Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling

Spear, Frank S.; Thomas, Jay B.; Hallett, Benjamin W.

doi:10.1007/s00410-014-1059-6

Cited by 100 publications

(108 citation statements)

References 20 publications

Supporting

Mentioning

101

Contrasting

Order By: Relevance

“…They calculated ΔT = 5-67°C and ΔG r = 0.7-5.8 kJ per mole of garnet. Spear et al (2014) also evaluated the degree of disequilibrium for the garnet-forming reaction. Using a combination of quartz in garnet barometry (QuiG) and thermodynamic modeling for garnet zone metamorphic rocks in eastern Vermont, they calculated a ΔT of 10°C, a pressure overstep of 0.6 kbar, and a ΔG r of 2 kJ per mole of garnet.…”

Section: Discussionmentioning

confidence: 99%

“…Using a combination of quartz in garnet barometry (QuiG) and thermodynamic modeling for garnet zone metamorphic rocks in eastern Vermont, they calculated a ΔT of 10°C, a pressure overstep of 0.6 kbar, and a ΔG r of 2 kJ per mole of garnet. For the staurolite-kyanite zone in the same metamorphic terrain and for a blueschist sample from Sifnos, Greece, Spear et al (2014) calculated a ΔT of 50°C, a pressure overstep of 2-5 kbar, and a ΔG r of 10-18 kJ per mole of garnet. Although diverse methods have been used to infer the degree of disequilibrium for the garnetforming reaction, the calculated values suggest that in all cases the garnet nucleated and grew after a significant overstep from equilibrium conditions.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Diffusion-controlled growth and degree of disequilibrium of garnet porphyroblasts: is diffusion-controlled growth of porphyroblasts common?

Miyazaki

2015

Prog. in Earth and Planet. Sci.

View full text Add to dashboard Cite

Rate-limiting processes and the degree of disequilibrium during metamorphic mineral growth are key controls on the rate of dehydration and hydration in the Earth's crust. This paper examines diffusion-controlled growth and the degree of disequilibrium of garnet porphyroblasts in the Tsukuba metamorphic rocks of central Japan. The analyzed porphyroblasts have irregular and branching morphologies with clear diffusional haloes, indicating that they grew in a diffusion-controlled regime. Mathematical analysis shows that the dominant wavelength of the interface of a garnet porphyroblast is dependent on the extent of supersaturation (Δζ), which is an index for the degree of disequilibrium. Using the calculated upper and lower limits of the dominant wavelength, the value of Δζ is estimated to be 0.05 × 10 −1 -0.16, which corresponds to a Gibbs free energy (ΔG r ) overstep of 0.9-27 kJ per mole of garnet (12 oxygen atoms) and a temperature overstep (ΔT) of 1.7-50°C. Using the average value of the dominant wavelength, the following results are obtained: Δζ = 0.15 × 10 −1 , ΔG r = 2.7 kJ per mole of garnet, and ΔT = 5°C. These values bring into question the importance of diffusion-controlled growth of garnet porphyroblasts, as highly irregular and branching garnet porphyroblasts are rare in most metamorphic belts. After significant overstepping for the nucleation of garnet, the garnet porphyroblasts grow at a high degree of disequilibrium. However, a high degree of disequilibrium under diffusion-controlled growth would be characterized by diffusional instability. The results indicate that garnet porphyroblasts that lack an irregular and branching morphology may grow at a high degree of disequilibrium under interface-controlled growth, provided they are set in a medium where the diffusion and supply of constituent elements are sufficient, such as a sufficient volume of metamorphic fluid.

show abstract

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Diffusion-controlled growth and degree of disequilibrium of garnet porphyroblasts: is diffusion-controlled growth of porphyroblasts common?

Miyazaki

2015

Prog. in Earth and Planet. Sci.

View full text Add to dashboard Cite

show abstract

“…Literature data may also need to be corrected for reaction overstepping (Spear et al, 2014). It will be important to further assess whether the maximum PT conditions truly reflect the maximum conditions during burial or that they were rather locked in during exhumation rather than burial.…”

Section: Discussionmentioning

confidence: 99%

Mafic High‐Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings

Keken

Wada

Abers

et al. 2018

Geochem Geophys Geosyst

View full text Add to dashboard Cite

The oceanic crust that enters a subduction zone is generally recycled to great depth. In rare and punctuated episodes, however, blueschists and eclogites derived from subducted oceanic crust are exhumed. Compilations of the maximum pressure‐temperature conditions in exhumed rocks indicate significantly warmer conditions than those predicted by thermal models. This could be due to preferential exhumation of rocks from hotter conditions that promote greater fluid productivity, mobility, and buoyancy. Alternatively, the models might underestimate the forearc temperatures by neglecting certain heat sources. We compare two sets of global subduction zone thermal models to the rock record. We find that the addition of reasonable amounts of shear heating leads to less than 50 °C heating of the oceanic crust compared to models that exclude this heat source. Models for young oceanic lithosphere tend to agree well with the rock record. We test the hypothesis that certain heat sources may be missing in the models by constructing a global set of models that have high arbitrary heat sources in the forearc. Models that satisfy the rock record in this manner, however, fail to satisfy independent geophysical and geochemical observations. These combined tests show that the average exhumed mafic rock record is systematically warmer than the average thermal structure of mature modern subduction zones. We infer that typical blueschists and eclogites were exhumed preferentially under relatively warm conditions that occurred due to the subduction of young oceanic lithosphere or during the warmer initial stages of subduction.

show abstract

“…6). Recent reinterpretation based upon an alternate model (Spear et al 2014) also yielded apparent garnet nucleation 70-90 °C hotter than the equilibrium garnet isograd, and although the pre-garnet growth history is poorly constrained, any possible path to the point at which garnet eventually grew reflects a similarly large overstepping of equilibrium garnet-growth reactions. The large crystal size and relatively low maximum temperature of the figurE 5.…”

Section: Incongruities Between Calculated and Observed Phase Equilibriamentioning

confidence: 96%

Beyond the equilibrium paradigm: How consideration of kinetics enhances metamorphic interpretation

Carlson

Pattison

Caddick

2015

American Mineralogist

View full text Add to dashboard Cite

The equilibrium model of prograde metamorphism, in which rocks are regarded as departing only negligibly from equilibrium states as they recrystallize, has generated a wealth of petrologic insights. But mounting evidence from diverse approaches and observations over a range of scales has revealed that kinetic impediments to reaction may prevent metamorphic rocks from attaining rock-wide chemical equilibrium along their prograde crystallization paths. To illustrate the resulting potential for inaccurate interpretation if kinetic factors are disregarded, we briefly review several case studies, including: out-of-sequence, metastable, and displaced isograds in contact aureoles; paragenetic sequences documenting overstepped, disequilibrium reaction paths; patterns of compositional zoning in garnet demonstrating partial chemical equilibrium; petrologic incongruities between observation and thermodynamic prediction; and inhibited reaction progress revealed by petrologically constrained numerical simulations of garnet crystallization. While the equilibrium model provides an indispensable framework for the study of metamorphic systems, these examples emphasize that all reactions require departures from rock-wide equilibrium, so all rocks must traverse kinetically sensitive reaction paths during recrystallization. Mindfulness of the potential significance of kinetic influences opens new avenues for petrologic investigation, thereby enhancing both analysis and interpretation.

show abstract

Overstepping the garnet isograd: a comparison of QuiG barometry and thermodynamic modeling

Cited by 100 publications

References 20 publications

Diffusion-controlled growth and degree of disequilibrium of garnet porphyroblasts: is diffusion-controlled growth of porphyroblasts common?

Diffusion-controlled growth and degree of disequilibrium of garnet porphyroblasts: is diffusion-controlled growth of porphyroblasts common?

Mafic High‐Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings

Beyond the equilibrium paradigm: How consideration of kinetics enhances metamorphic interpretation

Contact Info

Product

Resources

About