Dating metamorphic rocks


The radioactive parent elements used to date rocks and minerals are: Radiometric dating using the naturally-occurring radioactive elements is simple in concept even though technically complex.If we know the number of radioactive parent atoms present when a rock formed and the number present now, we can calculate the age of the rock using the decay constant.As radioactive Parent atoms decay to stable daughter atoms (as uranium decays to lead) each disintegration results in one more atom of the daughter than was initially present and one less atom of the parent.The probability of a parent atom decaying in a fixed period of time is always the same for all atoms of that type regardless of temperature, pressure, or chemical conditions. The time required for one-half of any original number of parent atoms to decay is the half-life, which is related to the decay constant by a simple mathematical formula.In areas of collision between subduction of the cool oceanic lithosphere perturbs the thermal regime in such a way that high pressures can be obtained at relatively low temperatures, thereby generating blueschists and eclogites (high-pressure facies series) from ocean-floor basalts transported down the subduction zone.Continued subduction of these rocks to great depth may eventually result in either (1) rising temperatures and partial melting of subducted rocks or (2) the melting of hydrated peridotite created by fluids released from metamorphic reactions in the subduction zone that rise into the overlying mantle wedge.Well-developed paired metamorphic belts are exposed in Japan, California, the Alps, and New Zealand.



Most of the high-pressure rocks that have been studied from Japan, California, New Caledonia, the Alps, and Scandinavia record maximum pressures of 10–20 kilobars, corresponding to subduction to depths of approximately 35–70 kilometres.Although the processes that formed each of these mountain belts are broadly similar, in almost all such crustal events at different times and places, there is uniqueness as well as conformity to a general pattern.Metamorphic events in the Alps, the Urals, and the Himalayas all show specific differences: to unravel such differences and their significance is one of the major tasks of metamorphic petrology.This is termed ultrahigh-pressure metamorphism (UHPM). These pressures are particularly noteworthy in that they are recorded in rocks derived from sedimentary rather than basaltic protoliths.

Because of the low density, and hence greater buoyancy, of sediments relative to basalts, many geologists have argued that sediment subduction must be a rather limited process; the coesite-bearing metapelites (metamorphosed pelites) provide important evidence that sediment subduction can and does occur under certain circumstances.

They are the rocks involved in the cyclic processes of erosion, sedimentation, burial, metamorphism, and mountain building, events that are all related to major convective processes in Earth’s mantle.