The Role of Volatile Elements in Megaspherulite Formation: Essential to the Recipe or Icing on the Cake?
Abstract
More than thirty megaspherulites, some more than 3 m in diameter, occur in an extensively hydrated unit of rhyolite obsidian in Silver Cliff, CO. The obsidian contains water contents ranging from 5-7 wt.% (Smith et. al., 2001). High volatile content likely suppressed crystal nucleation, allowing the growth of a small number of large crystals. Smith et. al. (2001) concluded that the spherulites grew under highly disequilibrium conditions, just above or just below the glass transition temperature. To assess the growth of these unusual features, we produced a high-resolution 3D model of the megaspherulite-bearing outcrop using a combination of ground- and aerial-based photogrammetry.
Several series of obsidian samples were collected from the margin of the spherulites, to a distance of up to 4 meters, in order to examine variations in water content around the perimeter of each one. FTIR analysis of the first samples collected for this study yielded water concentrations of 4-5 wt.%. On average, the ratio of OH- to H2O is 1:4, which suggests the last equilibration temperature would have been ~400 °C (Ihinger et. at., 1999). During heating, measurements by Uniaxial Creep Viscometry and Differential Scanning Calorimetry both initially show glass transition-like behavior at ~400˚C. After heating to ≥700˚C, sample mass decreased by ~5 wt.%, consistent with near-total degassing but without vesiculation. There has been debate in the literature as to whether the unit is a subaerial flow or a sill that quenched to glass. Though originally mapped as a flow, the unit shows no flow banding or vesiculation, although patches of brecciated obsidian are visible. It is unlikely that a rhyolitic obsidian can effusively erupt at the surface with ~5 wt.% primary water. The unit would be expected to have erupted explosively. Thus, it is more likely an intrusion. Considering water content and chemical composition, the melt should have stayed at pressures of at least 150 MPa to avoid degassing (Liu et. al., 2005). Therefore, emplacement depth could have been as shallow as ~5-6 kilometers.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51I0177C
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 8430 Volcanic gases;
- VOLCANOLOGY;
- 8450 Planetary volcanism;
- VOLCANOLOGY