Quantitative temperature‐time information from retrograde diffusion zoning in garnet: constraints for the <i>P–T–t</i> history of the Central Black Forest, Germany

A model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory. The model is quantitatively evaluated for cation diffusion profiles in garnet. Important model parameters were calibrated empirically using diffusion zoning of Ca in garnet from the Pikwitonei Granulite Domain, a terrane for which the thermal history has been well characterized. The results are used: (i) to empirically test diffusion parameters for Mg and Fe(II) and (ii) to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet‐bearing rocks. The thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates. The results overlap within uncertainties in all cases, but result that are based on Fe(II) and Mg chemical‐diffusion profiles are up to three times more precise than those acquired by conventional methods. The remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high‐grade metamorphic rock. The tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression.

Section: Chemical Diffusion a Tool For Thermometrymentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

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Peak metamorphic temperatures from cation diffusion zoning in garnet

Smit

Scherer

Mezger

2013

“…Whereas the zoned rim reflects the progressive equilibration under the new P–T conditions, the unzoned core of the crystals probably preserved garnet composition at the temperature peak (e.g. Tracy, 1982; Loomis, 1983; Bohlen, 1987; Robinson, 1991; Weyer et al , 1999).…”

Section: Petrography and Mineral Chemistrymentioning

confidence: 99%

Palaeoproterozoic high‐pressure granulite overprint of the Archean continental crust: evidence for homogeneous crustal thickening (Man Rise, Ivory Coast)

Pitra

Kouamelan

Ballèvre

et al. 2010

International audienceThe character of mountain building processes in Palaeoproterozoic times is subject to much debate. Based on the discovery of high-pressure granulites in the Man Rise (Côte d'Ivoire), several authors have argued that Eburnean (Palaeoproterozoic) reworking of the Archean basement was achieved by modern-style thrust-dominated tectonics. A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and PT pseudosections calculated with thermocalc suggest granulite facies conditions of ~ 13 kbar, 850 °C and <7 kbar, 700800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A SmNd garnet whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems

“…Both thermal (diffusional) relaxation of growth zoning profiles and retrograde zoning developed at the rims of formerly homogenous high‐temperature phases have been used to obtain time constraints on the cooling/exhumation history of metamorphic rocks (e.g. Chakraborty & Ganguly, 1991; O'Brien & Vrána, 1995; Spear & Parrish, 1996; Duchene et al ., 1998; Weyer et al ., 1999; Ganguly et al ., 2000). The effects of late diffusion on geothermobarometry have been discussed, e.g.…”

Section: Introductionmentioning

confidence: 99%

Constraints on the duration of high‐pressure metamorphism in the Tauern Window from diffusion modelling of discontinuous growth zones in eclogite garnet

Dachs

Proyer

2002

An eclogite sample from the Grossgockner region of the Hohe Tauern, Austria contains garnet with a pronounced compositional discontinuity between a Mn‐rich core and an Fe‐rich rim. This jump in composition was caused by a garnet‐consuming reaction followed by growth of the garnet rim + omphacite and marks the prograde transition from epidote–amphibolite to eclogite facies metamorphism. Garnet growth ended at peak metamorphic conditions of 570 °C, 17 kbar, but intracrystalline diffusion continued until about 450 °C, 4 kbar on the retrograde path. This garnet overgrowth texture represents a natural diffusion couple and a time span of 1 Myr was calculated from the diffusion profile developing out of the original sharp compositional step. For typical crustal densities, this time corresponds to a minimum average velocity in the range 4.6–7.4 cm yr−1 (for vertical movement), which is one of the fastest exhumation rates reported. The diffusion of all divalent cations of four profiles was modelled, both analytically and numerically. Both approaches gave comparable results, but the times computed for each element were always discrepant up to a factor of 2. Variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits. Numerical modelling does not require the simplifications introduced in the analytical approach. On the other hand, error propagation was computationally unfeasible with this method.