Deformation Mechanisms and Strain Localization in the Mafic Continental Lower Crust

Chapter 4 has been published in Lithos as Degli Alessandrini et al. (2017) with title: Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province). Co-authors of this publication are Luca Menegon (University of Plymouth), Nadia Malaspina (University of Milano Bicocca), Arjan Dijkstra (University of Plymouth) and Mark Anderson (University of Plymouth)The rheology and strength of the lower crust play a key role in lithosphere dynamics, influencing the orogenic cycle and how plate tectonics work. Despite their geological importance, the processes that cause weakening of the lower crust and strain localization are still poorly understood. Through microstructural analysis of naturally deformed samples, this PhD aims to investigate how weakening and strain localization occurs in the mafic continental lower crust. Mafic granulites are analysed from two unrelated continental lower crustal shear zones which share comparable mineralogical assemblages and high-grade deformation conditions (T > 700 °C and P > 6 Kbar): the Seiland Igneous Province in northern Norway (case-study 1) and the Finero mafic complex in the Italian Southern Alps (case-study 2). Case-study 1 investigates a metagabbroic dyke embedded in a lower crustal metasedimentary shear zone undergoing partial melting. Shearing of the dyke was accompanied by infiltration of felsic melt from the adjacent partially molten metapelites. Findings of case-study 1 show that weakening of dry and strong mafic rocks can result from melt infiltration from nearby partially molten metasediments. The infiltrated melt triggers melt-rock reactions and nucleation of a fine-grained (< 10 µm average grain size) polyphase matrix. This fine-grained mixture deforms by diffusion creep, causing significant rheological weakening. Case-study 2 investigates a lower crustal shear zone in a compositionally-layered mafic complex made of amphibole-rich and amphibole-poor metagabbros. Findings of case-study 2 show that during prograde metamorphism (T > 800 °C), the presence of amphibole undergoing dehydration melting reactions is key to weakening and strain localization. Dehydration of amphibole generates fine-grained symplectic intergrowths of pyroxene + plagioclase. These reaction products form an interconnected network of fine-grained (< 20 µm average grain size) polyphase material that deforms by diffusion creep, causing strain partitioning and localization in amphibole-rich layers. Those layers without amphibole fail to produce an interconnected network of fine grained material. In this layers, plagioclase deforms by dislocation creep, and pyroxene by microfracturing and neocrystallization. Overall, this PhD research highlights that weakening and strain localization in the mafic lower crust is governed by high-T mineral and chemical reactions that drastically reduce grain size and trigger diffusion creep.University of Plymout

Apatite Microstructures and its Volatile Composition in Eucrites

Apatiteis a common phosphate mineral in planetary materials known to contain appreciable amounts of volatiles (F, Cl, OH) [e.g. 1-4]. As such, apatite has recently been of significant interest in assessing the volatile evolution of various bodies within the Solar System via in-situ analysis (e.g. [5-7]). Whilst these works account for the textural context of the apatite grain and the surrounding mineralogy, less attention has been given to understanding as to how the structure of grains may be influenced by metamorphism and shock deformation. Electron Backscatter Diffraction (EBSD) analyses provide structural information at the μmand sub-μmlength scales. In extraterrestrial samples, it is largely used to interpret larger-scale plastic deformation [8] and shock deformation in geochronometers such as zircon and baddeleyite [e.g. 9-11]. Importantly, these studies highlight the importance of understanding deformation at the μm-scale when interpreting complex U-Pbdata and the mobility of Pb, a moderately volatile element. As yet, there have been no studies of how deformation-induced microstructures may influence the abundance and isotopic composition of volatiles in apatite in eucrites. In this study we investigate the microstructure of apatite grains in eucritesfor which H and Cl isotopic composition have been previously reported [12, 13], in order to explore the relationship between crystallographic features of apatite and its volatile content and isotopic composition in eucritesof different shock grades

The microstructure and mechanical properties of microwave-heated lunar simulants at different input powers under vacuum

To achieve a sustainable human presence on the Moon, it is critical to develop technologies utilising the local resources (a.k.a. in-situ resource utilisation or ISRU) for construction and resource extraction. In this study, we investigate the viability of microwave heating of two lunar soil simulants (JSC-1A and OPRH3N) under vacuum conditions, to simulate a lunar surface environment compared to previous studies performed at atmospheric pressure. All simulants are thermally treated in a bespoke 2.45 GHz microwave apparatus using three input powers: 1000 W, 600 W and 250 W. The microstructures and mechanical properties of the microwaved samples are analysed to identify their potential applications. Our key findings are: (i) higher input powers generate materials in shorter fabrication times with higher mechanical strengths and higher yields despite the same total energy input; (ii) the microstructures of the microwaved samples under vacuum are very different from those under atmospheric conditions due to the widespread vesicles/bubbles; and (iii) different heating rates caused by different input powers can be utilised for specific ISRU purposes: higher input powers for extra-terrestrial construction and lower input powers for resource extraction. Findings from this study have significant implications for developing a microwave-heating payload for lunar ISRU demonstration missions

Investigating the microwave heating behaviour of lunar soil simulant JSC-1A at different input powers

For a sustainable human presence on the Moon, it is critical to develop technologies that could utilise the locally available resources (a.k.a. in situ resource utilisation or ISRU) for habitat construction. As the surface soil is one of the most widely available resources at the Moon, we have investigated the viability of microwave heating of a lunar soil simulant (JSC-1A). JSC-1A was thermally treated in a bespoke microwave apparatus using 2.45 GHz frequency, using five different microwave powers in the range of 250 W to 1000 W. The structural properties of the resulting products were analysed to determine whether their microstructures and mechanical strengths differ under different input powers; and whether input power plays a crucial role in triggering thermal runaway, for identifying the optimum power for developing a microwave-heating. Our key findings are: (i) the higher input powers (800 W and 1000 W) generate the highest yields and microstructures with the greatest mechanical strengths, at the shortest fabrication times, and (ii) thermal runaway improves the microwave heating efficiency despite the rapid increase in temperature, once it is triggered. Our findings are of key importance for developing a microwave-heating payload for future lunar ISRU demonstration missions, contributing towards 3D printing-based extra-terrestrial construction processes

The shock response of apatite and its effect on volatiles in eucrites

The abundance and isotopic composition of volatile elements in meteorites are critical for understanding planetary evolution given their importance in a variety of geochemical processes (e.g. [1]). As such, there has been significant interest in the mineral apatite, which is known to contain appreciable amounts of volatile elements. Because shock-induced deformation is pervasive in meteorite samples, it is important to determine if the process has affected their volatile abundance and isotopic composition. Electron Backscatter Diffraction (EBSD) analyses provide crystallographic information at the µm and sub-µm length scales. Recently, greater complexity in microscale deformation features with increasing shock pressure has been observed in phosphate minerals [2–4] and its effect on the abundance and isotopic composition of H has been explored [5]. In this study we investigate the potential linkage between µm-scale shock structures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using EBSD combined with NanoSIMS volatile abundance and isotopic composition of hydrogen and chlorine

Formation of β-Ti phase during L-PBF processing of martensitic NiTi

Shape memory activations in martensitic NiTi are highly influenced by the processing technique used. Laser Powder Bed Fusion (L-PBF) is a metal additive manufacturing method being investigated for NiTi processing. The rapid transient thermal effects involved in L-PBF processing are found to cause microstructural anomalies which can result in unfavourable mechanical and functional properties. In this paper, anomalous hard protrusions observed in L-PBF processed NiTi microstructure were investigated in detail. The spatial distribution and cause of these protrusions were analysed. Studies were conducted using EDX, BSE, EBSD, TEM, DSC, AFM and nanoindentation, to identify the compositions, crystal structures, thermal characteristics and nanomechanical properties of these protrusions. The protrusions were identified to be hard β – Ti phase