Project

Discipline

Archean crust; Central Alps; Crustal fluids; Halogens; Plate tectonics; Volatiles; Apatite; Microanalysis of apatite; SIMS; Fluid tracers; Fluid cycle in the crust

Lead
Fluids (Flüssigkeiten & Gase) sind von grundlegender Bedeutung für die Entwicklung der Erdkruste. Sie haben einen direkten Einfluss auf deren Stabilität, Dichte und Zusammensetzung. Zudem kommt den Fluids eine wichtige Bedeutung bei der Bildung von Erzlagerstätten zu. Ein Problem besteht in der direkten Nachweisbarkeit der Fluids. Die Kenntnisse über den Kreislauf von Fluids in der Erdkruste ist jedoch grundlegend, um ihre Evolution rekonstruieren zu können und um Aussagen darüber zu treffen, wie und weshalb Anreicherungen von gewissen Elementen in hohen Konzentrationen erfolgen (z. B. Erzlagerstätte).
Lay summary
Inhalt und Ziel des Forschungsprojekts Das in der Erdkruste verbreitete Mineral Apatit hat die ungewöhnliche Fähigkeit, Spurenelemente wie Halogene (Cl, F, Br, I), S, C und H2O einzubauen. Dies sind Elemente die in Fluids transportiert werden. Verschiedene Konzentrationen von diesen Elementen sind charakteristisch für die Herkunft der Fluids. Das Ziel dieses Projekts ist es, neue mikroanalytische Methoden zu entwickeln, die es erlauben, diese Elemente in Apatit zu messen. Weiter wird mittels den neuen Methoden und einem besseren Verständnis über die Zusammensetzung von Fluids in der Erdkruste angestrebt, den geologischen Zeitpunkt zu bestimmen, wann Fluids mittels Subduktionszonen in die tiefere Erdkruste transportiert wurden. Dies würde konkrete Hinweise liefern, wann sich die moderne Plattentektonik entwickelt. Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts Der neue analytische Ansatz, Halogene und flüchtige Bestandteile in Apatit genau zu messen, um dann Rückschlüsse über die Herkunft von Fluids in der Erdkruste zu ziehen, wird wichtige Informationen generieren, um den Fluid-Kreislauf in der Erdkruste besser verstehen und quantifizieren zu können. Die neue analytische Fähigkeit wird danach genutzt, um verschiedene umstrittene geologische Probleme zu untersuchen und kontroverse Theorien, wie zum Beispiel der Beginn der modernen Plattentektonik, zu testen.

Direct link to Lay Summary

Last update: 17.05.2019

Active participation

This project will use microanalyses of halogens and volatiles in the accessory mineral apatite to constrain the nature and composition of fluids that assisted in the transformation of the Earth’s continental crust over time. Fluids are a fundamental agent in the evolution of the Earth’s crust, directly impacting its stability, composition, density, rheology, and triggering seismic activity. Mass transport in the crust via fluid flow is responsible for the concentration of trace elements, which can in some cases locally enrich the crust in economically important elements, such as Au. However, despite the importance of fluids in the Earth’s crust, many of the processes involving fluids are poorly understood, as fluids are fugitive and often only indirect evidence of their activity is preserved in the rock record. Halogen (Cl, Br, I, and F) and volatile (CO2, SO3, H2O) components of minerals are important records of past fluid properties, fluid origins, and fluid-rock geochemical exchange. Crustal halogen and volatile reservoirs are poorly constrained, yet play an important role for developing global halogen and volatile flux models. Such models are fundamental for our general understanding how different geochemical reservoirs behave and what their roles are for the evolution of the crust. Furthermore, monitoring halogen distribution and partitioning into fluids is crucial for understanding mass transfer capabilities of metamorphic fluids, formed, for example, during metamorphic devolatilization reactions. Currently though, the precise measurement of halogens and volatiles is difficult, particularly due to a lack of widely available, well characterized standard materials. Therefore, in order to understand more about the Earth’s crust through past fluid-rock interaction, an important goal of this project is to develop robust standards and to refine micro-analytical procedures for precisely quantifying fluid tracers in the mineral apatite. The primary technique will be Secondary Ionisation Mass Spectrometry (SIMS), but other potentially suitable micro-analytical techniques will also be explored. The mineral apatite will be targeted because it is ubiquitous in crustal rocks and is able to host the entire range of halogens as well as H2O, SO3, CO2, and other important trace elements (e.g., REEs). The geochemical inventory of apatite will be used for constraining fluid properties, to gain understanding of the volatile and halogen behaviour in the crust during metamorphism, and the formation of Earth’s early crust. For example, one component of this research will involve quantifying halogens and volatiles in apatite hosted by Archean igneous rocks from the Pilbara Craton, which were emplaced over a key time interval thought to involve a geodynamic transition to modern-style (subduction zone) plate tectonics. The quantification of halogens and volatiles in these rocks that may have formed in arc settings will test if subduction zone fluids were involved in their genesis and therefore provides a new test for the onset of modern plate-tectonics. Results of this study might permit halogens to become an important tracer for understanding in what geodynamic setting crust has formed.This novel approach, involving innovative measurements of halogens and volatiles preserved in apatite as a proxy for crustal fluids will be developed to trace crust formation via different tectonic processes (e.g., via the presence of subduction zone fluids); and also has potential for studying enigmatic fluid involved in a variety of orogenic ore deposits; and may even have application for extra-terrestrial processes.