Controls on metamorphic equilibration: the importance of intergranular solubilities mediated by fluid composition

Carlson, William D.; Hixon, J. D.; Garber, Joshua M.; Bodnar, Robert J.

doi:10.1111/jmg.12113

Cited by 26 publications

(33 citation statements)

References 61 publications

(100 reference statements)

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“…The surface chemistry implies that Mn is transported preferentially to garnet, perhaps sourced from nearby biotite, along grain boundaries between quartz and muscovite. This is apparently at odds with suggestions of relatively sluggish intergranular diffusion of Mn (Carlson, Hixon, et al, ).…”

Section: Discussionmentioning

confidence: 99%

“…The transport of chemical components within rocks is recognized as a rate limiting process during metamorphism that facilitates the growth and dissolution of minerals and allows equilibrium to be established between phases (Carlson, Hixon, Garber, & Bodnar, ; Mueller, Watson, & Harrison, ). Volume diffusion within minerals is known to be relatively slow in metamorphic conditions as demonstrated by the preservation of growth zoning of cations in many metamorphic porphyroblasts (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

Dempster

Symon

Chung

2017

Journal Metamorphic Geology

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Novel approaches to garnet analysis have been used to assess rates of intergranular diffusion between different matrix phases and garnet porphyroblasts in a regionally metamorphosed staurolite‐mica‐schist from the Barrovian‐type area in Scotland. X‐ray maps and chemical traverses of planar porphyroblast surfaces reveal chemical heterogeneity of the garnet grain boundary linked to the nature of the adjacent matrix phase. The garnet preserves evidence of low temperature retrograde exchange with matrix minerals and diffusion profiles documenting cation movement along the garnet boundaries. Garnet–quartz and garnet–plagioclase boundaries preserve evidence of sluggish Mg, Mn and Fe diffusion at comparable rates to volume diffusion in garnet, whereas diffusion along garnet–biotite interfaces is much more effective. Evidence of particularly slow Al transport, probably coupled to Fe3+ exchange, is locally preserved on garnet surfaces adjacent to Fe‐oxide phases. The Ca distribution on the garnet surface shows the most complex behaviour, with long‐wavelength heterogeneities apparently unrelated to the matrix grain boundaries. This implies that the Ca content of garnet is controlled by local availability and is thought likely to reflect disequilibrium established during garnet growth. Geochemical anomalies on the garnet surfaces are also linked to the location of triple junctions between the porphyroblasts and the matrix phases, and imply enhanced transport along these channels. The slow rates of intergranular diffusion and the characteristics of different boundary types may explain many features associated with the prograde growth of garnet porphyroblasts. Thus, minerals such as quartz, Fe‐oxides and plagioclase whose boundaries with garnet are characterized by slow intergranular diffusion rates appear to be preferentially trapped as inclusions within porphyroblasts. As such grain boundary diffusion rates may be a significant kinetic impediment to metamorphic equilibrium and garnet may struggle to maintain chemical and textural equilibrium during growth in pelites.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

Dempster

Symon

Chung

2017

Journal Metamorphic Geology

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“…Spear and Daniel 1998) highlights another instance in which a purely equilibrium-based interpretation would be in error (Carlson et al 2015, and references therein). As seen in Figure 5, porphyroblasts that grew in locally quartz-rich, mica-poor environments (crystals A, B, C, with quartz-rich layers identified by black zones in the Mg map) exhibit patchy overprint zoning for Mn, Fe, and Mg, incorporating layering from the precursor; but for Ca and Y, these same crystals display smooth concentric zoning, reflecting rock-wide equilibration.…”

Section: Evidence From Mineral Zoningmentioning

confidence: 94%

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

Carlson

Pattison

Caddick

2015

American Mineralogist

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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.

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“…Carlson, 2010;Carlson et al, 2015) but that Al 2 O 3 mobility is commonly rate limiting in texture development (e.g. Previous work shows that the relative intergranular transport rates of major mineral-forming species can vary with fluid availability and composition (e.g.…”

Section: Methodsmentioning

confidence: 99%

Shooting at a moving target: phase equilibria modelling of high‐temperature metamorphism

Guevara

Caddick

2016

Journal Metamorphic Geology

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Thermodynamic modelling and calculation of P–T pseudosections are commonly employed for quantifying the P–T evolution of metamorphic rocks. A key assumption involved in interpreting a P–T pseudosection is that the bulk‐rock composition used is representative of the effective bulk composition (EBC) from which apparently equilibrated mineral assemblages grew. Choosing an EBC can be difficult in cases where the rock has evolved significantly throughout the P–T history and has become domainal for whatever reason (e.g. loss of fluid and/or melt), particularly at suprasolidus conditions. During partial melting, melt migration may not only change the bulk composition by melt loss but also may generate local variations due to the variable consumption/loss of melt from domain to domain to create volumes of rock that were once internally equilibrated in the presence of a grain boundary melt, but which departed from equilibrium as inter‐granular mobility was slowed by local reductions in melt volume. As well as careful consideration of an EBC, the results of thermodynamic modelling are highly dependent on the specific thermodynamic data set and solution models used, as updates to these data sets may lead to substantially different calculated phase equilibria. This contribution addresses: (1) how consideration of evolving EBCs at multiple scales of observation can be used to resolve the history of complex high‐grade rocks, and (2) how use of different thermodynamic data sets and a–x models (i.e. thermocalc ds5.5 v. ds6) can result in different interpretations of metamorphic evolution. This study investigates the evolution of a mineralogically heterogeneous and texturally complex hand sample of granulite from the Gruf Complex (Central Alps). At the hand‐specimen scale, an EBC can be identified and used to constrain the P–T conditions at which the ‘whole rock’ was last in mutual equilibrium, in the presence of intergranular melt that has subsequently been lost or consumed. Smaller macrodomains (~cm scale) and microdomains (~mm scale) can be identified that represent subsequent evolution during and after melt channelization and loss, and P–T pseudosections can be calculated for the compositions of these domains. Using this approach reveals that the sample experienced a clockwise P–T path marked by near‐isothermal decompression following attainment of peak UHT conditions (~960 °C, 8.5 kbar). The approach enables construction of a P–T history of a rock for which P–T pseudosections are otherwise difficult to interpret. Thermodynamic modelling using ds6 yields similar results to those stated above, but suggests: (1) near‐isothermal decompression occurred over a wider pressure range (~0.5 kbar v. 1.5 kbar), and (2) that not all microdomains record this part of the P–T evolution.

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Controls on metamorphic equilibration: the importance of intergranular solubilities mediated by fluid composition

Cited by 26 publications

References 61 publications

Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration

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

Shooting at a moving target: phase equilibria modelling of high‐temperature metamorphism

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