In situ Rb-Sr mica petrochronology: a transformative approach to characterizing tectonic processes

The radiogenic decay of 87Rb to 87Sr is an important “clock” that geologists use to measure the age of rocks and minerals. However, these measurements have traditionally required time-consuming lab work on pooled mineral separates. Several technical advances have recently alleviated this problem by enabling in-place (in situ) Rb-Sr measurements, but there are still barriers to their widespread use. These include a lack of appropriate reference materials, as well as questions about what Rb-Sr dates record in different rocks. This proposal addresses these problems in two ways. First, the researchers will develop two Rb-Sr reference glasses to significantly expand the use and application of Rb-Sr dating. They will also determine Rb-Sr dates from three well-studied rock suites, to provide key context for similar data elsewhere. This technique is novel because it allows us to accurately date mica, one of the most common minerals in Earth's crust. It also facilitates the measurement of rock Rb-Sr dates in less than an hour, far shorter than the traditional method (weeks to months). This project is led by middle and early career scientists and supports the professional development of two undergraduate and two graduate students. It additionally promotes scientific collaboration between two rural institutions in the northeastern US. The scientific goals of this proposal are two-fold. First, the researchers will produce two mica glasses (muscovite + biotite) and characterize them for major + trace elements and Rb-Sr isotopes. This includes high-precision isotope-dilution techniques as well as a community-wide, multi-instrument proficiency testing scheme (G-Probe). Their mica glass reference materials will be freely and widely disseminated, filling a critical analytical gap and spurring further development of the Rb-Sr technique in other labs, which will inevitably lead to other significant discoveries. As part of this effort, a data reduction scheme (DRS) for processing laser ablation Rb-Sr data will be further developed to include mass-shifted isotopes from multicollector instruments and downhole fractionation assessment. Second, they will determine in situ mica Rb-Sr dates by single- and multi-collector LA-ICP-MS/MS from three well-characterized locales representing endmember tectonic cases: the Mooselookmeguntic contact aureole, ME (USA); the Franciscan Subduction Complex, CA (USA); and the Ivrea-Verbano Zone, S. Alps (Italy). By performing campaign-style mica Rb-Sr geochronology in three tectonic case studies, within the context of other textural and geochemical data collected during this work, the researchers will test the behavior of the Rb-Sr system across a range of geologically relevant conditions. 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.