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Contributed by John P. Grotzinger, May 5, 2016 (sent for review March 18, 2016); reviewed by Jon Blundy, Robert M. Hazen, and Harry Y. McSween)
Significance
Tridymite, a SiO2 mineral that crystallizes at low pressures and high temperatures (>870 °C) from high-SiO2 materials, was detected at high concentrations in a sedimentary mudstone in Gale crater, Mars. Mineralogy and abundance were determined by X-ray diffraction using the Chemistry and Mineralogy instrument on the Mars Science Laboratory rover Curiosity. Terrestrial tridymite is commonly associated with silicic volcanism where high temperatures and high-silica magmas prevail, so this occurrence is the first in situ mineralogical evidence for martian silicic volcanism. Multistep processes, including high-temperature alteration of silica-rich residues of acid sulfate leaching, are alternate formation pathways for martian tridymite but are less likely. The unexpected discovery of tridymite is further evidence of the complexity of igneous petrogenesis on Mars, with igneous evolution to high-SiO2 compositions.
Abstract
Tridymite, a low-pressure, high-temperature (>870 °C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has ∼40 wt.% crystalline and ∼60 wt.% X-ray amorphous material and a bulk composition with ∼74 wt.% SiO2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (∼17 wt.% of bulk sample), tridymite (∼14 wt.%), sanidine (∼3 wt.%), cation-deficient magnetite (∼3 wt.%), cristobalite (∼2 wt.%), and anhydrite (∼1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (∼39 wt.% opal-A and/or high-SiO2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides−perchlorates−chlorates), and has minor TiO2 and Fe2O3T oxides (∼5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a “Lake Gale” catchment environment can account for Buckskin’s tridymite, cristobalite, feldspar, and any residual high-SiO2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.
Footnotes
- ↵1To whom correspondence may be addressed. Email: grotz{at}gps.caltech.edu or richard.v.morris{at}nasa.gov.
Author contributions: R.V.M., D.T.V., D.F.B., R.G., S.J.C., D.W.M., S.M.M., A.S.Y., J.P.G., T.F.B., and J.-M.M. designed research; R.V.M., D.T.V., D.F.B., R.G., S.J.C., E.B.R., D.W.M., S.M.M., R.T.D., A.H.T., A.S.Y., J.P.G., C.N.A., T.F.B., J.A.C., D.J.D.M., J.D.F., K.V.F., J.F., T.G.G., and J.-M.M. performed research; R.V.M., D.T.V., D.F.B., R.G., S.J.C., E.B.R., D.W.M., S.M.M., R.T.D., A.H.T., A.S.Y., J.P.G., C.N.A., T.F.B., J.A.C., D.J.D.M., J.D.F., K.V.F., J.F., T.G.G., J.-M.M., E.M.S., and S.P.S. analyzed data; and R.V.M. and J.P.G. wrote the paper.
Reviewers: J.B., Bristol University; R.M.H., Carnegie Institution of Washington; and H.Y.M., The University of Tennessee, Knoxville.
The authors declare no conflict of interest.
Data deposition: CheMin and APXS experiment data records and CheMin diffraction patterns have been deposited with the NASA Planetary Data System at pds-geosciences.wustl.edu/msl/msl-m-chemin-4-rdr-v1/mslcmn_1xxx/ for the CheMin data and pds-geosciences.wustl.edu/msl/msl-m-apxs-4_5-rdr-v1/mslapx_1xxx/ for the APXS data.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1607098113/-/DCSupplemental.
Freely available online through the PNAS open access option.
Tridymite evidence for silicic volcanism on Mars
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