Application of in situ analytical techniques in the resolution of petrological problems associated to plutonic and vulcanic environments

• Grant number: 09/07307-9
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: August 01, 2009
• End date: July 31, 2010
• Field of knowledge: Physical Sciences and Mathematics - Geosciences - Geology
• Principal Investigator: Valdecir de Assis Janasi
• Grantee: Adriana Alves
• Host Institution: Instituto de Geociências (IGC). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

The chemical variations recorded in single crystals have been used as an important tool in igneous petrology, since they can monitor the pathways of magma crystallization. Crystals from different magmatic systems are alos different in terms of their isotopic composition and are thus reliable sensors of the compositions of their sources and of the processes involved in the generation and evolution of their parent melts and, in the case of xenocrysts, also of the host magmas. Recent advances in microanalytical techniques, allowing the analysis of isotopes and trace elements with a spatial resolution of 1 micrometer, have led to the precise identification and to the quantitative modelling of these processes. A fundamental requirement for this approach is the careful textural control of the sampled material, with the determination of the temporal succession of events recorded in the crystal; because of this, great advances have been achieved in recent years in petrographic studies, using, besides the convencional methodologies of optical mineralogy, also imaging by electron microscopy and cathodoluminescence.The crystal isotope stratigraphy in feldspar crystals, especially plagioclase (early forming mineral in many basaltic and granitic systems), has gained importance in petrogenetic studies since the end of the 1990s, and the methodology is currently being applied to a wide spectrum of problems such as the identification of open system processes (e.g., magma mixing; contamination) in magma chambers with varied compositions and evolutive histories.This project will be mostly developped at the Instituto de Geociencias, Universidade de São Paulo, where analytical protocols by LA-Q-ICPMS were recently put into routine. It will focus on the combined use of chemical zoning and crystal isotope stratigraphy (Sr-Sr and Pb-Pb isotopy)in plagiclase and alkali feldspar crystals aiming to solve two petrogenetic problems in different environments, vulcanic (diabase dikes of the Paraná-Etendeka Magmatic Province) and plutonic (Salto rapakivi Granite). The crystals will be used to identify: (1) processes responsible for the generation of enclaves with varied composition (basic and acid) and desequilibrium structures in granitic systems, and (2)contamination processes in basalt systems, and their consequences for the petrogenesis and dating of these rocks.