Recent Studies of Serpentinite Occurrences in the Oceans: Mantle-Ocean Interactions in the Plate Tectonic Cycle

摘要

Serpentinite occurrences in the oceans are primarily associated with major fault structures. The serpentinization of peridotites can occur on regional scales as continental rifts open and new ocean basins form. The process of seawater incursion deep into faulted oceanic crust at slow-spreading mid-ocean ridges during periodic amagmatic phases produces bands of serpentinized peridotite tens of kilometers long. Emplacement of these rift-related serpentinites is primarily by vertical tectonism and not diapirism as originally supposed. Serpentinite may form at the ends of mid-ocean rift segments as a consequence of low-angle faulting that results in unroofing of the peridotite protoliths, much as metamorphic core complexes are exposed in continental extensional environments. Deep faults along transforms, the tips of propagating ridges (Hess Deep), or other extensional features such as Kings Trough permit seawater to penetrate the crust and upper mantle to form serpentinite masses. The serpentinites of convergent margins can form simply by seawater interaction with fault-exposed peridotites deep on the inner slope of the trench or in high-relief fault traces within the overriding plate. Some, however, differ fundamentally from those of ocean-continent transition regions and mid-ocean ridges by having formed as a consequence of the interaction of peridotites with fluids of slab origin. The process of serpentine mud volcanism in supra-subduction zone environments is a relatively newly recognized mechanism for the formation of large volumes of chaotic deposits of melange-like serpentinite masses. These may be the origin of some of the world's most complex and voluminous serpentinite terranes of former convergent margins. They occur world-wide and may be recognized by careful analysis of the sheared and unconsolidated matrix serpentines and the variously metamorphosed ultramafic and mafic clasts entrained within the mud flows.