Whole-rock oxygen isotope ratios as a proxy for the strength and stiffness of hydrothermally altered volcanic rocks

Abstract

Hydrothermal alteration is considered to increase the likelihood of dome or flank collapse by compromising stability. Understanding how such alteration influences rock properties, and providing independent metrics for alteration that can be used to estimate these parameters, is therefore important to better assess volcanic hazards and mitigate risk. We explore the possibility of using whole-rock δ 18 O and δD values and water contents, metrics that can potentially track alteration, to estimate the strength (compressive and tensile) and Young's modulus (i.e. "stiffness") of altered (acid-sulfate) volcanic rocks from La Soufrière de Guadeloupe (Eastern Caribbean). The δ 18 O values range from 5.8 to 13.2‰, δD values from -151 to -44‰, and water content from 0.3 to 5.1 wt%. We find that there is a good correlation between δ 18 O values and laboratory-measured strength and Young's modulus, but that these parameters do not vary systematically with δD or water content (likely due to their pre-treatment at 200 °C). Empirical linear relationships that allow strength and Young's modulus to be estimated using δ 18 O values are provided using our new data and published data for Merapi volcano (Indonesia). Our study highlights that δ 18 O values can be used to estimate the strength and Young's modulus of volcanic rocks, and could therefore be used to provide parameters for volcano stability modelling. One advantage of this technique is that δ 18 O only requires a small amount of material, and can therefore provide rock property estimates in scenarios where material is limited, such as borehole cuttings or when sampling large blocks is impracticable.

The thermal conductivity of unlithified granular volcanic materials: The influence of hydrothermal alteration and degree of water saturation

“…In total, 19 rock blocks were collected from different locations around the volcano (sampling locations are shown in Figure 1). These blocks have been previously used in recent studies focused on the influence of hydrothermal alteration on the compressive and tensile strength (Heap et al, 2021(Heap et al, , 2022b(Heap et al, , 2022c and the thermal properties (Heap et al, 2022a) of volcanic rocks.…”

Petrographic and geochemical evidence for a complex magmatic plumbing system beneath Bagana volcano, Papua New Guinea

“…We measured oxygen isotopic compositions at the University of Cape Town, South Africa, using a Thermo DeltaXP mass spectrometer. We prepared samples for analysis using the methods described in Heap et al (2022). Duplicates of the internal quartz standard MQ were run with unknowns and used to calibrate the raw data to the Standard Mean Ocean Water scale, using a δ 18 O value of 10.1 ‰ for MQ (calibrated against the quartz standard NBS-28).…”

“…Given the lack of visible alteration on our lava samples, we do not consider these positively enriched δ 18 O values to reflect hydrothermal alteration (c.f. Heap et al, 2022).…”

Porosity, strength, and alteration – Towards a new volcano stability assessment tool using VNIR-SWIR reflectance spectroscopy

“…Measuring rocks in-situ additionally bypasses complex sample retrieval issues when assessing the mechanical heterogeneity of a volcano, such as highly altered or fractured material which by their nature are often delicate, friable, and difficult to prepare for laboratory analysis. Recent efforts in this direction have shown that rock physical and mechanical properties can be estimated using the percentage by weight (wt.%) of secondary alteration minerals (Heap et al, 2021b), rapid, non-invasive reflectance spectroscopy (Schaefer et al, 2021), and whole-rock oxygen isotopes (Darmawan et al, 2022;Heap et al, 2022) and cation exchange capacity (Revil et al, 2022).…”