Retention of inherited Ar by alkali feldspar xenocrysts in a magma: Kinetic constraints from Ba zoning profiles

Renne, Paul R.; Mulcahy, Sean R.; Cassata, William S.; Morgan, Leah E.; Kelley, Simon P.; Hlusko, Leslea and Njau, Jackson (2012). Retention of inherited Ar by alkali feldspar xenocrysts in a magma: Kinetic constraints from Ba zoning profiles. Geochimica et Cosmochimica Acta, 93 pp. 129–142.

DOI: https://doi.org/10.1016/j.gca.2012.06.029

Abstract

⁴⁰Ar/³⁹Ar dating of volcanic alkali feldspars provides critical age constraints on many geological phenomena. A key assumption is that alkali feldspar phenocrysts in magmas contain no initial radiogenic ⁴⁰Ar (⁴⁰Ar*), and begin to accumulate ⁴⁰Ar* only after eruption. This assumption is shown to fail dramatically in the case of a phonolitic lava from southern Tanzania that contains partially resorbed xenocrystic cores which host inherited ⁴⁰Ar manifest in ⁴⁰Ar/³⁹Ar age spectra. Magmatic overgrowths on the xenocrysts display variable oscillatory zoning with pisodic pulses of Ba enrichment and intervals of resorption. Ba concentration profiles across contrasting compositional zones are interpreted as diffusion couples. Inferred temperature time histories recorded by these profiles reveal significant variations between phenocrysts. Combined with Ar diffusion kinetics for alkali feldspars and magma temperature inferred from two feldspar thermometry, the results indicate that >1% inherited 40Ar can be retained in such xenocrysts despite immersion in magma at ~900°C for tens to >100 years. In cases where the age contrast between inherited and magmatic feldspars is less pronounced, the age biasing effect of incompletely degassed xenocrysts may easily go undetected.

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