Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?

Putnis, Andrew; Moore, J. Casey; Prent, Alexander; Beinlich, Andreas; Austrheim, Håkon

doi:10.1016/j.lithos.2021.106413

Cited by 17 publications

(26 citation statements)

References 109 publications

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“…This reaction reduces the amount of ferric iron in pyroxene, in agreement with the observed trend in the pyroxene compositions (Table 2). Putnis et al (2021) showed that the granulite pyroxene host amphibole lamellae along the cleavage planes. This mechanism has been discussed by several scientists and identified as another possibility to incorporate OH within a pyroxene grain (e.g., Maresch et al 2009;Buseck 1980, 1981).…”

Section: Clinopyroxenementioning

confidence: 99%

How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

Kaatz

Reynes

Hermann

et al. 2022

Contrib Mineral Petrol

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Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H2O in the form of OH groups. H2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H2O, pyroxene from ~ 50 to ~ 310 µg/g H2O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H2O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H2O increase from ~ 450 to ~ 2500 µg/g H2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

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Section: Clinopyroxenementioning

confidence: 99%

How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

Kaatz

Reynes

Hermann

et al. 2022

Contrib Mineral Petrol

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“…The low-strain domains are composed either of preserved granulite or eclogite that has formed statically; however, in most cases, they feature a mixture of the two covering the entire spectrum from non-eclogitized to completely eclogitized. When eclogitization occurs statically (see Zertani et al 2019 for details on static vs. dynamic eclogitization in Holsnøy) the reacted volume typically forms finger-like domains, that are often parallel to the pre-existing granulite foliation (e.g., Jamtveit et al 2000;Putnis et al 2021;Zertani et al 2019). Similar to the shear zones exposed in the same area the fluid is thought to initially enter the rock volume along fractures and be channelized along shear zones.…”

Section: Geological Contextmentioning

confidence: 99%

“…On the outcrop scale, the main growth direction of this eclogitization feature is parallel to the preexisting granulite foliation. A slice adjacent to the sample presented here was recently described in detail by Putnis et al (2021), and our sample description thus focuses on those features relevant to our results.…”

Section: Outcrop Descriptionmentioning

confidence: 99%

“…However, fluid flow in those examples is intimately linked with ductile deformation. Nevertheless, it has been demonstrated that fluid migration through crustal rocks also occurs without associated deformation by viscous creep processes (e.g., Jamtveit et al 2000;John and Schenk 2003;Mørk 1985;Plümper et al 2017a;Putnis et al 2021). In essentially impermeable rocks fluid flow is attributed to the generation of permeability by hydraulic or reaction-induced fracturing, densification reactions, diffusion along grain and phase boundaries, and interface-coupled dissolution and precipitation reactions (e.g., Ague and Axler 2016;Harlov et al 2011;Hövelmann et al 2010;Jamtveit et al 2000;John and Schenk 2003;Oliver 1996;Plümper et al 2012Plümper et al , 2017bPutnis 2009;Putnis and Austrheim 2010;Putnis and John 2010).…”

Section: Introductionmentioning

confidence: 99%

“…Similarly, fluid migration in feldspar has been linked with transient porosities that ensure that fluids are provided to the reaction front (Plümper et al 2017a). Further, fluid migration into dry rocks was suggested to be driven by mineral reactions that cause disturbances in the local stress field leading to microfracturing (Jamtveit et al 2000) or local pressure variations (Putnis et al 2021).…”

Section: Introductionmentioning

confidence: 99%

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Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks

Zertani

John

Brachmann

et al. 2022

Contrib Mineral Petrol

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Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P–T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.

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Preservation of granulite in a partially eclogitized terrane: Metastable phenomena or local pressure variations?

Cited by 17 publications

References 109 publications

How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks

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