Using Chemical Affinities to Understand Disequilibrium Textures of Plagioclase Preserved in Magmatic Systems. Issue 10 (19th May 2021)

• Record Type: Journal Article
• Authors: Huggins, Ellyn G.; Ruprecht, Philipp; Ghiorso, Mark S.
• Is Part Of: Geophysical research letters. Volume 48:Issue 10(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-19
• Subjects: disequilibrium -- MELTS -- microlites -- plagioclase; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL092884
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Volcanic textures are strongly related to magma ascent histories. Degassing of a volatile‐rich magma can drive plagioclase microlite growth on eruptive timescales, but such growth can be inhibited when ascent is sufficiently fast. Thus, the absence of microlites is used to determine the minimum ascent rates. However, open‐system processes, such as magma recharge, prior to decompression can also suppress microlite growth, possibly extending the timescales for ascent. Here, we explore the trade‐offs between degassing‐driven growth and reheating‐driven suppression of microlite formation. We develop a quantitative model on the basis of the chemical affinity that accounts for disequilibrium driving crystal growth or dissolution. We test decompression scenarios to understand how net‐crystallization varies with different pressure‐temperature ascent histories for the 2008 Chaitén eruption. Degassing‐driven growth and reheating‐driven dissolution are balanced for ascent rates of 0.002–0.006 MPa/s (∼0.08–0.25 m/s) from 200 MPa storage depth and reheating by 50 °C–70 °C, significantly reducing the minimum ascent rates. Plain Language Summary: Understanding how fast magma ascends during an eruption is critical for volcanic hazard mitigation. One way to determine the speed is derived from the minerals carried in the volcanic rocks. As magmas ascend to the surface, pressure decreases and gasses that were initially dissolved in the magma, such as H2 O or CO2, are released. This process can drive the growth of small crystals, known as microlites. The lack of microlites in the rocks that formed from explosive silicic eruption is commonly interpreted to represent a magma ascent so fast that there is insufficient time to form microlites. Alternatively, if the magma is disturbed prior to eruption by the addition of hotter magma (also known as recharge), microlite formation can also be suppressed. Thus, any model that uses the lack of microlites to determine magma speed needs to consider the effects of hot recharge. Here, we present a method that calculates the effect that hot recharge may have on the formation of microlites during an eruption. We present a range of alternative scenarios that can suppress microlites, many of those extending to much slower magmatic ascent speeds. Thus, these results may provide new insights on how magmas ascend during an eruption. Key Points: Magma ascent‐driven microlite growth can be suppressed by open system processes as an alternative to kinetic limitations Experimentally determined ascent rates from microlite‐free assemblages represent maximum ascent rates estimates Transient changes in the degree of disequilibrium can be calculated through the thermodynamic potential, the chemical affinity