Solution to the volume problem in serpentinization

Kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized, or unserpentinized, peridotite. It is found adjacent to shear zones in some serpentinites. Kernels are commonly rectangular, with length/width ratios of 1 to 3. The boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross-fractures. The cross-fractures, which contain either picrolite or chrysotile asbestos, taper out in the core and widen in the rim. They are almost always perpendicular to the core-rim boundary and point into the core of the kernel. In this sense they are radially distributed in the rim. The kernel pattern can be observed in thin section, in outcrop, and on the megascopic scale of tens of metres. Cross-fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel. As the serpentinization front penetrates into the kernel, the serpentinized peridotite expands. However, the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume. As serpentinization proceeds, more expansion occurs, extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen. These fractures are subsequently filled by serpentine minerals. The resulting pattern consists of a rim of serpentinite, cut by serpentine veins, that surrounds a core of peridotite. Measured densities of cross-fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite.