GSA Data Repository 2016038
Early hydrothermal carbon uptake by the upper oceanic crust:
Insight from in situ U-Pb dating
Laurence A. Coogan, Randall R. Parrish, and Nick M.W. Roberts
This supplementary information consists of three parts:
S1. Background information about the study sites
S2. Analytical techniques and data reduction
S3. Table of all data used in age determination as an Excel file.
S1. Study locations
DSDP Sites 163/164: DSDP Sites 163 and 164, drilled on Legs 16 and 17, are in the equatorial
central Pacific (van Andel et al., 1973; Winterer et al., 1973). Based on the seafloor age maps of
Muller et al. (2008), Sites 163 and 164 have crustal age of 80.8 and 109.0 Myr respectively
(throughout the crustal ages are taken from GeoMapApp: http://www.geomapapp.org/). The age
for Site 163 is consistent with the Campanian biostratigraphic age derived from the deepest
portion of the 276 m of sediment overlying basement in this hole (van Andel et al., 1973). Site
164 is overlain by 264 m of sediment but there is poor control on the biostratigraphy in the basal
sediments; these are at least Albian in age (Winterer et al., 1973), consistent with the assigned
age.
DSDP Site 307: DSDP Site 307, drilled on Leg 32, is in the NW Pacific; basement is overlain by
307 m of sediment (Larson et al., 1975). Based on the seafloor age maps of Muller et al. (2008)
the crustal age is 148.3 Myr. Based on the radiolarian assemblage the basal sediments are
Valanginian to Berriasian (132.5 to 145 Myr; Larson et al., 1975) consistent with the assigned
crustal age.
DSDP Sites 417/418: DSDP Sites 417D and 418A are in the western Atlantic, with Site 417D
about 8 km north of Site 418A (Donnelly et al., 1979). Based on the seafloor age maps of Muller
et al. (2008) Sites 417D and 418A have crustal age of 120.0 and 119.9 Myr respectively and the
basement is covered by 352 and 323 m of sediment respectively (Donnelly et al., 1979). At both
sites biostratigraphy suggests that the oldest sediments are Aptian (Donnelly et al., 1979)
consistent with the assigned crustal age.
DSDP Site 543: DSDP Site 543A is in the western Atlantic just outboard of the deformation
front associated with the Less Antillies subduction zone (Biju-Duval et al., 1984). Based on the
seafloor age maps of Muller et al. (2008) the basement age is 80.8 Myr, consistent with the
Campanian age derived from biostratigraphy (Bergen, 1984). The 411 m of sediment overlying
this site has been thickened due to deformation and does not reflect the abyssal sedimentation
history.
DSDP Site 595B: DSDP Site 595B lies just to the north of the Osbourn trough in the western
Pacific outboard of the Tonga-Kermadec trench (Billen and Stock, 2000). Based on the seafloor
age maps of Muller et al. (2008) crust here has an age of 90.3 Myr however the local tectonics
are complex with two orthogonal spreading directions in this area potentially increasing the
uncertainty on the model age. The Muller et al. (2008) crustal age is somewhat younger than the
total fusion 40Ar-39Ar ages of 96.8±0.6 and 101.5±0.6 for two ferrobasalts from this site (R.
Duncan pers. comm. reported in Montgomery and Johnson, 1987). Because of this uncertainty in
the crustal age in the figures we assign a crustal age of 95 ± 10 Myr to be conservative. This area
of the Pacific has a very slow sedimentation rate with only 68 m of sediment covering this site.
Troodos ophiolite: The Troodos ophiolite formed ~91.6 Myr ago based on U-Pb dating of
zircon from plagiogranites (Mukasa and Ludden, 1987) in a supra-subduction zone setting. It is
perhaps the only ophiolite that preserves a typical seafloor low-temperature (i.e. off-axis)
alteration history with extensive preservation of volcanic glass and no overprint of highertemperature (obduction related) metamorphism (e.g. Gillis and Robinson, 1990).
S2. Analytical techniques and data reduction
Samples consisting of small chips of carbonate, all reasonably clear, were mounted in
epoxy, polished and cleaned prior to insertion into a laser ablation cell (Fig. DR1). In general,
methods of U-Pb analysis closely resemble those used for routine U-Pb zircon dating. Calcite
was analysed for U-Pb geochronology mainly using a Nu Instruments Attom HR sector-field
single-collector inductively coupled plasma mass spectrometer (SC-ICP-MS) following the
method of Mottram et al. (2014) and Li et al. (2014). Laser ablation was performed with a New
Wave Research UP193ss laser ablation system, using a New Wave Research ‘large-format’ cell,
or a New Wave Research 193UC with a TV2 cell; both feature a moveable cup with an ablation
volume of ca. 3-4 cm3 which, combined with <1 m of tubing to the plasma torch, leads to a
washout time of < 1 second. The ablation parameters were a 100 μm static spot, a repetition rate
of 10 Hz, a fluence of ~5-6 Jcm-2, a 15 second washout period between analyses, and a 30
second ablation time. Data processing used the time-resolved analysis on the Nu Instruments’
software, an in-house Excel spreadsheet for data reduction, including subtraction for gas blank
counts on Pb isotopes, and uncertainty propagation, and Isoplot (Ludwig, 2003) for age
determination. All data are plotted at the 2σ confidence interval.
The Nu Attom ICP-MS is used in peak-jumping mode with measurement on a
MassCom secondary electron multiplier (SEM). The following masses are measured in each
sweep: 202Hg, 204Pb+Hg, 206Pb, 207Pb and 238U. Each data integration records 80 sweeps of the
measured masses, which roughly equates to 0.4 seconds.
A standard-sample-bracketing technique was used to normalise the data using the WC-1
calcite that has been measured by isotope dilution to yield an age of ~254±7 Ma (2σ); this
material has ~5 ppm U and is approximately 90% radiogenic lead (although this is slightly
variable). This procedure involved calculation of U-Pb normalisation factors based on the 254
Ma age of the WC-1 calcite using an assumed initial 207Pb/206Pb of 0.83±0.02. The
normalisation is not sensitive to the initial ratio due to the highly radiogenic lead within the WC-1
calcite. It was analysed at regular intervals during each session, before and after groups of
unknown analyses. The method of normalisation is to bracket based on a stable portion of an
analytical session, which may vary from an entire day, to several blocks within one day.
Because the reference material used for normalisation is heterogeneous (~2.75%), small
(<~2%) variations in the normalisation procedure may add some bias to the final age. The
reproducibility of the reference material is propagated to the final age uncertainty, which
should account for this effect, but it is worth noting this caveat to the method.
207
Pb/206Pb
normalisation used NIST 614 glass and values from Woodhead and Hergt (2001).
Overall reproducibility achieved on the calcite in-house reference material was ≤4% (2
standard deviations) for U-Pb ages and this value is propagated onto the datapoint uncertainties
as an excess variance. Plots of the WC-1 standard, after normalisation, are shown in Fig. DR2
for the three analytical sessions of this study over three years.
Regressions of data from each sample were performed after eliminating most data with
signals of Pb so weak that uncertainties in ratios were ~20% (1σ). Data with no U but higher Pb,
were generally retained with an assumed U/Pb of zero, as these allow a good measurement of the
initial lead isotope ratio. Intersections with the Tera-Wasserburg concordia were calculated
using Isoplot (mostly Model 1, occasionally Model 2). In the final ages the additional 2.75% (2σ)
uncertainty of the WC-1 standard was added in quadrature, and all final ages are quoted at 2σ in
Table 1. Only one of the samples was anchored to a value (0.83±0.03) of common lead isotopic
ratio (Table S3) because of its lack of any unradiogenic datapoints.
S3. Interpretation of ages
There are some samples that clearly have excess scatter; this may arise by more than one
reservoir of initial common lead, or sampling zones that may have been altered or undergone
more than one growth stage. It could also arise from the inherent difficulty of measuring very
small Pb signals approaching the background uncertainty in Pb counts in the blank gas because
there is temporal variation the magnitude of the gas blank counts. This latter aspect cannot
always be fully accounted for in uncertainty propagation of low-Pb samples and therefore may
lead to some bias and additional unquantifiable uncertainty in some analyses. Most samples,
however, form a good array with low MSWD, suggesting but not proving that the arrays are
mainly a simple mixture of radiogenic ingrowth with initial common lead with a single age of
growth.
In some cases the ages appear slightly older than the age of the host oceanic crust. The
measurements, normalized to the WC-1 calcite appear satisfactory and we have no reason to
suspect the measurements or uncertainties on arrays of data, but in cases such as 595B-2R1,
595B-3R2, or 2012-CL26 (i.e. the bottom row in Fig. 1), which are relatively non-radiogenic and
involve large extrapolation to calculate an age, the uncertainties have to be treated as a
minimum. It is possible, but not likely, that a compositional or textural bias of samples relative
to the standard could introduce additional bias, but we have no way of evaluating this as of yet,
given that we only have one calcite reference/normalization material. On the basis of our wider
measurement experience of carbonate using this WC-1 material, including some by isotope
dilution, if there is any compositional or textural bias in measuring an unknown against the WC1 material, it is likely to be a few percent bias at most, but again this cannot be fully evaluated at
the present. This remains a caveat using this method.
Figure DR1. Photographs of carbonate grains showing analysis pits. Larger pits show the preablation to clean the surface. Smaller pits are older analyses (2014) that have been partially
polished away during preparation for the next analytical session. Brighter images are from the
2011 analytical session. Scale bars are 1 mm in all parts.
2011 WC-1 measurements
2014 WC-1 measurements
2015 WC-1 measurements
Figure DR2. Tera Wasserburg plots for the carbonate standard from each analytical session
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