nd isotopes - uc santa cruzapaytan/290a_winter2014/pdfs/lecture... · provenance studies with nd...
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t = 106 Byr
eNd = [(143Nd/144Ndsample / 143Nd/144NdCHUR)] –1] x 104
Nd Isotopes
eNd = (143Nd/144Nd)sample
(143Nd/144Nd)CHUR
-1 x 104 [ ]
higher eNd = more radiogenic = mantle-derived
lower eNd = less radiogenic = continental crust
Nd Isotopes
Ancient cratons Negative eNd values “Nonradiogenic”
Archean Canadian shield
eNd < -20
Young volcanogenic rocks Positive eNd values
“Radiogenic”
Pacific arc volcanics eNd : 0 to +10
Provenance studies with Nd isotopes
Terrigenous sources are the predominant source of Nd to seawater
Geological distribution of crustal ages are
reflected by seawater eNd values
Frank, 2002
North Pacific
Deep Water
eNd = -4
North Atlantic
Deep Water
eNd = -13.5
Circumpolar
Deep Water
eNd = -8
Indian Ocean
Deep Water
eNd = -8
Modern Deep Waters Nd Isotopes
Albarède and Goldstein (Geology, 1992) modified by Goldstein and Hemming (TOG, 2003)
Core-top Sediment Record Nd Isotopes
Nd Isotopes Nd Isotopes
• Quasi-conservative water mass tracer
* Residence time < ocean mixing time
* Continents predominant source
* Hydrothermal Nd removed at ridge
• Source regions
• Geological Processes
von Blanckenburg (Science, 1999)
Modern Water Masses
Albarède et al., 1997
Archives of Nd Isotopes
• Fe-Mn oxides – Crusts – Nodules – Coatings
•Phosphates - Fish teeth
Nd Isotopes
1 mm
Nd isotopic composition of terrigenous inputs
(Jeandel et al., 2007)
-26
-10
-14
Nd from Piepgras & Wasserberg, 1987 and Lacan & Jeandel, 2004
Burton et al. (1997) Burton and Vance (2000)
Piotrowski et al., 2005
Nd and Sr cross-plots from Franco Marcantonio
Pb isotopes
Four isotopes are measured, one primordial and three stable daughter products
• 204Pb 1.4%
• 238U 206Pb t1/2 = 4.47 * 109 yr 24.1%
• 235U 207Pb t1/2 = 0.71 * 109 yr 22.1%
• 232Th 208Pb t1/2 = 14.01 * 109 yr 52.4%
All different ratios reported:
208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 207Pb/208Pb, 206Pb/207Pb
Older Rocks
Relative Abundance:
More Radiogenic
Individual ratios provide slightly different information based on starting
compositions, but can generally be interchanged
More 206,207,208 Pb {
Lead isotopes are most commonly measured using thermal ionization mass
spectrometry more recently with MC-ICP-MS.
There is only one nonradiogenic isotope, instrument fractionation cannot be
corrected, thus, care must be taken when running a sample, Tl is used for correction.
Pb must be separated for most environmental work (in a clean lab) because it is very
easily contaminated
U-Pb, Th-Pb, and Pb-Pb isotopic ratios may be used in age dating and petrogenetic
tracing of igneous, metamorphic, and hydrothermal rocks.
Since there is a divergence in chemical behavior between uranium, thorium, and their
daughter elements, many geological processes can lead to extensive fractionation of the
various isotopes
Lead ratios used to identify pollution sources in surface water and the atmosphere
210Pb used to date recent deposition of snow, lake sediments, etc. 210Pb has a half-
life of 22.3 years, allowing dating within the past 100 years. It is also useful in
determining changes in ambient environmental conditions.
Lead isotopes can be used in archaeology to date ores used in artifacts.
Pollution
Greenland Snow
Pb Deposition in Ocean
29
Hydrothermal Input
(localized)
Remainder of river sourced Pb is entrained
in water masses
Scavenging
along coast
Aerosols
Deposition into ocean,
typically through rainout
ITCZ
Seawater Pb a mixture
of these sources
By understanding the Pb mass balance, we can then utilize the short half life
of Pb in the ocean to look at regional changes in dust sources, circulation
variations, and the prevalence of hydrothermal inputs downcore
Four phases measured in this study
30
Pb adsorbed onto
FeMn surfaces-
recording
seawater
Deposition of
detrital fraction in
sediment column
Bulk source
sediments
Pb from the
soluble fraction
of dust
Results: Different fractions of source sediments have different isotopic compositions
- Different fractions of
the same sample have
different Pb isotopic
compositions
- Chinese and New
Zealand Loess are
distinct
18.5 19 19.5
38.6
38.8
39
39.2
39.4
206Pb/204Pb
20
8P
b/2
04P
b
Chinese Loess Seawater Leach 1
Chinese Loess Acetic Acid Leach 2
Chinese Loess 0.25 M HCl Leach 3
Chinese Loess Acetic Acid Leach 3
New Zealand Seawater Leach 1 Chinese Loess Bulk 2
Chinese Loess Detrital Fraction 1
Chinese Loess Detrital Fraction 2
New Zealand Detrital Fraction 1
Chinese Loess Detrital Fraction 3
New Zealand Loess Bulk 4
1 This study 2 Jones et al., 2000 3 Ling et al., 2005 4 Stancin et al., 2008
Leaching
studies
Detrital
fractions
Bulk
sediment
Chinese
Loess
Detrital
Chinese
Loess
Bulk
New
Zealand
Detrital
New
Zealand
Bulk
Loess
Soluble
New
Zealand
Seawater
Soluble
Chinese
Loess
Error bars are smaller than the symbols for all plots
Records of 206Pb/204Pb (a,b), 207Pb/204Pb (c,d), 208Pb/204Pb (e,f) and
εNd(t) (g,h) from sites
738 and 689
respectively. Red filled
circles represent
seawater extracted
from Fe–Mn
oxyhydroxides for Pb
isotopes and fossil fish
teeth for Nd isotopes,
black filled squares
represent detrital
silicates. Blue shaded
region indicates the
EOT and dashed
vertical lines represent
steps 1 and 2 of the
two-step Eocene–
Oligocene glaciation1.
High rates of chemical weathering in the late Eocene Basak and Martin 2012
Compiled by van de Flierdt et al., 2003
Hf Isotopes Hf Isotopes
• Quasi-conservative water mass tracer
* Radiogenic Isotope
* Residence time < ocean mixing time
* Continents rivers and dust
* Hydrothermal?
Reflect:
* Source regions + water mass mixing
Similar to Nd but we know less about it
t = 106 Byr
eHf = [(176Hf/177Hfsample / 176Hf/177HfCHUR)] –1] x 104
ɛHf
Hf Isotopes
eHf = (176Hf/177Hf)sample
(176Hf/177Hf)CHUR
-1 x 104 [ ] higher eHf = more radiogenic = mantle-derived
lower eHf = less radiogenic = continental crust
Hf Isotopes
Analysis with TIMS or MC-ICPMS
Instrumental mass fraction corrected to 179Hf/177Hf = 0.7325 applying an
exponential mass fractionation law.
All 176Hf/177Hf ratios normalized to the accepted value for JMC 475 of 0.28216
Modern Sea Water
The Hf isotopic compositions
range from εHf=−3.1 in surface
waters of the Labrador Sea to
+4.4 in AABW of the Cape Basin.
Patterns of Nd and Hf with depth
are not consistent (different
distribution in rocks and minerals
resulting in different weathering
characteristics).
Provide information on proximal
weathering processes and
sources that supply Hf to the
ocean.
Hf isotopic composition of
seawater reflects water mass
mixing only on a basin wide
scale.
J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010)
540–557
Dust also contributing Hf