Website by Marco A. Lopez-Sanchez - Last update: 2024-01-12

The OUTCROP project at a glance

The OUTCROP project stands for

From the Lower Crust to the mantle: elastic properties, anisotropy, and water content of the Cabo Ortegal complex

This is a research project funded by the PCTI-Asturias (Spain) started in December 2021 and led by Marco A. Lopez-Sanchez at the University of Oviedo. We aim to determine the average seismic properties (wave speeds & anisotropy) and water content of an ancient volcanic arc root section (lower crust and mantle), a key building block in the assembly of the continental crust where these parameters are not well constrained. To determine the seismic properties, we use a two-step approach combining direct measurements on rocks exhumed from the lower crust and the mantle and determining average rock properties (density, mineral content) at depth using thermodynamic equilibrium modelling. For water tracing, we will use FTIR on nominally anhydrous minerals (NAMs). The core team is shared between the University of Oviedo (Asturias, Spain) and the IACT-CSIC in Granada (Spain) with collaborations from Geosciences Montpellier (France).

Why the OUTCROP project?

The lower crust has become the focus of recent attention as we now better understand that its properties are relevant to understand earth dynamics, the chemical origin of crustal rocks, the assembly of the continental crust, the role of fluids at depths, and seismology. Despite this, the lower crust of subcontinental and transitional tectonic plates remains the least known section of the Earth's "rigid" outer layer (aka the lithosphere). Located at depths beyond the current limit of drilling (~15-40 km), only geophysical (indirect) methods can probe the current structure (radial and lateral) of the lower crust. Achieving high-resolution images and composition of the depth lithosphere from seismic or magneto-telluric data requires precise knowledge of rock properties such as density, wave speeds and anisotropy or water content at those depths. We aim to determine these properties in a volcanic arc root section, a tectonic setting where these parameters are not well constrained.

Although there is no technology yet capable of making direct measurements at lower crust/mantle depths, Earth scientists can explore the composition and structure of the Earth at these depths by studying deep rocks brought to the surface during tectonic and volcanic processes. For example, exhumed rock fragments during volcanic eruptions, known as xenoliths, or portions of the lower crust (granulitic terrains) and mantle brought to the surface during tectonic events. Xenoliths pose clear limitations as they provide no information on the lithosphere structure and tend to under-represent some lithology types. Granulitic terrains provide information on how the properties of depth rocks vary radially and laterally in a limited portion of the lithosphere, making them ideal for this task.

The presence of tectonically emplaced granulite terranes at the Earth’s surface is limited and scattered worldwide. Besides, these terrains also pose challenges. In many cases, the original geodynamic setting is not well known and subject to speculation. Likewise, the original structure and mineralogical composition may be obscured by deformation and metamorphic recrystallisation that might induce important chemical and/or physical changes in the rocks modifying their original properties. This always raises the question as to whether the rocks observed at the present surface are representative of those that once resided at deep, where pressures exceeded 0.6 GPa. Another desirable but rare feature to find in such terrains is the crust-mantle transition (i.e. the Mohoroviĉić discontinuity or Moho). This is a sharp seismic discontinuity in the Earth where the P-wave speed increases from ~7 to 8 km s^-1 with broad implications for lithospheric strength models and seismic interpretation. To sum up, an ideal granulitic terrain for determining seismic properties should meet the following criteria: (1) allow a systematic study of their properties (fairly good rock exposure), (2) include the crust-mantle transition, and (3) allow reconstruction of its properties over time (i.e. mineral/microstructure changes induced by the exhumation process are easily identifiable).

Water content affects several physical properties of rocks such as melting temperature, rheology, diffusion, elastic/seismic properties, and electrical conductivity. It is also key for a correct estimate of P and T based on thermodynamic modelling (i.e. water fugacity). At depths below 15 km, water is mostly contained within hydrous minerals, such as mica and amphibole, that normally become unstable at the elevated pressures and temperatures typical of the lower crust. Indeed, the continental lower crust and mantle are generally considered dry (i.e. no free fluids at phase boundaries). During exhumation, water-unsaturated rocks react with any available water to produce hydrous minerals that may obscure the hydrated or dry nature of the pristine rock at depth. The determination of trace amounts of water (OH & H) in the crystalline structure of persistent nominally anhydrous minerals (NAMs) is a gateway to prove the existence of water at these depths, provided that NAMs behave as closed systems during exhumation. Despite having this option available, the water content in the lower crust remains highly unconstrained due to measurement limitations in NAMs: a low water content and FTIR orientation biases caused by the high anisotropy of most minerals forming the lower crust. We aim to develop new procedures to overcome these limitations within the project.

In a nutshell, the central goal of the OUTCROP project is to determine the typical seismic properties and water content of a volcanic arc root section by studying an exhumed granulite and mantle section with exceptional features. Average rock properties will be determined by combining direct measurements in rock samples, the radial and lateral distribution of these properties within the terrane, and thermodynamic equilibrium modelling. With this knowledge, we will be able to answer questions that have direct applications for geophysicists (modelling the state of deformation and strain rates in the lithosphere), seismologists (e.g. infer composition & structure from seismic data, how mineralogical changes and reaction fronts affect the seismic response) and petrologists (understanding the processes that make up the lithosphere), and boost the state of the art of different techniques that serve to constrain seismic anisotropy and water content of deep rocks.

How will we do it?

The project is subdivided into five work packages summarized in figure 1 below.

Figure 1. Synoptic board summarizing the research methodology, general and specific goals (“work packages”), and their links.

Seismic properties of deep rocks (> 15 km) can be measured using two approaches: direct laboratory measurements using high-temperature and high-pressure (HT-HP) devices or calculated from mineral content and rock microstructure using averaging schemes. Laboratory measurements are challenging. First, the number of measures allowed by HT-HP devices is usually fewer than that required to characterize the full elastic tensor that allows estimating the seismic wave speeds in any direction, i.e. the anisotropy. Second, the range in applied pressure and temperature in many of these devices does not cover lower crustal and mantle conditions, e.g. pressures are usually limited to < 0.5 GPa (lower crust > 0.6 GPa) and only room temperatures apply for setups that allow restoring the full elastic tensor. Another main limitation of the laboratory approach is that the tiny size (millimetre scale) of the sample required by high-pressure devices often prevents measurements on representative elementary rock volumes, especially for the lower crust and mantle rocks where the grain size is usually in (or close to) the centimetre range. At pressures above ~0.6 GPa, seismic wave speeds are no longer influenced by extrinsic factors such as microcracks and pores (the high confining pressures cause pores/cracks to collapse) and the bulk rock density and the crystal preferred orientation (CPO) of anisotropic minerals are the two main variables at play controlling the seismic wave speeds and anisotropy. This makes the approach based on modelling very suitable for the study case, where confining pressures exceeded 1.0 GPa (see Fig 2).

Determine the seismic properties using averaging schemes

We use a two-step approach combining direct measurements on rocks from the exhumed lower crust and lithospheric mantle, and modelling the average rock seismic properties at depth. This approach requires three different work packages (WPs 2, 3 and 4 in figure 1). Firstly, measuring the crystallographic orientation of all the main mineral phases composing the rocks. This will be done by making wide-coverage orientation maps using electron backscattering (EBSD). Secondly, to establish the modal proportions of the different minerals and thus the density of the rocks lying at these depths at different stages of their evolution. For this, we will combine different chemical analysis techniques and use the Perple_X software for thermodynamic modelling. Finally, we will apply different seismic property averaging schemes using Matlab (MTEX) and own Python codes by jointly analysing the crystallographic orientation and thermodynamic equilibrium data together with the distribution of the different rocks.

We also aim to determine water content in persistent nominally anhydrous minerals or NAMs using Fourier Transform Infrared (FTIR). The determination of trace amounts of water (hydroxyl & hydrogen) in NAMs is challenging due to measurement difficulties with traditional FTIR. First, due to the low water content of common NAMs that have an H₂O-carrying capacity of ~100-1000 ppm, and second, because FTIR spectroscopy have orientation biases caused by the high anisotropy of most NAMs forming the lower crust (i.e. the signal depends partly on the crystallographic orientation of the mineral). To overcome these limitations, we propose to combine the use of crystallographic orientation mapping using EBSD and FTIR measurements coupled with the development of free and open-source codes for the processing of these data considering the orientation bias.

The target section

The Cabo Ortegal Complex (COC) is a well-exposed metamorphic terrane in north-western Spain (Fig. 2a). It includes high-P and high-T rocks surrounded by arc-volcanic and oceanic-derived rocks. The high-P and high-T unit is condensed in a sequence of ~1 km thick that contains four major mappable rock units (Fig. 1b). So far, most studies focused on the geological history of the COC terrane rather than their physical properties with few exceptions (Brown et al. 2009; Ábalos et al. 2011 Llana-Fúnez and Brown, 2012).

The general structure, local composition, deformation/metamorphic sequence, and geochronology is fairly studied. Petrologic and chemical criteria indicate that the high-P Garnet (Opx-free) granulites and the underlying mantle rocks belong to a transitional (island-arc root) lower crust section (e.g. Gil-Ibarguchi et al. 1990 Galán and Marcos, 1997; Moreno et al., 2001; Tilhac et al., 2016). The most striking feature of the COC section is that it exposes a full transitional section from the lower crust to the mantle (i.e. the Moho discontinuity lies in situ) which is a rare occurrence for a continental or transitional section. The focus on this particular section is twofold: the seismic properties of this type of transitional lithosphere differ significantly from those of the continents (the Moho appears as a fuzzy discontinuity) and they have been proposed as one of the places where the continental Moho can originate, hence its major scientific interest in geosciences.

Figure 2. (a) Location of the Cabo Ortegal complex. (b) Geological map of the HP-HT units showing the rock sequence and peak P-T conditions. (c) Geological cross-section along the Uzal massif after Marcos et al. (d) Simplified P-T grid with eclogite, granulite and amphibolite facies for Si-rich mafic compositions. Coloured field corresponds to the HP granulites stability field. Mineral content in equilibrium listed in each field. The inferred metamorphic path and events of the HP rocks are in red.

The COC rock sequence records changes in mineralogy and microstructure due to the exhumation. Peak pressure and temperature conditions, metamorphic events, and mineral assemblages are summarized in figure 2d. Mantle rocks show pervasive serpentinization with the original microstructure locally preserved. According to previous studies, lower crustal rocks record three major metamorphic events (Fig. 2d). Eclogites and high-P granulites in contact with mantle rocks, referred to as the lower member of Bacariza formation and mainly composed of the so-called pyrigarnites (Vogel, 1967) and hornblendites, record the highest pressure conditions (event M1). Later transformed into high-P granulites (event M2) characterized by the presence of garnet, clinopyroxene, ±plagioclase and the lack of orthopyroxene (Fig. 2d). Plastic deformation causing the layering and the strong resetting of preferred orientation of the minerals mainly occurred at the transition from M2 to M3 event. Data on water content on NAMs formed during the high-P high-T metamorphism is yet to be reported. Overall, the local preservation of high-pressure mineral assemblages and microstructure and the state-of-the-art on the general COC metamorphism make these rocks an exceptional target to study how the seismic response varies during exhumation of the lower crust. We will focus the collection of samples on the Uzal/Ouzal area (Fig. 2b), where the lower crust section and the Moho outcrops with the best conditions.

Publications

Available soon

Codes and databases

PyRockWave (in active development): a Python-based tool for the analysis of elastic properties of rocks and minerals and modelling wave speeds based on physical properties (elastic tensor, density, crystal preferred orientation) and averaging schemes

more info: https://marcoalopez.github.io/PyRockWave/

The Mineral Elastic Database (MED) is a database project on elastic properties of common rock-forming minerals born out of the OUTCROP project. This database differs from other existing ones in that it is a reactive database, i.e. a database consisting of functions that return the elastic properties of minerals under specific conditions set by the user. The idea is to provide an up-to-date, well-documented database of elastic properties with rigorous tracking of changes (key to reproducibility). You can see an example of how to access the database using the PyRockWave codes here: https://github.com/marcoalopez/PyRockWave/blob/main/src/example_database.ipynb.

Both the PyRockWave tool and the database arose from the need to process data from the OUTCROP project, but will continue to be developed independently of this project in the future.

Talks

Invited keynote speaker of MTEX Workshop 2023 (Freiberg, Germany, 2023-03-14)

https://mtex-toolbox.github.io/workshop23

Datasets & multimedia content

Available soon

Core team

Marco A. Lopez-Sanchez (PhD in Geology, 2013, Oviedo) is a fixed-term hired researcher at the Department of Geology of the University of Oviedo. He has six years of postdoctoral experience, four of which have been in the Manteau et Interfaces research group at Geosciences Montpellier (France) as a CNRS researcher including an MSCA-COFUND grant. Marco is an expert in microstructure and texture (CPO) analysis of solid materials (rocks, minerals, alloys) using Electron Backscatter Diffraction (EBSD), image analysis, and digital image correlation (DIC) techniques, the development of in situ and ex situ experimental methods, and modelling rock elastic properties (e.g. seismic wave speeds and anisotropy). He also has extensive experience in programming (Python, Matlab) and code development for data analysis (https://github.com/marcoalopez). By 2022, he begins a new stage in his career as a researcher in Spain to develop the project OUTCROP as principal investigator.

Personal website: https://marcoalopez.github.io/

Sergio Llana-Fúnez (PhD in Geology, 1999, Oviedo) is a lecturer in geodynamics in the Department of Geology at the University of Oviedo expert in rock deformation, crustal structure and tectonics of orogens, and microstructural analysis. After his PhD, Sergio spent nearly ten years in several postdoctoral positions (ETH Zürich, University of Manchester and University of Liverpool) including MSCA and NERC Postdoctoral grants. He returned to Oviedo holding a Ramón y Cajal fellowship in 2009. He led 10 research projects and was the Head of the Department of Geology at the University of Oviedo between 2016 and 2018. Sergio has published five papers on the Cabo Ortegal complex, two of which related to its seismic properties, several geological field guides of the area, and co-authored a detailed geological map of the Cabo Ortegal complex in 2002.

Personal website: http://orcid.org/0000-0002-8748-5623

José A. Padrón-Navarta (PhD in Geology, 2010, Granada) is a tenured scientist at the IACT-CSIC (Spain) and a CNRS researcher at Géosciences Montpellier (France) expert in phase equilibria and metamorphism of ultramafic rocks, fluid-rock interactions, and NAMs. After his PhD, he spent nearly four years in several postdoctoral positions (UGR, ANU Research School of Earth Sciences, Geosciences Montpellier) including an MSCA global fellowship. He obtained a full-time researcher position at the CNRS in 2014 (Géosciences Montpellier) and moved to IACT-CSIC (Granada, Spain) in 2020 with an RyC fellowship. Within the OUTCROP project he supervises FTIR analysis, metamorphic interpretation and thermodynamic modelling.

Personal website: https://www.iact.ugr-csic.es/personal/perfil/jose-alberto-padron-navarta/

Collaborators

Andréa Tommasi (PhD in Geology, 1995, Montpellier) is a CNRS senior researcher in Earth Sciences at Geosciences Montpellier specialising in the processes controlling the deformation of the Earth's interior, from the crystal to the plate tectonic scale, and the relations between mantle flow and the anisotropy of its physical properties. Within the OUTCROP project she contributes with her in-depth knowledge of mantle processes..

Personal website: http://www.gm.univ-montp2.fr/PERSO/tommasi/deia-us.html

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