20-EPN-007: Investigating mantle heterogeneity through high spatial resolution mineral Pb and Nd isotopic analyses
Virtual visit by George Cooper, Cardiff University, Wales (UK) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 22 February – 09 December 2022 (10 days remote access)
Report Summary: The traditional approach of measuring the isotopic compositions of mid-ocean ridge basalts (MORB) is problematic because MORB is homogenised prior to eruption, and therefore does not record the full heterogeneity of the mantle source. To overcome this problem, we developed low- concentration coupled Pb-Nd isotope analysis of minerals at high spatial resolution to assess the isotopic heterogeneity of melts delivered to Earth’s oceanic crust and hence that of the depleted upper mantle. We acquired small volume Pb and Nd isotope analyses from minerals in gabbroic cumulates from fast-spreading oceanic crust at Hess Deep using the Thermo Scientific TRITON Plus at the Vrije Universiteit in Amsterdam. We measured minerals from 27 samples (Nd from 25 cpx and 19 plag, Pb from 18 plag) covering the full stratigraphic depth (4350 m to 25 m) of the Hess Deep oceanic crust. Our study reveals that Pb isotopes from primitive plagioclase domains show greater heterogeneity than Nd isotopes from plagioclase and clinopyroxene, validating the new coupled Pb-Nd isotopic approach. The Pb data do not vary systematically with depth but do show a departure in 207Pb/204Pb away from the NHRL and across the main trend of East Pacific Rise MORB that may indicate cumulate-melt mixing throughout the crust or the involvement of an exotic mantle source.
Read the full scientific report, with kind permission from George Cooper.