Nature vs. Nurture: Does H2O, fO2, or the parental composition determine calc-alkaline vs. tholeiitic trends?

Tuesday, 15 December 2020: 15:30
Bradley William Pitcher, Oregon State University, CEOAS, Corvallis, OR, United States, Guilherme A R Gualda, Vanderbilt University, Department of Earth and Environmental Sciences, Nashville, TN, United States and Mark S Ghiorso, OFM Research, Redmond, CA, United States
Recorded Presentation
Abstract:
Many volcanic arcs and individual volcanoes erupt both calc-alkaline (CA) and tholeiitic (TH) magmas. Although experiments suggest a correlation between high H2O and CA differentiation, the relative importance of oxidizing vs. hydrous conditions for CA differentiation has not been established. Furthermore, the time-consuming nature of experiments inhibits the investigation of the sensitivity of liquid lines of descent (LLDs) to small changes in magmatic conditions (H2O, fO2, P), over a wide range of values, for a variety of initial parent compositions.

We took a Monte Carlo approach, using MELTS to model LLDs of seven parental compositions from four magmatic systems (Thingmuli, Iceland; Newberry, Oregon; Seguam and Augustine, Alaska). For each parental composition, we completed ~2,000 MELTS runs, each with a different constrained fO2 (FMQ-1 to FMQ+3), initial H2O (0.1-6 wt. %), and P (100-500 MPa, isobaric). We created a new tholeiitic index (THI) to quantify the degree of Fe-enrichment of each LLD. This allows us to plot, for each P and parent composition, a 3-D surface of THI as a function of H2O and fO2 and to assess, with unprecedented precision, the effect of various intensive parameters on the differentiation of magmas.

In general, under oxidizing conditions (FMQ > 0.5), magma will evolve along a CA LLD (THI<1), regardless of H2O (>0.5 wt. %) or parental composition. However, we find that Fe-rich (TH-like) parent magmas will evolve along a more Fe-enriched TH trend than CA-like parents at low H2O and fO2, and along a more Fe-depleted CA trend than CA-like parents at moderate fO2 (FMQ+1 to FMQ+2). In addition, more Al-saturated parents (higher ASI) can evolve along TH LLD at higher H2O (2 wt. % vs. 0.5 wt. %). Thus, our results suggest that although the production of both CA and TH magmas at a single volcano does not require a heterogeneous source, differing parents could evolve along vastly different LLDs given the exact same subvolcanic storage conditions.

<< Previous Abstract | Next Abstract >>