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.
In 2012, I was awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at the University of Tennessee. The research focused on the unique geochemical features observed in mantle xenoliths, which are "foreign rock fragments" carried to the surface from deep within Earth’s mantle. Specifically, the study investigated petrological features and volatile (He-N-Ne) isotope characteristics in continental cratonic xenoliths, in an attempt to reconstruct how continents initially formed and the subsequent chemical evolution of the sub-continental lithospheric mantle (SCLM). Subduction of oceanic crust has occurred continuously since the Archean, transporting volatiles bound in surface material into the convecting mantle. This dynamic process enables mass transfer between the surface, mantle, and continents. Cratonic regions formed from the amalgamation of subducted material throughout Earth history, and thus can provide insight into past subduction processes. However, remarkably little is known about volatiles in the SCLM. 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. In our study, xenolith samples from the Siberian Craton were selected because they display strong ancillary evidence (e.g., oxygen isotope anomalies) for a recycled crustal protolith. We measured major and trace elements, and He-N-Ne isotopes in a suite of xenoliths samples, 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. The main thrust of our research focused on the geochemistry of the Siberian SCLM, due to its incredibly unique geochemical feautes. Major and trace element results were presented in the journal Lithos (Howarth et al., 2014). Manuscripts detailing the He and Re-Os systematics of the Siberian suite are currently under review in the same journal (Barry et al., 2014; Pernet-Fisher et al., 2014). In addition, a manuscript is now under review in Geochimica et Cosmochimica Acta (Day et al., 2014), which presents a global overview of He systematics in the SCLM. A fifth manuscript, dealing primarily with the N-Ne-Ar systematics of these samples is currently being prepared. Results from these studies show that Siberian peridotites are strongly enriched in volatile elements (e.g., He, Ne, N, Li and halogens), and isotopic evidence suggests a subduction-related source. For example, helium isotope and abundance characteristics from the oldest xenoliths cannot be explained by simple time-integrated radiogenic ingrowth models and instead require a significant metasomatic input, which we interpret to be crustal-derived fluids. In contrast, younger Siberian samples retain high 3He/4He, and show evidence that the Siberian SCLM has also been modified by (plume-derived) basaltic fluids. In addition to Siberia, we have also investigated eclogites and peridotites from other stable cratonic regions, including the Beni Boussera Peridotite Massif, Morocco; Diavik, Canada, Kilbourne Hole/San Carlos, Somerset Island, Canada; South Africa (Roberts Victor, Kao, Premier, Bultfontein), and China. Furthermore, we have investigated a number of non-cratonic SCLM localities including xenoliths from Etna, the western USA, Australia, Antarctica, Cameroon, and Europe. Furthermore, this study has provide an excellent opportunity to engage both students and science educators. Specifically, the GK-12 outreach program has enabled me to interact with local high school teachers and update them on the details of our research; bringing cutting edge geoscience research into the classroom. Furthermore, I was lucky enough to have the opportunity to mentor three different undergraduates during my tenure at the University of Tennessee - Planetary Geosciences Institute. These ambitious young students assisted in mineral separation and sample preparation and gained practical experience in a geoscience laboratory, which will help prepare them for graduate school in the future. In addition I helped co-teach several graduate level seminars (i.e., Mantle Petrology Seminar – 2012; Phase Equilibria Seminar – 2013; Thermodynamics Seminar – 2014) taught chiefly by Prof. Taylor. More broadly, the results from these studies were presented at four international conferences (i.e., Goldschmidt 2013; DINGUE 2013; AGU 2013; Goldschmidt 2014) and the most recent results will be presented in the fall of 2014 (GSA 2014; AGU 2014).
