Does deformation lead to misinformation? How much can granitic rocks deform before accessory minerals are geochemically disturbed?

Washington State University, Pullman WA

Investigators

Abstract

How Earth’s early crust formed and evolved is one of the most debated questions in Earth Science. A major problem is that all of these ancient rocks have experienced at least one episode of alteration, deformation and/or metamorphism. So no pristine rocks (protoliths) exist. However, many studies have used whole rock geochemistry and isotope tracers, and more recently the isotope signatures of accessory phases as important anchor points for models of how the crust has evolved and grown over time. Various theories and processes have been proposed for the formation of Earth's earliest stable crust. The debate over interpreting these data revolves around a core inquiry: whether the deformation and metamorphism of crustal rocks alters certain geochemical markers commonly used in reconstructing crustal history. The primary goal of this study is to test the hypothesis that certain elements and isotopes in deformed (ancient) rocks accurately reflect the original geochemical signatures of their protolith. This hypothesis will be tested by evaluating if the isotope signatures of whole rock and accessory minerals remain unchanged with increasing strain, and hence faithfully record the original protolithic (isotope) composition, or if deformation leads to the disturbance of isotope systems on a mineral and/or whole rock scale. Three different sequences of rocks, varying in degrees of deformation, will be studied to understand the extent to which deformation may alter their geochemical signatures on a whole rock, and (sub)-mineral-scale. These sequences range from initially undeformed (protolith) rocks to highly deformed ones, which closely resemble the world's oldest rocks. The findings of this study will help researchers use and interpret geochemical, particularly isotopic, data from deformed ancient rocks. This research will support student training, international collaboration, and continued development of analytical facilities at the Peter Hooper GeoAnalytical Lab and the Radiogenic Isotope & Geochronology Lab at Washington State University. Two PhD students (one female), as well as undergraduate students will be trained in field work and cross-disciplinary research spanning from mineralogy to structural geology to geochemistry. Isotope signatures in whole rock samples and in some accessory minerals, such as zircon, apatite, allanite, and titanite, are frequently used to reconstruct the formation and evolution of the Earth’s crust. However, virtually all Archean and Proterozoic rocks have experienced one or more episodes of deformation and metamorphism following their emplacement, and it is unclear to what degree these rocks retain their original (i.e., protolithic) isotope ratios after experiencing these tectono-thermal events. This research combines a geochemical and structural approach to understand to what degree granitic rocks can be deformed without changing their protolithic geochemical fingerprint at a whole rock- and (sub-)mineral-scale. This project will focus on testing a primary hypothesis: that whole rocks and accessory minerals in variably deformed granitic rocks faithfully retain their original geochemical isotope signatures and hence can be used to reconstruct long-term crustal processes, such as the formation of Earth’s earliest stable crust. The following key questions will be investigated: 1) what accessory minerals are involved in deformation-induced mineral reactions? 2) does deformation lead to open-system processes, and if so, how does this vary with degree of deformation? and 3) to what extent do Lu-Hf, Sm-Nd, and Rb-Sr isotope systematics in accessory minerals of variably deformed rocks faithfully preserve their initial isotope compositions? Three different field localities in which the same granitic bodies are exposed across strain gradients, spanning from undeformed to highly deformed will be studied. Via comparison of isotope data of the above systems on an accessory mineral- and whole rock-scale between deformed granitic rocks and their protoliths can be tested if deformed rocks retain their original isotope signatures on different scales (mineral vs. whole rock) or if the above isotope systems become disturbed upon deformation and hence cannot be trusted to reflect the protolithic composition. High-spatial resolution in-situ isotope and elemental analyses via EPMA and LA-(MC)-ICP-MS will be conducted on all major and accessory phases in order to gain an in-depth understanding of how minerals geochemically communicate with each other during deformation and if and to what degree trace elements are mobilized and redistributed on different scales. The results of this study will offer key understandings into how geochemical and particularly isotopic information from deformed and metamorphosed rocks can be interpreted and applied to reconstruct large-scale crustal processes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.