Michael Brown
Although the number of geoscientists who practice petrology has decreased, the discipline remains an important specialty. Unraveling the record of the depth-temperature-time evolution of the lithosphere is necessary if we are to understand the geodynamic evolution of Earth. Therefore, it is probably no surprise that at least 1,250 articles identified by the key words metamorphism or metamorphic were published in 2000, and most of them were written in English.
Thus, these highlights can be no more than a personal view of the hot
areas where new depth was revealed. Dominating the metamorphic petrology
literature during 2000 were the questions: How long are periods of metamorphism?
At what rate does burial and exhumation occur? How deep? Under what temperature?
and, How much fluid was present?
[To left: The mineral assemblage sapphirine
and quartz (sapphirine is the blue mineral included in transparent quartz)
is one of the assemblages diagnostic of ultra-high-temperature metamorphism.
It indicates minimum conditions of about 1,040 degrees Celcius at about
10 kilobar.This example is from the Anápolis-Itauçu Complex,
part of the Neoproterozoic Brasília Fold Belt
exposed in Goiás, Central Brazil.
Long dimension of field view is 0.3 millimeters.]
How long? How fast?
An essential element in depth-temperature-time, or P-T-t, path studies, is time. It has proven difficult to measure the length of prograde evolution of metamorphic rocks (metamorphism at a higher temperature and pressure than the rock previously experienced) — in part because of the intrinsic difficulty of preserving a record as temperature increases to the peak in high-grade rocks and, in part, because of the difficulty of relating an “age” to a P-T datum. Simpson and co-authors (Geology, v. 28, p. 403-406) and Foster and co-authors (Earth and Planetary Science Letters, v. 181, p. 327-340) provide examples of the power of an integrated approach — in this case, using mineral assemblages and mineral chronometry.
Is faster better? Harris and co-authors (Chemical Geology, v. 162, p. 155-167) used geochemistry and accessory phase thermometry to argue that melt segregation in the Himalaya may have occurred within 100 years, that ascent through 10 kilometers of crust may have been achieved in one day and that even the largest granite could have been emplaced in approximately 10 years. These rates contrast with the time scale for prograde heating, which, because it likely exceeds 1 million years, means that heat flow is the rate-determining step of crustal melting.
Oliver and co-authors (Geology, v. 28, p. 459-462) demonstrate that the classic Barrovian and Buchan metamorphism in Scotland was contemporaneous with delivery of detrital metamorphic minerals to the southern Uplands flysch basin.
These authors argue that the entire Grampian episode lasted about 15 million years, and that high-grade metamorphic rocks were exposed at the surface within about 8 million years of peak metamorphism. This classic episode of tectonometamorphism was brief and exhumation was fast.
Perchuk and Philippot (International Geology Review, v. 42, p. 207-223) use geospeedometry, based on the kinetics of diffusive processes, to argue for rates of change of several hundreds of degrees Celsius per million years, and of several centimeters per year along the P-T-t path of eclogites. They also argue that tectonometamorphic events were short-lived.
What goes down must come up: How deep, how fast and how?
Ultra-high-pressure metamorphism (UHPM) has been a focus of activity and controversy for more than two decades. At least three thematic issues were published in 2000 that include papers on UHPM: Journal of Geodynamics, v. 30, p. 1-283; Journal of Metamorphic Geology, v. 18, p. 123-219; and The Island Arc, v. 9, p. 258-455). The difficulty of establishing beyond a reasonable doubt the P-T conditions of UHPM is illustrated by the papers of Green and co-authors (Journal of Geodynamics, v. 30, p. 61-76) and Trommsdorff and co-authors (Journal of Geodynamics, v. 30, p. 77-92) concerning the enigma of the Alpe Arami peridotite. The suggestion that these rocks equilibrated at pressures greater than 10 gigapascals gained some support at the end of the year in the paper presented by Paquin and co-authors at the American Geophysical Union Fall Meeting (Eos, v. 81, p. F1104).
How hot do rocks get?
We have known for some time that the Napier Complex in Antarctica records evidence for perhaps the highest temperatures of regional-scale, ultra-high temperature metamorphism (UHTM) on Earth. However, it was still startling to see the paper by Harley and Motoyoshi (Contributions to Mineralogy and Petrology, v. 138, p. 292-307), in which the authors reported evidence for greater than 1120EC, as recorded by aluminum zoning of orthopyroxene in a sapphirine quartzite. Why startling? We have yet to understand fully how regions of lower crust can be heated to such extreme temperature. In comparison, papers by Satish-Kumar (Journal of Geology, v. 108, p. 479-486) on UHTM in granulites from southern India, by Moraes and Fuck (Journal of Metamorphic Geology, v. 18, p. 345-358) on UHTM from central Brazil, and by Barboza and Bergantz (Journal of Petrology, v. 41, p. 1307-1327) on metamorphism and anatexis in the contact aureole of the Mafic Complex in the Ivrea Zone, report peak temperatures that seem ordinary.
How wet is the crust?
Three years ago, Craig Manning’s review in Geotimes contrasted the existence of aqueous brines in deep drill holes in continental crust with petrologic evidence for dry lower crust. The role of fluids in lower-crustal processes remains controversial, and Wannamaker, Yardley and Valley (Journal of Geophysical Research, v. 105, p. 6057-6068) debated the issue following the 1997 paper by Yardley and Valley (Journal of Geophysical Research, v. 102, p. 12,173-12,185). This issue is important to the melt vs. brine argument, competing explanations for high conductivity measured in some sections of deep crust; and to the melt loss vs. carbon-dioxide-flushing models for the origin of granulites. The mechanism of metamorphic fluid flow continued to receive much attention throughout 2000, and I highlight here the papers by Skelton and co-authors (Contributions to Mineralogy and Petrology, v. 138, p. 364-375) and by Cartwright and Buick (Contribution to Mineralogy and Petrology, v. 140, p. 163-179 and Journal of Metamorphic Geology, v. 18, p. 607-624).
Such was the year in metamorphism. Half a century ago we were concerned with equilibrium thermodynamics and how to pinpoint a P-T datum. Today metamorphism is providing the key data to understanding the geodynamic evolution of Earth’s lithosphere.
Brown is Director of the Laboratory for Crustal Petrology and Professor
in the Department of Geology at the University of Maryland, College Park.
E-mail: mbrown@geol.umd.edu
For a longer version of this story that includes a review of metamorphic
research in the Himalaya, visit www.geotimes.org
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