Antigorite polysomatism: behaviour during progressive metamorphism

Abstract

Antigorite forms a polysomatic series of discrete compositions that are chemographically colinear with chrysotile/lizardite, Mg₃Si₂O₅(OH)₄ and talc, Mg₃Si₄O₁₀(OH)₂. The compositional variations of antigorite correspond to discrete changes in the lattice parameter, a. A complete suite of antigorites, collected from a cross-section representing increasing metamorphic grade through the Swiss and Italian Alps, has been studied by optical and transmission electron microscopy (TEM). The specimens within this suite range from those formed near the lower stability limit of antigorite (250 °C) through to those formed near its breakdown temperature (550 °C). The lower grade samples belong to the regionally metamorphosed upper Pennine Ophiolites of the Oberhalbstein-Malenco area, while higher grade antigorites were obtained from regionally metamorphosed Malenco serpentinites. The highest grade samples are also from Malenco. They underwent a later contact metamorphism within the thermal aureole of the Bregaglia Intrusive. The lattice parameter a of antigorites evolves from longer (60 Å) to shorter (35 Å) values with increasing metamorphic grade. However, individual antigorites almost invariably show a heterogeneous distribution of a periodicities with higher values close to grain boundaries or reaction fronts and lower values towards the grain centers. The crystal-chemical evolution of antigorite, expressed by reduction in a, is usually accompanied by increased crystallinity. With the TEM, this is seen as an increase in crystallite size and a decrease in the number of crystal defects (twinning, polysomatic disorder, modulation dislocations, wobbling, offset). The structural and compositional evolution of antigorite requires intracrystalline diffusion and reconstructive transformations at relatively low temperatures. Therefore, the process of evolution is sluggish. Equilibrium is frequently not attained, and relics of longer a periodicities can be observed. In addition, relics of chrysotile may be observed in high-grade metamorphic rocks of the Malenco area, in which antigorite coexists with new-formed olivine. Only at one locality is there evidence of “equilibrium”: antigorite formed at 435 °C has a=43 Å; it shows very little variation in the a periodicity, and it is characterized by a homogeneous annealing texture. A geothermometer based upon a periodicities, as proposed by Kunze (1961) has limited practical applicability.