2020 | Fluids, melts and pressure changes during fracturing of the lower crust

Fracture and fault systems are potentially important for both generation and migration of melts out of the lower crust. Fractures can propagate in confined rock masses when fluid pressures are sufficiently high that crack tip stresses exceed the fracture toughness, KIC (Irwin, 1957). In this case, principals of elasticity dictate that the fractures have minimum apertures, of order of 10-4 times their lengths (Nüchter, 2017). We aim to explore the fluid pressure drops that occur within such opening fractures (Moulas et al., 2014; Korenaga, 2017) and the effects that they have on the fluids and melts they contain. We need to first assess the pressure changes that accompany fracture formation. We propose to do this by first examining the geometries of a network of fractures within lower crustal rocks to verify that they display realistic apertures compared to length, then to compute the pressure changes that would be consistent with the observed geometric changes. We will also measure the pressures that were actually realised inside the fractures, through microthermometric and compositional study of fluid inclusions within vein-filling materials (cf. Toy et al., 2010). The Balmuccia Peridotite and surrounding gabbros and metapelites in granulite facies that out-crop in the Ivrea-Verbano Zone, NW Italy, are dissected by orthogonal fracture sets, containing both serpentine and solidified melts (Fig. 1). They thus present a good test case for this analy-sis, which could then potentially be applied to other outcrops of lower crustal rocks such as the Valle Fértil-Sierra Famantina complex in Argentina (Ducea et al. 2017) as part of an SFB pro-posal building on TeMaS outcomes.

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