Thesis Defense: Michael Lara, Ph.D. Candidate
Student: Michael Lara
Department: Earth, Environmental and Planetary Sciences
Defense Date: June 20th, 2022
Time: 1:30 p.m.
Location: KWGL 123
Effects of slab-derived hydrous silicate melts and H2O-CO2 fluids on mantle wedge melting
In the canonical model of subduction zone magmatism, H2O derived from subducting oceanic lithosphere fluxes the mantle above, significantly decreases the peridotite solidus, and induces partial melting in the mantle wedge. While H2O undoubtably plays a major role in melt generation in subduction zones, many lines of evidence suggest that natural slab-derived fluxing agents are not pure H2O, but rather hydrous fluids or melts with potentially high concentrations of dissolved silicates and/or CO2. Thus, the aim of this thesis is to explore the melting systematics mantle wedge peridotite which has been fluxed by a MORB-derived hydrous silicate melt (Chapter 2) and H2O-CO2 fluids (Chapters 3&4) by use of high pressure-temperature experiments.
Reaction between depleted peridotite and MORB-derived hydrous silicate melt at T < 1150 °C produces low degree melts which are rich in SiO2 and Al2O3, poor in FeO*- and MgO-and resemble natural high Mg# andesites (Chapter 2). Melting reactions during low-degree, H2O-saturated melting are incongruent, consuming opx and producing olivine + SiO2-rich melts. As extent of melting increases and the free fluid phase is consumed, a spectrum of basaltic andesites to basanites are produced. Comparison of experimental partial melts from this and other hydrous peridotite melting studies with natural primitive arc magmas suggests that melting of peridotites with varying bulk compositions but with 2.5 – 4.2 wt.% H2O can reproduce the major oxide spread and trends of primitive arc magmas globally. In contrast to melts produced from nominally CO2-free hydrous peridotite systems, reaction between peridotite and H2O-CO2 mixed fluids with XCO2 [= molar CO2 / (CO2 + H2O)] > 0.10 produces melts at 1200-1350 °C which are poorer in SiO2 and richer in CaO than all primitive arc melts found globally, suggesting an upper XCO2 limit ~ 0.10 in fluids inducing melting in mantle wedges (Chapters 3&4). A new subduction zone mass balance model which applies this experimentally derived upper XCO2 limit suggests that at least 34-86% of CO2 entering subduction zones bypasses the sub-arc melt generation zone and is subducting to the convicting mantle.