Pressure-Temperature-Time Paths of Metamorphism

Abstract

Chronological information is indispensable for the detailed analyses of a temperature-pressure-time path of metamorphic rocks and for modeling of whole metamorphic processes from subduction to exhumation. The succession of prograde and retrograde metamorphic reactions should have been recorded by chemical zoning and reaction texture of metamorphic minerals including epidote group minerals and titanite. These minerals, containing substantial amounts of ThO₂ (up to 2.5 wt%) and UO₂ (up to 0.6 wt%), are useful for providing reliable chronological information on individual metamorphic reactions. CHIME (chemical Th-U-total Pb isochron method) is a nondestructive age determination method having high spatial resolution. Although the CHIME coupled with EPMA (electron probe micro-analyzer) is a powerful tool for subgrain dating of Th-and/or U-rich minerals, the EPMA is hard to analyze Pb in young epidote group minerals as well as titanite. The detection sensitivity of the micro-PIXE (Proton Induced X-ray Emission) analysis, a proton analogue of EPMA, is an order of magnitude higher than that of EPMA. Thus, the CHIME coupled with micro-PIXE will make it possible to obtain subgrain age data of epidote group minerals and titanite as young as 10Ma. Our analytical capabilities for the age determination are much greater than seemed possible ten years ago, they become certainly possible powerful tools for Earth Sciences as EPMA has been for the last twenty-five years.