Knowledge of active slip systems and element distribution in deforming minerals is important in understanding deformation processes. Pyrite is such a common mineral in many ore deposits (e.g. auriferous deposits) and in shear zones that a detailed understanding of the mechanisms steering its deformation is necessary. Due to its resistant, pyrite commonly preserves microstructural evidence of both brittle and low-temperature crystal-plastic deformation. The study of intragrain sub-structures within naturally deformed pyrite grains offers the potential, therefore, to investigate the remobilization of chemical elements within the crystal lattice induced by strain. In this study, we combine microstructural observations by electron back-scattered diffraction analysis (EBSD) with pyrite elemental mapping by laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) to study crystal plastic deformation and element mobility in pyrite from a naturally deformed quartz-sulphide vein associated with a strike-slip fault that cyclically experienced transient differential stress and pore pressure oscillations. Initially, LA-ICP-TOFMS imaging was used to image sets of otherwise invisible intragrain sub-structures as highlighted by the structurally controlled accumulation of specific elements (e.g. Co, Ni, Cu, Sn, Ag, As, Sb, Pb). EBSD analysis was subsequently performed in the areas analysed by TOFMS with the aim to study the nature of the intragrain sub-structures. EBSD data show that these chemically defined intragrain sub-structures correspond to low-angle boundaries. Cumulative misorientation profiles across pyrite grains suggest that low-angle boundaries are both related to healed fractures and growth features but also indicate a continuous cumulative lattice misorientation up to >5°. Boundary trace analysis suggests that fracturing is competing with tilt boundary rotation and pressure-solution to accommodate strain and locally concentrate specific chemical elements.
Marchesini, B., Menegon, L., Prando, F., Keresztes Schmidt, P., Garofalo, P.S., Schwarz, G., et al. (2019). Evidence of low-temperature plasticity in naturally deformed pyrite: a LA-ICP-TOFMS-EBSD combined approach (Olkiluoto Island, Finland).
Evidence of low-temperature plasticity in naturally deformed pyrite: a LA-ICP-TOFMS-EBSD combined approach (Olkiluoto Island, Finland)
Marchesini B.
;Garofalo P. S.;Viola G.
2019
Abstract
Knowledge of active slip systems and element distribution in deforming minerals is important in understanding deformation processes. Pyrite is such a common mineral in many ore deposits (e.g. auriferous deposits) and in shear zones that a detailed understanding of the mechanisms steering its deformation is necessary. Due to its resistant, pyrite commonly preserves microstructural evidence of both brittle and low-temperature crystal-plastic deformation. The study of intragrain sub-structures within naturally deformed pyrite grains offers the potential, therefore, to investigate the remobilization of chemical elements within the crystal lattice induced by strain. In this study, we combine microstructural observations by electron back-scattered diffraction analysis (EBSD) with pyrite elemental mapping by laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) to study crystal plastic deformation and element mobility in pyrite from a naturally deformed quartz-sulphide vein associated with a strike-slip fault that cyclically experienced transient differential stress and pore pressure oscillations. Initially, LA-ICP-TOFMS imaging was used to image sets of otherwise invisible intragrain sub-structures as highlighted by the structurally controlled accumulation of specific elements (e.g. Co, Ni, Cu, Sn, Ag, As, Sb, Pb). EBSD analysis was subsequently performed in the areas analysed by TOFMS with the aim to study the nature of the intragrain sub-structures. EBSD data show that these chemically defined intragrain sub-structures correspond to low-angle boundaries. Cumulative misorientation profiles across pyrite grains suggest that low-angle boundaries are both related to healed fractures and growth features but also indicate a continuous cumulative lattice misorientation up to >5°. Boundary trace analysis suggests that fracturing is competing with tilt boundary rotation and pressure-solution to accommodate strain and locally concentrate specific chemical elements.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
