The Origin of Precious Metals on Earth

Tim Elliott

Bristol Isotope Group, School of Earth Sciences, University of Bristol

Precious Metals

PGEHighly siderophile elements (HSE)

Geochemical Reservoirs

Bulk Earth

(Chondritic)

Bulk Silicate Earth

Core

Highly Siderophile Element (HSE) Abundances

Walker 2009

Depletion of Siderophile Elements

Drake and Righter 02

Geochemical Reservoirs

Bulk Earth

(Chondritic)

Bulk Silicate Earth

Core

Dmetal-silicate

Most HSE compatible during mantle melting

Surprisingly Abundant Precious Metals

Drake and Righter 02

Wood et al 06

Deep, high temperature core formation?

Higher P experiments

Mann et al 11

Late Veneer

• Addition of chondritic material post core formation

• Requires ~0.5% by mass of Earth by such late meteorite bombardment

Siderophile element abundances

Drake and Righter 02

NASA imageFessler and Spudis LPI

Timing of Core Formation

Rudge et al 2010

‘Late’ Bombardment

Image from Elements Magazine

Late Veneer

• Material certainly added post core formation

• Need ~20:1 Earth:Moon accretion

• Can this amount of material be delivered without being added to core?

Late Veneer vs High T core

• Internal or external source of precious metals

• Sample Earth before Late Veneer?

The oldest part of the planet

Jack Hills,Yilgarn Block, W. Australia

Wilde et al 01

Zircon agesWilde et al 01

Archaean Greenland

Kamber et al 02

Istaq Gneisses, Greenland

Istaq Gneisses, Greenland

Kamber et al 02

Extinct 146Sm

142Nd/144Nd142Nd =

142Nd/144NdSample

Ames Nd-1 x10000

146Sm - 142Nd (t1/2 ~ 103 67Ma)

Oldest Terrestrial Rocks

Caro et al 2006

Isua, Greenland~3.8Ga

Precious Metal Content of Earliest Mantle?

• Need to see through melting, crystallisation and metamorphism

• Hard to constrain moden mantle Platinum Group Elements (PGE) contents even with samples of mantle

• Need a robust proxy for PGE• Isotopic tracer of terrestrial vs extra-terrestrial

PGE?

W proxy of Precious Metals

PGE

Extinct 182Hf

182W/184W182W =

182W/184WSample

NIST SRM 3163-1 x10000

182Hf – 182W (t1/2 ~ 8.9Ma)

Jacobsen 06

Earth’s Core(today)

Influence of Late Veneer on 182W

•Multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), Thermo Finnigan Neptune

W isotopic measurements

182W variability in modern mantle?

• Entrainment of core material into mantle via plumes may lower 182W

• High 186Os/188Os of Hawaiian plume attributed to core component (Walker and Brandon)

• Outer core should have low 182W ~-1.9 and much W

Jacobsen 06

Earth’s Core(today)

Coffin 2006

Large Igneous Provinces

Deep-seated plumes

Montelli et al 2006

Mantle 182W variability

Large Igneous Provinces

‘Deep rooted’ OIB

Istaq Gneisses, Greenland

3.8Ga Greenland(Isua)

182W

Modern

2Ga Granitoid

Mantle W isotope evolution

Influence of Late Veneer on 182W

Origin of Precious Metals

• Homogeneity of 182W of modern mantle leaves no room for precious metal deliver from core to mantle

• Contrast of 0.15 182W between modern and Isua rocks entirely compatible with Late Veneer

• Current inventory of precious metals delivered to Earth by meteorite bombardment post core formation

• 20x higher flux to Earth compared to Moon a lucky chance of the random nature of accretion (Botke et al 2011)