Document Type

Thesis

Date of Degree Completion

Fall 2019

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Wendy Bohrson

Second Committee Member

Gerhard Wörner

Third Committee Member

Chris Mattinson

Abstract

The Magma Chamber Simulator (MCS; Bohrson et al., 2014) was used to produce new thermodynamic models that constrain and quantify recharge and fractional crystallization (RFC) processes for the Old Cone (OC; 52-20 ka; 56-66 wt.% SiO2) and Healing Flows (HF; >8 ka-recent; 58-64 wt.% SiO2) eruptive stages of Parinacota volcano, northern Chile. Using the MCS, over 200 RFC models were produced that constrain host magma initial conditions, including pressure, fO2 and H2O, and host and recharge magma masses, temperatures and compositions. A Rhyolite Domes (RD; 47-40 ka; 74 wt.% SiO2) lava serves as the silicic host magma in model runs for the subsequent OC stage, whereas the most silicic HF sample serves as the HF’s starting composition. Lavas from the youngest eruptive stage, the Ajata Flows (10-3 ka, Upper Ajata; 53 wt.% SiO2; Lower Ajata; 57 wt.% SiO2), represent mafic recharge into the system. Best-fit MCS results, which were determined by direct comparison of model results with observed major and trace element and 87Sr/86Sr data of whole-rocks and mineral phases, suggest a magma storage zone at relatively shallow depths (~4 to 11 km) and that both the host and recharge magmas had low initial H2O (<1.5 wt. %). Modeling also indicates that OC and HF magma formation was dominated by recharge (up to ~5:1 recharge to host magma ratio for the OC and 2:1 ratio for the Healing Flows) with little to no crystal removal between recharge events and near liquidus recharge temperatures during the time of mixing, consistent with the crystal-poor nature of samples from the Lower and Upper Ajata stages. Thus, mixing to eruption timescales are likely short because significant magma cooling is precluded. Because magma from the OC and HF stages do not systematically become more mafic through time, it is interpreted that the magma storage zone consists of a complex of dikes and sills, where each magma batch receives a variable proportion of recharge. Assuming that all melt was erupted following mixing and that minimal fractional crystallization occurred, MCS results yield intrusion rate estimates of ≤0.7 km3/ka for the OC stage and ≤1.4 km3/ka for the HF stage. These results show an increase in intrusion rate through time, which likely correlates to the observed increase in eruption rate at Parinacota (Hora et al., 2007). Modeling studies like this one that constrain recharge masses and rates provide insight into how recharge affects the behavior (e.g., eruption rate and recharge to eruption timescales) of a volcano, and this insight, in turn, may inform hazard mitigation for volcanoes those pose risks to nearby communities.

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