2019 | Bulk seismic properties of mantle wedge peridotites

We aim to refine our understanding of the seismic anisotropy of peridotites, which are a component of mantle wedges. Results will provide important constraints for seismological studies (e.g. shear-wave splitting analyses, seismic tomography) necessary to more precisely to validate models of melt migration through these materials (cf. Stewart et al., 2016). To adequately model bulk seismic properties of any heterogeneous rock mass we need to combine those properties of representative samples of all constituent lithologies in a realistic way. This can be done if we know the geometric arrangement of the representative lithologies. We will explore this idea in a relatively lithologically and structurally simple system, namely the Balmuccia peridotite. This rock mass comprises host lherzolites which have varying proportions of pyroxene:olivine, and different fabrics, and a series of cross-cutting pyroxenite dikes. There are also dunite bodies, which in most peridotites are interpreted as channels through which basaltic partial melts migrated. Thus both compositional variation and fabric variation in these rocks primarily reflect migration of melts extracted out of those bodies. The rocks are comparable to those that might be encountered in mantle wedges. Associated work already in progress (University of Otago Master’s project by Mr. Michael Ofman) is determining seismic properties of a representative suite of lithologies using numerical simulations based on crystallographic orientation maps, and laboratory measurements (cf. Zhong et al., 2014). In this research we will determine the 3D distribution of dikes and ‘bulk host rock’ in Balmuccia, using a combination of existing field maps and new detailed maps of selected areas. We will then build a 3D geological model, and use this to combine the seismic properties to determine the bulk behaviour of the body. These results will also be useful to constrain the geometry of the Balmuccia Peridotite body at depth allowing better targeting of proposed ICDP project “DIVE: Drilling the Ivrea-Verbano zonE’ (Pistone et al., 2017).

Stewart, E. Lamb, W., Newman, J., Tikoff, B. 2016. The petrological and geochemical evolution of early forearc mantle lithosphere: an example from the Red Hills Ultra- mafic Massif, New Zealand. Journal of Petrology 57(4), 751-776.
Zhong, X., Frehner, M., Kunze, K., Zappone, A. 2014. A novel EBSD-based finite-element wave propagation model for investigating seismic anisotropy: application to Finero Peridotite, Ivrea-Verbano Zone, Northern Italy. Geophysical Research Letters 41, 7105-7114.