Black Butte Dacitic Dome, Ca: A Study of Ascent Rates Using Amphibole and Plagioclase Reactions and Crystal Size Distribution
Abstract
Magma ascent rate can be quantified petrologically utilizing measurement of (1) decompression-driven amphibole breakdown rims, (2) compositional changes in plagioclase phenocryst rims and (3) crystal size distribution (CSD) of groundmass plagioclase microlites, that are known to have grown during ascent and cooling. Measurements of these parameters in both natural and experimental samples allows for an investigation into the relative rates and correlations between amphibole breakdown, plagioclase growth (of both phenocrysts and microlites) and magma ascent. Black Butte, Ca provides a good opportunity to study the effects of ascent rate as determined using the above three methods. Black Butte consists of four overlapping dacitic domes on the southwest flank of Mt. Shasta. At Black Butte the natural phase assemblage consists of amphiboles with well developed breakdown rims, homogeneous An74 plagioclase phenocryst cores decreasing monotonically to An55 rims, and extensive groundmass plagioclase microphenocryst growth. Amphibole breakdown rim widths in natural samples are 30-45 μ m, consistent over all four domes. It is important to note that unlike any other studied dome, these samples exhibit a lack of variable rim thicknesses in any one section, indicating little to no magma mixing in the conduit. Plagioclase phenocryst rim widths, defined by a compositional change, are 10-30 μ m. Textural analysis of the groundmass indicates that crystallization of plagioclase microlites accounts for ~57 volume%, with little variation between samples. CSD plots are approximately log-linear over the size range 1-250 μ m. The characteristic microphenocryst size, volumetric number density, and growth rate are 33μ m, 1.5E-5, ~1.3E-5, respectively. Isothermal, constant rate decompression (P1=200 MPa, P2=2 MPa, dP/dt=6 MPa/day) experiments were run to determine the conditions necessary to produce the rims measured in the natural samples. The starting material was a partially crushed Black Butte rock which was brought to conditions believed to have existed in the magma storage region prior to ascent (870° C, 200 MPa) and held for 48 hours to remelt and homogenize the holocrystalline groundmass. 30 day decompressions were found to best replicate the plagioclase rim widths and compositions. These runs also produced comparable amphibole rim widths, although previously determined phase equilibria indicates these were only formed over the final 17 days when the pressure was below 100 MPa. Preliminary analysis of groundmass microlite crystallization indicates a compositional match with the phenocryst rims discussed above. The uniform amphibole rim width throughout all Black Butte samples is consistent with magma ascent from the storage region in a similar way throughout the eruption. Plagioclase phenocrysts rim widths are also remarkably similar from crystal to crystal again implying a similar ascent rate throughout the eruption. Any long pause in the ascent would be recorded as a leveling off of the An decrease, rather than the observed monotonic decline. Experimental results suggest the magma experienced a steady ascent rate, rising from ~100 MPa to the surface in ~17 days. Information on ascent from deeper in the storage region can be gleaned from study of the plagioclase phenocryst and microlite growth records. Use of these three techniques in concert allows for deduction of a longer ascent history since plagioclase growth records more of the ascent than amphibole breakdown.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.V21B0974M
- Keywords:
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- 8439 Physics and chemistry of magma bodies