Continental margin magmatic arcs (such as the Andes or the Cascades) are the most spectacularly complex processing zones in the solar system. It is common for a planet to make basalt but it is quite difficult to make granite. Continental margin arcs are the much-studied expression of the wholesale differentiation of our planet, yet first-order characteristics of these systems remain the subject of fierce debate. The magmas that these systems produce commonly include melt (sometimes multiple immiscible liquids), phase populations that change in a dynamic feedback, and a smorgasbord of volatiles, both dissolved and exsolved. The physical and chemical parameters of magmas, such as viscosity and volatile fugacity, can vary over many orders of magnitude during the life-cycle of a single system. Even temperature and pressure are always moving targets. I am drawn to complex problems with a variation of scales; I like to think about weaving these scales together into a rich, orders-of-magnitude type model of how the world works. It is a great paradox that in order to understand continental-scale processes, you have to investigate at the scale of individual mineral crystals. I work at scales from microns to hundreds of kilometers.

Exhumed portions of ancient magmatic systems provide snapshots of variation through tectonic environment, host crust and time (to name a few) that can be compared to elucidate shared characteristics fundamental to continental arc magmatism. Increase in complexity in the crust through geologic time necessitates an understanding of the nature of the pre-existing crust into which an arc is built and the cyclical nature of deformation and magmatism at the margins of continental land masses. My strategy is to work in differentially exhumed extinct magmatic systems, or “tilted arc-crustal sections.” This has taken me to fields from the steppes of the Gobi desert to the vineyards of northern Italy. I am currently focusing on micro-analysis of trace element and radiogenic element-rich magmatic accessory phases by secondary ionization mass spectrometry (SIMS) and I employ supporting analytical techniques, including thermal ionization mass spectrometry, electron probe, gas-source mass spectrometry and scanning electron microscopy.

My active research addresses three inter-related topics: 1. Understanding the connections between different crustal levels in magmatic arcs, 2. Characterizing signals from accessory minerals & their implications for geochronology, and 3. Investigating volatile & redox budgets of magmatic systems.

Transverse Ranges/Joshua Tree National Park, Southern California

Mid crust to deep crust: influence of lower crustal dynamics on continental magmatism during the life-cycle of an arc: The Transverse Ranges tilted crustal section is a segment of the Mesozoic Cordilleran arc exhumed to mid-crustal levels, revealing compositionally and structurally variable crust, including a mid-crustal sheeted complex that is likely analogous to the mid-crustal low velocity zone in the modern Andean arc. Plutons in this crustal section also preserve copious inherited zircons from the arc lower crust. Work combining zircon geochronology and trace element geochemistry with whole-rock geochemistry revealed that the lower crust of this arc was the main driver for variations in magmatic pulse intensity, composition and upper crustal structure (Economos et al., in review, JPet). Batholiths in this region were likely generated by the introduction of fertile crustal material into the magma source zone by back-arc shortening.
Some inherited zircons also reveal the trace element signature of co-crystallization with garnet, which is a major driver in dynamic arc processes such as delamination due to its large negative ΔV of formation. I am currently using SIMS to study trace element characteristics of zircon in a Mojave lower crustal xenolith population and in garnet bearing rocks from deep crustal exposures to explore the conditions of garnet formation in the lower crust and its dynamic and chronologic relation to Cretaceous magmatism.

Trace element budgets in crystallizing magmas: Trace element measurements in accessory phases are leading to a leap in understanding of magmatic conditions and processes similar to that precipitated by electron probe for major phases. This work is just beginning to clarify some of the fundamental magmatic processes occurring during crystallization. Large datasets of in-situ zircon, apatite and titanite trace elements reveal characteristic magmatic trajectories that indicate fractionation, mixing, thermal perturbations, and overall tectono-magmatic environment (Barth et al. in press, Geology).

Redox budgets in magma systems – Variations in fO2 in magmas control the speciation of sulfur, which can in turn cause sulfur isotope fractionation in S bearing phases. The mineral apatite contains S in concentrations up to 2500 ppm and preserves oscillatory magmatic zoning in S concentration. By utilizing the unique spatial resolution of SIMS, I have found significant S isotopic fractionation in core-rim transects of these apatite crystals (Economos, 2012 AGU abstract). The results of this work have implications for debates about the conditions of crystallization in melt generation zones of continental margin arcs and whether magmatic processes can account for the fO2 variations between arc magmas and MORBs.

Ivrea Zone, Sesia Valley, Italy

Volatile budgets in bimodal volcanic systems– A recently funded National Geographic proposal will initiate several years of planned work in the Permian “Sesia Valley supervolcano,” a Yellowstone-type volcanic system that sat atop the famous Ivrea crustal section in northern Italy. A multi-institutional collaboration of American and Italian workers will investigate volatile budgets in ignimbrites and sub-volcanic plutons as well as mafic underplates through the study of zircon O isotopes and trace elements by SIMS.

Quick, et al., 2009

Gobi-Tienshan Intrusive Complex, Southwestern Mongolia

The Gobi-Tienshan intrusive complex (GTIC) was identified by workers from the Czech Geological Survey as a possible tilted section through a continental margin arc during their 1:200,000 scale mapping of the area. The result of three field expeditions to this remote field area was the fundamental characterization of tectonic and magmatic environment. Our 1:100,000 and 1:25,000 scale mapping, Al-in-Hbl barometric analysis and geochronological work has supported this hypothesis, and revealed a 15 degree tilt of the complex. The GTIC was found to be 300-292 Ma, therefore likely generated by a single phase of arc-building. Deepest exposures (10-13 km) are very complex mingling zones with widely variable compositions. Mid-level exposures (10-4 km) are larger, more homogeneous plutons with organized sheeting of granites through large granodiorite plutons, and shallow plutons (4-0 km) that are porphyritic with groundmasses that grow finer grained as they approach the volcanic contact. A volcanic pile at least 1 paleo-km thick is preserved. Economos et al., 2012

Initial chemistry has revealed that this calc-alkaline, med- to high-K complex has variable source material, including some crustal material about 100 my older that the intrusion. In keeping with most magmatism in southern Mongolia, the complex is dominated by manlte-like isotopic signatures.

Preliminary work on the compositional and chronological relationship between the volcanic section and underlying plutons reveals a system that underwent cyclical thermal events, likely associated with the injection of mafic magmas.

Chandman Massif, Altai Range, Central Mongolia

The Chandman Massif is a Carboniferous pluton in the central Altai range in Mongolia. At the junction of three hypothesized terranes, the age and nature of magmatism in the Chandman massif is important to the first order tectonic interpretation of central Mongolia. Mapping revealed that this area experienced amphibolite facies metamorphism of the Chandman Khayrkhan crystalline complex (intruded by the Chandman Massif), exhumation to mid-crustal levels, juxtaposition against the greenschist facies metasedimentary formation, intrusion of the Chandman Massif under tectonic strains that continued through the solidification of plutons, and late block-style rotation related to motion on recent faults. Age and geological constraints identify the Chandman Massif as an intrusion of substantially younger age than the “Caledonian” association into which it was previously placed. It is thus far the only arc-type intrusion in the earliest “Hercynian” age range identified in the Gobi-Altay Terrane. Its metamorphic and magmatic history of migmatization followed by intrusion of metaluminous and peraluminous plutons are similar to those of rocks to the west, in the Tseel Terrane, and may be its easternmost counterpart. Economos et al., 2008

Half Dome Lobe, Tuolumne Intrusive Suite, Central Sierra Nevada, CA

The southern Half Dome lobe is a 2 km x 5km lobe that extends from the southern margin of the (in)famous Tuolumne Intrusive Suite in the Central Sierra Nevada. Our mapping revealed that this lobe is normally zoned with gradational transitions between units, and major oxide geochemistry revealed that it spans an SiO2 variation as large as that of the whole batholith. Trace element trends of Rb, Sr, Ba, and REE patterns were all suggestive of fractionation, and initial Sr and Nd isotopic values were invariant across the lobe. These patterns are consistent with fractionation and difficult to explain by mixing models. The cause of extreme SiO2 variation over a very short distance remains a mystery. Considering the unprecidented level of detail of our geochemical understanding of the Tuolumne Intrusive Suite, further petrologic modeling of data from the Half Dome lobe and the main batholith is needed to uniquely differentiate between the predicted compositional variation from mixing and fractionation processes. Economos et al., 2010