# Transient and periodic brittle deformation of eclogites during intermediate-depth subduction

Kirkland S. Broadwell, Michele Locatelli, Anne Verlaguet, Philippe Agard, Mark J. Caddick

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Kirkland S. Broadwell, Michele Locatelli, Anne Verlaguet, Philippe Agard, Mark J. Caddick. Transient and periodic brittle deformation of eclogites during intermediate-depth subduction. Earth and Planetary Science Letters, 2019, 521, pp.91-102. 10.1016/j.epsl.2019.06.008. insu-03595771

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Distributed under a Creative Commons Attribution 4.0 International License
Transient and periodic brittle deformation of eclogites
during intermediate-depth subduction

Broadwell, K.S.<sup>a*</sup>, Locatelli, M.<sup>b</sup>, Verlaguet, A.<sup>b</sup>, Agard, P.<sup>b</sup>, Caddick M.J.<sup>a</sup>

<sup>a</sup>Dept. of Geosciences, Virginia Tech, 926 West Campus Drive, Blacksburg, VA 24061, USA

<sup>b</sup> Sorbonne Universités, UPMC Univ. Paris 06, CNRS, Institut des Sciences de la Terre de Paris
(ISTeP), 4 place Jussieu, 75005 Paris, France

* corresponding author email address: sbkirk7@vt.edu
Brecciated eclogite clasts from the Monviso ophiolite in the Western Alps fossil subduction zone preserve healed fractures within garnet crystals. These fractures are divided into two distinct populations based on chemical and textural criteria. Diffusion modeling on profiles across fractures identifies a characteristic diffusive timescale for each population, with a mean calculated difference between population timescales of ~2 Myr. We interpret this relative difference in diffusional timescales to represent the maximum period between individual fracturing events during prograde subduction at eclogite facies *P-T* conditions. The period between the two distinct fracturing events is likely controlled by transient fluid pressure and/or strain rate increases during prograde subduction. An increase in pore fluid pressure may be related to the dehydration of adjacent portions of the down-going plate during prograde subduction, while an increase in strain rate may be related to seismicity at shallower depths in the subducting slab. The absolute duration for diffusion, as constrained by diffusion modeling, implies extremely short timescales near peak *P-T* conditions. This suggests that the subducting slab potentially experienced multiple periods of seismic activity immediately prior to detachment and subsequent exhumation from intermediate depths. The presence of similar healed fractures in garnets from eclogites is relatively common in the rock record, implying that similar transient and periodic processes may be relatively common during subduction worldwide and through time.

**Keywords: Episodic Metamorphism, Diffusion, Subduction, Eclogites, Seismicity, Rheology**
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# 1. Introduction

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Metamorphic rocks can preserve evidence for tectonic processes that operate on extremely short timescales (<< 1 Myr.), such as pulsed fluid flow, seismicity, partial melting, and rapid thrusting (Ague and Baxter, 2007; Camacho et al. 2009; Harris et al. 2000; Rubatto et al. 2013).

The recognition of these and other short-timescale processes has significant consequences for our understanding of the physical and chemical evolution of the Earth via tectonic processes and the suitability of equilibrium thermodynamics to model these processes. Chemical and isotopic zoning in minerals is often used to identify the preserved record of short-timescale processes.

However, the identification and interpretation of textures as indicative of short-timescale processes is not always straightforward, due to overprinting, subsequent deformation and/or thermal re-equilibration. Additionally, the temporal resolution of the analytical method used to constrain a timescale may be too large to distinguish between multiple, short-lived events, versus a single, longer event (e.g. Harris et al. 2000).

Recent research has highlighted the importance of seismicity at intermediate depths (~50 – 300 km) in subduction zones, which has been linked to dehydration reactions in both crustal and mantle rocks (e.g. Agard et al. 2018; Hacker et al. 2003). Evidence for paleo-intermediate depth seismicity in the rock record has been argued for based upon the presence of pseudotachylyte in subducted continental crust (Austrheim et al. 1996), ocean crust (John and Schenk, 2006) and lithospheric mantle (Scambelluri et al. 2017). Claims for intermediate depth seismic activity have also been made on the grounds of eclogite facies breccias in Fe-Ti metagabbros (Angiboust et al. 2012; Locatelli et al. 2018). Rhythmic zoning in garnets from the Franciscan complex has recently been interpreted to represent an indirect record of paleoseismicity in subducted
70 metasedimentary rocks (Viete et al. 2018). These and other studies imply that while some
71 intermediate depth seismic activity involves a strong and dry protolith, at other times it appears
72 to involve relatively weak and hydrous lithologies.

73 Multiple hypotheses have been put forward, and no clear consensus yet reached, to explain
74 the generation of intermediate depth seismicity, including transformational faulting, ductile shear
75 instability, dehydration embrittlement and stress transfer (e.g. Hacker et al. 2003; Ferrand et al.
76 2017 and references therein). While seismic activity has been temporally linked to focused fluid
77 flow, there is still considerable debate on whether an influx of fluid causes brittle deformation, or
78 if channelized fluid flow is facilitated by fracturing (Hertgen et al. 2017). Other variables, such
79 as fluid production rate and permeability evolution, are also likely to control rheological
80 behavior in these settings (e.g. Miller et al. 2003).

81 There is an absence of direct quantitative constraints from the rock record on the periodicity
82 of intermediate depth seismic activity, irrespective of causal mechanism. This is due in part to a
83 paucity of recognizably suitable samples that preserve direct and easily interpretable chemical
84 and textural evidence for even a single intermediate depth seismic event. Additionally, protracted
85 deformation in shear zones typically leads to obscuring and overprinting of the evidence for early
86 seismic events. Recently, it has been suggested that single rock volumes may record multiple
87 seismic cycles (e.g. Viete et al. 2018). However, at the time of writing, direct estimates on the
88 timescales of such recurrence is lacking.

89 Here, we report on healed fractures within garnet crystals from the Monviso ophiolite – one
90 of the few localities where a record of intermediate depth seismicity and associated pulsed fluid
91 flow has been recognized (Angiboust et al. 2011; Locatelli et al. 2018). Eclogites from the
92 Monviso ophiolite preserve textural evidence for multiple brecciation events from the outcrop to
thin section scale (Locatelli et al. 2018). In this contribution, we document the occurrence of
multiple texturally and chemically distinct healed fractures populations within garnets from
eclogite breccia clasts. We also utilize preserved chemical diffusion profiles in the healed
fractures to constrain the timescale between successive short-lived, brittle deformation events.
This allows us to place quantitative temporal constraints on the period of this fracturing deep in
subduction zones for the first time. These temporal constraints are then discussed in relation to
the broader subduction and exhumation history of Monviso, with application to subduction zones
worldwide.

## 2. Geologic Setting

### 2.1 Geologic Setting

The Monviso ophiolite represents a relatively coherent and complete slice(s) of Jurassic
Tethyian oceanic crust that was subducted and subsequently exhumed during the Eocene-
Oligocene Alpine orogeny (Figure 1a, 1b) (e.g. Lombardo et al. 1978, Rubatto and Hermann,
2003). Today, Monviso is overlain by the Schistes Lustrés paleo-accretionary wedge to the
northwest and underlain by the ultra-high pressure (UHP) Dora Maira continental massif to the
southeast, separated from both by extensional shear zones (Figure 1c) (Agard et al. 2002; Agard
et al. 2009 and references therein).

Geochronological studies have constrained the timing of peak burial metamorphism in
Monviso to ca. 45 – 46 Ma (Duchêne et al. 1997; Rubatto and Angiboust 2015; Rubatto and
Hermann, 2003). Age constraints for Dora Maira indicate a somewhat younger age (ca. 35 – 40
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Ma) for peak burial metamorphism to a depth of ~120 km (e.g. Castelli et al. 2007; Chopin et al. 1984; Gebauer et al. 1997; Rubatto and Hermann 2001).

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Monviso is itself divided into two main tectonic units – the blueschist facies Monviso Unit and the eclogites facies Lago Superiore Unit, which are separated from each other by a major ductile shear zone referred to as the Upper Shear Zone (USZ) (Figure 1c) (e.g. Angiboust et al. 2011; 2012). The Lago Superiore unit is cut by two major shear zones (Intermediate and Lower Shear Zones – ISZ and LSZ; Angiboust et al. 2011; 2012), separating metabasaltic and metagabbroic dominated units and a basal serpentinite-talc schist-rich unit. The LSZ separates the serpentinite sole from the gabbroic section (both Mg-Al and Fe-Ti metagabbros) of the ophiolite, and is documented along strike for ~15km with a thickness ranging from tens of m to over 100 m. The LSZ contains serpentinite and talc-chlorite schists with subordinate cm-thick magnesite-rich layers and embedded gabbroic, metasedimentary, and ultramafic blocks (Locatelli et al., 2018).

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Peak P-T conditions for the Lago Superiore unit are estimated to be ~2.8 GPa and ~580°C, as constrained by mineral thermobarometry and thermodynamic modeling (Angiboust et al. 2011; Angiboust et al. 2012; Groppo and Castelli, 2010; Locatelli et al. 2018; Schwartz et al. 2000). Additional details on the mineralogy, petrology, geochemistry, and field relations of Monviso are presented elsewhere and the reader is referred to these studies for a more detailed description (e.g. Angiboust et al. 2011, Locatelli et al. 2018).

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## 2.2 Eclogite Breccias

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Numerous 'eclogite breccias' are found among the blocks within the LSZ (Angiboust et al. 2012; Locatelli et al. 2018). These blocks are predominantly made up of Fe-Ti metagabbros with subordinate Mg-Al metagabbros (<20%) that range in size from ~1 m up to ~20 m and are typically wrapped by a strongly foliated serpentinite-talc-tremolite-rich matrix (Figure 1d). The Fe-Ti metagabbros are often brecciated, while the Mg-Al metagabbros are typically foliated. The peak assemblage within each Fe-Ti metagabbro clast is typical of mafic eclogites – garnet, omphacite, and rutile, plus accessory phases, and the eclogitic fabric is dominantly mylonitic with a foliation defined by rutile stringers and omphacite (Figure 1e). Garnets within individual clasts contain inclusions of rutile, omphacite, and lawsonite pseudomorphs. Mg-Al metagabbros, not discussed further here, contain mostly Cr-rich omphacite, clinozoisite, rutile, and lawsonite pseudomorphs. The eclogite clasts are themselves entrained within 3 distinct matrix generations (M1 – M3), which are relatively undeformed (Figure 1e) (Locatelli et al. 2018). The M1 matrix is characterized by omphacite, M2 matrix by omphacite and garnet, and M3 matrix by omphacite, garnet (pseudomorphed), lawsonite and talc. Together, the different matrices are interpreted to represent successive brecciation events and fluid influx from both internal (M1 and M2) and external (M3) sources. All breccia blocks are surrounded by a metasomatic rind consisting of a talc-amphibole-chlorite assemblage. The matrices and metasomatism is interpreted to record the progressive detachment of the entire unit from the down-going slab and incorporation into a serpentinite-dominated subduction channel along the plate interface, followed by subsequent exhumation (Locatelli et al. 2018). There has been significant debate concerning the tectonic setting for the formation of the Monviso eclogite breccias (i.e. seafloor vs. subduction zone). Here, we emphasize that the presence of fractured and offset garnets with garnet infilling healed fractures necessitates that garnet was stable prior to, during, and
immediately after fracturing took place. Thus, the co-stability of garnet and omphacite in this
lithology is both necessary *and* sufficient evidence for an eclogite-facies origin for these
structures. The interested reader is referred to Locatelli et al. (2018) for a more thorough and
detailed discussion.

Garnets within breccia clasts contain distinct healed fractures, which are the subject of the present study and will be discussed in more detail below (Figure 1f). These healed fracture patterns were first identified by Angiboust et al. (2011) and are present in both chlorite - bearing eclogites from the LSZ and relatively anhydrous eclogites in block cores, as well as from blocks located in the ISZ (approximate locations shown in Figure 1c). Importantly, while both garnets from the ISZ and LSZ display similar healed fracture chemistries, Angiboust et al. (2011) note that only garnets from the LSZ have evidence for *multiple* healed fracture sets. Below, we present a detailed description of the texture and chemistry of these healed fractures.

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3. Materials: Microstructural and Chemical Analyses

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SEM-EDS and BSE images of multiply fractured garnets were acquired using a Zeiss Gemini
SEM. All spot analysis for quantitative traverses were collected using a CAMECA SX-5
microprobe, operating with a 10 nA beam current, 15 keV accelerating voltage, dwell time of 20
s, and a step size ranging from 1 to 0.5 microns. The following standards were used: albite (Na),
orthoclase (K, Al), garnet (Fe,Si,Mg,Ca), MnTiO₃ (Ti, Mn), Cr₂O₃ (Cr). All EPMA analyses
were conducted at ISTeP in Paris, France.

BSE images reveal a complex network of dark, linear features within garnet interiors (Figure
2b, 2c) (for comparison, see Angiboust et al. 2011), which we interpret to represent healed
184 fractures (Figure 2a). These typically cross the interior of garnet crystals in all directions (at least 185 from the perspective of observation). In some cases, they appear faint and slightly curved, 186 whereas in other places they appear to be more well-defined in the BSE image and are relatively 187 straight. The darkest healed fractures correlate with a dark BSE rim (typical thickness ~10 µm) 188 around portions of the garnet crystals. The thickness of the fractures, estimated from the BSE 189 images, is typically ~ 5 µm. The thickest healed fracture has an estimated thickness of 18 µm, 190 while the thinnest observable healed fracture is 2 µm in diameter. Importantly, these healed 191 fractures are only found in garnets within breccia clasts. Investigated garnets in the M2 and M3 192 matrices that surround the clasts do not display the healed fractured morphology.

193 The healed fractures fall into two chemical populations, differentiated mainly by their Mn (3 194 mol % vs. 8 mol %) and Fe (66 mol% vs. 62 mol%) contents, while the Mg (10 mol% vs. 11 195 mol%) and Ca (20 mol% vs. 19 mol%) compositions are nearly identical (Figure 2e, 2e; Table 1). All healed fractures are characterized by increases in Mg and Mn and decreases in Fe and Ca 196 relative to the host garnet (Table 1). A comparison with Locatelli et al. (2018) shows that the 197 low-Mn healed fracture compositions are comparable to Grt1a and Grt1b, while the high-Mn 198 fractures are distinctively enriched in Mn relative to other garnet compositions (Figure 2f; Figure 199 SM1). 200

201 The low-Mn fractures appear to mostly be contained within garnet cores, often terminating 202 within the interior of the garnet crystal. In some cases, the ends of these healed fractures are 203 truncated by a high-Mn garnet rim, suggesting that prior to the high-Mn rim growth, the healed 204 fracture may have cut through the entire garnet crystal. In contrast, the high-Mn fracture 205 population is dominantly through-going and is clearly correlative, both texturally and 206 chemically, with a high-Mn rim along the edge of garnet crystals. Almost all high-Mn healed
207 fractures transect entire garnet crystals and cross-cut the low-Mn healed fractures (Figure 2c).
208 The high-Mn healed fractures also texturally correlate with a second generation of omphacite
209 that has a vein-like morphology (Figure 2d).

210 Taken together, the textural and chemical observations suggest two distinct fracturing events,
211 with the high-Mn healed fractures younger than and cross-cutting the low-Mn healed fractures
212 (Figure 3a, 2b), with both fracturing events occurring prior to and possibly coincident with the
213 formation of the M1 matrix. We envision (at least) two possible scenarios for the development of
214 the fractures and subsequent growth of garnet of a different composition within the former
215 fracture (Figure 3c). In scenario 1, the garnet instantaneously fractures and is then held open by
216 asperities on the fracture surface, allowing a free fluid phase to infiltrate into the crack and
217 precipitate new garnet within the open fracture. In scenario 2, the fracture immediately closes
218 following fracturing, but with a thin film of fluid coating the surface of the former fracture. This
219 fluid subsequently diffuses into the surrounding host garnet, which reequilibrates to a new
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## 4. Methods

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### 4.1. *Diffusion Modeling*

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Garnet crystals were selected for diffusion modeling following careful petrographic analysis
and chemical characterization using electron beam methods.

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We measured multiple healed fractures from 3 garnet crystals from 2 samples (1 garnet from
sample LSZ 14-53; 2 garnets from sample LSZ 63-15B) for a total of 41 healed fracture profiles.

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Each analyzed garnet crystal is contained within a mm-scale mylonitic eclogite "clast" that is surrounded by a matrix corresponding to later brecciation. For each healed fracture profile, a corresponding step function was assumed as an initial pre-diffusional profile from which intracrystalline diffusion is modeled (Figure 4a). We note here that this assumption provides a maximum timescale for the resulting measured profile to develop: any deviation from the idealized step profile involving a shallower overall slope for the concentration profile would result in a shorter calculated timescale.

For our diffusion model, we utilized the 1-D finite difference code from Caddick et al. (2010), in which relaxation of an assumed initial zoning profile is controlled by *P*, *T*, composition and time. Coupled diffusion of Fe, Mg, Ca and Mn is calculated with a multi-component model using several published diffusivities for divalent cations in garnet (Carlson, 2006; Chakraborty and Ganguly, 1992; Vielzeuf et al. 2007). The code was run repeatedly for a range of timescales for each profile and the goodness of fit between the final modeled profile and measured crystal compositions (in terms of mole fraction almandine, pyrope, grossular and spessartine) was then assessed by calculating the square root of the mean of the errors (RMSE) (Figure 4b). The timescale corresponding to the minimum RMSE (i.e. best-fit) for each profile was taken as the most likely timescale over which diffusion operated following fracture formation.

Calculations were initially run for diffusion at a fixed *P-T* condition corresponding to a peak temperature of either 550°C or 580°C, using the characteristic temperature of diffusion (*T*<sub>C</sub>) concept of Chakraborty and Ganguly (1992), (in this case, *T*<sub>C</sub> = 535°C or 560°C), and at a pressure of 2.1 GPa. A second set of models calculated diffusivity along a *P-T* path which was based on previous work on the Monviso ophiolite (Locatelli et al. 2018).
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## 5. Results

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The best-fit diffusional timescale for each healed fracture profile was grouped into the two texturally and chemically distinct healed fracture populations (low-Mn and high-Mn). The final uncertainty for each population was calculated with a gaussian error propagation. For the fixed P-T model run at 530°C (i.e. corresponding to a peak temperature of 550°C), using the diffusivities from Chakraborty and Ganguly (1992) and Vielzeuf (2007), the low-Mn fractures experienced a longer calculated post-fracturing intracrystalline diffusion timescale (2.8 Ma ± 1.8 Ma) compared to the high-Mn fracture population (0.5 Ma ± 0.4 Ma) (Figure 5a, Table 2).

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Each calculated timescale represents the time that each fracture spent near peak P-T conditions (i.e. the time between the fracturing event and cooling below an effective closure temperature for diffusion). Thus, the difference in calculated timescale between the two fracture populations represents the maximum time between the two fracturing events. This results in a maximum calculated period between the formation of the low-Mn healed fractures and the high-Mn healed fractures of 2.3 Ma ± 1.8 Ma. Even accounting for likely uncertainties, the diffusion modeling results strongly suggest that the two texturally and chemically distinct healed fracture populations are also temporally distinct.

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Figure 5 also highlights that the high-Mn healed fracture population has a much smaller spread in calculated timescales (0.4 Ma) and corresponding stronger probability density peak, relative to the low-Mn healed fracture population (1.8 Ma). The low-Mn healed fracture population has a broad, shallow peak with a maximum probability density peak of only ~0.2 and appears to display a secondary peak close to 5 Ma. This may indicate the possibility of a third,
older set of low-Mn healed fractures. However, textural and chemical criteria do not provide a clear indication that these fractures are distinct from the rest of the low-Mn healed fracture population. An alternative possibility is that this secondary hump may be related to a preferred orientation of fractures relative to measured profiles (See Discussion below). Several measured profiles also maintain some residual “structure” compared to the model profiles (the lowest RMSE value is still a “relative” best fit for a diffusional timescale), which is likely due to irregular asperities along the fracture surface that originated during initial fracturing (Figure 4; Figure 3c).

An increase in characteristic diffusion temperature from 530°C to 560°C (corresponding to an increase in peak temperature from 550 °C to 580°C) results in a decrease in the total timescale on the order of ~70% for the fixed P-T model (Figure 5b, Table 2). The corresponding calculated timescales for the low-Mn and high-Mn healed fracture populations for the higher temperature fixed P-T model are 0.9 Ma ± 0.6 Ma and 0.2 Ma ± 0.1 Ma, respectively, with a calculated period of 0.7 Ma ± 0.6 Ma.

The “P-T path” model, using the diffusivities from Chakraborty and Ganguly (1992) and Vielzeuf et al. (2007) and a P-T path based upon Locatelli et al. (2018), results in calculated timescales of 2.1 Ma ± 1.3 Ma for the low-Mn healed fractures and 0.4 Ma ± 0.3 Ma for the high-Mn healed fractures (Figure 5c). These timescales correspond to a calculated period of 1.6 Ma ± 1.3 Ma.

The same fixed P-T model run at ~530°C, but with diffusivities from Carlson (2006), gives significantly shorter calculated timescales (Figure 5d). The low-Mn healed fracture population has a calculated timescale of only 0.3 Ma ± 0.2 Ma, whereas the high-Mn healed fracture population has a nearly identical calculated timescale 0.2 Ma ± 0.2 Ma. The timescales of the
299 two fracture populations are thus indistinguishable from one another within error, corresponding 300 to a calculated period of only 0.1 Ma ± 0.2 Ma. The shape of the probability density functions is 301 also noticeably different from models using other diffusivity data. The high-Mn fracture 302 population displays a secondary “hump” and a peak that is nearly identical to the low-Mn 303 fracture population curve. The overall goodness of fit values are comparable to the other models 304 that use diffusivities from Chakraborty and Ganguly (1992) and Vielzeuf (2007).

305 Overall, the fixed P-T model run at 530°C with the diffusivities from Chakraborty and 306 Ganguly (1992) and Vielzeuf (2007) provides the longest timescale (2.78 Ma) and the largest 307 period (2.26 Ma). This fixed P-T model is broadly similar to the P-T path model that utilizes the 308 same diffusivities and peak temperature (low-Mn timescale of 2.06 Ma and period of 1.65 Ma). 309 The model that uses the Carlson diffusivities reduces the calculated timescales and period by 310 over an order of magnitude, which is similar to the effect of increasing the peak temperature by 311 30°C. 312

312 Any calculated diffusional timescale is dependent on several variables, including 313 temperature, pressure, diffusivity, dislocation density, and H₂O fugacity (e.g. Zhang et al. 2019). 314 However, while the absolute values for the calculated diffusional timescales may change 315 depending on these variables, our major result does not – the high-Mn healed fractures, which 316 cross-cut the low-Mn healed fractures, yield a shorter best-fit diffusional timescale (Figure 5, 317 Table 3). This relative difference in diffusional timescale is always consistent with the textural 318 relationships, even in models in which the calculated diffusive timescales overlap within 319 uncertainties (i.e. Figure 5d).

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321 **6. Discussion**
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Textural observations show that a series of high-Mn fractures cross-cut an older, low-Mn
healed fracture population. The diffusion modeling results are consistent with these textures,
suggesting that the low-Mn healed fracture population has a longer timescale for diffusional
reequilibration than the high-Mn healed fracture population (maximum Δt = 2.3 Myr.). We
interpret the difference in diffusional timescale recorded by the low-Mn and high-Mn healed
fracture populations to represent the maximum period between two successive fracturing events
that occurred during prograde subduction at eclogite facies P-T conditions. Below, we discuss
these temporal constraints in the context of the subduction history of Monviso, implications for
eclogite rheology, and their usefulness for constraining and interpreting other short timescale
processes in the rock record.

6.1. Subduction History of Monviso and Possible Fluid Sources

The healed garnet fractures are a direct record of multiple brittle deformation events during prograde subduction. Importantly, all healed fractures are located in garnets contained within breccia clasts. This implies that these brittle deformation events took place during prograde subduction, prior to the detachment of the slab, km-scale brecciation along the LSZ, and exhumation of the ophiolite (Angiboust et al. 2011; Locatelli et al. 2018). Figure 6 provides a summary of these events in the context of the entire subduction history of Monviso.

Initial eclogitization of the Fe-Ti metagabbros likely occurred close to 2 GPa and 500°C,
transforming the Fe-Ti metagabbros from a hydrous, blueschist facies assemblage into eclogites
(Locatelli et al. 2018). This eclogitization was accompanied by ductile deformation, resulting in
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a well-developed foliation (Figure 1e). Subsequently, yet prior to peak P-T conditions, at least two fracturing events occurred, separated in time by a *maximum* of ~2 Myr. After each fracturing event, the fractures healed to garnet of a slightly different composition than the host garnet. The low-Mn healed fracture compositions are comparable to both the Grt1a and Grt1b compositions from Locatelli et al. (2018). These fractures post-date the formation of the mylonitic garnets (Grt1a) and thus likely equilibrated with Grt1b. The high-Mn healed fractures are compositionally similar to Grt1b, yet noticeably enriched in Mn (Figure 2f; Figure SM1). The higher Mn content may be related to either partial garnet dissolution or the dissolution of a Mn-bearing phase such as chlorite during fracturing.

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We emphasize that the healed garnet fractures in the eclogite breccia clasts formed prior to peak P-T conditions and widespread brecciation and matrix formation. As such, they represent a process occurring prior to or during incipient detachment of the slab. Given a conservative average subduction rate of ~1 cm/yr and considering that the garnet crystals must have formed prior to both sets of fractures that cross-cut the crystals, these timescales indicate that garnet was essentially inert during the final ~20 km of burial. That is, the only record of the final ~20 km of burial appears to be the two sets of healed garnet fractures.

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After the first fracturing event, and possibly coincident with second fracturing event, the M1 matrix formed, close to peak P-T conditions (Figure 6; Locatelli et al. 2018). Considering the calculated diffusional timescale for the high-Mn healed fracture population of ~0.5 Myr near peak P-T conditions, it is likely that the period between the formation of the high-Mn healed fractures and the M1 matrix was ≤ 0.5 Myr. Formation of the M2 and M3 matrices occurred after M1 matrix formation, representing full activation of the LSZ and complete incorporation of

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367 the Fe-Ti metagabbros into the serpentinite-rich shear zone during exhumation, as the Lago
368 Superiore tectonic slice returned along the subduction plate interface (Agard et al. 2018).

369 Healed garnet fractures with a different composition than the host garnet implies the presence
370 of a free fluid phase, at least transiently, during and after fracturing and prior to the main
371 brecciation events described in Locatelli et al. (2018) (Figure 4). Three likely possibilities for the
372 source of this fluid are 1) Fe-Ti metagabbros 2) Mg-Al metagabbros and 3) serpentinites. *In-situ*
373 oxygen isotopes strongly suggests that any fluid involved in the healing of the fractures and the
374 high-Mn rims was locally derived (Rubatto and Angiboust, 2015), so we tentatively favor either
375 Fe-Ti or Mg-Al metagabbros as a potential source of this fluid. However, we also note that that
376 later brecciation and metasomatism appears to involve significant contributions from
377 serpentinite-derived fluid (Locatelli et al. 2018). This would be consistent with the observations
378 of Locatelli et al. (2018), who showed that most eclogite breccias are close to a major lithologic
379 contact that may have acted as a fluid focusing structure, possibly channeling the fluid from
380 dehydration reactions in adjacent lithologies and facilitating rapid and significant pore fluid
381 pressure increases in the Fe-Ti metagabbros, either promoting fracturing of the Fe-Ti
382 metagabbros or facilitated by and following the fracturing.
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## 6.2. Rheological Implications

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The presence of two distinct fracture populations is unequivocal evidence for repeated
fracturing during prograde subduction at eclogite facies conditions. Moreover, the presence of
healed fractures in garnet cores within mylonitic eclogite clasts documents the activity of both
brittle and ductile deformation at eclogite facies conditions for the Monviso Fe-Ti metagabbros,
prior to complete brecciation. Plastic deformation mechanisms (e.g. dislocation creep) are usually expected to dominate at depths of >70 km. However, transient increases in either strain rate or pore fluid pressure, or both, can induce a transient switch from ductile to brittle deformation (Angiboust et al. 2015; Wex et al. 2018). For example, Angiboust et al. (2015) argue that strain rate increases of 2 orders of magnitude and pore fluid pressure increases are required to explain a switch from ductile to brittle deformation in a cataclastic zone in the Dent Blanche thrust. Similar arguments were made by Wex et al. (2018) to describe multiple generations of pseudotachylyte and mylonites in lower crustal units from the Musgrave Block. Importantly, these studies document a repeated cycling between ductile and brittle deformation.

We create a rheological profile to test if geologically reasonable strain rates and pore fluid pressures are compatible with brittle deformation in eclogites at these P-T conditions and to calculate the required differential stresses for fracturing (Figure 7). This calculation employs an eclogite flow law from Jin et al. (2001) based upon experiments on a natural eclogite. Assuming temperatures of ~550°C and ~70 km depth (pressure ~ 2 GPa), a strain rate is $10^{-14} s^{-1}$ and a pore fluid pressure ratio of ~ 0.4, a differential stress of ~1.1 GPa would be required to exceed the brittle failure envelope. Strain rates would need to be lower than ~$10^{-16} s^{-1}$ to allow for ductile deformation at these conditions. At a given strain rate ($10^{-14} s^{-1}$), higher pore fluid pressure ratios will reduce the yield stress for brittle deformation, with a pore fluid pressure ratio of ~0.8 reducing the differential stress to ~400 MPa, which is less than half the value obtained when assuming hydrostatic pore fluid conditions. A value of the pore fluid pressure ratio near unity (essentially lithostatic pore fluid pressures), reduces the required differential stress for brittle failure to ~200 MPa. These calculations suggest that reasonable strain rates ($10^{-14} s^{-1}$ – $10^{-16} s^{-1}$) and pore fluid pressures (0.4 – 0.8) can and should lead to brittle failure in mafic eclogites at
413

these *P-T* conditions, with switching between ductile and brittle regimes possible given slight
changes in either pore fluid pressure ratios or strain rates. Given geochemical evidence that
suggests the presence of a free fluid phase (Locatelli et al. 2018), we estimate differential
stresses of ≤ 400 MPa during fracture formation.

414

415

416

417

Microtextures suggest that omphacite may have experienced brittle deformation coeval with
garnet. Omphacite with a vein-like morphology can be seen infilling fractures from earlier
omphacite generations in some clasts, correlative with some healed garnet fractures (Figure 2d).
In areas where there is little evidence for fracturing, later healing and ductile deformation likely
erased some evidence for brittle deformation. Thus, it is possible that omphacite may have
undergone brittle deformation coeval with garnet and that this record of brittle deformation has
since been erased by later recrystallization mechanisms.

418

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425

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427

428

429

430

431

432

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434

Healed garnet fractures and “fragmentation” of garnets similar to those in the Monviso Fe-Ti
metagabbros are described in other paleo-subduction zones, including elsewhere in the Western
Alps and Norway (e.g. Austrheim et al. 2017; Konrad-Schmolke et al. 2007). This suggests that
brittle deformation in mafic eclogites that experienced similar P-T conditions may be relatively
common to subduction zones worldwide and through time, given similar strain rates and fluid
pressures as experienced by the Monviso Fe-Ti metagabbros. A hotter geothermal gradient
would likely result in plastic deformation (Figure 7), so a relatively cold subduction geothermal
gradient may be required for widespread brittle deformation at these depths (e.g. Kim et al.
2018). Other parameters, such as fluid pressure variations, viscosity contrasts between the slab
and mantle and increased mechanical coupling, are also likely to play an important role in
determining the mode of deformation during subduction (Agard et al. 2018).
Often, but not always, healed garnet fractures in high-pressure rocks are interpreted to be associated with paleoseismicity (and only if they are identified and interpreted as healed fractures). Examples include the Sesia zone, where garnet is fragmented, displaced, and pulled apart during synseismic loading and post-seismic creep (Trepmann and Stöckhert, 2002), in the Bergen Arcs, where the presence of psseudotachlyte unequivocally documents paleoseismic activity, (Austrheim et al. 1996; Austrheim et al. 2017), and in the Mt. Emilius klippe (e.g. Hertgen et al. 2017). In these examples, a switch to brittle deformation is interpreted to be induced by a transient increase in strain rate and related to seismic activity at shallower depths, with focused fluid flow occurring after fracturing. Using this interpretative framework, the two sets of healed fractures in Monviso garnet crystals may be interpreted to represent two earthquakes that occurred at intermediate or shallower depths and allowed for channelized fluid to follow initial fracturing (e.g. Austrheim et al. 2017; Trepmann and Stöckhert, 2002; Wex et al. 2018).

However, the fracturing documented here is not focused within a single fault plane and lacks any identifiable pseudotachlyte. Rather, it appears to involve previously intact rock volumes away from a main fault plane. This is consistent with recent proposals made by Jamtveit et al. (2018), who suggests that large volumes of rock can be affected by repeated seismic activity in the lower crust. This may indicate that evidence for past seismic activity may found in locations that do not preserve clear evidence for past seismic faulting but may rather be preserved in otherwise "ordinary" metamorphic rocks.

Alternatively, the transient strain rate increase may be aseismic in origin. Deformation mechanisms such as episodic aseismic creep are expected to significantly contribute to plate motions (e.g. Heki et al. 1997), yet there are few documented examples of processes involving
458
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short term deformation with high strain rates, fluid pressure fluctuations, and high differential
stresses identified in the rock record.

460
461
462
463
464
465
466
467
468

In either case, two likely explanations for the dearth of evidence for transient and periodic
processes in the rock record is that the chemical and textural evidence in the rock record has
either been misinterpreted, or that the evidence (i.e. healed fractures) is often erased by later
healing and reequilibration. The diffusion modeling results in this study show that given slightly
longer timescales near peak temperatures, or slightly higher peak temperatures, the petrologic
record of these fracturing events would be erased.

6.3. *Significance of multiple fracturing events*

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478
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While few studies currently exist, the early consensus is for timescales of periods for fluid
pulses of 10⁴ – 10⁶ years, which is generally compatible with the results of this study. The
frequency of dehydration embrittlement, likely dependent on both heating and burial rates and
the kinetics of dehydration reactions and fracture-healing processes, has been estimated to be
~10⁵ – 10⁶ yrs. (Hacker et al. 2003; Taetz et al. 2018). A characteristic period for ductile shear
instabilities of 10⁴ yrs. in shear zones has been proposed based upon numerical modeling studies
(Kelemen and Hirth, 2007). A period of 10³ – 10⁴ yrs. for fluid pulses leaving the slab has also
been estimated based upon U-series isotope systematics (e.g. Yokoyama et al. 2002).
Meanwhile, individual channelized fluid flow events have been shown to be extremely brief,
with calculated timescales ranging from ~10¹ – 10³ yrs. (e.g. Camacho et al. 2005; John et al.
2012; Penniston-Dorland et al. 2010; Taetz et al. 2018).
480

This study documents the first direct evidence in the rock record where a period has been calculated for multiple brittle deformation events at intermediate depths in a subduction zone.

481

482

Other localities where multiple fracture sets, sometimes intermingled with ductile deformation structures, may also be fruitful locations at which to apply the methodology used in this study to directly constrain the period of these cycles.

483

484

The recognition of repeated instances of brittle deformation with characteristic periods, possibly related to intermediate depth seismicity may then be used as a potential distinguishing factor among different causal mechanisms for intermediate depth seismicity in the rock record.

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489

### 6.4. *Episodic Metamorphism – the norm or the exception?*

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491

The results of this study, in combination with the growing body of evidence from numerous other studies, demonstrate an intimate relationship and strong feedbacks between the seismic cycle and metamorphic processes that are only beginning to be explored (i.e. Agard et al. 2018; Jamtveit et al. 2018; Hacker et al. 2003). Metamorphism occurs in response to both rapid, stochastic and slow, gradual processes. A picture that is currently emerging for many orogenic settings involves superposition of transient processes upon slow, gradual, change (e.g. Ague and Baxter, 2007). The ongoing challenge is to identify this superposition of short and long timescale processes in the rock record and to successfully integrate these observations with other disciplines (seismology, geodynamics, etc.) to more fully capture the rock record of short-duration metamorphic processes. This requires knowledge of the coupling between tectono-thermal drivers and interpretable metamorphic recrystallization.

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500

501

502
503 **7. Conclusions**
504

505 Multiple healed fracture sets in garnets are interpreted to represent successive brittle 506 deformation events during prograde subduction of the Monviso ophiolite. Diffusion modeling on 507 each healed fracture population constrains the maximum period between each fracture 508 population to be on the order of ~ 2 Myr. Moreover, the younger, high-Mn fracture population 509 formed ≤ 0.5 Myr prior to the M1 matrix, close to peak P-T conditions. Higher estimated peak 510 temperatures result in shorter calculated absolute timescales and periods. Application of a 511 rheological profile for cold subduction of mafic eclogites suggests that brittle deformation may 512 be a common process in subduction at intermediate depths. In the case of Monviso, the ophiolite 513 experienced a long period of ductile and brittle deformation, with these cycles occurring on the 514 order of 2 Myr or less. This highlights that the deformation mechanisms present in the slab 515 change from ductile to brittle over short periods of time and are not merely a function of P-T 516 conditions. That is, metamorphism is not simply a slow, gradual, equilibrium-controlled process, 517 but is also transient and episodic. Variables such as pore fluid pressure fluctuations, rheological 518 heterogeneity, and transient strain rate increases may in fact be the normal operating condition 519 for subduction zones even at intermediate depths. These results demonstrate that these transient 520 processes can repeatedly affect the same rock volumes over relatively short timescales. 521

521
522 **Acknowledgements**
523

523
524 This study was conducted as part of the E-FIRE project, supported by NSF Grant #1545903. We
525 thank the EPMA and SEM lab technicians at Sorbonne Université for their help in data
collection. We also thank the Metamorphic Processes research group at Virginia Tech and the E-FIRE research network for helpful and insightful discussions. Two anonymous reviews and editor Rebecca Bendick are also thanked for their comments and suggestions, which significantly improved this manuscript.

530

531

## Figure Captions

532

533 **Figure 1.** Geologic setting and field relations. (a) Schematic cross section of a subduction zone. MEQ = megathrust earthquake, ETS = episodic tremor and slip, IDS = Intermediate Depth Seismicity. Stars indicate locations of seismicity and blue arrows indicate locations of dehydration reactions. (b) Schematic map of western Europe. The Western Alps is marked by the red box. (c) Cross section of Monviso ophiolite, modified from Locatelli et al. (2018). USZ = Upper Shear Zone, ISZ = Intermediate Shear Zone, LSZ = Lower Shear Zone. Purple diamonds represent approximate locations of documented clast garnets with healed fractures. (d) Field occurrence of Monviso eclogite breccia blocks. (e) Close-up view of eclogite breccia block from (d) highlighting the variably rotated mylonitic foliation of individual Fe-Ti metagabbro breccia clasts. (f) BSE image of fractured garnet from mylonitic clast in (e). Healed fractures are darker in BSE relative to host garnet.

544

545 **Figure 2.** (a) Photomicrograph of fractured garnet from sample LSZ 1453. (b) SEM-BSE image showing a complex series of healed fractures within the garnet from (a). Notice that the garnet is also broken up and displaced by omphacite (black in image). Red lines indicate the location of electron microprobe traverses (c) SEM-EDS Mn map of garnet from (a) and (b). Notice that
high-Mn fractures correlate with Mn-rich garnet rim (d) Composite Ca-Na-Mg SEM-EDS map of garnet in (a), (b), and (c). Notice also that a second generation of Mg-rich omphacite (light blue according to color scheme) is correlative with the high-Mn healed fractures in garnet (dark blue according to color scheme; compare with (c). (e) Chemistry of healed garnet fractures (Mg # vs. mole fraction Mn) as a function of fracture diameter and garnet #. (f) Ternary plot with healed garnet fractures, compared to garnet compositions analyzed in Locatelli et al. (2018).

**Figure 3.** (c) Cartoon illustrating model for multiple fracture-healing cycles in garnet crystals. (a) BSE image of a fractured garnet crystal. White box refers to sketch in (b). (b) Sketch of cross-cutting relationships between low-Mn and high-Mn healed fractures. (c) Cartoons of two fracture-healing models. Model 1 involves a fracture held open by surface asperities. Model 2 involves a thin fluid film whetting the former fracture surface, which then diffuses into the host garnet. For compilation of all fracture profiles, see Figure SM3.

**Figure 4.** (a) Example of electron microprobe traverse across a healed fracture. Dots correspond to microprobe analyses. Dashed line corresponds to assumed step-function initial profile. Solid line corresponds to the best-fit model profile. (b) Example of best-fit criteria used to estimate diffusional timescale for each fracture. The square root of the mean of the errors (RMSE) is calculated at each model time step, with the lowest value taken as the most likely timescale over which diffusion operated following fracture formation.
**Figure 5.** Probability-density distribution plots of results from diffusion modeling. The best-fit timescale for each fracture (corresponding to the minimum RMSE value) is grouped by fracture chemistry (high-Mn vs. low-Mn). The high-Mn fracture population distribution is shown in red and the low-Mn fracture population distribution is shown in blue. The periodicity is calculated from the difference in the mean best-fit timescale between the two fracture populations. Diffusivity values for Mg, Mn, and Fe taken from Chakraborty and Ganguly (1992), and Ca from Vielzeuf et al. (2007). (a) Fixed *P-T* at 530°C, using the diffusivities of Chakraborty and Ganguly (1992) for Mn, Mg, and Fe and Vielzeuf et al. (2007) for Ca. (b) Fixed *P-T* at 560°C, using the diffusivities of Chakraborty and Ganguly (1992) for Mn, Mg, and Fe and Vielzeuf et al. (2007) for Ca. (c) *P-T* path proposed by Locatelli et al. (2018). Diffusivity values for Mg, Mn, and Fe taken from Chakraborty and Ganguly (1992), and Ca from Vielzeuf et al. (2007). (d) Fixed *P-T* at 530°C with diffusivities from Carlson (2006).

583

**Figure 6.** Summary *P-T* diagram. *P-T* path for Monviso and matrices 1,2, and 3 taken from Locatelli et al. (2018). Schistes Lustres *P-T* constraints taken from Agard et al. (2001). Monviso *P-T* constraints taken from Messiga et al. (1999), Schwartz et al. (2000), Agard et al. (2009), and Angiboust et al. (2012). Dora Maira *P-T* path taken from Rubato and Hermann (2001). Insert on the lower left corner is a schematic cross-section of a subducting slab. Numbers in the cross-section refer to numbers on the *P-T* path for the Monviso ophiolite.

590

**Figure 7.** Rheological profile for Monviso Fe-Ti metagabbros eclogites for a geothermal gradient of 8 °C km⁻¹. Frictional strength envelope calculated with a coefficient of friction of 0.7, following Byerlee (1978) and Sibson (1974). Eclogite flow law from Jin et al. (2001). Blue
594 rectangle represents depth estimates for *P-T* conditions during fracturing. Insert shows
595 rheological profile assuming a geothermal gradient of 25 °C km⁻¹ for comparison.

596

597 **Table 1. Summary of microprobe data**

598

599 **Table 2. Diffusion Modeling results**

600

601 **Table 3. Diffusion Results Summary**

602

603

604 **References**

605

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740 Mineralogy and Petrology 154, 153–170. https://doi.org/10.1007/s00410-007-0184-x

741 Viete, D.R., Hacker, B.R., Allen, M.B., Seward, G.G.E., Tobin, M.J., Kelley, C.S., Cinque, G.,

742 Duckworth, A.R., 2018. Metamorphic records of multiple seismic cycles during

743 subduction. *Science Advances* 4, eaaq0234. https://doi.org/10.1126/sciadv.aaq0234

744 Wex, S., Mancktelow, N.S., Camacho, A., Pennacchioni, G., 2018. Interplay between seismic

745 fracture and aseismic creep in the Woodroffe Thrust, central Australia – Inferences for
746 the rheology of relatively dry continental mid-crustal levels. Tectonophysics.

747 https://doi.org/10.1016/j.tecto.2018.10.024

748 Yokoyama, T., Nakamura, E., Kobayashi, K., Kuritani, T., 2002. Timing and trigger of arc

749 volcanism controlled by fluid flushing from subducting slab. Proceedings of the Japan

750 Academy. Ser. B: Physical and Biological Sciences 78, 190–195.

751 https://doi.org/10.2183/pjab.78.190

752 Zhang, B., Li, B., Zhao, C., Yang, X., 2019. Large effect of water on Fe–Mg interdiffusion in

753 garnet. Earth and Planetary Science Letters 505, 20–29.

754 https://doi.org/10.1016/j.epsl.2018.10.015

755

756 **Appendix A. Sensitivity Testing**

757

758 The exact orientation of the electron microprobe traverse relative to the orientation of the

759 fracture will affect the calculated timescale, with oblique traverses producing an erroneously

760 over-diffused apparent profile and an artificially longer diffusive timescale. Unsurprisingly,

761 some of the model profiles gave extremely long durations (up to 10 Myr.), which are statistical

762 outliers in our dataset (4 outliers in entire dataset). Despite these few outliers, the large number

763 of profiles measured in our study show that most are minimally affected by an oblique cross-

764 section effect. In fact, the population-wide sampling method that we employ enables easy

765 identification of these outliers. The averaging of individual calculated timescales (excluding

766 outliers) into a total population and subsequently taking the mean timescale is taken as a

767 representative average diffusive timescale of each fracture population.
The interaction volume of each microprobe spot analysis will produce a spatial averaging (convolution) of the compositional profile due to the relatively close spacing of the microprobe spot analyses (~0.5 – 1µm). We use the equations derived in Ganguly et al. (1988) to calculate the convolution effect for our measured profiles, using a convolution factor value of 0.48 (Borinski et al. 2012; Ganguly et al., 1988). The results suggest that there is minimal effect on our measured profiles (Figure SM2). This is in agreement with Ganguly et al. (1988), who estimate that for a step size of ~0.5 µm, the convolution effect is insignificant beyond a diffusion profile length on the order of 10 µm (i.e. a sharper compositional profile would lead to a greater convolution effect). Importantly, any spatial averaging from the convolution effect will tend to artificially smooth out all resultant profiles, leading to artificially longer timescales for a given profile. As other assumptions in our model (initial step profile, traverse obliquity, peak temperature estimate) also make our calculations a maximum timescale, we do not consider the convolution effect to have a significant effect on our main results relative to these other uncertainties.

782

783

**Figure SM1.** Ternary plot in Fe-Mg-Ca (mol%) space with healed garnet fractures, compared to garnet compositions analyzed in Locatelli et al. (2018).

786

**Figure SM2.** Deconvolution calculations, following Ganguly et al. (1988). (a) and (b) show two different initial starting profiles. (a) represents a step-function profile and (b) represents a profile that has a shallower slope due to diffusional modification of the initial step-function in (a).
790 Profile (a) is slightly convoluted, as shown by the green dashed line, while in (b) there is no
791 noticeable convolution.

792
793 **Figure SM3.** Compilation of complete dataset of healed fracture profiles and best-fit diffusion
794 model results (Static *P*-*T*, 530 °C, using the diffusivities of Chakraborty and Ganguly (1992) for
795 Mn, Mg, and Fe and Vielzeuf et al. (2007) for Ca).

796
797 **Supplementary Material References**
798

798
799 Borinski, S.A., Hoppe, U., Chakraborty, S., Ganguly, J., Bhowmik, S.K., 2012. Multicomponent
800 diffusion in garnets I: general theoretical considerations and experimental data for Fe–Mg
801 systems. Contributions to Mineralogy and Petrology 164, 571–586.
802 https://doi.org/10.1007/s00410-012-0758-0

803 Ganguly, J., Bhattacharya, R., Chakraborty, S., 1988. Convolution effect in the determination of
804 compositional profiles and diffusion coefficients by microprobe step scans. American
805 Mineralogist 73, 901–909.

806
807
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>
Fixed P-T (530°C), Vielzeuf and
Chakraborty and Ganguly

<figure><img src="image_17.png" /></figure>

Best-Fit Timescales

Low-Mn fractures: 2.8 ± 1.8 Ma

High-Mn fractures: 0.5 ± 0.4 Ma

$$
\Delta t = t_{\text{low-Mn}} - t_{\text{high-Mn}} = 2.3 \pm 1.8 \text{ Ma}
$$

Maximum timescale between
fracture populations

Younger,
high-Mn
fracture
population

Older distribution of
at least one
population of low-Mn
fractures

n = 7

n = 34

Fixed P-T (560°C), Vielzeuf and
Chakraborty and Ganguly

<figure><img src="image_18.png" /></figure>

Best-Fit Timescales

Low-Mn fractures: 0.9 ± 0.5 Ma

High-Mn fractures: 0.2 ± 0.1 Ma

$$
\Delta t = t_{\text{low-Mn}} - t_{\text{high-Mn}} = 0.7 \pm 0.6 \text{ Ma}
$$

n = 34

n = 7

P-T path, Vielzeuf and Chakraborty
and Ganguly

<figure><img src="image_19.png" /></figure>

Best-Fit Timescales

Low-Mn fractures: 2.1 ± 1.3 Ma

High-Mn fractures: 0.4 ± 0.3 Ma

$$
\Delta t = t_{\text{low-Mn}} - t_{\text{high-Mn}} = 1.6 \pm 1.3 \text{ Ma}
$$

n = 34

n = 7

Fixed P-T (530°C), Carlson

<figure><img src="image_20.png" /></figure>

Best-Fit Timescales

Low-Mn fractures:
0.3 ± 0.2 Ma

High-Mn fractures:
0.2 ± 0.2 Ma

$$
\Delta t = t_{\text{low-Mn}} - t_{\text{high-Mn}} = 0.1 \pm 0.2 \text{ Ma}
$$

n = 34

n = 7
<figure><img src="image_21.png" /></figure>

Temperature(°C)

Depth(km)
<figure><img src="image_22.png" /></figure>

Increasing pore
fluid pressure

<figure><img src="image_23.png" /></figure>

Increasing
strain rate

<figure><img src="image_24.png" /></figure>

25°C km⁻¹

<figure><img src="image_25.png" /></figure>

Differential Stress (MPa)

Depth range of Monviso eclogites

8°C km⁻¹
<table><thead><tr><th>Fracture #</th><th>Diameter</th><th>X_Fe</th><th>X_Mg</th><th>X_Mn</th><th>X_Ca</th></tr></thead><tbody><tr><td colspan="6"><strong>Garnet 1</strong></td></tr><tr><td>1a</td><td>3.1</td><td>0.59</td><td>0.11</td><td>0.02</td><td>0.28</td></tr><tr><td>2</td><td>3.05</td><td>0.62</td><td>0.1</td><td>0.01</td><td>0.27</td></tr><tr><td>3a</td><td>4.24</td><td>0.62</td><td>0.12</td><td>0.02</td><td>0.24</td></tr><tr><td>3b</td><td>5.45</td><td>0.6</td><td>0.15</td><td>0.02</td><td>0.23</td></tr><tr><td>5a</td><td>3.05</td><td>0.69</td><td>0.08</td><td>0.03</td><td>0.2</td></tr><tr><td>5b</td><td>3.06</td><td>0.68</td><td>0.07</td><td>0.04</td><td>0.21</td></tr><tr><td>5c</td><td>6.11</td><td>0.65</td><td>0.09</td><td>0.03</td><td>0.23</td></tr><tr><td>5d*</td><td>18.31</td><td>0.62</td><td>0.14</td><td>0.08</td><td>0.16</td></tr><tr><td>7</td><td>5.2</td><td>0.67</td><td>0.08</td><td>0.03</td><td>0.22</td></tr><tr><td>8a</td><td>5.04</td><td>0.67</td><td>0.08</td><td>0.03</td><td>0.22</td></tr><tr><td>8b*</td><td>9.58</td><td>0.61</td><td>0.11</td><td>0.08</td><td>0.2</td></tr><tr><td>8d*</td><td>8.57</td><td>0.63</td><td>0.11</td><td>0.08</td><td>0.18</td></tr><tr><td>9*</td><td>7.33</td><td>0.62</td><td>0.11</td><td>0.09</td><td>0.18</td></tr><tr><td>10b</td><td>5.59</td><td>0.68</td><td>0.07</td><td>0.03</td><td>0.22</td></tr><tr><td>11</td><td>6.03</td><td>0.69</td><td>0.14</td><td>0.03</td><td>0.14</td></tr><tr><td>12*</td><td>6.6</td><td>0.63</td><td>0.12</td><td>0.08</td><td>0.17</td></tr><tr><td>14b</td><td>4.55</td><td>0.66</td><td>0.09</td><td>0.03</td><td>0.22</td></tr><tr><td>15a</td><td>6.58</td><td>0.67</td><td>0.12</td><td>0.02</td><td>0.19</td></tr><tr><td>15b</td><td>4.05</td><td>0.68</td><td>0.1</td><td>0.02</td><td>0.2</td></tr><tr><td>18</td><td>5.13</td><td>0.65</td><td>0.14</td><td>0.03</td><td>0.18</td></tr><tr><td>19</td><td>8.15</td><td>0.64</td><td>0.16</td><td>0.03</td><td>0.17</td></tr><tr><td colspan="6"><strong>Garnet 2</strong></td></tr><tr><td>3</td><td>5.08</td><td>0.66</td><td>0.09</td><td>0.03</td><td>0.22</td></tr><tr><td>4a</td><td>6.22</td><td>0.68</td><td>0.08</td><td>0.02</td><td>0.22</td></tr><tr><td>4b</td><td>4.15</td><td>0.68</td><td>0.08</td><td>0.02</td><td>0.22</td></tr><tr><td>5</td><td>4.61</td><td>0.68</td><td>0.15</td><td>0.05</td><td>0.12</td></tr><tr><td>6</td><td>5.02</td><td>0.73</td><td>0.11</td><td>0.02</td><td>0.14</td></tr><tr><td>7*</td><td>3.55</td><td>0.62</td><td>0.11</td><td>0.08</td><td>0.19</td></tr><tr><td>8</td><td>2.54</td><td>0.65</td><td>0.09</td><td>0.05</td><td>0.21</td></tr><tr><td>10a</td><td>4.03</td><td>0.63</td><td>0.09</td><td>0.04</td><td>0.24</td></tr><tr><td>10b</td><td>6.04</td><td>0.64</td><td>0.08</td><td>0.04</td><td>0.24</td></tr><tr><td>12*</td><td>2.12</td><td>0.62</td><td>0.07</td><td>0.1</td><td>0.21</td></tr><tr><td>13a</td><td>3.97</td><td>0.68</td><td>0.1</td><td>0.04</td><td>0.18</td></tr><tr><td>13b</td><td>8.44</td><td>0.74</td><td>0.06</td><td>0.01</td><td>0.19</td></tr><tr><td>13c</td><td>5.46</td><td>0.69</td><td>0.09</td><td>0.03</td><td>0.19</td></tr><tr><td colspan="6"><strong>Garnet 3</strong></td></tr><tr><td>3d</td><td>4.91</td><td>0.69</td><td>0.17</td><td>0.02</td><td>0.12</td></tr><tr><td>Average low-Mn</td><td>4.96</td><td>0.66</td><td>0.1</td><td>0.03</td><td>0.2</td></tr><tr><td>Average high-Mn</td><td>6.29</td><td>0.62</td><td>0.11</td><td>0.09</td><td>0.19</td></tr></tbody></table>

<footnote>* high-Mn fracture</footnote>
<table><thead><tr><th>Fracture #</th><th>Static P-T(530°C),1</th><th>Static P-T(530°C),2</th><th>P-T path,1</th><th>Static P-T (560°C),1</th></tr></thead><tbody><tr><td colspan="5"><strong>Garnet 1</strong></td></tr><tr><td>1a</td><td>1.6</td><td>0.06</td><td>1.2</td><td>0.4</td></tr><tr><td>2</td><td>0.62</td><td>0.04</td><td>0.6</td><td>0.26</td></tr><tr><td>3a</td><td>4.1</td><td>0.2</td><td>2.9</td><td>1.2</td></tr><tr><td>3b</td><td>6.6</td><td>0.35</td><td>4.6</td><td>2</td></tr><tr><td>5a</td><td>2.35</td><td>0.2</td><td>1.3</td><td>0.5</td></tr><tr><td>5b</td><td>1.05</td><td>0.25</td><td>0.8</td><td>0.4</td></tr><tr><td>5c</td><td>2.18</td><td>0.38</td><td>1.9</td><td>0.7</td></tr><tr><td>5d*</td><td>1.13</td><td>0.43</td><td>0.9</td><td>0.4</td></tr><tr><td>7</td><td>3.25</td><td>0.3</td><td>2.3</td><td>1</td></tr><tr><td>8a</td><td>3.4</td><td>0.2</td><td>2.4</td><td>1</td></tr><tr><td>8b*</td><td>0.5</td><td>0.09</td><td>0.4</td><td>0.2</td></tr><tr><td>8d*</td><td>0.68</td><td>0.34</td><td>0.5</td><td>0.2</td></tr><tr><td>9*</td><td>0.93</td><td>0.27</td><td>0.7</td><td>0.3</td></tr><tr><td>10b</td><td>5.95</td><td>0.28</td><td>4.5</td><td>1.8</td></tr><tr><td>11</td><td>0.96</td><td>0.27</td><td>0.7</td><td>0.31</td></tr><tr><td>12*</td><td>0.18</td><td>0.09</td><td>0.2</td><td>0.07</td></tr><tr><td>14b</td><td>1.64</td><td>0.11</td><td>1.3</td><td>0.55</td></tr><tr><td>15a</td><td>1.7</td><td>0.18</td><td>1.3</td><td>0.55</td></tr><tr><td>15b</td><td>1.76</td><td>0.44</td><td>1.25</td><td>0.55</td></tr><tr><td>18</td><td>2.08</td><td>0.46</td><td>1.6</td><td>0.7</td></tr><tr><td>19</td><td>4.45</td><td>0.38</td><td>3.25</td><td>1.4</td></tr><tr><td colspan="5"><strong>Garnet 2</strong></td></tr><tr><td>3</td><td>1.8</td><td>0.34</td><td>1.65</td><td>0.7</td></tr><tr><td>4a</td><td>5.55</td><td>0.35</td><td>4.15</td><td>1.8</td></tr><tr><td>4b</td><td>3.23</td><td>0.4</td><td>2.4</td><td>1.05</td></tr><tr><td>5</td><td>1.25</td><td>0.68</td><td>0.95</td><td>0.4</td></tr><tr><td>6</td><td>3.15</td><td>0.54</td><td>2.15</td><td>0.95</td></tr><tr><td>7*</td><td>0.18</td><td>0.06</td><td>0.1</td><td>0.05</td></tr><tr><td>8</td><td>0.58</td><td>0.1</td><td>0.41</td><td>0.18</td></tr><tr><td>10a</td><td>3.15</td><td>0.1</td><td>2.25</td><td>1</td></tr><tr><td>10b</td><td>5.65</td><td>0.25</td><td>4.2</td><td>1.85</td></tr><tr><td>12*</td><td>0.08</td><td>0.02</td><td>0.06</td><td>0.03</td></tr><tr><td>13a</td><td>0.73</td><td>0.23</td><td>0.54</td><td>0.23</td></tr><tr><td>13b</td><td>0.93</td><td>0.17</td><td>0.68</td><td>0.29</td></tr><tr><td>13c</td><td>4.35</td><td>0.72</td><td>3.5</td><td>1.5</td></tr><tr><td colspan="5"><strong>Garnet 3</strong></td></tr><tr><td>3d</td><td>3.93</td><td>0.55</td><td>2.85</td><td>1.25</td></tr></tbody></table>

1 = Diffusivities from Chakraborty and Ganguly, 1992;Vielzeuf et al. 2007

2 = Diffusivities from Carlson, 2006

* high-Mn fracture
<table><thead><tr><th></th><th></th><th>Static P-T(530°C),1</th><th>Static P-T(530°C),2</th><th>P-T path,1</th><th>Static P-T (560°C),1</th></tr></thead><tbody><tr><td rowspan="2">low-Mn</td><td>Mean</td><td>2.78</td><td>0.3</td><td>2.06</td><td>0.88</td></tr><tr><td>Standard Deviation</td><td>1.75</td><td>0.18</td><td>1.27</td><td>0.55</td></tr><tr><td rowspan="3">high-Mn</td><td>Mean</td><td>0.52</td><td>0.19</td><td>0.41</td><td>0.18</td></tr><tr><td>Standard Deviation</td><td>0.4</td><td>0.16</td><td>0.31</td><td>0.14</td></tr><tr><td>Δt</td><td>2.26</td><td>0.12</td><td>1.65</td><td>0.7</td></tr><tr><td></td><td>Standard Error</td><td>1.8</td><td>0.24</td><td>1.31</td><td>0.56</td></tr></tbody></table>