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Optimization of lithium diffusion modelling in plagioclase: implications for pre-eruptive timescales assessment

Simone Costa1, Matteo Masotta2, 3, Fabio Colle4, Pier Paolo Giacomoni2, 3, Claudia D'Oriano1, Patrizia Landi1

  • Affiliations: 1Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa (INGV-PI), Via Cesare Battisti, 53, 56125 Pisa, Italy; 2Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56126 Pisa, Italy; 3Centro per l'Integrazione della Strumentazione Università di Pisa (CISUP), via Lungarno Pacinotti 43, 56126 Pisa, Italy; 4Dipartimento di Scienze Chimiche, della Vita e della Sostenibilitá Ambientale (SCVSA), Plesso Geologico, Università degli studi di Parma, Via Università 12, 43121 Parma, Italy;

  • Presentation type: Talk [Invited]

  • Presentation time: Thursday 08:30 - 08:45, Room S150

  • Programme No: 1.1.1

  • Theme 1 > Session 1

Abstract

Lithium diffusion modelling in plagioclase is used to retrieve timescales, varying from seconds to hours, of fast pre-eruptive processes, such as decompression-induced degassing. Effects related to diffusion anisotropy and random orientation of the exposed crystal surface can impact models results yielding to underestimation of Li diffusion timescales. In order to evaluate these effects, we applied Li diffusion modelling to core-to-rim profiles acquired by LA-ICP-MS on different sections of 8 plagioclase crystals from Stromboli volcano (Italy). The crystal sections include the (010) basal section intersecting the core of the crystals and multiple (3 to 4) parallel sections oriented perpendicular to (010), subsequently exposed at distance steps of ~65--270 µm starting from the section intersecting the core of each crystal. The model accounts for Li analytical uncertainty and, through bootstrapping, finds a best-fit timescale in a set of 1000 time values. Longer best-fit timescales were obtained in section perpendicular to (010) intersecting the core of each crystal. Depending on diffusion coefficient (D), best-fit timescales range from 129 to 1134 sec (D1 = 4.5e-11 m2 s-1) and from 803 to 7405 sec (D2 = 6.7e-12 m2 s-1). Timescales determined in other crystal sections are 58--94% shorter, meaning that large underestimates can derive from random crystal surfaces. By comparing these estimates with Li diffusion timescales obtained from previous studies at Stromboli, we discuss the influence of the proper crystal section, D and error propagation. The presented methodology allows for a more robust assessment of Li diffusion timescales in plagioclase and is applicable to other volcanoes.