Before explaining about the feldspathoid Leucite (KAlSi2O6), mineral found surrounding the melt crust of Veas-01 Iron Rock, let us tell you briefly about the basis of the 40-Ar/39-Ar Method.Potassium-Argon Dating
The 40-Ar/39-Ar method of dating rocks has its foundations in the potassium-argon (K-Ar) isotopic dating method, a widely used technique for measuring numerical ages on mineral and rocks. Since the K-Ar method was developed over 30 years ago, it has been applied to a diverse range of geological samles to help elucidate many important geological problems of local, regional, or global significance. Particularly notable successes, dependent largely upon dating by the K-Ar method, include the development of the geomagnetic polarity time scale and the numerical calibration of the Phanerozoic geological or relative time scale.
The method is based upon the occurrence in nature of the radioactive isotope of potassium, 40-K, which has a half life of 1250 million years (Ma). This isotope of potassium has a dual decay to 40-Ca and to 40-Ar, and the branch yielding radiogenic argon (40-Ar*) as daughter product provides the basis for the K-Ar dating technique through its accumulation over geological time. In the simplest case of an igneous rock, for example, an unaltered lava, the K-Ar method normally yields an age that is equal to the time that has elapsed since its eruption and cooling. At the high temperature of a magma, the argon contained within the melt will tend to equilibrate with the ambient gas phase, which is likely to be atmospheric in composition at or near the Earth’s surface. Thus, argon that partitions into the melt during its generation in the source region for the magma, and possibly significantly enriched in radiogenic argon, is expected to exchange with argon of atmospheric compositio as it approaches the surface. If complete equilibrium is attained, then effectively all trace of preexisting radiogenic argon that may have been present will be lost. However, subsequent to cooling of the lava, the 40-Ar* generated from the decay of 40-K begins to accumulate quantitatively within the crystal structures of the minerals comprising the rock. At ambient temperature, the radiogenic argon remains trapped within the crystals indefinitely because of its relatively large atomic size of about 1.9 Amstrong. Subsequent measurement of the amount of parent 40-K and daughter 40-Ar* contained within the rock or mineral, combined with the known rate of dacay of 40-K to 40-Ar*, enables an age to be calculated, reflecting the time since eruption and cooling the lava.
The K-Ar dating method was found to give reliable ages on many rapidly cooled igneous rocks, but in some cases it was noted that ages on potassium-bearing minerals from the same rock, whether igneous or metamorphic, were discordant. This initially puzzling phenomenon is relatively well understood in terms of differences in diffusion behavior for radiogenic argon in different mineral structures during slow cooling or during thermal events subsequent to original crystallization. As the radiogenic argon is trapped within crystal lattices as neutral atoms, increased temperature causes diffusive transfer, with the rate of diffusion increasing exponentially with temperature. Thus, rocks that ahve experienced elevated temperatures after crystallization may partially or completely lose accumulated radiogenic argon from their constituent minerals, depending upon the diffusion behavior, the temperature, and te time involved. A K-Ar age therefore may register the time since crystallization and cooling below a critical temperature, the time since cooling after a metamorphic event, or an intermediate age that does not date a particular event, but simply reflects partial diffusion loss of radiogenic argon during a metamorphism. These aspects of the K-Ar dating method can be partly explored by judicious choice of samples for measurement, but can be much more fully exploited using the 40-Ar/39-Ar dating technique to decipher the detailed thermal history of a given region.
The K-Ar method is one of the most versatile and widely applied of the various geochronometers available for dating rocks, and with the advent of the 40-Ar/39-Ar technique the applications are being progressively broadened. In part this is because potassium is the eighth most abundant element in the Earth’s continental crust, comprising about 1 wt% (Taylor and McLennan, 1985). Mineral in which potassium is an essential element are fairly common in nature, and include many of the micas and the potassic alkali faldspars. Potassium also is present in a range of other minerals as a major or minor element, so that the K-Ar method, in principle, is applicable to many rocks and individual minerals. Another reason for its popularity as a dating method is that, with current techniques, there is a very high sensitivity for detection of radiogenic argon. In favorable circumstances, the technique can be applied to igneous rocks as young as a few thousand years, with no older limit in terms of the physical measurements.
Argon Nomenclature
Atmospheric Argon: Argon with the isotopic composition of that found in the present-day atmosphere.
Radiogenic Argon: Argon formed from in situ dacay of 40-K in a rock or mineral.
Trapped Argon: This refers to the argon that is trapped or incorporated within a rock or mineral at the time of its formation or during a subsequent event. For terrestrial samples, the trapped argon component commonly, but not necessarily, has atmospheric composition, and the assumption generally is made that all such argon is atmospheric in composition with 40-Ar/36-Ar = 293.5, and although this commonly is so, there are exceptions. In extraterrestial samples, the trapped argon is very different in compsition from atmospheric argon, often having 40-Ar/36-Ar about 1.
Neutron-Induced Argon: Argon produced in a sample during irradiation in a nuclear reactor, owing to neutron interactions on chlorine, potassium, and calcium.
Extraneous (including excess and inherited) Argon: In those cases in which trapped argon in terrestrial samples has 40-Ar/36-Ar major than 295.5, the value of this ratio in atmospheric argon, the additional 40-Ar commonly is referred to as extraneous argon (cf. Damon, 1968; Dalrymple and Lanphere, 1969). Excess argon is that component of 40-Ar incorporated into samples by processes other that by in situ radioactive decay of 40-Ar. Inherited argon probably is best defined as that 40-Ar, essentially radiogenic, introduced into a rock or mineral sample by physical contamination from older material.
Feldspathoids
Leucite and Nepheline are the only two minerals of diverse group that have been used with any degree of success for K-Ar dating.
Leucite (KalSi2O6) is a relatively rare mineral (see the left micrograph that was took over Veas-01), found mainly in potassium-rich, silica undersaturated lavas, although it has been recognized in some hypabyssal rocks. It is unknown in plutonic rocks, but there is evidence in some rocks from deep-seated environments, as well as in some hypabyss
al and volcanic rocks, that it initially crystallized from the magmas, but is now represented by a mixture of alkali feldspar and nepheline, termed pseudoleucite, or by alteration products. Because of its very high potassium content (up to 17.9%) with only limited substitution of sadium for potassium, leucite is a good candidate for K-Ar dating, but its rarity and the ease with which it alters mean that it has not been used extensively form dating purposes. Studies in which leucite has been employed to date rocks, mainly volcanic, include those by Evernden and Curtis (1965), Radicati di Brozolo et al. (1981), and Tingey et al. (1983). The limited evidence available suggests that Leucite is a reliable geochronometer for datin rocks not subsequently reheated. Experiments on leucite in vacuo by Evernden et al. (1960) showed that most of the argon is lost in 1 or 2 days at about 550ºC, probably owing to its transition from pseudoisometric to isometric structure.
al and volcanic rocks, that it initially crystallized from the magmas, but is now represented by a mixture of alkali feldspar and nepheline, termed pseudoleucite, or by alteration products. Because of its very high potassium content (up to 17.9%) with only limited substitution of sadium for potassium, leucite is a good candidate for K-Ar dating, but its rarity and the ease with which it alters mean that it has not been used extensively form dating purposes. Studies in which leucite has been employed to date rocks, mainly volcanic, include those by Evernden and Curtis (1965), Radicati di Brozolo et al. (1981), and Tingey et al. (1983). The limited evidence available suggests that Leucite is a reliable geochronometer for datin rocks not subsequently reheated. Experiments on leucite in vacuo by Evernden et al. (1960) showed that most of the argon is lost in 1 or 2 days at about 550ºC, probably owing to its transition from pseudoisometric to isometric structure.
Because fo the high potassium content of leucite, relatively large amounts of radiogenic argon are produced per unit time, and this, taken together with the modest amount of atmospheric argon found associated with the mineral, means that it is particularly useful for dating very young rocks. Thus, Tingey et al. (1983) measured leucite separated from two samples of leucite lava on Gaussberg, a volcano on the coast of East Antarctica, and obtaines conventional 52 +/- 3 ka and 59 +/- 2 ka, respectively; the proportion of radiogenic argon to total argon ranged up to 42%. The most comprehensive dating study on leucite so far made was by Radicati di Brozolo et al. (1981), who measured 40-Ar/39-Ar age spectra on separates of this material from pyroclastics in the Albion Hills volcanic complex of the Roman province in Italy. Excellent results were obtained, showing essencially flat age spectra, with a mean age of 358 +/- 8 ka for five of the samples. Biotite from three of the samples gave 40-Ar/39-Ar results concordant with the coexisting leucite, but one sample yielded an older apparent age of 380 +/- 22 ka and the fifth sample gave a youger age of 278 +/- 15 ka. Measurements by the Rb-Sr method on leucite, biotite, and pyroxene on three samples gave ages that are less precise but in good agreement with the 40-Ar/39-Ar leucite ages. Villa (1986), however, reported the presence of excess argon in some leucites from the Roman volcanic province.
Nepheline is a relatively common mineral in alkaline rocks, occurring as a primary phase in many igneus rocks; it is also present in some metamorphic rocks. Although its composition often is close to Na3Kal4Si4O16, more sodic or potassic compositions are found, especielly in volcanic rocks. Nepheline has not been widely used for K-Ar dating, but it appears to yield reliable ages and is quite retentive of radiogenic argon from studies made by Mcintyre et al. (1966) on rocks from the Precambrian Grenville Province in Canada. York and Berger (1970) reported a sigle 40-Ar/39-Ar total fusion age on nepheline concordant with its K-Ar age. Similarrly, Shanin et al. (1967) and Gerling et al. (1969) found that nepheline was suitable for K-Ar dating.
