An evolutionary model and classification scheme for nephrite jade based on veining, fabric development, and the role of dissolution-precipitation.

Although nephrite jade has been collected and treasured since the Stone Age, we lack a clear understanding of how it forms during deformation and metasomatism in shear zones. Using microstructural analysis of samples from Taiwan, California, and New Zealand, we propose a conceptual model for the evolution of nephrite jade that distinguishes four nephrite types based on mode of formation and textural characteristics: (1) primary (type 1a) or folded (type 1b) vein nephrite, (2) crenulated nephrite (type 2), (3) foliated semi-nephrite (type 3), and (4) nodular or domainal nephrite (type 4). We interpret the texture of our analysed samples to represent snapshots of a progressive textural evolution similar to that experienced by other deformed and fine-grained metamorphic rocks that develop under fluid-present, greenschist-facies conditions. Our observations suggest that types 2 and 3 nephrite can evolve from vein nephrite (type 1) by the development of crenulated and foliated metamorphic fabrics, during which the most important deformation process is dissolution-precipitation. However, development of metamorphic fabrics can be interrupted by transient brittle deformation, leading to the formation of type 4 nephrite that is characterised by nodular or angular clasts of nephrite in a nephritic matrix.

Dynamic earthquake rupture preserved in a creeping serpentinite shear zone.

Laboratory experiments on serpentinite suggest that extreme dynamic weakening at earthquake slip rates is accompanied by amorphisation, dehydration and possible melting. However, hypotheses arising from experiments remain untested in nature, because earthquake ruptures have not previously been recognised in serpentinite shear zones. Here we document the progressive formation of high-temperature reaction products that formed by coseismic amorphisation and dehydration in a plate boundary-scale serpentinite shear zone. The highest-temperature products are aggregates of nanocrystalline olivine and enstatite, indicating minimum peak coseismic temperatures of ca. 925 ± 60 °C. Modelling suggests that frictional heating during earthquakes of magnitude 2.7-4 can satisfy the petrological constraints on the coseismic temperature profile, assuming that coseismic fluid storage capacity and permeability are increased by the development of reaction-enhanced porosity. Our results indicate that earthquake ruptures can propagate through serpentinite shear zones, and that the signatures of transient frictional heating can be preserved in the fault rock record.