Collaborative Research: Use and Abuse of Zircon Thermometry - Integrating Modeling, Trace Element Chemistry and Isotopes to Maximize the Use, Limit the Abuse

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Zircon (ZrSiO4) is arguably the single most powerful mineral for elucidating the evolution of the continental crust. The proposal will take advantage of the ability of zircon to record the temperature history of magma generated in the crust. We will specifically target granites produced in the period of earth history at approximately 1.05 billion years ago (the Grenville orogenic event). Our prior research demonstrated that this is a unique episode for generation of large volumes of exceptionally hot (900-1100 oC) granitic magma. The fingerprint of these hot granites includes high zirconium contents and conspicuously high modal abundance of zircon that does not contain xenocrysts of prior zircon (i.e., the zircon is not recycled from other crustal rocks and represents a primary chemical/mineralogic feature of the granites). Preliminary chemical analysis of the zircon in the hot granites demonstrates that the zircon also has high titanium contents, which prove high zircon crystallization temperatures. Computer modeling of the crystallization history of the high zirconium granites also demonstrates that zircon begins crystallization at ~1000 oC. The important insight from these preliminary results regarding the evolution of the Earth?s crust is that there must be unique lithospheric conditions in order to generate such hot granites. The proposed research will target for study a much larger suite of granites from the Grenville Blue Ridge Province in eastern North America (Shenandoah massif, Virginia) where we have identified a terrane-wide occurrence of high zirconium granites. The purpose of the research is to more thoroughly elucidate the temperature history of the Grenville granite suite and examine in detail the chemical and textural relationship of zircon to the bulk rock chemistry and modal mineralogy, in order to more precisely ascertain the conditions at which the zircon crystallized. We will conduct whole rock geochemical analysis, crystallization modeling using the program rhyoliteMELTS, cathodolomuninescence imaging of individual zircon grains to explore for potential xenocrysts, and ion microprobe analysis of trace elements and isotopes in zircon. The research will be conducted by graduate and undergraduate students from the collaborating institutions, providing critical training for future geoscientists.