The (U-Th)/He picking and grain measurement facilities were funded through JSG start-up funds to Stockli and are also used for U-Pb (LA-ICP-MS and TIMS) and other geochronologic sample preparation.Automated He Extraction Line for (U-Th)/He Dating The UT (U-Th)/He laboratory houses three state-of-the-art, all metal, ultra-high vacuum noble gas extraction and purification lines for measuring 4He.The quadrupole He mass spectrometry systems consist of the following principle components: 1) One Photonmachine 30W Diode Laser and two Photonmachine 75W Diode lasers and a U. Laser continuous-mode Nd-YAG lasers for total fusion He laser extraction, ideal for single-crystal work (see House et al., 2000), 2) two separate all-metal extraction lines equipped with computer-controlled pneumatic Nupro valves and pumped by a combination of ion, turbo and rough pumps, 3) precise volume aliquot systems for spiking sample gas with 3He for isotope dilution, 4) precise volume aliquot systems for delivering a 4He standard with separate depletion tank systems to monitor 4He tank depletion, 5) gas purification system consisting of two SAES NP10 getters and two Janis cryogenic trap capable of separating He from other gases by variable temperature release at 16-37K, and 6) two Blazers Prisma QMS-200 quadrupole mass spectrometer for measuring 3He/4He ratios.Our helium isotope dilution procedure allows very low-blank ( *reference age is published (U-Th)/He age; sample used by several laboratories as intra- and inter-laboratory age standard.The (U–Th)/He age determinations were supported by NSF EAR 0105981. We thank Mike Villenueva and Joann Wartho for their helpful reviews. Comments by Lyons, Swanson, Mark Cloos, Todd Housh and Richard Ketcham have greatly improved this manuscript.
Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings.
The stereomicroscopes have both transmitted (polarized) and reflected light capabilities.
Prior to loading samples into Pt sleeves, all grains are digitally photographed using a Nikon digital Color View® camera and all digital pictures are archived.
Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon.
Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less.