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Documenting the processes and timescales of magma formation and diversification and defining the locations, shapes, volumes, and phase states of magma storage and transport zones rely on data produced by novel analytical techniques and state-of-the-art experimental methods. Computational modeling effectively links these critical tools. The Magma Chamber Simulator (MCS) is an internally consistent thermodynamic open system model that uses experimental constraints from rhyolite MELTS (Gualda et al. 2012, Ghiorso & Gualda, 2015) to compute paths of open system magmas that evolve via processes including crystallization, magma mixing, cumulate/mush entrainment, and host-rock assimilation. MCS results yield elemental, isotope, mass, and thermal characteristics of melt ± crystals ± volatiles in "resident" magma, crustal wallrock (melt and solids), recharge magma, and entrained material. To model the petrochemical evolution of igneous rocks related by open-system processes, one typically runs 200+ models that vary initial compositions, pressures, and temperatures of magma, wallrock, etc. Comparison of model results with whole rock, mineral, and melt inclusion chemical data and other constraints (e.g., thermobarometry) yield interpretations about igneous processes at a range of scales—from how crust forms and evolves to processes responsible for in situ geochemical records of crystals—and allows assessment of epistemic and aleatoric uncertainties. Two examples of computational studies will illustrate MCS's utility and flexibility. (1) Modeling of historical basalts at Mt. Etna (Italy) provides evidence for variable degrees of melting of metasomatized mantle, followed by magma recharge and assimilation of partial melts of carbonate-flysch crust. (2) MCS models reproduce whole rock and mineral data of plagioclase-rich basalts at Steens Mountain (USA) through entrainment of gabbroic mush that likely formed in early stages of Columbia River Basalt magmatism. To enhance understanding of trans-lithospheric magma systems, future work on MCS will prioritize (i) building a post-processing environment that utilizes select statistical methods to inform "best-fit" models and to quantitatively assess uncertainty, and (ii) increasing modeling efficiency by adding automated modeling capabilities.
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Modulation of zircon solubility by crystal–melt dynamics
Zircon dating is commonly used to quantify timescales of magmatic processes, but our appreciation of the consequences of internal magma body dynamics lags behind ever-increasing analytical capabilities. In particular, it has been shown that crystal accumulation and melting of cumulates by recharge-delivered heat may affect melt chemistry within magma bodies. We considered the effect of such processes on zircon solubility in highly evolved silicate melts of diverse chemical affinities. Our modeling shows that in most cases cumulate melting perpetuates the zircon saturation behavior of the first melts emplaced at shallow storage levels. Once cumulate melting is established, the ease of saturating in zircon is controlled by cumulate mineralogy, with a particular effect of the amount of cumulate zircon and its availability for resorption. The fidelity of zircon as a recorder of magma system history thus depends on both the system’s chemical affinity and mineralogy, and the history itself.
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- Award ID(s):
- 1735512
- PAR ID:
- 10163285
- Date Published:
- Journal Name:
- Geology
- ISSN:
- 0091-7613
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Trace element concentrations and ratios in zircon provide important indicators of the petrological processes that operate in igneous and metamorphic systems. In granitoids, the compositions of zircon have been linked to the behaviour of garnet and plagioclase—pressure-sensitive minerals—in the source during partial melting. This has led to the proposal that Europium anomalies in detrital zircon are linked to the depth of crustal melting or magmatic differentiation and are a proxy for average crustal thickness. In addition to the mineral assemblage present during partial melting, Eu anomalies in zircon are also sensitive to redox conditions as well as magma evolution during extraction, ascent, and emplacement. Here we quantitatively model how rock type, mineral assemblages, redox changes, and reaction sequences influence Eu anomalies of zircon in equilibrium with silicate melt. Partial melting of metasedimentary rocks and metabasites yields felsic to intermediate melts with a large range of Eu anomalies, which do not correlate simply with pressure (i.e. depth) of melting. Europium anomalies of zircon associated with partial melting of metasedimentary rocks are most sensitive to temperature whereas Eu anomalies associated with metabasite melting are controlled by plagioclase proportion—a function of pressure, temperature, and rock composition—as well as changes in oxygen fugacity. Furthermore, magmatic crystallization of granitoids can increase or decrease Eu anomalies in zircon from those of the initial (anatectic) melt. Therefore, Eu anomalies in zircon should not be used as a proxy for the crustal thickness or depth of melting but can be used to track the complex processes of metamorphism, partial melting, and magmatic differentiation in modern and ancient systems. Secular changes of Eu/Eu* from the zircon archive may reflect a change in thermal gradients of crustal melting or an increase in the reworking of sedimentary rocks over time.more » « less
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null (Ed.)Abstract The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ18O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ18O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ18O magma [δ18O(WR) = 9.8‰] toward lower values is shown by high-δ18O zircon cores (7.8‰) and lower δ 18O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ18O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ18O in garnets from the western side of the pluton are consistently higher (avg = 7.3‰) relative to the west (avg = 5.9‰). These δ18O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high-δ18O magma partially melted low-δ18O wallrock and was subsequently contaminated near the current level of emplacement by higher δ18O melts. Collectively, the comparison of δ18O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small <100 km2 plutons that are intimately associated with metasedimentary wall rocks and often surrounded by later and larger metaluminous tonalite and granodiorite plutons. The general associations suggest that early-stage arc magmas sample crustal heterogeneities in small melt batches, but that with progressive invigoration of the arc, such compositions are more effectively blended with mantle melts in source regions. Thus, peraluminous magmas provide important details of the nascent Sierran arc and pre-batholithic crustal structure.more » « less
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Abstract Whether and how subduction increases the oxidation state of Earth's mantle are two of the most important unresolved questions in solid Earth geochemistry. Using data from the southern Cascade arc (California, USA), we show quantitatively for the first time that increases in arc magma oxidation state are fundamentally linked to mass transfer of isotopically heavy sulfate from the subducted plate into the mantle wedge. We investigate multiple hypotheses related to plate dehydration and melting and the rise and reaction of slab melts with mantle peridotite in the wedge, focusing on electron balance between redox-sensitive iron and sulfur during these processes. These results show that unless slab-derived silicic melts contain much higher dissolved sulfur than is indicated by currently available experimental data, arc magma generation by mantle wedge melting must involve multiple stages of mantle metasomatism by slab-derived oxidized and sulfur-bearing hydrous components.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G47405.1
