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Metamorphism
Donna L. Whitney

Metamorphic rocks now exposed at Earth's surface have experienced a vast range of temperatures and pressures during metamorphism in continental and oceanic settings. Metamorphism is therefore closely linked with other processes that involve heat and mass transfer, including magmatism, erosion, geochemical cycling and tectonic exhumation. In recent years, much research has focused on the interactions and feedbacks among these processes, particularly between metamorphism and tectonic and surface processes.

Metamorphism, heat and earthquakes

Metamorphic petrologists have long been fascinated by the hydrothermal alteration of mantle rocks to serpentinite. Most studies have taken place on land — for example, studies of serpentinite lenses in mountain belts — but serpentinization is a much more significant process at oceanic ridges. The discovery of peridotite at the seafloor, including some that are hosts to hydrothermal systems, has generated debate about the role of metamorphic hydration reactions on the heat budget, fluid chemistry and geobiology of oceanic ridges.

One idea is that the heat released by exothermic serpentinization at slow-spreading divergent zones may drive hydrothermal systems, as presented by Gretchen L. Früh-Green and colleagues in the July 25, 2003, Science. This is a controversial idea: Doug Allen and W.E. Seyfried in the March 15, 2004, Geochimica et Cosmochimica Acta, argued, based on experimental results, that serpentinization does not account for the observed vent fluid compositions or temperatures.

Serpentine-bearing rocks have also gained new significance in studies of subduction zones. Oceanic crust, hydrated at or near the sea floor, will dehydrate during subduction. Recent papers (including Hacker et al., Journal of Geophysical Research, v. 108, p. 2030; Yamasaki and Seno, Journal of Geophysical Research, v.108, p. 2212) built on prior work by Simon Peacock (2001) and documented a spatial correlation between the predicted location of dehydration reactions (including deserpentinization) and the locations of subduction zone earthquakes.

Metamorphism and surface processes

There has also been much interest in recent years in the coupling between metamorphism, tectonics and surface processes in active orogens, for example, the role of erosion in exhuming metamorphic rocks and driving metamorphic reactions, including crustal melting. The western and eastern ends of the Himalaya Mountains have been a focus of most of these studies, but investigations of other major river systems that cross the Himalaya have also demonstrated a dynamic relationship between erosion and exhumation of metamorphic rocks (Vannay et al., Tectonics, v. 23).

Patterns and rates or crystallization

Feedback between metamorphism and deformation has been a topic of longstanding interest, and the theme of recent conference sessions, such as "Understanding Coupled Metamorphic and Deformational Processes" at last year's meeting of the Geological Society of America and "Rates and pattern formation during metamorphic and igneous crystallization" at this year's meeting of the European Geophysical Union. Renewed interest in metamorphic textures focuses on integrating compositions and microstructures to better understand metamorphic crystallization processes, such as garnet nucleation and growth (Dobbs et al., Contributions to Mineralogy and Petrology, v. 146, p. 1). Studies of metamorphic crystal growth are also central to investigations of rates and timing of petrologic and tectonic processes, such as Joerg Hermann and Daniela Rubatto's integration of monazite geochronology with petrologic information from garnet compositions and textures in the December 2003 Journal of Metamorphic Geology.

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Whitney is a professor in the Department of Geology and Geophysics at the University of Minnesota-Twin Cities. E-mail: dwhitney@umn.edu.

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