M. B. Wyatt

Publications (25)195.31 Total impact

• Article: Identification of Highly Silicic Features on the Moon

  T. D. Glotch, P. G. Lucey, J. L. Bandfield, B. T. Greenhagen, I. R. Thomas, R. C. Elphic, N. E. Bowles, M. B. Wyatt, C. C. Allen, K. L. Donaldson Hanna, D. A. Paige
  [show abstract] [hide abstract]
  ABSTRACT: Diviner has detected several features on the Moon with highly silicic compositions. These geologic settings of these features suggest formation of highly evolved lithologies from both extrusive volcanic and intrusive processes.
  02/2010; 41:1780.
• Article: Mars Exploration Rover Geologic traverse by the Spirit Rover in the Plains of Gusev Cater, Mars

  L.S. Crumpler, S.W. Squyres, R.E. Arvidson, J.F. Bell, D. Blaney, N.A. Cabrol, P.R. Christensen, D.J. DesMarais, J.D. Farmer, R. Fergason, [......], M.C. Payne, J.W. Rice, S.W. Ruff, M. Sims, S.D. Thompson, N. Tosca, A. Wang, P. Whelley, S.P. Wright, M.B. Wyatt
  Geology. 01/2010; 33:809-812.
• Article: TES and GRS Compositions of the Martian Surface: Evidence for Igneous and Secondary Chemical Fractionation Processes.

  M. B. Wyatt, H. Y. McSween
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  ABSTRACT: TES and GRS provide unique and complementary insights into martian surface compositions. TES measures the composition of the upper hundred microns of the surface while GRS measures the upper few tens of centimeters. We examine TES oxide abundances of low-albedo surfaces and compare distributions to GRS element abundances to constrain the relative roles of igneous and alteration processes on Mars. The bulk variability of compositions measured by TES is accounted for by two spectral endmembers (ST1 and ST2). TES oxide abundances (wt. %) (SiO2, Na2O, K2O, CaO, MgO, FeO, Al2O3) are calculated by combining compositions of spectral endmembers in proportion to their relative modeled abundances (vol. %). Recent GRS studies report Si, K, Fe, Th, and K/Th for 'regions' dominated by TES ST1 (RT1) and ST2 (RT2) materials. The most significant TES chemical trends are higher abundances of FeO for ST1 (ST1 15.2 % vs. ST2 12.4 %) and higher abundances of SiO2 for ST2 (ST2 57.9 % vs. ST1 53.9 %). Results from OMEGA are in agreement with TES oxides. OMEGA pyroxene maps are closely correlated with the distribution of ST1 (High FeO) while ST2 materials lack evidence of mafic bands and are consistent with an enrichment of high-silica phases. GRS RT2 chemistries have higher abundances of FeO (RT2 20.1 % vs. RT1 17.6 %), K, and Th compared to RT1. Abundances of SiO2 (RT1 44.7 % and RT2 45.8 %) and K/Th ratio do not show significant spatial variations. Chemical trends from TES and GRS appear to be in disagreement. TES ST1 is enriched in FeO while GRS RT1 is depleted in FeO. TES ST2 is enriched in SiO2 while GRS RT2 shows no enrichment in SiO2. One can account for these discrepancies, and constrain igneous and alteration processes, by considering the sampling depth differences between TES and GRS. The constant K/Th ratio across RT1 and RT2 is not consistent with subaqueous or deep subaerial aqueous weathering of basalt as K would fractionate from Th. Fractional crystallization and subduction zone magmatism could enrich K and Th, however GRS does not detect an enrichment of Si as would be expected. The lack of any significant enrichment in SiO2 between GRS RT1 and RT2 indicates that evolved volcanics (andesites) are not present in high-abundances within the upper few tens of centimeters at global scales. The favored model from the GRS team is thus initial bulk differentiation processes on Mars producing compositionally distinct magma source regions in the mantle. RT1 and RT2 basaltic provinces with distinct trace element compositions could then be produced. However, the differences in SiO2 between TES ST1 and ST2 must be taken into consideration. Thin coatings or rinds of secondary high-silica phases (tens of microns) significantly affect the shape and position of absorptions in thermal emission spectra of basalt. Such coatings on Mars may form from near-surface ice and/or surface-atmosphere interactions with little to no water penetrating or cycling into the surface. Limited degrees of alteration in only the upper few tens of microns of the surface could affect TES derived chemistries and be undetectable to GRS due to a deep sampling depth. GRS and TES chemistries support: 1) Distinct magma source regions and basaltic compositions for ST1-RT1 and ST2-RT2 and 2) Thin secondary coatings or rinds of amorphous high-silica phases on ST2-RT2 basalt.
  AGU Spring Meeting Abstracts. 04/2007; -1:07.
• Article: Investigating the Role of Compositionally-Diverse Glasses in Interpreting Martian Chemistry and Mineralogy as Viewed by TES

  M. E. Minitti, M. B. Wyatt, V. E. Hamilton
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  ABSTRACT: Mineralogy and chemistry of the Martian surface are derived from Thermal Emission Spectrometer (TES) data by linear deconvolution using a spectral library containing the infrared spectra of a variety of phases [e.g. 1]. Obtaining successful results relies on utilization of a spectral library containing an accurate representation of the phases measured on the Martian surface. In order to explore the influence of a variety of glass compositions on the deconvolution of TES spectra, we synthesized five pristine glasses, obtained their thermal infrared spectra and conducted preliminary TES spectral deconvolutions using a spectral library containing the new glass spectra. The five new glasses have compositions relevant to Martian igneous processes. One basalt has a high-FeO, low-Al2O 3 composition linked to several Martian meteorites [refs in 3]. A second basalt is based on the Deccan Trap basalt that serves as a spectral analog to the TES Surface Type 1 (ST1) spectrum [2]. Similarly, one andesite is based on the Medicine Lake andesite that serves as a spectral analog to the TES Surface Type 2 (ST2) spectrum [2]. A second andesite represents the interstitial melt composition resulting from 90% crystallization of an anhydrous Martian meteorite basalt (described above) [3]. A dacitic glass represents the interstitial melt composition resulting from 80% crystallization of a hydrous Martian meteorite basalt [3]. Each glass was fused in either Pt or AgPd tubing vacuum sealed in SiO2 glass tubing for 72-96 hours. We collected thermal infrared spectra of the ≥500 μm fraction of the glasses at the Hawai'i Institute of Geophysics and Planetology with a ThermoElectron Nexus 470 FTIR interferometric spectrometer in thermal emission mode [4]. We obtained data from 200-4000 cm-1 at 2 cm-1 spectral sampling. Preliminary deconvolutions of ST1 and ST2 spectra indicate that the newly synthesized glasses may represent significant components in the Martian surface. ST1 and ST2 are modeled with high abundances of the andesitic interstitial melt endmember (17 vol% ST1 and 23 vol% ST2); ST2 is also modeled with 11 vol% of the analog TES basalt glass and 13 vol% of a high-silica and potassium glass [5]. Addition of the new glass spectral endmembers results in an overall increase in the total abundance of modeled glasses at the expense of plagioclase, pyroxenes and olivine. Modeling of distinct glass compositions does not necessarily indicate that each glass is present on the Martian surface. Different glass compositions and abundances may represent average glass chemistry much the way different plagioclase compositions are modeled to reflect average plagioclase composition in rocks. Average derived chemistries for glasses in ST1 (59 wt% SiO2 and 15.7 wt% FeO) and ST2 (64.2 wt% SiO2 and 10.5 wt%FeO) are influenced by inclusion of the new glass compositions in deconvolutions. Total derived bulk chemistries for ST1 and ST2, however, do not significantly change (e.g. ±1-2 wt% SiO2) with the addition of the new glass spectral endmembers. This likely reflects the partial substitution of chemically similar minerals with newly synthesized glasses. [1] M.S. Ramsey and P.R. Christensen (1998) JGR, 103, 577 596. [2] J.L. Bandfield et al. (2000) Science, 287, 1626 1630. [3] M.E. Minitti and M.J. Rutherford (2000) GCA, 64, 2535- 2547. [4] V.E. Hamilton and P.G. Lucey (2005) LPSC XXXVI, #1272. [5] M.B. Wyatt et al. (2001) JGR, 106, 14,711-14,732.
  AGU Fall Meeting Abstracts. 11/2006; -1:0067.
• Source

  Available from: Christian Schröder

  Article: Characterization and petrologic interpretation of olivine-rich basalts at Gusev Crater, Mars

  Y. McSween, M. B. Wyatt, R. Gellert, J. F. Bell, R. V. Morris, K. E. Herkenhoff, L. S. Crumpler, K. A. Milam, K. R. Stockstill, L. L. Tornabene, [......], J. E. Moersch, R. Rieder, S. W. Ruff, C. Schroder, P. A. de Souza, S. W. Squyres, H. Wänke, A. Wang, A. Yen, J. Zipfel
  Journal of Geophysical Research 01/2006; 111. · 3.02 Impact Factor
• Article: Evidence for magmatic evolution and diversity on Mars from infrared observations

  P. R. Christensen, H. Y. McSween, J. L. Bandfield, S. W. Ruff, A. D. Rogers, V. E. Hamilton, N. Gorelick, M. B. Wyatt, B. M. Jakosky, H. H. Kieffer, M. C. Malin, J. E. Moersch
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  ABSTRACT: Compositional mapping of Mars at the 100-metre scale with the Mars Odyssey Thermal Emission Imaging System (THEMIS) has revealed a wide diversity of igneous materials. Volcanic evolution produced compositions from low-silica basalts to high-silica dacite in the Syrtis Major caldera. The existence of dacite demonstrates that highly evolved lavas have been produced, at least locally, by magma evolution through fractional crystallization. Olivine basalts are observed on crater floors and in layers exposed in canyon walls up to 4.5 km beneath the surface. This vertical distribution suggests that olivine-rich lavas were emplaced at various times throughout the formation of the upper crust, with their growing inventory suggesting that such ultramafic (picritic) basalts may be relatively common. Quartz-bearing granitoid rocks have also been discovered, demonstrating that extreme differentiation has occurred. These observations show that the martian crust, while dominated by basalt, contains a diversity of igneous materials whose range in composition from picritic basalts to granitoids rivals that found on the Earth.
  Nature 07/2005; 436(7050):504-509. · 36.28 Impact Factor
• Article: Provenance and diagenesis of impure evaporitic sedimentary rocks on Meridiani Planum, Mars

  S. M. McLennan, J. F. Bell III, W. M. Calvin, P. R. Christensen, B. C. Clark, P. A. de Souza, W. H. Farrand, D. Fike, R. Gellert, A. Ghosh, [......], Z. Learner, M. C. Malin, H. Y. McSween Jr, J. Pocock, S. W. Ruff, S. W. Squyres, N. J. Tosca, W. Watters, M. B. Wyatt, A. Yen
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  ABSTRACT: Impure evaporitic sedimentary rocks preserved on Meridiani Planum preserve textural, mineralogical and geochemical evidence for a protracted and complex history of syndepositional through burial diagenesis dominated by phreatic meteoric conditions.
  Lunar and Planetary Science XXXVI, Lunar and Planetary Institute (2005). 02/2005;
• Article: Mineral Composition and Abundance of the Rocks and Soils at Gusev and Meridiani from the Mars Exploration Rover Mini-TES Instruments

  P. R. Christensen, M. B. Wyatt, T. D. Glotch, A D Rogers, S Anwar, R. E. Arvidson, J. L. Bandfield, D. L. Blaney, C Budney, W. M. Calvin
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  ABSTRACT: The Miniature Thermal Emission Spectrometer (Mini-TES) has provided remote measurements of mineralogy, thermophysical properties, and atmospheric temperature profile and composition of the outcrops, rocks, spherules, and soils surrounding the Spirit and Opportunity Rovers. The mineralogy of volcanic rocks provides insights into the composition of the source regions and the nature of martian igneous processes. Carbonates, sulfates, evaporites, and oxides provide information on the role of water in the surface evolution. Oxides, such as crystalline hematite, provide insight into aqueous weathering processes, as would the occurrence of clay minerals and other weathering products. Diurnal temperature measurements can be used to determine particle size and search for the effects of sub-surface layering, which in turn provide clues to the origin of surficial materials through rock disintegration, aeolian transport, atmospheric fallout, or induration. In addition to studying the surface properties, Mini-TES spectra have also been used to determine the temperature profile in the lower boundary layer, providing evidence for convective activity, and have determined the seasonal trends in atmospheric temperature and dust and cloud opacity.
  02/2005;
• Article: Mineralogy at Meridiani Planum from the Mini-TES Experiment on the Opportunity Rover.

  P R Christensen, M B Wyatt, T D Glotch, A D Rogers, S Anwar, R E Arvidson, J L Bandfield, D L Blaney, C Budney, W M Calvin, [......], A T Knudson, H Y McSween, G L Mehall, L K Mehall, J E Moersch, R V Morris, M D Smith, S W Squyres, S W Ruff, M J Wolff
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  ABSTRACT: The Miniature Thermal Emission Spectrometer (Mini-TES) on Opportunity investigated the mineral abundances and compositions of outcrops, rocks, and soils at Meridiani Planum. Coarse crystalline hematite and olivine-rich basaltic sands were observed as predicted from orbital TES spectroscopy. Outcrops of aqueous origin are composed of 15 to 35% by volume magnesium and calcium sulfates [a high-silica component modeled as a combination of glass, feldspar, and sheet silicates (approximately 20 to 30%)], and hematite; only minor jarosite is identified in Mini-TES spectra. Mini-TES spectra show only a hematite signature in the millimeter-sized spherules. Basaltic materials have more plagioclase than pyroxene, contain olivine, and are similar in inferred mineral composition to basalt mapped from orbit. Bounce rock is dominated by clinopyroxene and is close in inferred mineral composition to the basaltic martian meteorites. Bright wind streak material matches global dust. Waterlain rocks covered by unaltered basaltic sands suggest a change from an aqueous environment to one dominated by physical weathering.
  Science 01/2005; 306(5702):1733-9. · 31.20 Impact Factor
• Article: Soils of Eagle crater and Meridiani Planum at the Opportunity Rover landing site.

  L A Soderblom, R C Anderson, R E Arvidson, J F Bell, N A Cabrol, W Calvin, P R Christensen, B C Clark, T Economou, B L Ehlmann, [......], C Schröder, F P Seelos, J M Soderblom, S W Squyres, R Sullivan, W A Watters, C M Weitz, M B Wyatt, A Yen, J Zipfel
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  ABSTRACT: The soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.
  Science 01/2005; 306(5702):1723-6. · 31.20 Impact Factor
• Article: Mini-TES Derived Chemical Abundances at Gusev Crater and Meridiani Planum: Implications for Global Surface Compositions

  M. B. Wyatt, A. Ghosh, P. R. Christensen, H. Y. McSween
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  ABSTRACT: The Miniature Thermal Emission Spectrometer (Mini-TES) experiments provide remote measurements of mineral abundances and compositions at the Spirit and Opportunity landing sites of Gusev Crater and Meridiani Planum. Olivine basaltic sands and rocks are identified at Gusev Crater, along with variable amounts of fine-grained dust and other possible coatings. Olivine basaltic sands and coarse crystalline hematite, a clinopyroxene-rich basaltic rock, fine-grained dust, and outcrops composed of sulfates, hematite, and sheet silicates/glass are identified at Meridiani Planum. The occurrence of olivine basalt was predicted at both landing sites by observations from the orbiting Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and Mini-TES spectra of basaltic sands are very similar to a global average TES basalt spectrum in the overall spectral shape and positions of spectral features. Ground-truth observations of the mineralogy and chemistry of olivine basalt from the Mini-TES, Alpha Particle X-ray Spectrometer (APXS), and Mössbauer Spectrometer (MB) are significant because of the extensive distribution and high-abundance of olivine basalt on Mars and the inferred petrogenesis and evolution of a basaltic crust. In this study, we calculate bulk chemical oxide abundances of surface materials from Mini-TES derived mineral abundances and compositions and compare results to APXS and MB observations to examine the accuracy of thermal emission derived chemistry. We also examine methods to constrain APXS derived normative minerals using multivariate regression with Mini-TES and MB mineral abundances and compositions. We focus on comparisons of Mini-TES and TES derived chemistries of basalt to determine the limits to which we can constrain bulk compositions and the depth and degree of partial melting of their source regions. Local ground-truth observations of the bulk mineralogy and chemistry of a global basaltic unit provide insight to the degree of differentiation of the crust and mantle.
  AGU Fall Meeting Abstracts. 11/2004; -1:04.
• Source

  Available from: Deanne Rogers

  Article: Initial results from the Mini-TES experiment in Gusev Crater from the Spirit Rover.

  P R Christensen, S W Ruff, R L Fergason, A T Knudson, S Anwar, R E Arvidson, J L Bandfield, D L Blaney, C Budney, W M Calvin, [......], H Y McSween, G L Mehall, L K Mehall, J E Moersch, R V Morris, A D Rogers, M D Smith, S W Squyres, M J Wolff, M B Wyatt
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  ABSTRACT: The Miniature Thermal Emission Spectrometer (Mini-TES) on Spirit has studied the mineralogy and thermophysical properties at Gusev crater. Undisturbed soil spectra show evidence for minor carbonates and bound water. Rocks are olivinerich basalts with varying degrees of dust and other coatings. Dark-toned soils observed on disturbed surfaces may be derived from rocks and have derived mineralogy (+/-5 to 10%) of 45% pyroxene (20% Ca-rich pyroxene and 25% pigeonite), 40% sodic to intermediate plagioclase, and 15% olivine (forsterite 45% +/-5 to 10). Two spectrally distinct coatings are observed on rocks, a possible indicator of the interaction of water, rock, and airfall dust. Diurnal temperature data indicate particle sizes from 40 to 80 microm in hollows to approximately 0.5 to 3 mm in soils.
  Science 09/2004; 305(5685):837-42. · 31.20 Impact Factor
• Source

  Available from: Paulo A. de Souza

  Article: Basaltic rocks analyzed by the Spirit Rover in Gusev Crater.

  H Y McSween, R E Arvidson, J F Bell, D Blaney, N A Cabrol, P R Christensen, B C Clark, J A Crisp, L S Crumpler, D J Des Marais, [......], R V Morris, R Rieder, S W Ruff, P A De Souza, S W Squyres, H Wänke, A Wang, M B Wyatt, A Yen, J Zipfel
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  ABSTRACT: The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini-Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain </=25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. Mössbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust.
  Science 08/2004; 305(5685):842-5. · 31.20 Impact Factor
• Article: Soils of Eagle Crater and Meridiani Planum at the Opportunity rover Landing Site

  L.A. Soderblom, Anderson. R.-C, R.E. Arvidson, J.F. Bell, N.A. Cabrol, W. Calvin, P.R. Christensen, B.C. Clark, L. Richter, T. Economou, [......], C. Schröder, F.P. Seelos, J.M. Soderblom, S.W. Squyres, R. Sullivan, W.A. Watters, C.M. Weitz, M.B. Wyatt, A. Yen, J. Zipfel
  Science 01/2004; 306:1723-1726. · 31.20 Impact Factor
• Article: TES Observations of Chryse and Acidalia Planitiae: Multiple Working Hypotheses for Distributions of Surface Compositions

  M. B. Wyatt, J. L. Bandfield, Mcsween, H. Y, P. R. Christensen, J. Moersch
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  ABSTRACT: A gradation of surface units represents either (1) an influx of basaltic sediment from southern highlands, deposited on andesitic volcanics, or (2) incompletely weathered basalt marking the geographic extent of submarine alteration of basaltic crust. Additional information is contained in the original extended abstract.
  05/2002;
• Article: Compositions of Low Albedo Intracrater Materials and Wind Streaks on Mars: Examination of MGS TES Data in Western Arabia Terra

  J. L. Bandfield, M. B. Wyatt, P. Christensen, Mcsween, H. Y
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  ABSTRACT: Basalt and andesite surface compositions are identified within individual low albedo intracrater features and adjacent dark wind streaks. High resolution mapping of compositional heterogeneities may help constrain origin hypotheses for these features. Additional information is contained in the original extended abstract.
  02/2001;
• Source

  Available from: articles.adsabs.harvard.edu

  Article: Martian Surface Compositions from Mars Global Surveyor Thermal Emission Spectrometer Data: Mars Pathfinder Landing Site and Surrounding Regions

  M. B. Wyatt, J. L. Bandfield, McSween, H. Y., Jr, P. R. Christensen
  Meteoritics and Planetary Science Supplement. 06/2000; 35:171.
• Source

  Available from: articles.adsabs.harvard.edu

  Article: Recognizing Andesites on the SNC Parent Body: Mineralogy from Mars Global Surveyor

  M. B. Wyatt, V. E. Hamilton, McSween, H. Y., Jr, P. R. Christensen, A. Patchen, L. A. Taylor
  Meteoritics and Planetary Science Supplement. 06/1999; 34:123.
• Source

  Available from: brown.edu

  Article: THEMIS characterization of the MER Gusev crater landing site

  Keith A Milam, Karen R Stockstill, Jeffrey E Moersch, Harry Y Mcsween Jr, Livio L Tornabene, Amitabha Ghosh, Michael B Wyatt, Phillip R Christensen, K R Stockstill, J E Moersch, H Y Mcsween, L L Tornabene, A Ghosh, M B Wyatt, P R Christensen
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  ABSTRACT: 1] Gusev crater, previously interpreted as the depocenter for the Gusev-Ma'adim Vallis fluvio-lacustrine system, is a proposed landing site for one of the Mars Exploration Rovers (MER). Here we use new remote-sensing data from the Thermal Emission Imaging System (THEMIS) supplemented by data from the Thermal Emission Spectrometer (TES), Mars Orbiter Camera (MOC), and Mars Orbiter Laser Altimeter (MOLA) to characterize the geology of Gusev crater. Thermal infrared data from THEMIS and TES were used to map thermophysical units on the basis of relative albedos and diurnal temperature variations. THEMIS and MOC visible images were used to map unit morphologies and to estimate crater density ages. MOLA data were used to identify unit contacts and stratigraphic relationships. Various data were then combined to construct a new surface unit map and stratigraphy for units on the floor of Gusev. Seven surface units were identified in Gusev, mostly Hesperian in age, but with two showing evidence of later modification and redistribution. Five or more surface units and layering are present within the MER-A landing ellipse, attesting to the geologic diversity of this site. Surface units show features that could be consistent with fluvio-lacustrine, aeolian, and/or volcanoclastic deposition, but the spatial resolution of visible/infrared data does not allow for the identification of unambiguous volcanic or fluvio-lacustrine textures. However, a MER landing in Gusev may provide the opportunity to analyze multiple units, distinguish rock types, examine stratigraphic relationships, and shed light on the ancient depositional environment.
  J. Geophys. Res. 108(8078).
• Article: Provenance and diagenesis of the evaporite-bearing Burns formation, Meridiani Planum, Mars

  S.M. McLennan, J.F. Bell, W.M. Calvin, P.R. Christensen, B.C. Clark, P.A. de Souza, J. Farmer, W.H. Farrand, D.A. Fike, R. Gellert, [......], M.C. Malin, H.Y. McSween, J. Pocock, S.W. Ruff, L.A. Soderblom, S.W. Squyres, N.J. Tosca, W.A. Watters, M.B. Wyatt, A. Yen
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  ABSTRACT: Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.
  Earth and Planetary Science Letters.