. e, f, Evidence of liquid water: timing of valley network and outflow channel activity15,16 ages of key minerals formed by aqueous alteration (e) and important regional units with alteration minerals (f). Relative timing is determined using relative crater densities and stratigraphic relationships. Absolute ages of period boundaries5 have uncertainties of several hundred million years, inherent to extrapolation from cratering statistics99. NF, Nili Fossae; MV, Mawrth Vallis; VM, Valles Marineris; Carb., carbonates; Chl., chlorites; Sulph., sulphates. Ehlmann et al (2011) Aqueous Alteration Jeff Taylor

Evolution of Aqueous Environments during the First Billion Years of Mars History Figure 5 from Ehlmann et al (2011) Caption reads: Evolution of aqueous environments during the first billion years of Mars history. Locations of clay formation are indicated in green. Cold, arid and icy conditions characterized Mars during most of the Noachian, with clay formation mostly in a warmer, wetter subsurface environment. During the late Noachian/early Hesperian, volcanism was widespread and surface waters intermittently carved valleys, sustained lakes, transported sediments to basins and sustained near-surface weathering to form clays near the surface. Conditions from the late Hesperian to the present day have been cold and dry, resulting in the cessation of clay formation. Ehlmann et al (2011) Jeff Taylor Aqueous Alteration