METHODS

Samples used for chemical analysis were broken from thin sawn slabs of the respective specimens and carefully hand picked. After ultrasonic cleaning, washing in deionized water to remove sea salt, and drying, the samples were finely ground under acetone in a mechanical agate mortar and pestle. Analyses were performed by X-ray fluorescence (XRF) spectrometry and by inductively coupled plasma–atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS) using the CSIRO methods outlined in Miller et al. (this volume). Loss on drying (LOD; at 105C) and loss on ignition (LOI; at 1050C) were measured gravimetrically on separate powder aliquots for three samples. Two handpicked samples of altered dacite were too small for XRF, LOD, or LOI measurements. Table T1 lists preferred data, taking into account precision and sensitivity of the various methods.

Petrological observations were made on polished thin sections. Mineral compositions cited below were obtained with a Cameca Camebax microprobe, fitted with an energy dispersive spectrometer, at the Research School of Earth Sciences, Australian National University (Australia).

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