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Hematite Spherules in Basaltic Tephra Altered Under Aqueous, Acid-Sulfate Conditions on Mauna Kea Volcano, Hawaii: Possible Clues for the Occurrence of Hematite-Rich Spherules in the Burns Formation at Meridiani Planum, MarsIron-rich spherules (>90% Fe2O3 from electron microprobe analyses) approx.10-100 microns in diameter are found within sulfate-rich rocks formed by aqueous, acid-sulfate alteration of basaltic tephra on Mauna Kea volcano, Hawaii. Although some spherules are nearly pure Fe, most have two concentric compositional zones, with the core having a higher Fe/Al ratio than the rim. Oxide totals less than 100% (93-99%) suggest structural H2O and/or /OH. The transmission Moessbauer spectrum of a spherule-rich separate is dominated by a hematite (alpha-Fe2O3) sextet whose peaks are skewed toward zero velocity. Skewing is consistent with Al(3+) for Fe(3+) substitution and structural H2O and/or /OH. The grey color of the spherules implies specular hematite. Whole-rock powder X-ray diffraction spectra are dominated by peaks from smectite and the hydroxy sulfate mineral natroalunite as alteration products and plagioclase feldspar that was present in the precursor basaltic tephra. Whether spherule formation proceeded directly from basaltic material in one event (dissolution of basaltic material and precipitation of hematite spherules) or whether spherule formation required more than one event (formation of Fe-bearing sulfate rock and subsequent hydrolysis to hematite) is not currently constrained. By analogy, a formation pathway for the hematite spherules in sulfate-rich outcrops at Meridiani Planum on Mars (the Burns formation) is aqueous alteration of basaltic precursor material under acid-sulfate conditions. Although hydrothermal conditions are present on Mauna Kea, such conditions may not be required for spherule formation on Mars if the time interval for hydrolysis at lower temperatures is sufficiently long.
Document ID
20080026259
Acquisition Source
Johnson Space Center
Document Type
Reprint (Version printed in journal)
Authors
Morris, R. V.
(NASA Johnson Space Center Houston, TX, United States)
Ming, D. W.
(NASA Johnson Space Center Houston, TX, United States)
Graff, T. G.
(Arizona State Univ. Tempe, AZ, United States)
Arvidson, R. E.
(Washington Univ. Saint Louis, MO, United States)
Bell, J. F., III
(Cornell Univ. Ithaca, NY, United States)
Squyres, S. W.
(Cornell Univ. Ithaca, NY, United States)
Mertzman, S. A.
(Franklin and Marshall Coll. Lancaster, PA, United States)
Gruener, J. E.
(NASA Johnson Space Center Houston, TX, United States)
Golden, D. C.
(Jacobs Sverdrup Technology, Inc. Houston, TX, United States)
Robinson, G. A.
(Jacobs Sverdrup Technology, Inc. Houston, TX, United States)
Date Acquired
August 24, 2013
Publication Date
November 30, 2005
Publication Information
Publication: Earth and Planetary Science Letters
Publisher: Elsevier B.V.
Volume: 240
Issue: 1
ISSN: 0012-821X
Subject Category
Geophysics
Funding Number(s)
WBS: WBS 361426.04.05
Distribution Limits
Public
Copyright
Other

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