Download Chemical geodynamics of helium.

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. JACKSON1
1Department
of Earth Science, University of California, Santa Barbara, USA
([email protected])
Chemical Geodynamics of
3He, 129Xe, 182W, 142Nd, etc.
M.G. JACKSON1, S. MUKHOPADHYAY2
1Department
2Department
of Earth Science, University of California, Santa Barbara, USA
([email protected])
of Earth and Planetary Sciences, University of California–Davis, Davis, CA, USA
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
~20 km3 of oceanic crust subducted
annually (~5% of mantle’s mass in 3 Ga)!
Hofmann, Nature,1997
~1 km3 of sediment subducted annually
(mass of Africa + S. America in 3 Ga)!
The mantle
“zoo”: Origin of
the species?
Why all the
acronyms?
FOZO
Willbold and Stracke, 2002
Origin of the species
• 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)
Rizo et al., 2012
•
• All Hadean-generated 142Nd/144Nd heterogeneities
disappeared by 2.7 Ga (Debaille et al., 2013).
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!
129Xe (t1/2=15.7 Ma)
Mukhopadhyay, 2012
129I
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?
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.