Characterization of the chemical composition and 40Ar-39Ar systematics of intralaboratory standard MAC-83 biotite

Abstract

The composition and Ar systematics of biotite (710-355 µm) from MAC-83, a glassy, rhyodacitic crystal-lapilli tuff, have been characterized by electron microprobe and laser 40Ar-39Ar analysis. Fifty electron microprobe analyses demonstrate that the biotites are homogeneous in composition. Twenty-six laser total-fusion, 40Ar-39Ar analyses of the biotite, calibrated against Taylor Creek Rhyolite sanidine (28.34 Ma) and Fish Canyon Tuff sanidine (28.02 Ma), yielded a weighted mean age (based on the respective errors) of 24.36 ± 0.17 Ma. Mass spectrometric determinations of Ca/K and Cl/K were reproducible yielding Cl/K = 0.050 ± 0.002, Ca/K = 0.025 ± 0.040 (errors at 2s). Both conventional furnace and laser step-heating analysis of MAC-83 biotite reveals that it is characterized by minor excess argon, but that overall it yields a well-defined, internally concordant spectrum. These data indicate chemical homogeneity and regular', reproducible argon-release behaviour, implying suitability as a reference material for 40Ar-39Ar laser geochronology.

The Cenozoic History of Volcanism and Hydrothermal Alteration in the Central Andean Flat-Slab Region: New 40Ar-39Ar Constraints from the El Indio–Pascua Au (-Ag, Cu) Belt, 29°20′–30°30′ S

“…For each mineral separate, ~10 mg of material were wrapped in Al foil and stacked vertically into 11.5 cm long x 2.0 cm diameter containers, which were then irradiated with fast neutrons for 7.5 hours in the McMaster University Nuclear Reactor in Hamilton, Canada. 40 Ar-39 Ar flux monitors-MAC-83 biotite with a published age of 24.36 ± 0.17 Ma (2a) (Sandeman et al, 1999)-were inserted at ~0.5 cm intervals along the irradiation containers. Following irradiation, the samples and monitors were placed in small pits, ~2 mm in diameter, drilled in a Cu sample-holder.…”

Alunite and the role of magmatic fluids in the Tambo high-sulfidation deposit, El Indio–Pascua belt, Chile

“…All analyses were done with a Mass Analyzer Products MAP 216 mass spectrometer at the Queen's University 40 Ar-39 Ar laboratory. About 10 mg of each sample was irradiated for 7.5 h at the McMaster University nuclear reactor in Hamilton, Canada, using biotite standard Mac-83 as a radiation-flux monitor (24.36F0.17 Ma;Sandeman et al, 1999). J-values for individual samples were determined by second-order polynomial interpolation and, for the samples discussed in this article, were about 2.4Â10 À3 and varied by less than 0.02Â10 À3 .…”

40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile

“…Mineral separates were loaded into aluminum foil packets together with neutron flux monitors (24.36 Ma MAC-83 biotite of Sandeman et al, 1999) and stacked vertically into a 11.5 cm long and 2.0 cm diameter container. Samples were then irradiated with fast neutrons at the McMaster nuclear reactor (Hamilton, Ontario) in two 29-h irradiations.…”