Download Isotope Geochemistry of the Continents

Chapter 7
Time Scale
Age of Oldest
Rocks: 4.0 Ga
2.5 Ga
NeoArchean
EoArchean
In the Beginning…
Stephen Hui Museum
Univ. of Hong Kong
Global distribution of Archean rocks in modern continents.
Known (red), suspected (pink). Areas with rocks or zircons older than
3.6 billion years are labeled by name.
Acasta Gneisses: Oldest
Known Rocks
Age of the Acasta gneiss, zircon dated at 4.03 Ga.
Photo: Echos of Earth; Sue
Baugh & Lynn Martinelli
Remnant of Hadean Crust
Nuvvaugittuq Amphibolites
146Sm/142Nd
Nuvvaugittuq
Isochron

O’Neil et al. (2008), O’Neil et al. (2012)
and Roth et al. (2013) reported 142Nd
deficits in rocks form the Nuvvaugittuq
greenstone belt.

In a subset of these rocks, the
cummingtonite amphibolites, 142Nd/144Nd
correlates with Sm/Nd forming a psuedoisochron, implying 146Sm/144Sm of
0.00108, which in turn implies rocks from
97 Ma after start of solar system (i.e., at
4.36 Ga).

147Sm-143Nd

Highly controversial, but most likely
explanation is the correlation is a mixing
line formed by assimilation of pre-existing
Hadean crust by Eoarchean magmas.

Regardless, provides evidence of Hadean
crust.
are ≤3.8 Ga.
isochrons and zircon ages
Possible microfossils and traces of
13C-poor organic carbon recently
reported by Dodd et al. in Nature
Jack Hills Zircons
Photo: Echos of Earth; Sue
Baugh & Lynn Martinelli
Jack Hills Zircons
 The Jack Hills zircons occur
in a 3 Ga meta-sedimentary
conglomerate, surround by
the Narryer gneisses (3.653.3 Ga).

Most zircons 3.6-3.8 Ga but
a few are >4Ga. All have
complex histories.
 Hf isotope ratios measured
on these zircons have
negative εHf(t), indicating
crustal source.
 Hf isotope evolution projects
back as far as 4.46 Ga
(depending on what Lu/Hf
you assume).
Other Ancient Zircons
 Some zircons in the
Acasta gneiss have cores
as old as 4.2 Ga (not
shown).
 Others project back
suggesting other areas of
Hadean crust.
182W

182W

182W
Anomalies
anomalies have now been
reported in a number of Archean
rocks.
anomalies initially interpreted
as material into which a late
accretionary ‘veneer’
(undifferentiated chondritic and
therefore siderophile element-rich)
had not been mixed in.
 However, no negative anomalies


(indicative of the veneer) have been
found.
Samples with +ive εW have normal
platinum-group element abundances.
It now seems this phenomenon,
although indicative of early Earth
heterogeneity, is not related to a late
accretionary veneer.
142Nd

Anomalies
142Nd,
both positive and
negative, anomalies have
been reported in a number of
Archean rocks.
 Indicative of early terrestrial
differentiation and formation of
Hadean crust.
 Anomalies disappear by the
end of the Archean.
 Debate about the nature of
mantle convection in the
Archean:
 Plate tectonics?
 Stagnant-lid tectonics?
Pb Isotope Evidence of Early
Earth Differentiation
 Data from Neoarchean
Abitibi Greenstone belt
of 2.7 Ga age plot near
a ‘paleo’-2.7 Ga
Geochron – suggesting
differentiation event in
very early Earth history.
Subsequent Growth of
Continents
 How do we date continents?
 Do radiometric ages
necessarily record formation
of continental crust, or simply
resetting of chronometers?
 Work of Hurley and Rand
(1969) based on Rb-Sr
dating underestimated age of
North America.
Rates of Continent Growth
 A: Hurley & Rand
‘69
 B. Allègre &
Rousseau ’84
 C: Kramers &
Tolstikhin ’97
 D: Taylor &
McLennan ’95
 E. Armstrong ‘81
Compilation of U-Pb zircon ages by Condie & Aster, 2010
Mechanisms of Crustal
Growth
 Subduction zone volcanism
 Accretion of island arcs
 Plume-related volcanism
 Flood basalts
 Accretion of Oceanic Plateaus
 Underplating
 Rift-related volcanism
Schematic cross section of convergent, collisional, and extensional plate boundaries
associated with supercontinent cycle showing estimated amounts (in km3 yr−1) of
continental addition (numbers in blue above Earth surface) and removal (numbers in red
below...
P.A. Cawood et al. Geological Society of America Bulletin
2012;125:14-32
©2013 by Geological Society of America
Super Continents &
Continental Growth
 Zircon age populations
appear to correlate with
supercontinent formation.
 Some (Condie,
Hawkesworth) argue this
measure preservation of
juvenile crust in orogens,
rather than variable
production rates.
Cawood (2013) et al
GSA Bull.
The volumes of magma generated (blue line), and their likely preservation potential (red line)
based on relations outlined in Figure 8, vary through the three stages associated with the
convergence, assembly, and breakup of a supercontinent.
P.A. Cawood et al. Geological Society of America Bulletin
2012;125:14-32
©2013 by Geological Society of America
(A) Distribution of Hf model ages in 1376 detrital and inherited zircons sampled worldwide,
from which O isotopes have been measured (from Dhuime et al., 2012, and references
therein).
Cawood et al. argue that the continental volume was 60-70% of the
present one by 3 Ga, but only 10% of the present crust is that old.
P.A. Cawood et al. Geological Society of America Bulletin
2012;125:14-32
©2013 by Geological Society of America
Nd Model Ages & Evolution of
Southwestern US
Vishnu Schist of Grand
Canyon
Photo: Echos of Earth; Sue
Baugh & Lynn Martinelli
Crustal Residence Times
 Bennett and DePaolo
recognized three
Precambrian provinces in the
southwestern US based on
depleted mantle model ages.
Actual rocks are often much
younger.
 Pattern suggests assembly
from inside out – but it’s a bit
more complicated.
Initial Epsilons
 Examining initial epsilon
values, its likely that many
magmas were mixtures of
pre-existing crust and
mantle-derived magmas.
 All three provinces probably
formed between 1.8 and 1.65
Ga, but incorporating
variable amounts of preexisting crust.
 Most recent magmatism
appears to simply remelt
much older crust.
Hf in Lachlan Fold Belt
zircons
 Hf and O isotopes in two
granite suites from SE
Australia suggest mixing
between sedimentary
protolith and mantle-derived
magma in one case and
lower crustal gabbros in the
other.
Age Spectra of Lachlan
Zircons
 Age spectra of the detrital zircons
from Lachlan Fold Belt
metasediments dominated by
peaks at 450–600 Ma and 0.9–1.2
Ga.
 Hf model ages of zircons much
older, with peaks ~1.7–1.9 and
~2.9–3.1 Ga. The most mantlelike O isotope ratios tend to have
the older ages.
 Zircons with the lower δ18O, and
older Hf model ages represent
reworking deeper crustal material
that had never interacted with
water at low temperatures.
Bottom line, crust may be older than zircon ages indicate.
There may have been more old crust than thought.
Isotopic Composition of the
Continental Crust
Approaches
 Broad Sampling
 Composites
 Sediments
 Shale Composites are hybrid
 Rivers
 Including their sediments
Sr & Nd Isotope Composition of
Rivers & Sediments
Lower Continental Crust
 Sediments & Rivers sample
only the upper most crust
 Doesn’t representatively
sample lower crust.
 Lower crust samples come
from two sources:
tectonically emplaced
terrains and xenoliths
Lower Crustal Terrains
Pb Isotopes of Lower Crustal
Terrains
Pb Isotopes in Lower Crustal
Xenoliths
Terrestrial Pb Mass Balance
Pb Isotope Constraints on
Crustal Th/U
Sm/Nd in the Continental
Crust
 Sm/Nd in the continental
crust shows a fairly restricted
distribution.

143Nd/144Nd
shows a wider
distribution – a result of
varying crustal age.
143Nd/144Nd
and Sm/Nd in the
Crust
Estimating Crustal Growth from Crustal
Residence Times of Sediments
 In principle, examining the
relationship between
stratigraphic and Nd crustal
residence times should tell us
whether the rate of continental
growth is increasing or
decreasing.
 Cannibalistic nature of the
sedimentary cycle (sediments
are more likely to be eroded
than other rocks) complicates
things (line B).
 Could be consistent with
Armstrong constant
continental mass with
decreasing recycling rate.
Isotope Systematics of Mixing
We need to understand this before turning to Subduction Zones
Mixing Plots with 2 Ratios
 A general mixing equation is:
æqö
æ p ö æ p öæ q ö
A ç ÷ + B ç ÷ç ÷ + C ç ÷ + D = 0
è P ø è P øè Q ø
èQ ø
 If end members are designated 1
and 2 and have ratios (q/Q )1 and
(p/P)1, and (q/Q )2 and (p/P)2
respectively, then
æqö
æqö
A = Q2 P1 ç ÷ -Q1P2 ç ÷
è Q ø2
è Q ø1
B = Q1P2 -Q2 P1
æ pö
æ pö
C = Q2 P1 ç ÷ -Q1P2 ç ÷
è P ø2
è P ø1
æ pö æ q ö
æ pö æ q ö
D = Q1P2 ç ÷ ç ÷ -Q2 P1 ç ÷ ç ÷
è P ø2 è Q ø2
è P ø1 è Q ø1
 The curvature of the mixing line
will depend on the ratio r:
r=
Q1P2 Q1 / Q2
=
Q2 P1 P1 / P2
Mixing Plots of Ratios

In most instances, the denominator
in an isotope ratio is linearly
proportional to the elemental
concentration

Consequently r is related to the ratio
of concentrations in the endmembers:
r=
Q1P2 Q1 / Q2
=
Q2 P1 P1 / P2

Plot of an isotope ratio vs. inverse of
concentration will be linear for
mixing.

Note that an isochron is a ratio-ratio
plot with the same denominator in
both ratios.
 Hence mixing curves on such plots

are linear.
Plotting ratio against inverse
concentration can distinguish
isochrons from mixing lines.
Pb isotopes in subduction
zone volcanics
 Pb isotope ratios plot on steeper Pb-Pb slopes than oceanic
basalts and overlap sedimentary fields.
 Dick Armstrong noted this in 1971 and argued that
subducted sediments contributed to subduction zone
magmas.
Island Arcs and Adjacent
Sediments
Sr-Nd Isotope Systematics of
Subduction Zone Magmas
Isotopic Composition of
Sediments adjacent Lesser
Antilles
OPD Site 143
OPD Site 144
Lesser Antilles
Update
 Carpentier et al. (2008)
found much more radiogenic
Pb in U-rich ‘black shales’
from ODP 143 further south.
 Thus sediments seem to be
able to explain the very
radiogenic Pb in the
Southern Lesser Antilles.
10Be

10Be

10Be
in arc lavas
(t1/2 = 1.6 Ma) produced
by spallation in the
atmosphere, washed out by
rain, adsorbed on particles.
found in several arc
magmas, but not in other
kinds of magmas.
 Clear evidence of sediment
subduction
 Not present in arcs with
accretionary prisms such as
the Lesser Antilles and
Sunda.
Th Isotopes in Subduction
Zones
 Subduction zone
magmas typically have
U-Th isotope
systematics indicating
U-enrichment over Th.
 Inconsistent with
partition coefficients.
 Likely a result of
preferential transport of
soluble U6+ into magma
genesis zone from
(somewhat oxidized)
subducting slab.
Subduction Zone Mass
Balance
Isotope Geochemistry of the
Oceans
87Sr/86Sr
 Because its residence time in
the oceans is longer than the
mixing time, Sr isotope ratios
are uniform in the ocean at
any given time.
 They have varied over
geologic time as the relative
inputs from continental crust
and ’mantle’ (oceanic crust
and young basaltic rocks)
has varied.
eNd in seawater
 Nd has a short residence
time in seawater and eNd
varies between oceans.
 Can be used as a water
mass tracer.
eHf in seawater
 Hf also has a short residence
time in seawater and eHf
varies between oceans.
 Seawater forms a different
array than mantle and crust
(the mantle array)
 This is due to the ‘zircon
effect’
Changes through time
Mantle (ocean crust) input
Continental Input
Effect pf Glacial Cycles
Prelim Exam

Understand nucleosynthesis and how various nuclides were synthesized.

Understand nuclear structure and stability

Be able to do simple geochronological calculations

Interpret geochronological diagrams
 Including U decay series, extinct radionuclides, cosmogenic nuclides

Concept of closure temperature

Know which geochronological tool to use for which problem

Model ages and crustal residence times.

Mantle reservoirs, their characteristics, their evolution



How to interpret isotope data
Concepts of compatible-incompatible, lithophile-siderophile, etc.
Evolution of the continental crust

Ages, mechanisms of formation