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Enrichment of immobile elements in synmagmatic fractures

Michael Heap

Useful minerals containing rare Earth elements (REE) and metals are sourced from magma bodies, but exactly how these elements initially leave the magma is not well known. Here we present textural and chemical analyses of mineral-filled fracture bands within the rhyolitic Sandfell laccolith exposed in eastern Iceland. The fracture fillings showcase dynamic and complex textures and imply multiple energy levels during precipitation. The dominant mineral phases are Fe- and Mg-oxides, Mn carbonate, and La/Ce oxide. The textures they present are comb, laminate, radial, and a rounded reworked clastic texture filling the tips. Microtomography images of hand-samples show the fractures are stretched-penny shaped, and contain 80 vol% fillings and 20 vol% void space. The connectivity of fractures within one band is limited to 1-3 neighbours, via small oblique fractures joining two main fractures together. µXRF measurements revealed distinct halos of 0.8 wt% Fe depletion surrounding each facture, and within the fracture-fill a strong enrichment in an unusual suite of elements including Fe, Mn, Cl, Zn, Cr, Y, Ce, and La. This assemblage is puzzling, as many of these elements are typically carried by fluids which have strong alteration effects on the surrounding rock, and there is a lack of this kind of alteration at Sandfell. Our working hypothesis is that the formation of the fractures provided a degassing pathway through the impermeable magma. However, the nature and the composition of the magmatic volatiles are as yet unknown. The minimal connectivity between fractures (at hand-sample scale) suggests fluid would have travelled through the length of one to three fractures until intersecting with another fracture band system, and minerals precipitated along the way. Given the ubiquitous occurrence of the fracture bands within the laccolith, this small-scale process compounds into large amounts of mass transfer overall. The fractures at Sandfell may be a snapshot of the initial process of removing incompatible elements from silicic magma.

2022

The Permeability of Porous Volcanic Rock Through the Brittle-Ductile Transition

Michael Heap, Gabriel Gérard Dominique Meyer, Corentin Jean-Marie Rémi Noël, Marie Estelle Solange Violay

The permeability of volcanic rock controls the distribution of pore fluids and pore fluid pressure within a volcanic edifice, and is therefore considered to influence eruptive style and volcano deformation. We measured the porosity and permeability of a porous volcanic rock during deformation in the brittle and ductile regimes. In the brittle regime, permeability decreases by a factor of 2-6 up to the peak stress due the closure of narrow pore throats but, following shear fracture formation, remains approximately constant as strain is accommodated by sliding on the fracture. In the ductile regime, permeability continually decreases, by up to an order of magnitude, as a function of strain. Although compaction in the ductile regime is localized, permeability is not reduced substantially due to the tortuous and diffuse nature of the compaction bands, the geometry of which was also influenced by a pore shape preferred orientation. Although the evolution of the permeability of the studied porous volcanic rock in the brittle and ductile regimes is qualitatively similar to that for porous sedimentary rocks, the porosity sensitivity exponent of permeability in the elastic regime is higher than found previously for porous sedimentary rocks. This exponent decreases during shear-enhanced compaction toward a value theoretically derived for granular media, suggesting that the material is effectively granulating. Indeed, cataclastic pore collapse evolves the microstructure to one that is more granular. Understanding how permeability can evolve in a volcanic edifice will improve the accuracy of models designed to assist volcano monitoring and volcanic hazard mitigation.

AMER GEOPHYSICAL UNION2022

The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness

Michael Heap, Marie Estelle Solange Violay

The thicknesss of the brittle lithosphere—the outer portion of a planetary body that fails via fracturing— plays a key role in the geological processes of that body. The properties of both a planet and its host star can influence that thickness, and the potential range of those properties exceeds what we see in the Solar System. To understand how planetary and stellar parameters influence brittle lithospheric thickness generally, we modeled a comprehensive suite of combinations of planetary mass, surface and mantle temperature, heat flux, and strain rate. Surface temperature is the dominant factor governing the thickness of the brittle layer: smaller and older planets generally have thick brittle lithospheres, akin to those of Mercury and Mars, whereas larger, younger planets have thinner brittle lithospheres that may be comparable to the Venus lowlands. But certain combinations of these parameters yield worlds with exceedingly thin brittle layers. We predict that such bodies have little elevated topography and limited volatile cycling and weathering, which can be tested by future telescopic observations of known extrasolar planets.

2021

Enrichment of immobile elements in synmagmatic fractures

Michael Heap

2022