Potassium-argon dating of basaltic rocks

Abstract

To evaluate the reliability of K-Ar ages on fresh basaltic rocks, various fractions and whole rocks were dated. The samples were chosen to obtain a variety of petrographic types representing several petrologic kindred and cooling environments. The dates provided no evidence of argon leakage. One sample of an unusual rock type (a troctolite) gave an anomalously high age, suggesting excess radiogenic argon. Nepheline separates from two rocks yielded ages equivalent to the ages of biotite and pyroxene. Introduction. Basaltic rocks have been dated by means of the potassium-argon method by numerous workers. Erickson and Kulp [1961] obtained encouraging but erratic results from measurements on the Palisades diabase and Watchung basalt. Since this initial work, numerous ages have been determined on basaltic rocks [e.Most of the previous work has been done on basalt flows. or diabases. Our investigation includes very diverse petrographic rock types of basaltic composition representing a number of cooling environments. We have made at least two argon analyses on each of eighteen rocks and mineral fractions of essentially known age in order to evaluate the method. Our results indicate that whole-rock determinations onpetrographically suitable mafic rocks are as reliable as any mineral fraction that can be dated. Analytical techniques. The radiogenic argon is measured by the isotope dilution method using Ar • spikes. These spikes are prepared in large batches on a manifold, so that they all have the same pressure. This pressure is determined by calibration against a spike of highly purified commercial air argon. Samples are fused by induction heating in tungsten or columbium crucibles. Purification involves the following steps: 1. Dry ice trap. 2. Copper oxide at 600øC. 3. Liquid nitrogen trap. 4. Calcium at 900øC. 5. Adsorption on charcoal at liquid nitrogen temperature in a break seal for transfer to the mass spectrometer. The typical blank is •10 -• tool of atmospheric argon; the best are about 3 X 10 -•2 mol. The mass spectrometer is a Reynolds [1956] type with Faraday cup collector. Flow runs are used for the larger samples and static runs for the smaller ones. Corrections for fractionation are made in both cases. POTASSIUM-ARGON DATING OF BASALTIC ROCKS 1083 flame measurements. Details of both flame tech-

Near-surface geophysical mapping of an Upper Cretaceous submarine volcanic vent in Austin, Texas, USA

“…The Late Cretaceous igneous rocks of the Balcones Magmatic Province (BMP) are highly silica-undersaturated; in order of relative abundance the observed rock types include: mellite-olivine nepheline basanite (50%), olivine nephelenite (30%), phonolite (10%), and alkali basalt (<5%) (Spencer, 1969). Burke et al (1969) reported a K-Ar whole-rock age of 67.5 +/-1.5 Ma. Much more precise dating (U-Pb and 40Ar/39Ar) of the igneous rocks for BMP indicates that the igneous activity occurred in two discrete phases: older mafic volcanism occurred between 81.5 and 84.1 Ma, and younger felsic volcanism between 76.2 and 78.8 Ma (Griffin et al, 2010).…”

Fossil and K-Ar data for the age of the Antalya complex, S W Turkey

“…Analytical technique for potassium measurement was the combination of flame-photometry and precipitation described by Burke et al (1969). Table 1 gives the average of potassium measurements for each sample.…”

Geochronology of Bimodal Alkaline Volcanism in the Balcones Igneous Province, Texas: Implications for Cretaceous Intraplate Magmatism in the Northern Gulf of Mexico Magmatic Zone

“…Comparison of Techniques. The timing and duration of BIP magmatism has not been isotopically well constrained, although three previous studies have been conducted (Burke et al 1969;Baldwin and Adams 1971;Miggins et al 2004). The previous results, along with those of this study, are summarized in table 3 and shown in figure 6.…”

“…On the basis of the major, trace, and radiogenic isotope compositions of BIP igneous rocks, we have suggested that the location and style of magmatism in the BIP were intimately linked to an important lithospheric discontinuity (Griffin 2008;Griffin et al 2009); however, the temporal aspects are less clear because of the paucity of modern, high-resolution geochronological data. Previous studies of the BIP used 40 K /40 Ar techniques to date a number of volcanic centers (Burke et al 1969;Baldwin and Adams 1971), but only a few ages using higherprecision 40 Ar/ 39 Ar techniques are available (Miggins et al 2004), and no U-Pb zircon ages are reported. Although the BIP can be seen in the field to consist of many low-volume, monogenetic volcanoes and plugs, existing radiometric ages suggest protracted activity from 100-66 Ma, on the basis of the stratigraphic relationships of pyroclastic deposits and fossiliferous sediments (Barker 1996), to as short as 82-72 Ma, on the basis of 40 Ar/ 39 Ar geochronology (Miggins et al 2004).…”

“…Younger ages and more complex spectra (especially at the lowest-or highest-temperature steps) were obtained for amphibole and nephelineϩK-feldspar. Overall results from the nephelineϩK-feldspar dating experiment, coupled with the observation that these phases contain a very high proportion of the potassium in BIP samples (whole rock: 0.04-1.65 wt% K 2 O mafic and 3.05-5.34 wt% K 2 O felsic; nepheline: 2.44-12.53 wt% K 2 O; K-feldspar: 2.54-11.71 wt% K 2 O), indicate that whole-rock 40 Ar/ 39 Ar and 40 Ar/ 40 K dating of BIP samples is likely to yield ages that are significantly younger than the true eruption/intrusion/cooling age, explaining the younger ages of Burke et al (1969), Baldwin and Adams (1971), and Miggins et al (2004). The felsic samples are especially prone to age underestimation using whole-rock, groundmass, and nepheline-K-feldspar concentrates because nepheline and Kfeldspar comprise significant proportions of those rocks; the mafic rocks are somewhat less susceptible to these problems because nepheline makes up a much smaller proportion of the rocks and Kfeldspar is not present.…”