ANALYTICAL PROCEDURES

⁴⁰Ar/³⁹Ar total fusion and step heating experiments, ²³⁸U/²⁰⁶Pb ion microprobe analyses, and apatite fission track dating were undertaken on selected samples of igneous rocks recovered during Leg 180 to document their temperature-time history. Table T1 summarizes the samples analyzed in this study.

⁴⁰Ar/³⁹Ar analyses were conducted on plagioclase and clinopyroxene separates from two diabase samples from Sites 1109 and 1118 on the hanging wall and one gabbro sample from Site 1117 on the footwall. ⁴⁰Ar/³⁹Ar total fusion and step heat experiments were also conducted on single crystals of K-feldspar from two small rhyolite clasts from Sites 1110 and 1111. ⁴⁰Ar/³⁹Ar total fusion analyses were conducted on feldspar and biotite from a microgranite recovered at Site 1108.

High-purity mineral separates (>99%) were prepared from crushed and sized rock chips using conventional heavy liquid and magnetic separation techniques. Mineral separates were wrapped individually in Sn foil along with biotite standard GA1550 (97.9 Ma) used to monitor the neutron dose (McDougall and Harrison, 1999). Samples were vacuum sealed in super-silica quartz tubes and irradiated for 5 hr in position L-67 of the Ford reactor at the University of Michigan.

Argon analyses were performed in the noble gas laboratory at the University of Arizona. Extraction of gas from the samples was accomplished using a double-vacuum, resistance-heated tantalum furnace with temperature control via a thermocouple in contact with the bottom of the crucible and mounted on the outer (low) vacuum side of the furnace. Three SAES getters were used for purification of the extracted gas. Isotopic analyses were performed using a VG5400 mass spectrometer with an ion-counting electron multiplier. Machine mass discrimination and sensitivity were determined from repeated analysis of atmospheric argon. Data reduction was completed using in-house programs. Samples were corrected for blanks, neutron-induced interfering isotopes, decay of ³⁷Ar and ³⁹Ar, mass discrimination, and atmospheric argon, as well as H³⁵Cl, H³⁶Cl, and H³⁷Cl. Correction factors used to account for interfering nuclear reactions were determined by analyzing argon extracted from irradiated CaF₂ and K₂SO₄. All ages are calculated using the decay constants recommended by Steiger and Jäger (1977). Stated precisions for ⁴⁰Ar/³⁹Ar ages include all uncertainties in the measurement of isotopic ratios and are quoted at the 1- level. The errors do not include an error associated with the J parameter, which is <0.5%.

Zircon U/Pb ages were determined using the Stanford University sensitive high-resolution ion microprobe (SHRIMP RG). Zircon separates were prepared from clasts of microgranite (<75 µm) (Core 180-1108B-6R) and rhyolite (<106 µm) (Cores 180-1110B-3X and 180-1111A-16R) using conventional heavy liquid and magnetic separation techniques. Zircons were mounted in epoxy along with 1099-Ma standard AS57 from the Duluth anorthosite complex (Paçes and Miller, 1989). The mount was polished to expose midsections. Cathodoluminescence images were obtained to aid in spot location. Zircons from diabase Core 180-1117A-11R were analyzed in situ in thin section. These zircons were typically 10-15 µm in maximum dimension. Standards were analyzed from the second mount held in the source chamber and were interspersed with the in situ Core 180-1117A-11R analyses.

The operating conditions and techniques for the SHRIMP RG are similar to those described by Muir et al. (1996). Analyses were performed using a 6- to 7-nA primary O^- beam focused to a ~25-µm elliptical spot. A mass resolution of 7,000 M/M (full width; 10% maximum) was used to eliminate isobaric interferences. ²⁰⁶Pb⁺/U⁺ ratios were normalized against UO⁺/U⁺ and then calibrated against the AS57 standard. U and Th concentrations were normalized to the ANU SL13 standard (U = 238 ppm and Th = 20 ppm; ±20% variation). Common Pb was monitored with ²⁰⁴Pb, but ²⁰⁴Pb/²⁰⁶Pb is an insensitive monitor of common Pb in the young zircons analyzed. Ages were determined from the ²⁰⁶Pb/²³⁸U ratio at the concordia from an extrapolation through common Pb and the individual data points (²⁰⁷Pb correction). Thus, data on the Tera-Wasserburg plots use uncorrected Pb isotopic compositions. Common Pb, as determined from the ²⁰⁷Pb/²⁰⁶Pb ratio, was low in the epoxy mounts and substantially higher in the thin section. The relatively high common Pb in thin section is probably a result of cracks and grain boundaries containing common Pb rather than being intrinsic to the zircon. Nevertheless, data from all samples provided well constrained ages.

To constrain the lower-temperature thermal history of these samples, we attempted to separate out apatite for fission track analysis. Unfortunately, the lithologies recovered are mostly unfavorable for the presence of apatite and we only found apatite in datable quantities in the microgranite clast (Core 180-1108B-6R). Samples were prepared for fission track analysis using standard procedures (Fitzgerald et al., 1999).