CIDER impacts. -I started CIDER as student participant in 2006, when my thesis advisor (Hart) encouraged me to participate. CIDER was the most powerful experience of my graduate education. -Several publications emerged from interactions that I had at the CIDER workshop. One example: Raj and I were both students when we met at CIDER in 2006, and we have written 3 papers together. -First faculty position was directly a product of contacts I made at CIDER. -I had a post-doc participate in CIDER in 2014, and my students will be participants this summer. CIDER grandchildren? What about CIDER great-grandchildren?

Chemical Geodynamics of Helium M. G Chemical Geodynamics of Helium M.G. Jackson1 1Department of Earth Science, University of California, Santa Barbara, USA (jackson@geol.ucsb.edu)  

Chemical Geodynamics of 3He, 129Xe, 182W, 142Nd, etc. M.G. Jackson1, S. Mukhopadhyay2 1Department of Earth Science, University of California, Santa Barbara, USA (jackson@geol.ucsb.edu) 2Department of Earth and Planetary Sciences, University of California–Davis, Davis, CA, USA  

Rivers contribute > 85% of ocean floor sediment Generating a heterogeneous mantle Oceanic plates and sediment are injected into the mantle at subduction zones, returned to the surface in mantle upwellings (plumes?), and melted beneath hotspots. Wilson, 1989 Workman et al., (G-cubed, 2004 Rivers contribute > 85% of ocean floor sediment Hofmann, Nature,1997 Start the slide: Some remarkably enriched lavas just discovered in the Samoan hotspot give us some clues about 1. ONE PROCESS BY WHICH HETEROGENEITIES FORM and 2.) THE DYNAMICS OF THE MANTLE. ~20 km3 of oceanic crust subducted annually (~5% of mantle’s mass in 3 Ga)! ~1 km3 of sediment subducted annually (mass of Africa + S. America in 3 Ga)! “The geochemical signature of OIB originated in the upper mantle and crust through melting.” -Bill White, 2008

The mantle “zoo”: Origin of the species? Why all the acronyms? Willbold and Stracke, 2002 FOZO EM2: Recycled continental crust? “Metasomatism”? HIMU: Recycled oceanic crust? “Metasomatism”? EM1: A real “dog’s breakfast” of proposed origins: Pelagic sediment, lower continental crust, sub-continental lithosphere, “metasomatism”, etc., etc.

Big Problem: Survival of early-Earth reservoirs in a chaotic mantle? Davies, 2002 Brandenburg et al. (EPSL 2008)

Short-lived isotope system: 146Sm – 142Nd 146Sm 142Nd (t1/2=103 Ma) All Hadean-generated 142Nd/144Nd heterogeneities disappeared by 2.7 Ga (Debaille et al., 2013). Rizo et al., 2012

Oceanic peridotites sampling the convecting mantle Os-isotopic model ages in peridotites provide records of mantle depletion. Where are the Archean & Hadean depleted-mantle domains???? Lassiter et al., 2014

But there are heterogeneous, Hadean 129Xe/130Xe signatures in the modern mantle! 129I 129Xe (t1/2=15.7 Ma) Mukhopadhyay, 2012 PARADOX: 129Xe signatures preserved since the Hadean, but Hadean 142Nd and Hadean depleted mantle Os signatures have been “erased”.

…and there are heterogeneous, Hadean 182W/184W signatures in the modern mantle! 182Hf 182W (t1/2=8.9 Ma) Ontong Java Plateau (120 Ma) Baffin Is. (62 Ma) MORB Hawaii (Kilauea) Rizo et al. (Science, 2016) MAJOR PARADOX: 182W and 129Xe signatures preserved since the Hadean, but Hadean 142Nd and Hadean depleted mantle Os signatures “erased”.

Nd and Os are efficiently recycled back into the mantle, and recycled Nd and Os efficiently overprint Hadean 142Nd and Hadean depleted Os signatures. However, due to inefficient recycling of Xe and W, subduction does not completely overprint Hadean 129Xe and 182W signatures.

e (Fraction of element returned to mantle / fraction remaining in surface reservoirs)

Survival of early-Earth reservoirs in a chaotic mantle Survival of early-Earth reservoirs in a chaotic mantle? A role for recycling efficiency? The flux of an element back into the mantle must almost certainly influence whether early-formed isotopic heterogeneities of that element are preserved. Brandenburg et al. (EPSL 2008)

Looking back, and looking ahead Discovery of the survival of Hadean-formed heterogeneities in the Earth (129Xe, 182W) is an exciting development in geochemistry. How are early-formed heterogeneities preserved in the mantle for >4.5 Ga? 4 years ago I would have predicted these discoveries were impossible… we do not understand mantle mixing. Tackle this CIDER style: Geochemists: Clarify distribution of Hadean reservoirs, Need more 129Xe and 182W data on additional locations. Test “recycling efficiency” model by prospecting for Hadean signatures in other isotopic systems. Dynamicists: Evaluate relative roles of mixing/stirring versus recycling efficiency of different elements. Mineral physicists: Material properties (e.g., multi-phase rheology, viscosity and phase stability at ultra-high P) that influence mixing/stirring rates.