Electronic Supplementary Material for following manuscript:
Facies architecture and Late Pliocene-Pleistocene evolution of a felsic volcanic island, Milos, Greece
Andrew L Stewart, Jocelyn McPhie
New SHRIMP U-Pb ages for Milos
Geological context of samples
Zircon grains were separated and analysed from four samples (MIL 130, MIL 243, MIL 343 and MIL 365) taken from the basal pyroclastic series and complex of domes and lava flows on Milos. Sample MIL 130 is coherent porphyritic dacite from Triades, interpreted to be part of a submarine lava dome (Fig. 10A-log D). Sample MIL 243 is porphyritic dacite from a submarine cryptodome associated with mineralisation in the Triades (Fig. 10A-log D). Sample MIL 343 is coherent porphyritic dacitefrom the Kalogeros cryptodome (c.f. Stewart and McPhie 2003a) on the northern coast of Milos (Fig. 10C-log N). Sample MIL 365 is a rhyolitic pumice clast from the Filakopi Pumice Breccia (Fig. 10C-log N; Stewart and McPhie 2003).
Methods
Sample preparation and analyses were carried out at the Australian National University, Canberra, Australia Heavy-mineral concentrates were obtained from pulverised samples (typically varying in weight from 0.5 to 3 kg) using standard density and electromagnetic separation techniques. Zircons from all samples were mounted in epoxy, together with several chips of the FC1 and SL13 reference zircons, and polished for inspection and SHRIMP analysis. Backscattered electron (BSE) and cathodeluminescence (CL) images of all zircon crystals analysed were acquired with a scanning electron microprobe (SEM) to provide information about the internal structure of the sectioned grains and to target specific areas within the zircons suitable for determining the magmatic crystallisation age of crystals.
U-Pb isotopic analyses of the zircons were made using the SHRIMP RG. Zircon grains were selected at random for analysis, and in most cases all suitable grains were analysed. The data were collected during several 24-hour sessions; each analysis consisted of six scans through the mass range. The data have been reduced in a manner similar to that described by Williams (1998, and references therein), using the SQUID Excel Macro of Ludwig (2000). The Pb/U ratios have been normalised relative to a value of 0.1859 for the 206Pb/238U ratio of the FC1 (Duluth Gabbro) reference zircons, equivalent to an age of 1099 Ma (Paces and Miller 1993). Uncertainties given for single analyses (ratio and ages) are at the 1 σ level, however, the uncertainties in calculated weighted mean ages are reported as 95% confidence limits and include uncertainties associated with the reference zircon FC1.
Results
Each of the four dated zircon separates contains abundant equant to elongate grains, with euhedral pyramidal terminations. Some of the grains are fragments of what were originally doubly terminated crystals and appear to have been modified by post-crystallisation processes, giving rise to ragged or serrated grain outlines. Between 18 and 24 analyses were obtained per sample. All samples show a relatively simple age population which is interpreted to reflect the time of magmatic crystallisation. Virtually all of the zircons analysed contained significant amounts of common Pb, a feature not uncommon in mineralised terrains (Norman 2002). SHRIMP data for allfour samples are presented in Tables 1 to 4 and summarised in Fig .1
FIG 1. U-Pb SHRIMP data for the four samples plotted on probability plots. A SampleMIL 130 is porphyritic dacite. Twenty-four analyses form a single age population with aweighted mean age of 1.44 ± 0.08 Ma. B Sample MIL 243 is porphyritic dacite. Twenty analysesform a single age population with a weighted mean age of 2.18 ± 0.09 Ma. C Sample MIL343 is a moderately porphyritic dacite. Twenty-one analyses form a single age population with aweighted mean age of 2.70 ± 0.04 Ma. D Sample MIL 365 is juvenile rhyolitic pumice takenfrom the Filakopi Pumice Breccia. Eighteen analyses form a single age populationwith a weighted mean age of 2.66 ± 0.07 Ma.