Extended Method
Laboratory Processing
Calcite samples were selected and cut (in
~1cm3 blocks) to reveal internal
sections that cross-cut the veins. Subsequently, the calcite pieces were
mounted in 1-inch (2.5cm) round epoxy mounts using epoxy cure resin (5g
epoxy resin and 1.15g epoxy hardener) and ground (using 800 and 2000
grit sandpaper) and polished (using a 3μm polishing cloth with diamond
suspension fluid) to reveal a smooth surface.
Sample imaging was conducted at the British Geological Survey,
Nottingham, UK. Cathodoluminescence imaging was conducted with a
Technosyn 8200 MKII cold-cathode luminoscope stage attached to a Nikon
optical microscope with a Nikon long working distance lens, and equipped
with a Zeiss AxioCam MRc5 digital camera; vacuum and electron beam
voltage and current were adjusted as required to generate optimum
luminescence. Back-scattered electron and charge-contrast imaging were
conducted using a FEI QUANTA 600 environmental scanning electron
microscope (ESEM) with a working distance of 10 mm. BSE images were
recorded using a solid-state (dual-diode) electron detector, with a 20
kV electron beam accelerating voltage, and beam currents between 0.1 and
0.6 nA,. Charge Contrast Images were recorded using a FEI large-field
gaseous secondary electron (electron cascade) detector, with 20 kV
electron beam accelerating voltage, and beam currents of 1.2 to 4.5 nA.
LA-ICP-MS U-Pb spot-analysis
LA-ICP-MS analysis was conducted at the University of Adelaide using an
ASI resolution LR Laser Ablation System coupled to an Agilent 7900 mass
spectrometer in order to determine U and Pb concentrations. Large spot
sizes (110 microns) were selected in order to maximise the signals from
elements that were expected to have low concentrations. Only isotopes
necessary for U-Pb dating (43Ca,202Hg 204Pb,206Pb, 207Pb,208Pb, 232Th and238U) were measured during spot analysis in order to
maximise the dwell time on masses expected to have low abundance, such
as the isotopes of Pb. Standard-sample bracketing was used, with the
NIST614 glass reference material used for fractionation correction of
the Pb-Pb ratios, and the WC-1 calcite reference material (Age: 254.4 ±
6.4) for correction of the U-Pb ratios [Li et al. , 2014;Roberts and Walker , 2016; Roberts et al. , 2017]. An in
house calcite sample labelled ‘Prague’ of known stratigraphic age
(~424 Ma) was used as an accuracy check [Farkaš
et al. , 2016]. In more detail, a correction factor was calculated
based on the offset between the measured age and the known age of WC-1.
This factor was then used to correct both the ‘Prague’ secondary
standard and the unknowns.
LA-ICP-MS Elemental Mapping:
LAIPCMS elemental mapping was conducted to identify alteration and
different growth zones. Before elemental mapping, the surface of samples
was gently re-ground (using 2000 grit sandpaper) to just below the laser
ablation pits. Following this the surface was re-polished (using a 3μm
polishing cloth as before). Maps were created at the University of
Adelaide using an ASI resolution LR Laser Ablation System coupled to an
Agilent 7900 mass spectrometer (i.e. the same as U-Pb analysis). A
square laser beam of 91x91μm was used to create line rasters on selected
areas of calcite samples. Data reduction was conducted using Iolite
software [Paton et al. , 2011]. Elemental map data was
produced using the Monocle plugin for Iolite [Petrus et al. ,
2017]. In more detail, polygons, termed regions of interest (ROI)
[Petrus et al. , 2017] surrounding to ablation spots were used
to query elemental concentrations. Some spot analyses were removed based
on anomalous chemistry, particularly high Al and U.