date - Search and Discovery

Direct in situ Dating of Carbonates by LA-ICP-(MC)-MS and its Applications to Chronostratigraphy*
Randy Parrish1 and Troy Rasbury2
Search and Discovery Article #41269 (2014)**
Posted February 11, 2014
*Adapted from oral presentation given at AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013
**AAPG©2013 Serial rights given by author. For all other rights contact author directly.
1
University of Leicester & NIGL, British Geological Survey, Leicester, United Kingdom ([email protected])
SUNY at Stony Brook, Stony Brook, New York ([email protected])
2
Abstract
A viable method to provide absolute age constraints in carbonate strata has proven elusive. Direct dating of primary carbonate deposits,
and/or early marine cements would constrain rates of carbonate platform growth, allowing refinement of models of sea level and climate
change that might have influenced the carbonate depositional environment. Absolute age constraints on later diagenetic elements such as
meteoric and burial cements and tectonic veins is very important to a wide range of problems in both the hydrocarbon industry (such as burial
history and timing of fluid migration) and earth science more generally. U-Pb dating of carbonate has been shown to be viable but so far has
been restricted in application due to use of time consuming ID-TIMS methods that involve dissolution and processing of a sample of
carbonate. A major hurdle is that there is currently no a priori way to know if a sample has a viable U/Pb ratio, and many carbonates are not
datable.
We have successfully adapted LA-ICP-(MC)-MS methods (used routinely in zircon U-Pb dating) by using a well characterized 254+/-6 Ma
calcite standard (dated by ID-TIMS at NIGL and Stony Brook) to correct the U/Pb ratio for matrix effects. This approach allows in situ U-Pb
dating with uncertainties as little as ±4% at 95% confidence with minimal sample consumption. We have applied this method to the dating of
samples with average uranium concentrations (0.4-5 ppm) such as speleothems, paleosols, lacustrine carbonates, tufa, early marine cements
(in ammonite chambers), calcite veins from fractures in MORB and marine fossils. The age of dated materials so far ranges from Permian to
late Quaternary, as young as 250ky. Laser ablation allows us to quickly assess the U/Pb ratios of the variety of carbonate phases in a sample
to determine if the sample has potential for dating. Carbonate components with favorable U/Pb are subjected to in situ dating using textural
petrography and SEM/fluorescence imaging to guide the analysis of coherent zones of growth. Examples of these applications will be
presented.
The wealth of applications to explore in the future include structural geology and the dating of structural veins, post-depositional
cementation, and direct dating of biogenic aragonite (biogenic calcite usually being too low in uranium to be suitable). The future scope of
applicability of this method to earth science, sedimentary geology, and hydrocarbon exploration is very exciting.
Direct in situ
Direct
in situ Dating of Carbonates by LA
Dating of Carbonates by LA‐ICP‐(MC)‐
ICP (MC)
MS and its Applications to Chronostratigraphy
Randy Parrish
University of Leicester & NIGL, British Geological Survey
Troy Rasbury
SUNY, Stony Brook
Wouldn’tt we like to …
Wouldn
we like to
• Date calcite cement in carbonate rocks
l
b
k
g
p
,
• Determine when diagenesis took place, • Directly date fossils and other carbonate deposits speleothems paleosols etc
deposits, speleothems, paleosols, etc.
• Date calcite in structural settings – tension veins stretched belemnites calcite in fault
veins, stretched belemnites, calcite in fault rocks, slickenfibers, etc.
These applications have been nearly intractable, but there is real progress…..
Laser ablation high sensitivity
ICP-(MC)-MS
methods borrowed from U-Pb
U Pb
LA-ICP-MC-MS techniques
A vital ingredient
to an efficient method is …
g
standardisation during analysis –
g reference materials that are appropriate
pp p
using
U-Pb calcite standard: about 4ppm U, 251 Ma old
data-point error ellipses are 2
Tera-Wasserburg diagram
Y-intercept =
initial ‘common lead
0.6
0.5
Pb/2066Pb
462
Intercept
p at
0.4623±0.0065 Ma
MSWD = 3.5
7k
ky
0.4
0.3
207
This material is fantastic: almost no initial lead (<1ppb), ~100ppm uranium and not
uranium, and not altered, meaning little if any recrystallisation/
recrystallisation/ replacement during subsequent precipitation of i i i
f
younger calcite; the age via U‐Pb agrees with the U‐
series (U‐Th) date
0.2
0.1
1.6
0.0
0
0
3000
^
1.4 Ma
5000
0.8
^
1.0
7000
Data from high uranium Flowstone from Nahanni Park Yukon Territory
Nahanni Park, Yukon Territory
Data courtesy of Chris Smith, University of Bristol 2012
^
^
0.6
9000
11000
238
0.4
13000
15000
U/206Pb
‘Concordia’
Concordia intercept =
pure radiogenic lead,
ratio a function of age
17000
Early diagenetic calcite y
g
cements (photos: Q Li, 2011)
Large amount of calcite cements, less open deposit environment
Early diagenetic calcite
Regions
<100μm can
be targeted
and dated to
± 2-3% in
calcite
Septum
p
Hildoceras spp 1. Somerset, Bifrons Zone
Late diagenetic calcite
LA‐ICP‐MC‐MS U‐Pb data on early diagenetic cements
Ages of earliest
cements are 10-15 Ma
younger
y
g than
chronostratigraphic
ages
Data courtesy of
Qiong Li, 2011
data-point error ellipses are 2
0.8
Intercepts at
101.08±0.98
101
08±0 98 [±0.99]
[±0 99] & 4708±13 [±16] M
Ma
MSWD = 1.2
4800
207P
Pb/206Pb
0.6
DSDP 51-417D
51 417D 274R core
basalt host
4400
4000
04
0.4
3600
0.2
2800
1200
400 300
200
150
101
0.0
0
20
40
60
80
238U/206Pb
Fragments of calcite in vein hosted MORB, oceanic crust known to
be ~115 Ma old; this data demonstrates hydrothermal deposition of
calcite off the ridge axis for millions of years following generation
of oceanic crust
Data: L Coogan U of Victoria 2012
Pilot study
Palaeolake Olduvai
ca lcite crysta Is
La ke
margin
sediments
sodiments
t -N
10km
Lake carbonates
Detrital nodular calcite in
l
lacustrine-derived
t i
d i d sediments,
di
t
Olduvai Gorge, E Africa
100 μm
((E Rushworth 2010 data))
data-point error ellipses are 2s
0.8
0
8
Intercepts at Ma
Age: 1.89±0.10
2.39±0.12 & 4853±42 [±43] Ma
(±5%
MSWD2σ)
= 0.38
MSWD
S
0.38
04
0.4
207
In favourable
samples, calcite is
datable by U-Pb LAICP MS h
ICP-MS;
here, li
linear
arrays of data allow
a precise date with
very small
uncertainties
Pb/206Pb
06
0.6
0.2
0.0
0
1000
2000
238
U/
206
Pb
3000
4000
Test the potential resolution by dating crystals in
older and younger parts of the sequence
RHC
RHC
40Ar/39Ar ages'
Calcite crystal ages
-1.5 - 1.48 ± 0.05
1.623±0.033
1.79* -1.72 ± 0.003
1.89 ± 0.10
1.873 ± 0.086
1.85 ± 0.11
1.89 ± 0.12
1.839 ± 0.005 - 1. 79
1.917±0.037
1.900±0.059
1.98 ± 0.06 - 1.87 ± 0.01
2.022±0.033
Mean deviation of 0.103Ma with a 2-6% uncertainty
Age (Ma)
‘excess’ age attributable to high initial Uranium activity;
This excess activity is common in terrestrial waters• •
Primary aragonite and some calcite suitable for 87Sr/86Sr may
be datable for U-Pb
U Pb, depending upon degree of alteration
alteration,
common Pb content, and initial uranium content.
How widely applicable is this method?
How widely applicable is this method?
• C
Calcite cement in ammonite/fossil chambers
l it
ti
it /f il h b
• Calcite in veins in basalt, ophiolites • Carbonate lake sediments, and oolite‐like C b
t l k
di
t
d lit lik
concretionary grains
• Speleothems and flowstones 0.26‐>4 Ma
Speleothems and flowstones 0 26‐>4 Ma
• Fracture‐filled veins
• Tufas and paleosols, crusts on boulders
Tufas and paleosols crusts on boulders
• The above have been demonstrated
However it is early days of exploiting this method and
However, it is early days of exploiting this method and its application to sedimentary geology and chronostratigraphic applications
Structural Geology applications
Stretched belemnites, calcite infill
Tension gashes
brecciation with
calcite deposition
Cross-cutting veins
Other Applications …. •
•
•
•
•
•
Structural veins in foldbelts and Structural
veins in foldbelts and
thrusting
Stretched belemnites and other extensional structural features
C o o ogy a d t esca es o po os ty
Chronology and timescales of porosity occlusion in carbonate reservoir rocks gy
Better chronology in strata with non‐
diagnostic biostratigraphy
Molar tooth structure in Precambrian
Applications to strata of unknown age, (p
y)
in Precambrian (potentially)
Thank you for your interest in this work
rrp@nigl nerc ac uk (Parrish)
[email protected]
[email protected] (Rasbury)

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