"Fault pearls" - peculiar fault rocks and potential paleoseismic indicators

The Bisciarelle fault is a late-orogenic, brittle reverse fault cutting the lherzolite of the Voltri Massif, Ligurian Alps, Italy. This fault is co-genetic with quartz-carbonate faults hosting orogenic Au deposits of the area. The core of the Bisciarelle fault hosts two levels of large (size: 0.1-1.8 cm), well round, and rhythmically zoned dolomite aggregates set in a dolomite-chalcedony matrix. We call these aggregates fault pearls due to the morphological similarity with the mollusk pearls. We use field data, microtextural data, SEM-energy dispersive X-ray spectroscopy, and element imaging by Laser Ablation ICP Time-of-Flight Mass Spectrometry to document the textural, mineralogical, and chemical compositions of the pearl levels. The pearls are made of an alternation of thick dolomite bands and thin and massive dolomite bands that grew epitaxially. They are broadly coeval with chalcedony shear veins, and are cut by shear veins and surfaces. Elemental LA-ICPTOF-MS imaging shows that thin bands and chalcedony shear veins are enriched in a suite of components (e.g., SiO2, Al2O3, K2O, Mn, Zn, Ga, As, Sb, Ag, and In) with respect to thick bands. We propose that the pearl formed with a process similar to the so-called “transient” boiling in microfluidics. This process occurs within cavities when a liquid is instantaneously overheated and a vapor phase nucleates and expands up to explosive boiling, and so generates a myriad of vapor bubbles. Such process, which occurred during mixed mode fracturing in the fault, implies that the large/round pearls might reflect the liquid-vapor fractionation of chemical elements in a boiling hydrothermal fluid during seismic failure.