Hydrothermal Precipitation of Sanidine (Adularia) Having Full Al,Si Structural Disorder and Specular Hematite at Maunakea Volcano (Hawai'i) and at Gale Crater (Mars)

Abstract

Hydrothermal high sanidine and specular hematite are found within ferric‐rich and gray‐colored cemented basaltic breccia occurring within horizontal, weathering‐resistant strata exposed in an erosional gully of the Pu'u Poliahu cinder cone in the summit region of Maunakea volcano (Hawai'i). The cone was extensively altered by hydrothermal, acid‐sulfate fluids at temperatures up to ~400 °C, and, within strata, plagioclase was removed by dissolution from progenitor Hawaiitic basalt, and sanidine and hematite were precipitated. Fe₂O₃T concentration and Fe ³⁺/∑Fe redox state are ~12 wt.% and ~0.4 for progenitor basalt and 46–60 wt.% and ~1.0 for cemented breccias, respectively, implying open‐system alteration and oxic precipitation. Hydrothermal high sanidine (adularia) is characterized by full Al,Si structural disorder and monoclinic unit‐cell (Rietveld refinement): a = 8.563(19) Å, b = 13.040(6) Å, c = 7.169(4) Å, β = 116.02(10)°, and V = 719.4(19) ų. Hematite (structure confirmed by Rietveld refinement) is the predominant Fe‐bearing phase detected. Coarse size fractions of powdered hematite‐rich breccia (500–1000 μm) are dark and spectrally neutral at visible wavelengths, confirming specular hematite, and SEM images show platy to polyhedral hematite morphologies with longest dimensions >10 μm. Smectite and 10‐Å phyllosilicate, both chemically dominated by Mg as octahedral cation, are additional diagenetic hydrothermal alteration products. By analogy and as a working hypothesis, high sanidine (Kimberly formation) and specular hematite (Mt. Sharp group at Hartmann's Valley and Vera Rubin ridge) at Gale crater are interpreted as diagenetic alteration products of Martian basaltic material by hydrothermal processes.

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