Faculty

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Economos

Rita Economos

Assistant Professor
Igneous Petrology
Structural Geology

214-768-3313
reconomos@smu.edu

Research Interests:

-         Volcanic plumbing systems

-         Structures and physio-chemical properties of magmas

-         Continental arcs and evolution of continental crust

-         Isotope geochemistry; novel high-temperature stable isotope proxies

-         Accessory phase geochronology

-         Interaction of tectonism and magmatism

 

Current research projects:

 

-         Sesia Valley, Italy

 

The ~285 million year old Sesia Valley volcanic complex is rooted in a large granite body that may represent what is left of the pre-eruptive magma chamber. Below this lies a complex region of thinned continental crust, similar to that found in the west-central United States in Nevada and Arizona. There is evidence that melting of this crust generated magmas that formed the granites and volcanic eruptions. A scientifically critical feature of the section is the preserved thick sequence of gabbros that ponded at the base of the crust and provided the thermal engine that drove the system (Quick et al., 2009; Sinigoi et al., 2011). Detailed mapping of the Sesia Valley caldera has just been completed and the lower crustal gabbros have been a topic of ongoing study for more than a decade. However, the intervening connections are ripe for study, from the top of the gabbros, through the partially melted crust, into granite plutons and to the base of the caldera. This study is likely to yield fundamental constraints on material and thermal transport in this system with implications for the behavior of modern analogs.

 

-         Joshua Tree National Park, California, USA

 

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. 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.

 

-         Gobi-Tianshan Magmatic Complex, Mongolia

 

The Gobi-Tianshan Intrusive Complex in southwestern Mongolia preserves a tilted arc-crustal section that is exceptional in its preservation of a large package of adjacent, coeval volcanic rocks, in addition to mid- and upper-crustal plutons. Future studies in this field area will address the mis-match in signals between the compositional and chronological ‘snapshot’ preserved by a volcanic eruption and the more integrated signals preserved by slowly cooling plutons. We will utilize the trace element signals preserved in zircons as a novel way to track magmatic thermal and geochemical evolution and the connection between plutonic components and volcanic outputs.

 

-         Sulfur isotopes in Apatite crystals

 

Oxygen fugacity is an important parameter for arc-scale magmatic processes, crustal ore mineralization processes, and the distribution of economically valuable mineral deposits over geologic time. Sulfur is uniquely sensitive to variations in fO2 due to its large valence state transition (from sulfide [2-] to sulfate [6+], which occurs over a range of fO2 that is particularly pertinent to magmatic systems (ΔFMQ -2 to +2). The common magmatic accessory mineral apatite can preserve sub-micron scale sulfur concentration zoning generated during crystallization, confirming that S ions are highly immobile in the apatite crystal structure even at magmatic temperatures. High S concentrations and the high ionization potential of S allow for high precision in-situ measurement of S isotopes by ion microprobe. We have identified within-grain variations in granitic apatite samples of up to 3‰, and variations of 6‰ between apatite grains fro­­­m a single hand sample. We interpret the results of these measurements as a record of both magma mixing and Rayleigh fractionation during magma degassing. Future projects utilizing this technique will focus on evolution of fO2, magma recharge and degassing in sub-volcanic magma chambers, and economic mineralization applications.

 

-         Trace elements in accessory magmatic phases

 

Trace elements (as opposed to rock-forming elements such as Si, Ca, Mg etc.) are classic petrogenic indicators; much of what we understand about magmatic processes arose from the study of trace element distributions in crustal rocks. These elements are largely concentrated in small, ubiquitous minerals called accessory phases (e.g. zircon, apatite, titanite). The overall trace element budget of these phases is an area of study with great potential returns and widespread applications, as accessory phases: 1) Survive weathering and melting processes and are therefore representatives of rocks that are inaccessible or destroyed, 2) Contain chronological information due to concentration of radionuclides, 3) Preserve information about elusive but important magmatic parameters such as fO2, redox budget, and H2O content, and 4) are commonly zoned due to crystallization from an evolving magma and thus preserve a record of changing magmatic parameters.

 

Other current research interests:

My specialty in analyzing stable isotopes by ion microprobe has allowed me to get involved in some fascinating projects. Analysis by ion probe requires only micro-sampling, so it is ideal for working on the most rare, precious samples such as Martian meteorites. I’ve participated in several projects to explore the evolution of the Martian mantle and crust.

 

Past research projects:

-         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.

 

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

 

The southern Half Dome lobe is a 2 km x 5 km 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 unprecedented 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. 

 

Courses:

 

-         Structural Geology

 

CV

 

Graduate student opportunities:

I always want to hear from prospective graduate students. If my research sounds interesting to you and you are considering pursuing an M.S. or Ph.D. degree, send me an e-mail!