Precise 40Ar/39Ar dating of multiple potassic minerals constrain the age and rapid cooling history of the Walgidee Hills lamproite pipe, Kimberley Region, Western Australia, at 17.49 Ma

Abstract

To determine the emplacement age of the initial eruptive phase of the Walgidee Hills lamproite pipe in the Kimberley region of Western Australia, the veracity of utilizing multi-mineral dating of the non-standard potassium-bearing minerals potassic richterite, wadeite, jeppeite, and priderite and was tested by ⁴⁰Ar/³⁹Ar dating, along with the more traditionally-utilized potassium-rich mineral phlogopite. All minerals give excellent single-crystal plateau ages ranging from 17.82 ± 0.53 to 17.0 ± 1.6 Ma (2σ), and with age uncertainties ranging from ±0.14 Ma to ±1.6 Ma. The variation in precision is dependent on both the volume and K₂O content of the crystal analyzed. The mean age of the combined data set is 17.487 ± 0.083 Ma [± 0.086; inclusive of all sources of uncertainties] (n = 10; MSWD = 0.50; P = 0.87) and represents the initial crystallization age of the Walgidee Hills diatreme, the largest known and youngest diamondiferous lamproite in the world. The lack of age difference, despite the probable variation in mineral closure temperatures, confirms rapid cooling. Although most of the minerals analyzed are rare to absent in common magmatic rocks, some are widely distributed in alkaline complexes and in lamproite, kamapugite and kimberlite intrusions. Our results show these K-rich minerals are excellent for ⁴⁰Ar/³⁹Ar dating and can be used singly or in combination to obtain precise magmatic crystallization ages, and possible cooling rates, of ultrapotassic rocks. For a suite of potassic rocks generated in a plume setting, they can track the rate of plate motion.