METHODS

Secondary minerals were identified optically in thin section and by examination using backscattered electron imaging in scanning electron microscope and electron microprobe, coupled with compositional spectra from energy-dispersive X-ray analyses. Quantitative chemical analyses were performed with an automated Cameca CAMEBAX MBX microbeam with four wavelength-dispersive spectrometers and a JEOL JX-733 superprobe, both using a 10-nA beam current and 15-kV accelerating potential. Natural and synthetic mineral standards were used for calibration.

Secondary phyllosilicates and carbonates were scraped from veins for confirmation of mineralogy by X-ray diffraction analysis. Smear slides and oriented mounts were scanned at 2°2/min with CuK radiation in an automated Scintag diffractometer. Oriented mounts of phyllosilicates were analyzed after air drying and after saturation with ethylene glycol.

Oxygen was extracted from whole-rock powders and from secondary minerals by reaction with ClF₃ and converted to CO₂ gas for measurement of oxygen isotope ratios at the University of Michigan (Clayton and Mayeda, 1963). Phyllosilicates were dried at 110°C, stored overnight with P₂O₅, and degassed under vacuum at 150°-175°C for 6 hr prior to fluorination. Repeated extractions and measurements of samples and standards were reproducible within ±0.2. CO₂ from carbonates was liberated by dissolution in phosphoric acid, and carbon and oxygen isotope ratios were measured at the University of Michigan.

Sr isotope analyses and Sr concentration measurements were carried out at Woods Hole Oceanographic Institution (WHOI) using a VG 54 thermal ionization mass spectrometer. Samples were spiked with ⁸⁴Sr and dissolved in 2.5-N HCl (carbonates) and HF/HClO₄ (clays) in a Teflon beaker. Sr was separated in quartz columns with a 5-mL resin bed of AG50W-X12 200-400 mesh. ⁸⁷Sr/⁸⁶Sr are reported relative to National Bureau of Standards (NBS) 987 = 0.71024. Within-run precisions are reported in Table T1; external precision (2 ) of Sr isotope analyses at WHOI is usually 0.003%.