A PETROLOGICAL AND NITROGEN ISOTOPE STUDY OF CRUSTAL RECYCLING THROUGH TIME

Barry Peter, La Jolla CA

Investigators

Abstract

Dr. Peter Barry has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at the University of Tennessee. His study will focus on petrology and volatile (He-N-Ne) isotope characteristics of mafic xenoliths from cratonic and ocean island settings. Subduction of oceanic crust has occurred continuously since the Archean, effectively recycling materials into the convecting mantle and enabling mass transfer between the surface, mantle, and continents. Stable (e.g., nitrogen) and noble gas (e.g., helium and neon) isotopes are powerful tracers of mantle processes, and sensitive indicators of recycling and crustal inputs. Selected xenolith samples from the Cook Islands and the Siberian Craton display strong ancillary evidence (e.g., oxygen isotope anomalies) suggesting they are derived from a recycled crustal protolith. We will target xenolith samples for He-N-Ne isotopes, which have previously been age dated using Re-Os and Pb-Pb methods and span a large (~3 billion years) age-range, to assess time-integrated mantle variations that are potentially attributed to recycling. In addition, microprobe (major elements) and LA-ICP-MS (minor and trace elements) analyses will be conducted on the sample suite. We use a combined geochemical and petrological approach, as isotopic interpretations are most meaningful when bracketed by strong petrological constraints. This study has broad impacts for the overall understanding of mantle evolution and crustal recycling throughout Earth history. A central focus will be on nitrogen and how it has been transported between Earth's reservoirs throughout geological time. Nitrogen is important because it is the principle gas (~78%) in Earth's modern-day atmosphere. However, surprisingly little is known about nitrogen characteristics of the Earth's mantle - particularly ancient recycled reservoirs. We aim to determine the nitrogen isotope characteristics of rare xenolith samples that show evidence of crustal recycling and span a large age-range. This feat will be accomplished by utilizing a newly developed nitrogen isotope extraction and purification system, recently constructed at Scripps Institution of Oceanography. Furthermore, this study will provide an excellent opportunity to engage both students and science educators, through outreach programs that introduce them to laboratory procedures and updating them on the progress of current projects and bringing cutting edge geoscience research into the classroom. Furthermore, undergraduate and graduate students at the University of Tennessee - Planetary Geosciences Institute, will be mentored and involved in this study. Findings will be presented at international science conferences as well as in seminars at the University of Tennessee.