Lec 3 Major and Trace Element Geochemistry（Lec 3主要和微量元素地球化学）.pdf

Major and Trace Element Geochemistry Just as seismology is an important tool to image the earth’s interior, so too are chemical and isotopic compositions of igneous rocks that originate at great depths with the upper mantle and lower crust. • Importance of chemical compositions of igneous rocks – Petrogenesis of primary magmas • these reflect mineralogy and chemistry of the source rock – Differentiation of magmas • need to decipher shallow processes to infer deep source – Radiogenic isotopes • allow a time-integrated view of changing composition Major and Trace Element Geochemistry • Major elements – Comprise most of the rock – Expressed as weight (wt.) % oxides, each 0.1% – Analyzed by XRF, ICP-MS • Trace elements – Present in concentrations 0.1% – Expressed in ppm or ppb – Analyzed by XRF, ICP-MS, INAA • Volatile elements – H O, CO , SO 2 2 4 – Rare gases: He, Ar, Ne, etc. – Analyzed by spectroscopy or mass spectrometry • Radiogenic isotopes – Ratios of radiogenic to nonradiogenic isotopes of an element • recall isotopes of an element have same atomic no., but variable # of neutrons – Variations in ratios reflect differences produced over time by radioactive decay in source – Variations are extremely small: analyzed by magnetic sector mass spectrometry • Stable Isotopes – Lighter masses fractionated by geological processes – Analyzed by magnetic sector mass spectrometry Major and Trace Element Geochemistry • Variation Diagrams – Plot chemical differences and trends among related rocks (lavas = magmas?) • Only true for liquids (aphyric lavas and tephras) • Can define and help model products of partial melting and crys