Author: Sally Oberst
Last saved by: Tamzin Anderson
Company: Microsoft

Appendix 1: zircon age dating analytical methods
Zircons were separated using conventional crushing, hydrodynamic, magnetic and density discrimination techniques. A random selection of zircons were mounted in 25 mm epoxy resin discs and polished to reveal an equatorial section of the mineral grains. Representative cathodoluminescence images of the sample zircons are presented in Appendix 2. Zircons were randomly selected for analysis from each sample mount. Laser ablation U-Pb data were acquired using a Nu Instruments, Nu Plasma HR multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with a multi-ion-counting array. Data were collected using a single static acquisition with the configuration shown in Table A1. Instrumental parameters are detailed in Table A2. 
Ablation was conducted using a Nu Wave Research UP193SS (193 nm), Nd:YAG laser ablation system and an in-house designed low volume ‘zircon ablation cell’, based on the design of Bleiner & Gunther (2001). Ablated sample material was transported from the laser cell into the MC-ICP-MS using a continuous flow of 400 ml/min of He gas.
The MC-ICP-MS was tuned at the start of each analytical session using a 300 ppt solution of Tl-^235U  introduced via a Nu Instruments DSN-100 de-solvating nebuliser, using an ESI PFA-50 nebuliser tip. Ion counter (IC)-Faraday gains were determined for each of the three IC’s using a weaker 50 ppt Tl-^235U solution, jumping the ^205Tl peak through each IC, and comparing it to the equivalent Faraday signal. IC gains were of the order of 62, 72 and 79 % for IC0, IC1 and IC2 respectively, over the eight analytical sessions. Repeat measurement of the gains showed that these values were reproducible to within << 0.1 % (2SD) over an approximately 8 hour operating period.
Data were acquired using the Nu Instruments time resolved analysis (TRA) software. An instrument zero and on-peak zero were measured at the start of each run, each for a total of 60 seconds. After completion of the baselines, the laser was fired and sample data were acquired for approximately 40 seconds per ablation for a run of 10-15 (max 6) ablations. During the course of each acquisition the Tl-^235U solution was co-aspirated to allow for the measurement and correction of instrument induced mass bias and plasma induced elemental fractionation. ^202Hg was measured simultaneously, and used to correct for the isobaric interference of ^204Hg on the ^204Pb peak, (assuming ^204Hg/^202Hg = 0.229887).
The laser sampling protocol employed varied, depending upon sample size. For example, where samples were large enough, a dynamic sampling approach was taken (30 x 30 µm raster, using a 25 µm spot and 15 µm raster spacing). However, for the smaller grains found in some of the samples, a static sampling approach was taken using a 35-50 µm ablation spot. In all cases, samples were normalised to the relevant standard data ‘like-for-like’, according to spot size and fluence.
The internationally recognised 91500 zircon was used as a matrix matched standard and analysed at regular intervals throughout each session. Data were normalised according to the deviation of the average daily ^207Pb/^206Pb and ^206Pb/^238U values obtained for 91500. In the case of the latter, this is to correct for elemental fractionation in the plasma (dynamic ablation). Within-run laser-induced static elemental fractionation ablation is negligible due to ablation pit aspect ratios < 0.5. Normalisation of the ^207Pb/^206Pb ratio of 91500 corrects for any drift, or offset, from the gain value recorded previously for the ion-counters. Uncertainties for each ratio are propagated relative to the respective reproducibility of the standard, to take into account the errors associated with the normalisation process and additionally to allow for variations in reproducibility according to count rate of the less abundant ^207Pb peak.
All data were processed using an in-house spreadsheet calculation routine and are tabulated in Table A3. Data recording < 0.01 mV ^207Pb (i.e. below the detection limit) have been rejected and those reporting > 300 cps ^204Pb in the sample after correction for the isobaric interference of ^204Hg have been assessed and corrected for common-Pb where required (or rejected). All data are plotted on Tera-Wasserberg Concordia plots (Fig. 5). Frequency distribution plots (Fig. 6) have been generated on all data < 10% discordant. All quoted data point ages are ^206Pb/^238U ages unless otherwise stated.

Table A1. Configuration of the 'Zircon' collector block, used for LA-MC-ICP-MS U-Pb analysis at NERC Isotope and Geochemistry Laboratories.

Ex High | High 6 | | High 4 | High 3 | High 2 | High 1 | Axial | Low 1 | Low 2 | | IC0 | IC1 | Low 3 | IC2 | Low 4 | Low 5
238U | 235U | | - | - | - | - | - | - | - | | 207Pb | 206Pb | 205Tl | 204Pb&Hg | 203Tl | 202Hg

Table A2. Instrument and Laser operating parameters.

ICP | Laser
Model Name | Nu Instruments, Nu Plasma HR MC-ICP-MS | | Model Name | New Wave Research UP193SS laser ablation system
Forward Power | 1300 W | | Operating Wavelength | 193 nm
Reflected Power | < 1 W | | Max Output Energy | 8 J/cm^2
Cool Gas | 15 l/min | | Pulse Width | 3 ns
Aux Gas | 0.8 l/min | | Repetition Rate | 1-10 Hz
Nebuliser | ESI PFA-50 (50 ul/min uptake rate) fitted to a Nu Instruments DSN-100 | | Cell Volume | ~ 3 cm^3
Sample Cone | Nickel with 1.15 mm orifice | | Carrier Gas | He (flow rate ~ 400 ul/min)
Skimmer Cone | Nickel with 0.6 mm orifice | | Spot Size Range used | 25-50 um
Collector Types | 12 Faraday and 3 ion counters (ETP discrete dynode multipliers) | | |

Table A3. Analytical data obtained for detrital zircon populations within the Midland Valley sandstones. In separate file as pdf.