Wide Range of Post-mixing Cooling Timescales from Chaos Crags, Lassen Volcanic Center
Abstract
We use chemical zoning in olivine (Ol) to reconstruct timescales of pre-eruptive, post-mixing cooling at Chaos Crags (Lassen Volcanic Center, ca 1103 y). Our measurements test whether zoning was created by diffusion or crystallization. While both processes can create normal zoning of forsterite (Fo) content, we distinguish between them by comparing fast (Mg, Fe) and slow (Al, P, Ti) diffusing elements. Earlier sets of Ol composition profiles indicated very short timescales - on the order of seconds, which would mean that Ol grains were quenched upon contact with cooler magmas, and so would say nothing about the timescales of magma mixing. Low-Fo rims on Ol might instead represent rapid post-mixing crystallization due to increased undercooling. This distinction is crucial because timescales could be invalid if not truly obtained by post-crystallization diffusion within Ol, which allows for misinterpretations for a proximate eruption trigger. We measured 13 transects across 9 Ol grains (Fo57-81) derived from 4 different domes. While 6 transects seem viable to represent diffusion, the other 7 transects remain unclassified due to apparently cyclic behavior of P and low-Fo content. For our "viable" diffusion cases, we find concentrations of Al, P, and Ti appearing constant from core to rim despite spatially correlative changes in Fo content. Due to the expected slower diffusion rates of Al, P and Ti, we conclude that diffusion is possibly the dominant process that created the observed zoning in Fo content. In this case, we presume all components were homogenous upon mixing, but that the more rapidly diffusing Mg-Fe components reacted with a melt that had a much lower Mg# relative to the Mg# in which the Ol grains grew. Using DIPRA, Fo diffusion profiles return a wide range of timescales (1-865 days; median: 26 days). Given these results, it is difficult to say whether mafic recharge was the proximate eruption trigger but it is likely that a separate process caused Chaos Crags to reach eruption.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51F0124T
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 3651 Thermobarometry;
- MINERALOGY AND PETROLOGY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY