# Scales of fluid-rock interaction and carbon mobility in the deeply underplated and HP-Metamorphosed Schistes Lustrés, Western Alps

Gabe S. Epstein, Gray E. Bebout, Samuel Angiboust, Philippe Agard

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Gabe S. Epstein, Gray E. Bebout, Samuel Angiboust, Philippe Agard. Scales of fluid-rock interaction and carbon mobility in the deeply underplated and HP-Metamorphosed Schistes Lustrés, Western Alps. Lithos, 2020, 354, p.1416-1420. 10.1016/j.lithos.2019.105229. insu-03584807

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Scales of Fluid-Rock Interaction and Carbon Mobility in the
Deeply Underplated and *HP*-Metamorphosed Schistes Lustrés,
Western Alps

Gabe S. Epstein¹, Gray E. Bebout¹, Samuel Angiboust², and Philippe Agard³

¹Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania, 18015, U.S.A. (corresponding author; telephone: 610-758-1225; FAX: 610-758-3677; e-mail: gse216@lehigh.edu)

² Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France

³Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre Paris, ISTE P UMR 7193,
F-75005 Paris, France

second revision for *Lithos* (v. 9-21-19)

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**Abstract**

The Schistes Lustrés metasedimentary complex, exposed in the Italian and French Alps, is a regionally extensive unit of blueschist to eclogite facies marble, calc-schist and metapelite providing a record of sediment underplating and fluid-rock interaction at 40–65 km of a paleo-subduction zone. Limestones, carbonate-rich mudstones, and black shales in SE France and central Switzerland, herein used as a proxy for the unmetamorphosed Schistes Lustrés, bear a striking resemblance to sediments currently subducting into the E. Sunda margin.

The Schistes Lustrés, with estimated P-T conditions ranging from ~1 GPa and 350°C to ~2.3GPa and 550°C, have on a regional scale had their average carbonate $\delta^{18}$O$_{VSMOW}$ lowered to +20.4‰, relative to the average of +28.5‰ of their likely protoliths. This decrease in $\delta^{18}$O can be most easily explained by varying combinations of: (1) closed-system equilibration of carbonate O with O in silicate minerals in the same rocks (i.e., at the µm to cm scale); and (2) interaction between carbonates and H$_2$O-rich fluids released during prograde metamorphic devolatilization of the metapelitic rocks in the section (at up to km scales). Regional-scale decrease in carbonate $\delta^{18}$O in the Schistes Lustrés could have occurred without the need for infiltration by fluids generated outside of the Schistes Lustrés complex. More dramatic reductions in $\delta^{18}$O (as low as +8‰; average = +15.5‰) at/near major tectonic contacts, inferred to be fossil transient subduction interfaces, likely reflect deformation-enhanced infiltration by fluids derived in mafic or ultramafic rocks at greater depths.

Pressure solution and concomitant cleavage formation in the Schistes Lustrés presumably were associated with some carbonate removal, and higher-grade rocks show some evidence for decarbonation. However, within the Schistes Lustrés, C is deposited as carbonate in locally abundant veins and it has proven difficult to identify clear evidence of wholesale carbonate removal from the complex (i.e., at scales of kms to 10s of kms). This study demonstrates that large fractions of subducted sedimentary carbonate sections could persist to depths approaching those beneath volcanic fronts, if they are not accreted or underplated and depending on the extents to which they are infiltrated by H$_2$O-rich fluids capable of driving both decarbonation reactions and carbonate dissolution. Sediments such as these could supply C for additions to arc source regions or, with further subduction, convey C into the deeper mantle. The C removal and deposition reported by others for sites of particularly high fluid flux, along large fault systems, in zones of high fracture/vein density, and in metasomatized contacts with mafic/ultramafic rocks,
contrast with the relative retention and lower degrees of mobilization of C observed within km-scale packets of Schistes Lustrés away from zones of enhanced deformation and behaving as *relatively* closed systems. The fragmented and potentially lithologically biased nature of HP/UHP metamorphic suites greatly complicates attempts to derive fluid and C fluxes scalable to modern subduction margins.

**Keywords:** carbonate, devolatilization, Schistes Lustrés, stable isotopes, subduction, Western Alps

# 1. Introduction

Subduction of carbonate represents the largest carbon flux out of the crust-atmosphere system into the mantle and this C slow-cycle ingassing is in a relative steady state with additions of C to the atmosphere by volcanism (outgassing; see Dasgupta and Hirschmann, 2010; Dasgupta, 2013; Kelemen & Manning, 2015; Collins et al., 2015). Crude estimates of the total C flux into modern subduction zones are in the range of 39–96 Tg C/yr (1 Tg = $10^{12}$ grams), with 9–48 Tg/yr in sediments and the remainder bound in altered oceanic crust and ophicarbonates (Jarrard, 2003; Bebout, 2007; Dasgupta, 2013; Cook-Kollars et al., 2014; Kelemen & Manning, 2015; Collins et al., 2015; Clift, 2017). Of the sedimentary C currently subducting, about 80% is present as carbonate, with the remainder occurring as organic C (Bebout, 2007; Clift, 2017). Estimates of sedimentary inputs are based on the makeup of sections outboard of modern trenches and are often biased due to sparse drilling and inadequate knowledge of along-trench variation (see House et al., 2019). Carbon content in altered oceanic crust, largely as carbonate, is known to be extremely heterogeneous and to show huge variation related to spreading rates (see Collins et al., 2015).

The fate of C in subducting reservoirs during high-*P*, low-*T* metamorphism has been an area of active research in recent years. Mineralogical changes and fluid solubility during
prograde metamorphism presumably dictate the fraction of initially subducted C conveyed to
beneath volcanic arcs and any evolution in its isotopic composition. Accretion and forearc
underplating also can strongly influence the extent of deep subduction of C-bearing sediment
sections (Clift, 2017; House et al., 2019). The total flux of C subducted to post-arc depths is
largely unknown, with published estimates for C addition to the deeper mantle ranging from
$1 \times 10^{-4}$ Tg/yr (Kelemen & Manning, 2015) to 31 Tg/yr (Dasgupta & Hirschman, 2010). Studies in
which subduction inputs and outputs are compared make a wide range of assumptions regarding
the effects of metamorphism in forearcs on the fraction of initially subducted C retained to
beneath volcanic fronts.

Previous study of subduction zone fluid and C mobility based on *HP* and *UHP*
metamorphic rocks has been limited in scale to individual exposures, or several exposures,
disallowing assessment at km to 100s of km scales appropriate for considerations of margin-
scale mass transfer. Deeply underplated and *HP*-metamorphosed rocks of the Schistes Lustrés,
exposed in the Italian and French Alps, afford an examination of fluid-rock interactions, and
related C mobility, over a regional scale of 100s of km and in a part of the European Alps where
*HP/UHP* metamorphic rocks experienced limited amounts of exhumation-related overprinting
(Goffé and Chopin, 1986; Agard et al., 2001). However, the Schistes Lustrés reflects time-
transgressive stacking of packets of underplated metasedimentary rocks with complicated
tectonometamorphic histories and assembly (see Fig. 14 in Plunder et al., 2012).

Based on stable isotope work mostly restricted to areas in the Cottian Alps (see **Fig. 1**),
Cook-Kollars et al. (2014; cf. Henry et al., 1996) proposed that carbonate O isotope
compositions of the Schistes Lustrés complex largely reflect isotope re-equilibration of carbonate
and silicate fractions, with varying degrees of additional lowering of $\delta^{18}$O due to exchange of
carbonate with H₂O-rich fluids liberated from metapelitic rocks within the same complex. Work by Bebout et al. (2013) on the devolatilization of the metapelitic rocks, also focused largely on the Cottian Alps exposures, demonstrated that such rocks could have released 20–30 % of their H₂O along the prograde P-T paths represented collectively by the Schistes Lustrés sub-units and the *UHP* (coesite-bearing) rocks exposed at Lago di Cignana (Valtournenche, Italy).

We extended this previous work on the Schistes Lustrés in the following ways, allowing assessment at the regional scale of the generation and mobility of fluids and the extents of associated C mobility either via decarbonation reactions or carbonate dissolution. First, to better define the protoliths of the rocks in the Schistes Lustrés, we compiled a large dataset of carbonate δ¹⁸O and δ¹³C values for time-equivalent (Cretaceous) marine sediments from throughout Europe and the Atlantic Ocean (n = 905, Burkhard & Kerrich, 1988; Menegatti et al., 1998; Veizer et al., 1999; Föllmi et al., 2006). We included data for the actively subducting sediment section in the eastern part of the Sunda margin (e.g., those drilled at ODP Site 765; Plank & Ludden, 1992), as these sediments, and the tectonic setting in which they were deposited, are considered modern analogs to the Schistes Lustrés sedimentary protoliths and their depositional setting (see Trümpy, 2003).

We compared the compiled data for inferred protoliths with previously published analyses of Schistes Lustrés calcareous metasedimentary rocks (n = 370) from the Cottian Alps (Cook-Kollars et al., 2014), the Aosta Valley region of Italy (Cook-Kollars et al., 2014; Jaeckel et al., 2018), and Corsica (Cartwright & Buick, 2000) in an evaluation of change in isotopic composition during prograde metamorphism. To extend the regional coverage in this dataset, we obtained 99 new C and O isotope analyses, 35 of which are from a carbonate-rich section of the Schistes Lustrés exposed in the Val d'Isère region of France (Supplementary Table 1). A
comparison of the new and the previously published isotope data with peak metamorphic temperature estimates of Beyssac et al. (2002) derived by RSCM (Raman spectroscopy of carbonaceous material) provides evidence for an early-stage, prograde isotope reequilibration.

This approach allowed us to evaluate, at a regional scale, the hypothesis of Cook-Kollars et al. (2014) that carbonate δ¹⁸O values in the Schistes Lustrés can be explained by closed-system exchange between carbonate and coexisting silicate mineral phases (at the hand-sample scale), with additional shift in δ¹⁸O related to infiltration by H₂O-rich fluids derived from within-complex dehydration of more metapelitic rocks. The Val d'Isère section of the Schistes Lustrés was selected for a part of the new field and isotopic work because it is composed largely of metacarbonate ± quartz, with little metapelite to generate H₂O-rich fluid. Here, it was thus possible to test whether the present δ¹⁸O values reflect closed-system re-equilibration of the carbonate with silicate minerals in a part of the Schistes Lustrés for which there would be lowered expectation of interaction with metapelite-derived fluids.

Finally, we considered the evidence for the release and mobilization of C at the scale of individual outcrops (e.g., cleavage formation, multiple vein generations), as well as at the regional scale. We present 19 new major and trace element analyses across the Cottian Alps traverse (building on the dataset in Bebout et al., 2013) in an effort to better assess chemical changes associated with decarbonation and dissolution. We considered the degrees of decarbonation, in the context of the study of the Schistes Lustrés in the Cottian Alps by Cook-Kollars et al. (2014), in which the authors combined field and petrographic observations regarding mineralogy and mineral modes with calculations of devolatilization history using Perple_X. Also, we evaluated the extent of mobilization of carbonate via dissolution, at µm to
153 km scales, taking into account recently published calcite solubility data (see Kelemen & Manning, 2015).

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## 2. Geologic Background

### 2.1 Sedimentary and tectonic reconstructions

158 The Schistes Lustrés metasedimentary complex is exposed in the Western European Alps on the borders of France, Italy, and Switzerland, and on the French island of Corsica (Fig. 1).
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160 Correlative metasedimentary units extend into the Central Alps of NE Italy and Austria, where they are referred to as the Bündnerschiefer (Bearth, 1976; Deville et al., 1992; Lemoine, 2003; Garofalo, 2012). Pre-rift platform carbonate deposition began in the Early Triassic, with rifting and pelagic sediment deposition initiating in the latest Triassic (Lemoine et al., 1986). The majority of the deposition of the Schistes Lustrés occurred from the Late Jurassic through the latest Cretaceous within the basin of the Piemont-Ligurian Ocean, also called the Alpine Tethys (Smith, 1971; Deville et al., 1992; Trümpy, 2003). The Alpine Tethys formed between the European and African paleocontinents via oblique extension and was structurally linked to the opening of the Atlantic Ocean by a series of strike-slip fault zones (Bernoulli & Jenkyns, 1974; Coward & Dietrich, 1989). Although the two oceans were separated by a transform fault system, Cretaceous sedimentary stratigraphy has been correlated between them (Schole & Arthur, 1980; Brosse, 1982; Herrle et al., 2004).

172 Convergence of the European and African paleocontinents initiated during the Late Cretaceous and the Piemonte-Liguria oceanic lithosphere began subducting beneath Africa (Trümpy, 1975; Coward & Dietrich, 1989; Handy et al., 2010). Subduction continued until the middle Eocene when the two plates collided to begin formation of the Alps. The Alpine collision
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coincided with exhumation of the deeply subducted lithologies (Rubattto & Hermann, 2001;

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Agard et al., 2009; Beltrando et al., 2010).

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Subduction of the ancient Tethys basin was not continuous and constant and was instead
marked by major trench offsets during lateral migrations, collisions, and interface downstepping
(Rubatto et al., 1998; Agard et al., 2001; Handy et al., 2010). At depth, the subduction interface
was inferred to be transient in nature, with accretion of lower plate material to the hanging wall
causing reorganization of the locus of deformation (Trümpy, 1975; Angiboust et al., 2014a). The
sedimentary packages investigated in this and previous studies were all sourced from the
Piemonte-Liguria Ocean but were later accreted/underplated to the upper plate.

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The Schistes Lustrés complex is composed primarily of deep marine sediments but also
contains appreciable mafic and ultramafic material derived from Tethyan oceanic lithosphere
(Dal Piaz & Ernst, 1978; Lemoine & Tricart, 1986; Deville et al., 1992). Peak metamorphic
grade increases from west to east, with the broadest division separating the blueschist from the
eclogite facies. The highest-grade unit of the Schistes Lustrés complex is the Lago di Cignana
unit, inferred to have reached depths of ~100km (Reinecke, 1998; Groppo et al., 2009).
Extensive study of the metamorphic evolution of the Schistes Lustrés, particularly that exposed
in the Cottian Alps and Corsica, has revealed significant folding, transposition, and imbrication
of the unit, accompanied by devolatilization (Caron, 1977; Agard et al., 2001; Bebout et al.,
2013; Cook-Kollars et al., 2014; Vitale Brovarone et al., 2013, 2014).

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The Schistes Lustrés underwent two styles of deformation during subduction. Prograde
metamorphism led to devolatilization and related veining during subduction (Fig. 2a; Agard et
al., 2000); contemporaneous and later deformation variably transposed these veins and they fully
equilibrated with their host-rocks isotopically (see Henry et al., 1996; Cartwright & Buick, 2000)

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during strain-induced grain size reduction and recrystallization (in the presence of a sediment-
derived fluid). Acting separately on a larger scale, but overlapping in time with the transposition,
was large-scale stacking of thrust sheets of Schistes Lustrés against a buttress of overlying
continental crust (Trümpy, 1975; Agard et al., 2001, 2002; Plunder et al., 2012). The geometry
was similar to that of underplated units in an accretionary prism or a series of duplexes, but the
stacking occurred at depths of around 30–65km (Agard et al., 2001). Thus, the Schistes Lustrés
is present as a well-foliated schist (Fig. 2b), with vestiges of primary bedding preserved as cm-
to m-scale chemical heterogeneity. Where exposures of the schist are continuous, there is a
corresponding continuous increase in P-T conditions (Agard et al., 2001; Beyssac et al., 2002;
Gabalda et al., 2009; Plunder et al., 2012).

The preservation of large tracts of carbonate-bearing rock within the Schistes Lustrés (in
some cases, massive marbles) indicates appreciable retention of C during prograde
metamorphism to the peak P-T conditions represented (depths of up to ~65km). Evidence for
more significant C removal exists at or near the contact between Schistes Lustrés
metasedimentary rocks and underlying mafic or ultramafic rocks, but these contact zones are
typically volumetrically minor (scales of a few cm to ~300 m; Malvoisin et al., 2012; Ague &
Nicolescu, 2014; Barnes et al., 2014; Piccoli et al., 2016). The existence of carbonate- and
reduced (organic) C-bearing rocks (marbles, calc-schists, and metapelites) preserving maximum
pressures of 3 GPa at Lago di Cignana illustrates that some fraction of the C entering the Eocene
Alpine subduction zone reached depths similar to those beneath modern arcs (Groppo et al.,
2009; Frezzotti et al., 2011; Cook-Kollars et al., 2014).
The Schistes Lustrés are chemically similar to unmetamorphosed sediments exposed throughout W. Europe and previous authors have used these sediments as an inferred protolith composition during studies of devolatilization and element mobility in the schists. Busigny et al. (2003) and Bebout et al. (2013) relied on diagenetically altered, but unsubducted sediments of the northern Apennines, whereas Henry et al. (1996) relied on sediments from the Paris basin. As discussed by Schole & Arthur (1980) and Brosse (1982), Cretaceous marine sediments are comparable across the Atlantic and now-exposed Tethyan basins, with thick sequences of clay-rich limestone to calcareous mudstone present off the east coast of the United States (ODP Leg 171, Norris et al., 2001), the NE coast of Brazil (ODP Leg 207, Macleod, 2006), and throughout Europe (Brosse, 1982; Mennegati et al., 1998; Herrle et al., 2004; Föllmi et al., 2006).

The Schistes Lustrés also bear a striking resemblance to the E. Sunda margin section drilled at DSDP Site 261 and ODP Site 765 (Trümpy, 2003; Bebout et al., 2013; Cook-Kollars et al., 2014; House et al., 2019). This margin contains a Cretaceous to Pleistocene marine sediment section (138 Ma to 0.2Ma), composed of carbonate-bearing pelagic clays overlain by carbonate-rich turbidites (Dumoulin & Bown, 1992; Plank & Ludden, 1992). Lemoine (2003) compiled stratigraphic successions of the hemipelagic-pelagic background (HPB) for the Schistes Lustrés—the sections studied here contain some gaps in preservation but display similar trends (Fig. 3). The stratigraphy of the Schistes Lustrés protolith sections compiled by Lemoine (2003) is analogous to the lower part of the E. Sunda section (see Shipboard Scientific Party, 1990).
These stratigraphic sections, being simplified characterizations of background sedimentation, do not include detrital or tectonically emplaced components such as olistoliths, mafic lenses or within-section ophiolite fragments. Carbonate-rich pelagic sediments occur in the Upper Jurassic and the Late Cretaceous, whereas the intervening timespan was dominated by the deposition of
244 marls and shales. Importantly, within the Schistes Lustrés, black shale deposition was
245 intermittent and was punctuated by short intervals of carbonate sedimentation, equivalent to the
246 Palombini Shales of the Northern Apennines (Cornamusini et al., 2012). An exception was at the
247 boundary between the Early and Late Cretaceous (~110 – 90Ma) when there was deposition of
248 carbonate-poor black shale (Fig. 3; Herrle et al., 2004). It therefore appears likely that the
249 protoliths of the Schistes Lustrés section were initially clay-rich limestones with minor
250 siliciclastics and infrequent, thin black shale interlayers. Importantly, the clay component of
251 these limestones was variable and at times the dominant fraction in the sediment. Large sections
252 of pure limestone or siliciclastics are volumetrically minor in all Cretaceous sedimentary analogs
253 to the Schistes Lustrés.

254 2.3 Schistes Lustrés samples

255 The samples for which isotope data are presented in this paper are from multiple
256 localities spanning a P-T range of ~1–2.2 GPa and 350–550°C and are from five different
257 transects selected for their varying proportions of silicate:carbonate and lesser amounts of
258 intercalated metabasalt. Stratigraphic sections roughly corresponding to the sampled transects are
259 indicated in Fig. 3. The Schistes Lustrés in the W. Alps has previously been separated into at
260 least three major tectonic units, with contacts defined by peak-temperature estimates and
261 lithologic/structural variability; the upper, middle, and lower units (LPU, LPM, and LPL of
262 Plunder et al., 2012; see also Fudral et al., 1987, and Lagabrielle et al., 2015). These units
263 experienced differing prograde P-T-t paths and, following accretion, were juxtaposed during
264 exhumation (Agard et al., 2001; Angiboust et al., 2014a). Table 1 provides the peak-
265 metamorphic temperatures (obtained by RSCM) for samples of Schistes Lustrés from the various
266 localities.
Mineralogy is broadly uniform throughout the Schistes Lustrés: quartz, carbonate, phengite, albite, and variably graphitized carbonaceous material are ubiquitous, in widely varying proportions (Fig 2c), and common accessory phases include clinozoisite-epidote (some of it as replacements of prograde lawsonite), titanite, chlorite, sulfide, and tourmaline (see Henry et al., 1996; Bebout et al., 2013; Cook-Kollars et al., 2014). Some metamorphic index minerals also occur as related to bulk composition, increasing in grade these include: carpholite, chloritoid, lawsonite, and garnet (Agard et al., 2001; Bebout et al., 2013). Carbonate and/or quartz veins are ubiquitous, but highly variable in abundance, attesting to the presence of a free fluid at near-lithostatic pore pressure during metamorphism (Fig 2a).

The Cottian Alps traverse grades from carpholite-bearing blueschist-facies rocks to garnet-bearing eclogite-facies rocks across the contact between the LPM and LPL and shows a continuous P-T increase from 1.2 GPa and 350°C to 2.2 GPa and 450-500°C (Fig. 1; Agard et al., 2001). The carbonate abundance in these rocks is highly variable, with some sections nearly devoid of metacarbonate (Bebout et al., 2013). In the Cottian Alps, the abundance of mafic rocks increases in the highest-grade LPL unit.

The Val d'Isère region (Fig. 1) was sampled along a ~50km length of road. This field area is composed of km-scale packets of carbonate-rich Schistes Lustrés (marbles and calc-schists) and lies within the middle unit (LPM) of Plunder et al. (2012). The rocks in the Isère section, similar lithologically to that labeled “Maurienne” in Fig. 3, contain on average >~70 wt. % calcite, with a strikingly smaller proportion of metapelitic rocks than in the Cottian Alps traverse, and reached maximum temperatures of ~450°C (Plunder et al., 2012).

The Aosta Valley samples are from two transects within the Tsaté complex of the Schistes Lustrés, near the contact with the overlying Dent Blanche Tectonic System (also
investigated by Jaeckel et al., 2018). The transects are near the towns of Ollomont on the west and Breuil-Cervinia on the east (Fig. 1), and record P-T conditions ranging from ~1–1.5 GPa and ~400–475°C (Negro et al., 2013; Angiboust et al., 2014a). Eocene subduction was directed towards modern-day southeast (Stampfl & Hochard, 2009), therefore Breuil-Cervinia reached slightly higher maximum P-T conditions.

The Schistes Lustrés in Corsica is composed of lawsonite-bearing blueschist- and eclogite-facies metamorphosed rocks preserved as a complexly deformed stack of tectonic slices that experienced varying P-T paths (Caron, 1977; Caron & Delcey, 1979; Vitale Brovarone et al., 2013). The samples for which data are included in this paper are from blueschist-facies metasedimentary exposures that reached maximum P-T conditions ranging from ~1 GPa and 350°C (Bagliacone & Inzecca) to ~2.4 GPa and 550°C (Castagniccia, see **Table 1**; Cartwright & Buick, 2000; Vitale Brovarone et al., 2014).

## 3. Conceptual, Field, and Analytical Approach

One hypothesis tested in this study, building on the work of Cook-Kollars et al. (2014; also see Henry et al., 1996) and Jaeckel et al. (2018) is that, in sections of the Schistes Lustrés complex containing little metapelite, the effect of the devolatilization of this lithology (liberating H₂O-rich fluid) on the O isotope compositions of nearby marble and calc-schist is greatly diminished. In such rocks, any decrease in δ¹⁸O could instead be attributable to more closed-system (at hand-specimen scale) O isotope exchange of the carbonate with the coexisting silicate minerals in the same samples. As noted above, the Val d'Isère area afforded sampling of such a section. Conversely, in parts of the Schistes Lustrés containing abundant metapelite (e.g., along parts of the Cottian Alps traverse; see Agard et al., 2001; Bebout et al., 2013), the release of
H₂O-rich fluid in part evidenced by higher densities of transposed quartz-calcite veins (in some cases containing metamorphic indicator minerals such as carpholite and lawsonite; see Lefeuvre et al., 2018; also see Henry et al., 1996; Agard et al., 2000) resulted in larger shift in δ¹⁸O in neighboring carbonate-rich rocks.

The analyses reported in this paper are either for whole-rock powders, prepared in a mortar and pestle, or for microdrilled regions. Samples that were drilled are further separated into those that approximate the whole-rock carbonate:silicate ratio versus those that represent veins or carbonate-rich segregations, based on hand-specimen and petrographic inspection (Supplementary Table 1). Calcite δ¹⁸O and δ¹³C values were obtained following the procedure of McCrea (1950) involving production of CO₂ by reaction with 100% phosphoric acid (H₃PO₄). Whole-rock powders from the Val d'Isère region were weighed and 10–50 mg, depending on estimates of carbonate content, were reacted for 8–12hrs at 27°C. The resulting CO₂ was cryogenically purified on a glass vacuum extraction line before isotope analyses in dual-inlet mode on a Finnigan MAT 252 mass spectrometer at Lehigh University. Samples from other locations, some of them microdrilled, were weighed and 0.5–3.0 mg were reacted with 0.2 mL of 100% phosphoric acid for 3 hrs at 72°C, with the CO₂ then introduced into the mass spectrometer via He carrier gas using a Gas Bench II coupled with a Combi PAL autosampler (also at Lehigh University; description of this method in Collins et al., 2015). Regular analysis of a house standard and international standard NBS-19 allowed monitoring and correction of the data, resulting in a 1σ of 0.2‰ for both O and C isotope analyses. δ¹³C and δ¹⁸O values are reported relative to VPDB and VSMOW, respectively. Carbonate wt. % was calculated by comparing the mass-to-signal ratio of unknowns to the same ratio for the house standard.
335 Whole-rock major element analyses were determined by X-Ray Fluorescence (XRF) at 336 Franklin and Marshall College in Lancaster, Pennsylvania. Approximately 1 gram of sample 337 material was weighed, then heated at 950°C for 1.5 hours before being reweighed to determine 338 Loss on Ignition (LOI). The glass disks used for XRF analyses were created by mixing 0.4 grams 339 of sample powder with 3.6 grams of lithium tetraborate and melting in a platinum crucible. 340

340 Whole-rock trace element analyses were performed at Activation Laboratories Ltd., in 341 Ancaster, Ontario, Canada. With the exception of Li and B, trace elements were analyzed by 342 ICP-MS after a lithium metaborate fusion. Samples were digested in aqua regia and diluted to 343 250 ml volumetrically before being analyzed on a Varian Vista 735 or a Thermo ICAP 6500 344 ICP. Lithium was analyzed on a Varian Vista 735 ICP following total digestion of a 0.25 g 345 aliquot in hydrofluoric, nitric, and perchloric acids. For analyses of B concentrations, 1 gram of 346 material was irradiated in a nuclear reactor and measured for gamma ray output. Detection limits 347 for each trace element method are reported in **Table 3**, and more information regarding the 348 individual methods is provided at the following website: http://www.actlabs.com/. 349

350 In evaluating the new and existing O and C isotope datasets (see Figs. 4 and 5), we make 351 a number of simplifying assumptions to allow for comparison of isotope trends throughout the 352 Schistes Lustrés. We model the protolith as having an initial carbonate δ¹⁸O value of +27‰, as 353 this value best represents the δ¹⁸O value of Tethyan marine carbonates during the two most 354 significant carbonate deposition sequences (Fig. 3, Late Jurassic and Late Cretaceous; Voigt & 355 Hilbrecht, 1997; Godet et al., 2006; Duchamp-Alphonse et al., 2007). On Fig. 5, the diagonal 356 line labeled “calcite” represents the variation in calcite δ¹⁸O expected for closed-system 357 equilibration with the silicate fraction in the same samples, with δ¹⁸O plotted vs. the fraction of 358 carbonate in the samples. The quartz δ¹⁸O value for the low-carbonate endmember is set at 359
+19‰, this value is taken from minimum values for quartz coexisting with carbonate from the Bündnerschiefer (Hoernes & Friedrichsen, 1974, 1980), the Helvetic nappes (Burkhard & Kerrich, 1988; Henry et al., 1996), and Vermont (Schwarcz et al., 1970). In this figure, the carbonate:silicate ratio is variable, and the silicate fraction is assumed to be dominantly quartz (such that the silicate end-member has bulk δ¹⁸O of +19‰, and all other silicate phases are in equilibrium with quartz). Fractionation factors among quartz, calcite, phengite, and chlorite are calculated for a temperature of 395°C (Zheng 1993, 1999; Chacko et al., 1996), representing the lowest temperature determined for any of the studied sections (Ollomont: 396±15°C; Angiboust et al., 2014a), and consistent with calculations of fluid-aided O isotope exchange in the Schistes Lustrés (400–650°C; Cook-Kollars et al., 2014; Cartwright & Buick, 2000).

## 4. Results

### 4.1 Analyses of carbonate δ¹⁸O and δ¹³C

We present 99 new analyses from the Isére (n=35) and Aosta Valley regions (n=64), with the latter being further subdivided based on proximity to the interface (33 samples collected within 300m of the contact with the overlying Arolla Gneiss; **Supplementary Table 1**). These samples are plotted in **Fig. 4**, and additional information, including location and IGSN numbers, can be found in the supplementary file. **Figure 4** presents the O and C isotope compositions of the marine sediments (presumed protoliths) and the Schistes Lustrés. Samples from the internal nappes of the Tsaté complex, away from major contacts, have average δ¹⁸O and δ¹³C of +18.6 ± 1.3‰ and -1.2 ± 1.4‰ (mean ± standard deviation), while those samples near the contacts have average values of +15.2 ± 1.4‰ and -1.3 ± 1.5‰ (**Fig. 4**; blue squares and red circles,
respectively). In the carbonate-rich Val d'Isère section, the average values are +22.6 ± 1.5‰ and -0.7 ± 1.3‰ for δ¹⁸O and δ¹³C, respectively (blue-green triangles in **Fig. 4**).

Calculated carbonate abundances for individual samples analyzed in the present study range from ~4 wt. % to ~80 wt. % (Fig. 5; Supplementary Table 1). Isotope compositions and carbonate contents of veins are not included in Fig. 5 but are provided in Supplementary Table 1. Samples collected from within 300m of the Dent Blanche Thrust are not included in Fig. 5 — these samples are known to have been further shifted in δ¹⁸O by interaction with externally-derived fluids previously equilibrated with mafic or ultramafic lithologies (red-filled circles in Fig. 4; see Jaeckel et al., 2018).

## 4.2 Major and trace element analysis

Whole-rock major element compositions of the Schistes Lustrés are consistent with sedimentary mixture of three primary components: detrital quartz (SiO₂), clays (Al₂O₃-rich), and carbonate (Fig. 6a; Table 2). Al₂O₃ concentrations show a strong positive correlation with K₂O concentrations and are negatively correlated with CaO wt. % (Figs. 6b & 6c). Carbonate is the dominant host for CaO as shown in the plot of CaO vs. LOI, with increasing LOI corresponding to increased carbonate (and to a lesser extent clay+chlorite) concentrations (Fig. 6d). Table 2 also contains CO₂ and calculated H₂O concentrations, the former being determined from reaction with phosphoric acid and the latter determined by subtraction of CO₂ wt. % from the LOI.

Trace element concentrations also are consistent with a mixture of shale and carbonate, and in Fig. 6e are compared with concentrations in sediments outboard of the E. Sunda trench and metasedimentary rocks of the Bündnerschiefer (also see Table 3). The gray shading represents relative carbonate abundance, with the darker grays having the highest carbonate concentrations. The only chemical elements showing anomalous concentrations are Sr, Zr, and
403

Hf, with a negative Sr anomaly in the pelitic samples, and negative Zr, and Hf in the carbonate-

404

rich samples. Barium concentrations vary widely (~1 ppm to 800 ppm), perhaps indicating

405

interaction with fluids (see **Section 5**).

406

407

**5. Discussion**

408

**5.1 Protolith composition: ancient and modern analogs**

409

The average δ¹⁸O for 370 Schistes Lustrés carbonates is +20.4‰, with an average δ¹³C of

410

-1.1‰, values significantly different from those for the 905 marine carbonates (+28.5‰ and

411

+2.2‰, respectively; see **Fig. 4**). Variations in marine carbonate δ¹⁸O are due to variations in sea

412

surface temperature during the middle to late Mesozoic, whereas δ¹³C variations are due to

413

productivity of marine organisms and the variability of production and burial rates of organic C

414

(Weissert et al., 1998; Kump & Arthur, 1999). Trends within the datasets for individual suites of

415

marine carbonate are not shown in **Fig. 4**, but presumably reflect combinations of regional and

416

temporal isotope variability and varying degrees of diagenetic alteration (Vincent et al., 2004;

417

Föllmi et al., 2006).

418

Also included in **Fig. 4** are data from the E. Sunda trench region (from Plank & Ludden,

419

1992), thought to be a modern analog to the tectonic depositional setting of the pre-subduction

420

Schistes Lustrés (Trümpy, 2003; Bebout et al., 2013; Cook-Kollars et al., 2014). The marine

421

carbonates from the Cretaceous part of this section have average δ¹⁸O and δ¹³C of +28.8‰ and

422

+2.6‰, respectively (n = 6) similar to those of the marine sediments for which data are presented

423

in **Fig. 4**.

424

**Figure 6** provides a comparison of the major and trace element compositions of

425

sediments from the E. Sunda margin (Plank & Ludden, 1992) and those of metasedimentary
rocks from the W. Alps (Bündnerschiefer formation, Garofalo, 2012; this study). The overlap in the major element compositions of the marine sediments and the W. Alps suite further argues for some similarity in depositional environment. A few of the samples from the W. Alps extend to higher Al₂O₃ and K₂O concentrations than those from E. Sunda, perhaps reflecting slightly different sediment sources or modest mass transfer associated with diagenesis (loss of SiO₂ and gain of K₂O; Milliken, 2005). Loss on ignition (LOI) for the E. Sunda samples is somewhat higher, for a given CaO concentration, perhaps because of a greater abundance of hydrated diagenetic clays (Compton, 1992) and because the metamorphic units have experienced partial dehydration stabilizing micas. Trace element compositions also are broadly similar over all the rock types, with the exception of a negative Sr anomaly in the Schistes Lustrés pelitic samples, and negative Zr and Hf anomalies in the carbonate-rich samples (Fig. 6e; darker gray shading indicates higher carbonate abundance). The anomalies are most easily explained by preferential incorporation of Sr into carbonate and of Zr and Hf into the silicate fraction of the schists.

## 5.2 δ¹⁸O and δ¹³C trends at the nappe scale

During metamorphism, the sediments of the Schistes Lustrés, with very few exceptions, had their carbonate δ¹⁸O lowered to values of +16 to +25‰, considerably lower than values for marine carbonate, the latter in general in the range of +26 to +30‰ (see Figs. 4 and 5).

Interaction between carbonate and silicate phases, to produce these lower δ¹⁸O values, was first proposed for a small part of the Schistes Lustrés by Henry et al. (1996), who surmised that the δ¹⁸O value of carbonate and silicates would be controlled by their proportions in the sample.

However, as illustrated in the plot in Fig. 5, most of the carbonate in the Schistes Lustrés has δ¹⁸O lower than can be explained by closed-system exchange with the metapelites. Cook-Kollars et al. (2014) concluded that this further decrease in δ¹⁸O values reflects varying degrees of
449 interaction with H₂O-rich fluids produced from devolatilization reactions in nearby silicate-rich
450 metapelitic rocks (see the study of the metapelite devolatilization by Bebout et al., 2013). In this
451 thinking, carbonate in marbles and calc-schists in relatively metapelite-rich sections of the
452 Schistes Lustrés would be expected to see more of this H₂O-rich fluid and thus show greater
453 lowering in δ¹⁸O. Carbonate in sections of the Schistes Lustrés containing little of the
454 dehydrating metapelite would be less likely to experience fluid:rock interaction with these H₂O-
455 rich fluids and thus have carbonate δ¹⁸O more consistent with local-scale exchange with the
456 silicate in the same rocks.

457 The O isotope data for the Val d'Isère region (blue green triangles in Fig. 5), at which
458 there is little metapelitic material, fit well with the model of Henry et al. (1996) and Cook-
459 Kollars et al. (2018), indicating closed-system equilibration of the carbonate with silicate
460 minerals in the same rocks without further lowering in δ¹⁸O due to interaction of metapelite-
461 derived fluids. The expanded dataset presented here is thus consistent with, and adds support to,
462 the conclusions made by Cook-Kollars et al. (2014), that the δ¹⁸O of carbonate is controlled by:
463 1) closed-system exchange between carbonate and silicate, and 2) varying extents of interaction
464 with metapelite-derived fluids during devolatilization. A comparison of carbonate δ¹⁸O values to
465 RSCM temperature estimates indicates that the offset from marine values is of a similar
466 magnitude across all temperatures, but with some decrease in δ¹⁸O with increasing grade (see
467 Fig. 7). Cook-Kollars et al. (2014) suggested that this subtle decrease in δ¹⁸O is consistent with
468 that expected for across-grade equilibration (at peak metamorphic temperatures) of carbonate
469 with H₂O-rich fluid with δ¹⁸O reflecting a metapelitic source.

470 **Figure 8**, modified after Jaeckel et al. (2018), illustrates the present geometry and
471 makeup of the Schistes Lustrés complex, emphasizing the contacts among packets of largely
metasedimentary rock representing sediment sections with widely varying proportions of carbonate and silicate phases. The Schistes Lustrés reflects time-transgressive stacking of packets of metasedimentary rocks and the section has been thickened greatly by isoclinal folding (see Fig. 14 in Plunder et al., 2012). These units show a range of peak metamorphic grade and contacts among these packets are largely defined by mineral assemblages and RSCM temperatures (Agard et al., 2001; Plunder et al., 2012), except where they are directly juxtaposed with upper-plate quartzofeldspathic gneisses (see Angiboust et al., 2014a). The interfaces at which Schistes Lustrés is juxtaposed with the gneisses show field and O isotope evidence for extensive infiltration of fluids likely derived from an underlying mafic/ultramafic source (Jaeckel et al., 2018; see the black arrows in Fig. 8). However, the contacts among the other packets might also be expected to have served as zones of enhanced fluids flow (see the blue arrows in Fig. 8) and significant fluid flow might also occur within the upper parts of slabs and in detached slices of oceanic lithosphere (see Angiboust et al., 2014b; Vitale Brovarone et al., 2018).

Much of the variation in carbonate δ¹³C in the Schistes Lustrés is thought to have been imposed by partial equilibration with co-existing reduced C. Work by Cook-Kollars et al. (2014) and Kraft (2018; Kraft and Bebout, 2017) demonstrated that, for units peak-metamorphosed at temperatures below ~400°C, carbonaceous material in the Schistes Lustrés largely preserves marine organic δ¹³C near -22‰. Kraft (2018; Kraft and Bebout, 2017) presented normalizations of concentrations of reduced C to concentrations of Al, Ti, and Zr (similar approach taken for non-subduction-related rocks by Zhang et al., 2018). These ratios are uniform across the several lower grades along the Cottian Alps traverse (Fraiteve, Assietta, and Albergian; see **Table 1**), but show a hint of reduction for the highest grade unit, Finestre, and for metasedimentary rocks at Lago di Cignana but no obvious change at the interfaces for which Jaeckel et al. (2018)
demonstrated large shifts in δ¹⁸O. The work by Cook-Kollars et al. (2014) and Kraft (2018)

demonstrated that equilibration between carbonate and carbonaceous material was more

complete in units experiencing peak temperatures >400°C. Carbonate δ¹³C as low as -6‰ in the
Schistes Lustrés (see **Fig. 4**) is potentially indicative of this partial equilibrium process.

Alternatively, decarbonation can drive carbonate δ¹³C to lower values (see the discussion by
Collins et al., 2015). However, much of the Schistes Lustrés contains relatively few calc-silicate
phases indicating decarbonation reactions (Cook-Kollars et al., 2014; Jaeckel et al., 2018).

Analyses of veins and their host carbonates in the Schistes Lustrés were performed by
Henry et al. (1996) and Cartwright & Buick (2000; see the smaller dataset in Jaeckel et al.,
2018). These authors’ results indicate that vein carbonates are in isotopic equilibrium with their
host rocks, a conclusion that is consistent with internal fluid production and/or buffering of the
fluid by the host during vein formation (e.g., lawsonite breakdown in the Cottian Alps; Lefeuvre
et al., 2018). Cook-Kollars et al. (2014) observed no gradients in isotope compositions of
carbonate across individual cm-scale traverses of contacts between metapelite and
metacarbonate. This seemingly indicates control of the isotope compositions by fluids mobilized
over larger scales within the Schistes Lustrés (m- to km-scale, with fluids derived from the
metapelites interlayered with the carbonate-rich rocks), and not solely by local (cm)-scale
exchange among metapelite-metacarbonate interlayers.

Jaeckel et al. (2018) found that, near major tectonic contacts at the base of the Dent
Blanche Tectonic System, carbonates have δ¹⁸O as low as +8.6‰ consistent with interaction
with a fluid previously equilibrated with mafic or ultramafic rock (see also Piccoli et al., 2016;
Scambelluri et al., 2016). These δ¹⁸O values are consistent with open-system behavior near/along
the interface contact, the latter which could have acted as a conduit for fluids emanating from
deeper in the subducting slab (open and red-filled circles in **Fig. 4**). We stress that the total
volume of Schistes Lustrés displaying this open-system behavior is minor, and to date has only
been reported where the Schistes Lustrés is near or in direct contact with upper plate continental
gneiss (e.g. the Arolla Gneiss of the Dent Blanche Tectonic System).

522 *5.3 Evidence for carbon mobility*

Interaction between carbonate-bearing rocks and a hydrous fluid could result in
decarbonation reactions (formation of Ca-silicates at the expense of carbonate) or dissolution
reactions (uptake of carbonate by a previously CO2-undersaturated fluid; Kerrick & Connolly,
2001; Connolly, 2005; Gorman et al., 2006; Cook-Kollars et al., 2014; Ague & Nicolescu, 2014;
Kelemen & Manning, 2015; Gorce et al., 2019). In this section, we present end-member
scenarios for considering the mechanism and magnitude of C mobility within the Schistes
Lustrés and whether externally-derived fluids (i.e., not derived internally from devolatilizing
metapelites) are required to produce the observed δ18O shifts (see **Fig. 9**).

531
532
**Scenario 1: Internally-produced, metapelite-derived fluid leading to C removal by**
*decarbonation reactions*

There is no obvious trend of decreasing carbonate abundance with increasing grade
throughout the Cottian Alps, however, along the traverse, the increase in modal abundance of
calc-silicate phases with increasing grade reflects modest amounts of decarbonation presumed to
have resulted from interaction of these rocks with a H₂O-rich fluid (Figs. 9a and b; see the
related thermodynamic calculations by Cook-Kollars et al., 2014). The calculations by Cook-
Kollars et al. (2014; see Fig. 9 in that paper) indicate that, at extremely low XCO₂ (mole fraction
of CO₂ in a binary CO₂-H₂O fluid) presumably achieved/maintained via infiltration by an H₂O-
rich fluid, some of the more metapelitic rocks initially containing some carbonate (up to 15 wt.%
541 %) could have been completely decarbonated. The authors argued that the rocks contain fewer
542 calc-silicate phases than predicted by these calculations, suggestive that the calculated extents of
543 decarbonation were not experienced perhaps due to insufficient infiltration by sufficiently H₂O-
544 rich fluids. The persistence of carbonate to subarc or even greater depths during Rayleigh-like
545 decarbonation, without infiltration by externally-derived H₂O-rich fluid, has been extensively
546 modeled in other studies (see Kerrick and Connolly, 2001; Connolly, 2005; see the discussion by
547 Collins et al., 2015; Gorce et al., 2018).

548 For the highest grades in the Schistes Lustrés, having peak metamorphic temperatures of
549 ~550°C, Bebout et al. (2013) calculated the loss of 20-30 % of mineral-bound H₂O in
550 metapelites, mostly liberated from carpholite and/or chlorite (gray line in **Fig. 9b**). Fluid loss is
551 reflected in subtle decrease in whole-rock ratios of concentrations of lithophile elements Ba, Cs,
552 and Rb to K₂O and Li to Zr (**Fig. 9c**; also see the ion microprobe data for phengite in Bebout et
553 al., 2013). Upon interaction with impure metacarbonates, this fluid caused localized
decarbonation and led to the formation of Ca-bearing silicates, most notably garnet and
554 lawsonite/epidote — this is approximated by CaO excess in **Fig. 9b** (whole-rock CaO remaining
555 after removal of that residing in carbonate). Decarbonation, without additional isotopic effects of
556 infiltration, acts to decrease the δ¹⁸O and particularly the δ¹³C values of residual carbonate (see
557 the red line in **Fig. 4**, schematically indicating the result of a Rayleigh process). However, within
558 the Schistes Lustrés, the lowest carbonate δ¹³C values occur in samples where carbonaceous
559 matter preserves anomalously high δ¹³C values. This paired shift indicates that the main process
560 acting to decrease δ¹³C values at high grade was isotope exchange between carbonate and
561 reduced C (see Cook-Kollars et al., 2014; Kraft, 2018; Kraft and Bebout, 2017).
563

**Scenario 2: Internally-produced, metapelite-derived fluid leading to C-mobility by carbonate dissolution**

564

565

Carbonate dissolution could also have resulted in mobilization of C (e.g., during production of cleavage; see **Fig. 2c**) but on a hand-specimen scale, loss of C via this mechanism is difficult to assess due to large heterogeneity in the composition of the Schistes Lustrés inherited from its sedimentary protoliths (**Fig. 9a**). A low-carbonate metapelitic rock resulting from some removal of carbonate via dissolution would be quite difficult to distinguish from a rock with a lower-carbonate protolith (see **Fig. 6**). In this study, a thorough assessment of whole-rock major and trace element data for 36 samples, across grade (Cottian Alps traverse and including the rocks at Lago di Cignana), revealed no compelling signature of carbonate dissolution (see **Fig. 9c**). A pure carbonate removal scenario would presumably involve correlated reductions in whole-rock concentrations of CaO and CO₂ and presumably also of Sr and any other trace elements initially hosted by carbonate and behaving in the same way as CaO during fluid-rock interactions.

566

567

568

569

570

571

572

573

574

575

576

577

**Figure 9d** shows calculated fluid-to-rock mass ratios required to obtain the observed

578

579

580

581

582

583

584

585

carbonate δ¹⁸O value assuming interaction at 395°C between carbonate-bearing metasediment and pure H₂O with a δ¹⁸O value of +11.4‰ (i.e. in equilibrium with the silicate end-member assemblage of **Fig. 5**). These are minimum fluid-to-rock ratios as they assume infiltration only by fluids with this pristine +11.4‰ metapelitic δ¹⁸O value and thus no upstream modification in δ¹⁸O via interaction with carbonate-rich rocks within the Schistes Lustrés complex. This calculation indicates fluid-to-rock ratios mostly below 0.25:1 extending up to 0.8:1 in the most extreme cases. Solubility of C in H₂O over the conditions studied here is ~150–650 ppm (395–550°C and 1.6–2.0 GPa; Caciagli & Manning, 2003; Kelemen & Manning, 2015); given these
solubilities, a fluid-to-rock ratio of 0.8:1 corresponds to 1.0–4.3 wt.% carbonate dissolution from the most modified samples (as wt.% of the whole rock). Assuming closed-system interaction (i.e., no infiltration from pelitic sections), a silicate fraction in a mixed metapelite-metacarbonate rock containing 6 wt.% H₂O and undergoing 50% devolatilization, consistent with H₂O estimates for the Schistes Lustrés (gray line in **Fig. 8b**; Cook-Kollars et al., 2014), would not lead to complete dissolution unless the initial carbonate abundance was less than 1.6 wt.% (Fig. 9e, 550°C dissolution). Higher carbonate concentrations would reduce the H₂O content of the whole rock, thereby limiting the magnitude of dissolution. Together, **Figs. 9d and e** indicate that dissolution of carbonate in the Schistes Lustrés is not expected to exceed 5% (compared to whole rock wt.%), and is caused dominantly by infiltration of carbonate-undersaturated, metapelite-derived fluids into carbonate bearing horizons. Significant dissolution can progress only where large amounts of silicate-rich rock are intercalated with small lenses of carbonate-bearing metasediment. Cleavage formation presumably involved some dissolution of carbonate, perhaps extensive in some parts of the section (**Figs. 2b and 2c**; Davidson et al., 1998; Naus-Thijssen et al., 2010). However, at the microscopic to outcrop scales, carbonate-bearing veins and pressure shadows around porphyroblasts and quartz aggregates (**Figs. 2a and 2d**) seemingly reflect the *deposition* of carbonate.

Lastly, any internally-produced fluid, upon interaction with carbonate, would become saturated with respect to carbonate. The carbonate component of this fluid represents a minor amount of C-mobility but, where this fluid interacts with mafic-ultramafic lithologies, there could be deposition of carbonate (Piccoli et al., 2016; Scambelluri et al., 2016). While this does represent C-mobility out of the metasedimentary system, it could be a simple re-location of the C
and the volumetric significance of this process, at the scale of a subduction zone, has yet to be determined.

**Scenario 3: Decarbonation and/or dissolution due to influx of an externally-derived, mafic/ultramafic-sourced fluid**

Influx of an externally-derived fluid with a $\delta^{18}$O isotope composition initially not equilibrated with the Schistes Lustrés metapelites, but with some other lithology, could have affected the carbonate $\delta^{18}$O values (see Collins et al., 2015; Jaeckel et al., 2018) and, provided the fluid was H$_2$O-rich and CO$_2$-undersaturated, could have led to the mobilization of C. As discussed in previous sections in this paper, the O isotope data for the Schistes Lustrés do not require such fluid ingress except along major fault systems (red circles in **Fig. 4**; see also Jaeckel et al., 2018; Piccoli et al., 2016), and can be more easily explained by within-complex fluid-rock interaction. For interaction between the Schistes Lustrés carbonate-bearing rocks and an externally-derived fluid originally in equilibrium with mafic/ultramafic crustal rocks ($\delta^{18}$O ≈ +6‰), the calculated minimum fluid-to-rock ratios would be significantly lower than shown in **Fig. 9d**, and the smaller amounts of fluid infiltration could thus mobilize even less C via carbonate dissolution (or decarbonation).

The O isotope evidence of infiltration by farther-traveled fluids (e.g. Angiboust et al., 2014b; Jaeckel et al., 2018) occurs only within about 300m of the contacts. At these contacts, there is no clear indication of wholesale carbonate removal, but merely redistribution similar to that illustrated in **Fig. 2d**, involving dissolution of carbonate along cleavage surfaces and redeposition of carbonate as veins, pressure shadows, or as pervasive to diffuse zones of carbonation where these fluids interact with other lithologies (e.g. Piccoli et al., 2016). The
extremely heterogenous distribution of these features, and perhaps also bias in exposure,
complicates assessment of whether there was net loss from the rocks at m- to km scales.

### Scenario 4: No net C-removal; variations in carbonate:silicate directly reflect the Schistes Lustrés protoliths

Figure 3 demonstrates the inherent variability of the marine protolith to the Schistes Lustrés and supports the idea that this unit should have some sections that are devoid of carbonate (see also Fig. 9a). The carbonate-poor sections of the Schistes Lustrés are most easily explained by inheritance together with removal of small amounts (up to a few %) of carbonate (Fig. 9e; Cook-Kollars et al., 2014). The field and petrographic observations for the rocks from the Val d'Isère area indicate near complete retention of C in sections with only very minor components of metapelite (≥60% carbonate and <10 vol.% phengite; Figs. 4, 5, and 9d). Any fluid produced during dehydration in the metapelite present would act to dissolve some fraction of CO₂ but, considering the low solubility of carbonate at the conditions of these units, significant dissolution is unlikely (Newton and Manning, 2002; Caciagli & Manning, 2003; Facq et al., 2016). Ongoing work in this area, by the authors, is evaluating in more detail the possible correspondence of δ¹⁸O shifts in the less common metapelite-rich exposures in the Val d'Isère area.

Kelemen and Manning (2015) suggested that the presence of carbonate-free shale horizons in the Schistes Lustrés could reflect the complete removal of carbonate from a mixed carbonate-shale lithology. The abundant carbonate-poor metapelites are also explained by inheritance of compositions from their protoliths, which contain appreciable fractions of low-carbonate shaley material, particularly at the boundary between the Early and Late Cretaceous (see Fig. 3). We suggest that, whereas some removal of C did occur over a range of scales, via
decarbonation (see Cook-Kollars et al., 2014) and perhaps also via carbonate dissolution (see the discussion above), this removal was minor, and in only parts of the section, and is superimposed on the protolith-inherited wide range of lithologies. That is, explanation of all carbonate-poor horizons by massive carbonate dissolution in what were carbonate-rich rocks appears unnecessarily complicated given the variability in the protoliths.

## 5.4 Exhumation/retrograde overprint

There is evidence for modest late-stage fluid flow within the Schistes Lustrés in the form of quartz-carbonate veins at high angles to foliation (Agard et al., 2000). Late-stage veins are volumetrically minor when compared to transposed and/or boudinaged veins within the schists. Where present, vein carbonate in these late-formed veins has δ¹⁸O values similar or identical to those of carbonate within the host (Cartwright & Buick, 2000); consistent with fluid production from, or prior equilibration with, a metasedimentary source. While we did not perform a thorough analysis of fluid production during exhumation, the same processes described herein would apply, though with different fractionations associated with the temperatures of equilibration experienced along the exhumation-related P-T path of the rocks (Henry et al., 1996; Zheng, 1999). Similar to the prograde case, we find no evidence for interaction between the schists and a mafic-derived fluid during exhumation, except along the larger fault zones investigated by Jaeckel et al. (2018). The subduction-related rocks in the W. Alps, including those in the Schistes Lustrés, are well-known to have retained prograde, high-P/T mineral assemblages to a greater degree that in the Central and Eastern Alps and many if not most other HP suites. At the petrographic scale, the Schistes Lustrés investigated in this and our previous studies certainly show some exhumation-related overprinting of mineral assemblages, in some samples; however, many of the rocks preserve prograde assemblages and mineral chemistry (see
676 Agard et al., 2001; Plunder et al., 2012; Bebout et al., 2013). Based on known phase equilibria
677 for rocks of these compositions (e.g., Plunder et al., 2012), and the *P-T* path inferred for
678 exhumation (Agard et al., 2001), there is no reason to expect that the rocks dehydrated during
679 exhumation, releasing fluid—if anything, the rocks would have been expected to rehydrate (but
680 only where fluids were available).

681 *5.5 Bearing of inferred volatiles behavior in the Schistes Lustrés on efforts to trace and mass- balance volatiles at certain modern subduction margins*

683 Obviously, estimated fluid flux along any one or more of the larger-scale structures in the
684 Schistes Lustrés, or some other more local-scale metasomatic features within the complex, would
685 not be meaningful representations of whole-margin flux of volatiles at any stage of its evolution.

686 We instead view studies of the type presented in this paper as yielding information regarding
687 geometries and mechanisms of fluid flow, and local-scale fluid flux, in settings *analogous* to
688 those developing in *some* modern margins. For example, at the E. Sunda margin, a sediment
689 section somewhat similar to that represented by the Schistes Lustrés could very well be
690 underplating at depth and experiencing fluid-rock interactions of the type illustrated in our
691 studies (see the discussion in House et al., 2019). Study of the prograde metamorphism and
692 related devolatilization could inform us regarding the likely extents of volatiles loss in such a
693 section *not* underplated (or accreted at shallower levels) and thus retained to beneath volcanic
694 fronts and able to contribute volatiles to arc magmas generated at that margin (see Bebout et al.,
695 2013; Cook-Kollars et al., 2015). This information could thus help in attempts to reconcile
696 estimates of volatiles return in arcs with knowledge of seafloor inputs.

697

698

**6. Conclusions**
During subduction to depths of 50–65km, the carbonate in the Schistes Lustrés sediments equilibrated with coexisting silicates and, to varying extents, with fluids produced during devolatilization in metapelites within the complex (see Bebout et al., 2013; Cook-Kollars et al., 2014). The combination of these two processes resulted in regional-scale decrease in the δ¹⁸O values from values for unmetamorphosed marine carbonate protoliths near +26 to +28‰ to values of +18 to +25‰, with some subtle decrease in δ¹⁸O with increasing peak metamorphic grade. Involvement of externally-derived hybrid or mafic/ultramafic fluids would cause further lowering of Schistes Lustrés carbonate δ¹⁸O, as seen along the exposures of transient subduction interfaces of the Tsaté complex (Jaeckel et al., 2018).

The presence of discrete cleavage domains, variably transposed carbonate (± quartz) veins, carbonate in pressure shadows (Figs. 2a–d), and evidence for intense dynamic recrystallization (see Jaeckel et al, 2018) all support a model whereby carbonate in the Schistes Lustrés was continually strained, dissolved, and reprecipitated during progressive deformation and metamorphism. Cleavage formation and dissolution accompanying infiltration by carbonate-undersaturated, H₂O-rich fluid presumably resulted in some carbonate removal and higher-grade rocks experienced some C loss via decarbonation reactions (see Cook-Kollars et al., 2018). Elsewhere in the section, C was deposited as carbonate in locally abundant veins and in more metasomatized ultramafic rocks. Assessment of km-scale removal of C from the Schistes Lustres complex is complicated by its great lithologic heterogeneity related to the well-characterized protolith section. Samples with the lowest δ¹⁸O (~+16‰) can be accounted for by a fluid:rock mass ratio of less than 0.8:1 (Fig. 9d) insufficient to mobilize more than a few percent of the carbonate. In this scenario, the majority of the samples shown in Figs. 5 and 9d have lost less than 4.5% carbonate (as wt.% of whole-rock).
Of course, at face value, the Schistes Lustrés, and other carbonate-rich *HP* and even *UHP* metasedimentary suites (e.g., Cignana; see the description of other *UHP* marbles in Rumble et al., 2000) reflect the “survival” of a considerable fraction of initially subducting carbonate to depths approaching those beneath arc volcanic fronts. Prograde metamorphism and associated devolatilization and fluid-rock interaction that occurred in the Schistes Lustrés are likely occurring in modern subduction zones at which carbonate-rich sediment sections are entering the trench; examples include E. Sunda, Tonga, Vanuatu, and Central America margins (see the synthesis in Clift, 2017). Locally extensive C removal and deposition have been reported by others for sites of particularly high fluid flux (e.g., Ague and Nicolescu, 2015; Scambelluri et al., 2016), along large fault systems, in zones of high fracture/vein density, and associated with large fluid-rock interaction and lithological contrasts. This set of observations is in stark contrast with the larger degrees of retention of C observed within km-scale packets of Schistes Lustrés away from zones of enhanced deformation and experiencing smaller degrees of interaction with far-traveled fluids (see the discussions by Collins et al., 2015). *HP/UHP* metamorphic suites are fragmented and potentially biased records of ancient subduction, and modern subduction margins show large variation in dynamics (e.g., see Clift, 2017) and in the nature of the subducting lithosphere/sediment sections. However, studies of subduction zone metamorphic rocks can provide information regarding geometries and mechanisms of fluid flow, and estimates of local-to perhaps km-scale fluid flux, in settings similar to those developing in some modern margins.

741

## Competing or conflicting interests

The authors declare no competing or conflicting interests.

744
745 Acknowledgements

746 This project was largely supported by NSF grant OIA1545903 (to Bebout), with smaller
747 amounts of funding provided by NSF grant EAR-1119264 (to Bebout) and Lehigh University (to
748 Epstein). Additional support was provided by ZIP (Zooming in between plates), A European
749 Marie Curie Initial Training Network (to Agard and Angiboust). This is IPGP contribution
750 #40XX.
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1036 Zheng, Y.-F., 1993. Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates.
1037 Earth and Planetary Science Letters 120, 247-263.
1038 Zheng, Y.-F., 1999. Oxygen isotope fractionation in carbonate and sulfate minerals.
1039 Geochemical Journal 33, 109-126.
**Figure Captions**

**Figure 1.** Overview of the locations of Schistes Lustrés metasedimentary rocks discussed in this paper. All samples are blueschist to eclogite facies calcareous schists and metapelites. The sampling transects from Ollomont and Breuil-Cervinia are on the order of 1–2 km, whereas the Cottian Alps and Val d’Isère sections were sampled over ~50 km distances along often-winding roads. Western Alps and Corsica maps modified after Agard et al. (2009) and Cartwright & Buick (2000), respectively.

**Figure 2.** Field and petrographic evidence for at least local-scale redistribution of C. (A) Carbonate-quartz vein from Breuil-Cervinia, surrounded by carbonate-poor metapelite. (B) Development of discrete cleavage domain in the Val d’Isère sediment section, potentially controlled by a transposed bedding surface. (C) Reduced C-rich cleavage development within a calcschist from the Ollomont section. The cleavage domains are potential surfaces of dissolution. (D) Carbonate-deposition in pressure shadows of quartz and albite from the Breuil-Cervinia transect.

**Figure 3.** Representative stratigraphic sections for the Schistes Lustrés complex (modified after Lemoine, 2003). From left to right: the first 3 sections are from Corsica, sections 4 and 7 are low-grade equivalents to the Cottian Alps, section 8 is equivalent to Val d’Isère samples, and section 9 is the only approximation for the Tsaté complex in the Aosta Valley. These stratigraphic sections represent the hemipelagic-pelagic background (HPB; Lemoine, 2003) recorded in the Schistes Lustrés, not shown are detrital or tectonically reworked components such as the ubiquitous mafic lenses found within the complex. Successive episodes of dominantly argillite versus carbonate deposition are indicated with A’s and C’s on the left of the figure.

**Figure 4.** Diagram showing the $\delta^{18}$O and $\delta^{13}$C variations in the compiled dataset. The black dots labeled marine carbonate are taken to represent the isotope values of the protolith. Oxygen isotope values in carbonate from Val d’Isère (blue-green triangles) have an average offset of ~6‰ from the marine protolith, whereas regional Schistes Lustrés samples (blue and gray squares) are offset by an average of 8‰. Samples collected within 300m of major contacts with overlying crustal gneisses (open and red-filled circles) have $\delta^{18}$O values extending down to +10‰, and are on average offset by 13‰ compared to marine carbonate. The low $\delta^{13}$C values are controlled by some combination of C exchange between carbonate and organic C (Cook-Kollars et al., 2014; Kraft and Bebout, 2017) and decarbonation producing lawsonite, clinozoisite, garnet, or other Ca-bearing silicates (see the red line indicating the trend the residual carbonate would take during decarbonation; calculated as Rayleigh loss). Data for marine carbonate are from Menegatti et al. (1996) and Föllmi et al. (2006); data for the Schistes Lustrés are from Cartwright and Buick (2000), Cook-Kollars et al. (2014), and this study; data for the Schistes Lustrés near the interfaces are from Jaeckel et al. (2018) and this study. See **Table 1** for details regarding these sample suites.

**Figure 5.** Carbonate $\delta^{18}$O versus carbonate wt.% for samples of the Schistes Lustrés. The diagonal lines indicate the $\delta^{18}$O values of the various minerals in closed-system equilibrium in rocks over this full range of carbonate wt.%. All carbonate analyses would fall along the
red line labeled “calcite” with closed-system silicate-carbonate equilibrium as the only
process acting on the isotope compositions of these minerals. The deviation from these lines
is taken to indicate interaction between carbonate and a fluid produced during prograde
devolatilization. These fractionations are for a temperature of 395°C, but in reality the peak
temperatures achieved by these rocks ranged from ~300–550°C. To the right of the figure are
schematic stratigraphic sections inferred to represent the parts of the Schistes Lustrés for
which data are plotted in this figure. The Isère section, best explained by relatively closed-
system equilibrium and showing little evidence for fluid interaction, is the most carbonate-
rich section (blue-green triangles). Blue squares represent Tsaté complex samples from the
present study and gray squares are data from Cook-Kollars et al. (2014). Fractionation factors
are from Chacko et al. (1996) and Zheng (1993; 1999).

**Figure 6.** Comparison of Schistes Lustrés and equivalent metasedimentary rocks (the Bündnerschiefer) with modern sediments outboard of the E. Sunda trench, Indonesia. (A) The trend with the negative slope from high values of SiO₂ to high values of Al₂O₃ represents sandstone-shale mixtures of these samples. The positive slope trending away from the origin represents addition of carbonate, with these additions resulting in shift from the other negatively-sloped array. (B) CaO vs. Al₂O₃ with increasing CaO representing dilution of silicates by carbonate. (C) positive linear correlation between Al₂O₃ and K₂O indicating that the samples contain a range of clay or phengite abundance, for the E. Sunda and W. Alps samples, respectively. (D) CaO vs. loss on ignition (LOI), the non-zero intercept represents the H₂O content of the silicate fraction, whereas the positive trend indicates increased carbonate concentration. The E. Sunda samples generally have higher LOI at a given CaO wt% due to excess H₂O bound in clays. (E) multi-element diagram comparing the trace element abundance of the Schistes Lustrés samples with averages for the Bündnerschiefer and E. Sunda suites. Gray shading of the Schistes Lustrés samples indicates relative carbonate abundance, with darker colors having higher abundance. The overlap in the data suggests similarity in the compositions of these marine sediments with their subducted equivalents in the Western Alps. Additional data for the Schistes Lustrés are from Bebout et al. (2013). Data for the Site 765 sediment section are from Plank & Ludden (1992) and data for the Bündnerschiefer are from Garofalo (2013). Data for N-MORB, for normalizations, are from Hofmann (1988).

**Figure 7.** Temperature dependence of δ¹⁸O in the Schistes Lustrés complex, demonstrating a similar magnitude of isotope shift (from protolith values) over all temperatures but showing a subtle decrease in values with increasing metamorphic grade. Each temperature grouping samples Schistes Lustrés on a regional scale, and therefore smooths out any local isotope shifts associated with compositional variability. RSCM data were obtained from sources listed in **Table 1** and samples were binned in 50°C increments.

**Figure 8.** Sketch illustrating the likely structural setting and fluid-flow in the Schistes Lustrés during its underplating in the paleo-subduction zone (modified after Jaeckel et al., 2018). The various metasedimentary lenses are color-coded based on their carbonate to phyllosilicate ratio, which in turn controls the average value of δ¹⁸O in carbonate due solely to within-lens carbonate-silicate O isotope exchange. In these packets, further decline in δ¹⁸O was produced by localized infiltration by H₂O-rich fluids with δ¹⁸O inherited from metapelitic sources.
Near the subduction interface (within ~300m), more intense deformation afforded open-system flow of fluids from mafic to ultramafic derived sources, imparting even lower δ¹⁸O on the carbonate (see Jaeckel et al., 2018). However, more pervasive flushing by H₂O-rich fluids, driving extensive decarbonation (proposed by Gorman et al., 2006), did not in this case occur — such flushing presumably would also have driven more extensive carbonate dissolution.

**Figure 9.** Assessment of C mobility and fluid-rock interactions in the Schistes Lustrés along the Cottian Alps traverse and throughout the W. Alps. (A) Carbonate abundance (gray points and line) and carbonate δ¹⁸O values (blue open circles, right axis) with increasing grade along the Cottian Alps traverse and at Cignana. The black line represents the smoothed average, and gray points the actual values. All Cignana samples reached the same P-T conditions. (B) Calculated values of CO₂, H₂O (as LOI-CO₂), and excess CaO (that is CaO remaining after removal of carbonate). The clustering of excess CaO near zero indicates that modest decarbonation has occurred, except perhaps on the local scale and at the highest grades. Note the decrease in LOI-CO₂ with increasing grade, consistent with the calculations from Bebout et al. (2013; gray line). (C) Behavior of selected lithophile elements with increasing metamorphic grade, showing modest loss of Ba, Rb, and Cs compared to K₂O and loss of Li compared to Zr. (D) Calculation of the fluid-to-rock mass ratios required to produce the range of observed carbonate δ¹⁸O values in the Schistes Lustrés at 395°C, calculated using mixing equations of Banner and Hanson (1990; equations 24 and 25 assuming pure H₂O with initial δ¹⁸O of +11.4‰ (i.e., produced from, and in equilibrium with phengite). The gray lines represent the fluid-to-rock mass ratio mixing lines, symbols are the same as those used in Fig. 5. (E) Assessment of closed system dissolution at temperatures of 350, 450, and 550°C (blue, purple, and red lines respectively), assuming 50% devolatilization of a silicate fraction containing 6 wt% initial H₂O. Dashed lines indicate the maximum CO₂ dissolution (in wt.% of whole-rock) at temperatures of 350, 450, and 550°C; a sample containing ≤1.6 wt.% carbonate would undergo complete dissolution at 550°C. At higher carbonate abundances, the magnitude of dissolution decreases due to the lower abundance of H₂O in the system. The inset figure shows the fluid-to-carbonate mass ratio for all carbonate abundances, which becomes vanishingly for higher carbonate abundances. This figure does not take into account interaction with fluids derived from adjacent sections. The solubility data are from Newton & Manning (2002), Caciagli & Manning (2003), and Kelemen & Manning (2015). Additional data for the Schistes Lustrés are from Bebout et al. (2013), Cook-Kollars et al. (2014), and an unpublished dataset (G.E. Bebout, 2018).

**Supplementary Files**

**Supplementary Table 1.** Location information as well as δ¹⁸O and δ¹³C values for samples analyzed in the present study.
<figure><img src="image_2.png" /></figure>

1 - Cottian Alps

2 - Val d'Isère

3 - Ollomont

4 - Lago di Cignana

5 - Cervinia

6 - Corsica
<figure><img src="image_3.png" /></figure>

A

10 cm

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

B

25 cm

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

C

2 mm

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

D

500 µm

quartz +
albite

phengite
+ chlorite
CORSICA

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

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

<table><thead><tr><th>interface samples</th><th>Schistes Lustrés</th><th>protolith proxies</th></tr></thead><tbody><tr><td>● new analyses (n=33)<br>○ Jaeckel et al., 2018 (n=68)</td><td>▲ Val d'Isère (n=35)<br>■ Tsaté complex (n=31)<br>□ Tsaté, Cottian Alps & Corsica (n=314)</td><td>• marine carbonate (n=905)<br>○ East Sunda, site 765 (n=45)</td></tr></tbody></table>
<figure><img src="image_9.png" /></figure>

Schistes Lustrés whole rock

▲ Val d'Isère

■ Tsaté complex (this study)

☐ Cottian Alps (Cook-Kollars et al., 2015)

quartz

calcite

phengite

chlorite

≈ Val d'Isère

≈ Tsaté

≈ Cottian Alps

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

A

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

B

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

C

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

D

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

E

N-MORB normalized concentrations
<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>

30 km

40 km

Schistès Lustrés
(calcschists)

not to scale

Qz-Cal
veins

mafics

Average carbonate δ¹⁸O after
internal reequilibration

18 ‰

25 ‰

Increasing carbonate : phyllosilicate
<figure><img src="image_17.png" /></figure>

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

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

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

<figure><img src="image_21.png" /></figure>
<table><caption>**Table 1**</caption><caption>Summary of compiled datasets</caption><thead><tr><th></th><th>n</th><th>δ<sup>18</sup>O‰ (VSMOW) mean ± SD</th><th>δ<sup>13</sup>C‰ (PDB) mean ± SD</th><th>RSCM max T (°C)</th><th>references</th></tr></thead><tbody><tr><td><strong>Marine carbonates</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SE France</td><td>690</td><td>+28.2 ± 1.1</td><td>+2.4 ± 1.0</td><td>-</td><td>Herrle et al., 2004; Föllmi et al., 2006</td></tr><tr><td>Worldwide</td><td>159</td><td>+30.4 ± 1.0</td><td>+1.2 ± 1.3</td><td>-</td><td>Vezier et al., 1999</td></tr><tr><td>Switzerland</td><td>56</td><td>+25.5 ± 1.3</td><td>+1.6 ± 1.0</td><td>-</td><td>Burkhard & Kerrich, 1988</td></tr><tr><td></td><td><strong>905</strong></td><td><strong>+28.5 ± 1.7</strong></td><td><strong>+2.2 ± 1.2</strong></td><td>-</td><td></td></tr><tr><td><strong>IODP 765 data</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>E. Sunda</td><td>45</td><td>+30.2 ± 1.7</td><td>+0.9 ± 1.5</td><td>-</td><td>Plank & Ludden, 1992</td></tr><tr><td><strong>Schistes Lustrés</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Corsica</td><td>122</td><td>+20.3 ± 2.1</td><td>-1.5 ± 1.6</td><td>350-550</td><td>Cartwright & Buick, 2000; Vitale Brovarone et al., 2013</td></tr><tr><td>Bagliacone</td><td></td><td>+22.4 ± 0.7</td><td>0.0 ± 1.7</td><td>350</td><td></td></tr><tr><td>Inzecca</td><td></td><td>+21.3 ± 1.4</td><td>-1.0 ± 1.2</td><td>350</td><td></td></tr><tr><td>Castagniccia</td><td></td><td>+19 ± 1.9</td><td>-2.1 ± 1.4</td><td>490-550</td><td></td></tr><tr><td>Cottian Alps</td><td>132</td><td>+20.2 ± 1.96</td><td>-0.9 ± 1.6</td><td>350-500</td><td>Cook-Kollars et al., 2014; Beyssac et al., 2002</td></tr><tr><td>Fraiteve</td><td></td><td>+20.8 ± 1.5</td><td>-1.2 ± 1.7</td><td>350</td><td></td></tr><tr><td>Assietta</td><td></td><td>+20.1 ± 2.0</td><td>-1.4 ± 1.6</td><td>400</td><td></td></tr><tr><td>Albergian</td><td></td><td>+21.1 ± 2.0</td><td>-0.4 ± 1.1</td><td>450</td><td></td></tr><tr><td>Finestre</td><td></td><td>+20.1 ± 2.0</td><td>0.0 ± 1.0</td><td>500</td><td></td></tr><tr><td>Aosta</td><td>81</td><td>+19.5 ± 1.8</td><td>-1.2 ± 1.3</td><td>400-450</td><td>Jaeckel et al., 2018; this study; Negro et al., 2013; Angiboust et al., 2014</td></tr><tr><td>Val d'Isere</td><td>35</td><td>+22.7 ± 1.4</td><td>-0.8 ± 1.2</td><td>450</td><td>this study; Plunder et al., 2012</td></tr><tr><td></td><td><strong>370</strong></td><td><strong>+20.4 ± 2.1</strong></td><td><strong>-1.1 ± 1.5</strong></td><td></td><td></td></tr><tr><td><strong>Interface</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Aosta</td><td>101</td><td>+15.5 ± 1.9</td><td>-1 ± 1.2</td><td>400-450</td><td>Jaeckel et al., 2018; this study; Negro et al., 2013; Angiboust et al., 2014</td></tr></tbody></table>
<caption>**Table 2**</caption>

Whole rock major element and carbonate analyses for the Schistes Lustrés, all data reported as wt.%.

<table><thead><tr><th>grade*</th><th>Sample</th><th>SiO<sub>2</sub></th><th>TiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub>(T)</th><th>MnO</th><th>MgO</th><th>CaO</th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>P<sub>2</sub>O<sub>5</sub></th><th>LOI</th><th>Total</th><th>Carbonate</th><th>CO<sub>2</sub></th><th>H<sub>2</sub>O<br>calculated</th></tr></thead><tbody><tr><td>1</td><td>SL99-23B</td><td>20.46</td><td>0.07</td><td>2.53</td><td>3.25</td><td>0.14</td><td>1.19</td><td>39.57</td><td>0.14</td><td>0.23</td><td>0.07</td><td>32.11</td><td>99.78</td><td>70.80</td><td>31.13</td><td>0.98</td></tr><tr><td>1</td><td>SL99-19A</td><td>61.70</td><td>0.67</td><td>19.29</td><td>6.79</td><td>0.11</td><td>2.57</td><td>0.12</td><td>1.23</td><td>3.10</td><td>0.10</td><td>4.17</td><td>99.84</td><td>0.00</td><td>0.00</td><td>4.17</td></tr><tr><td>1</td><td>SL99-21A</td><td>32.60</td><td>0.10</td><td>2.85</td><td>1.80</td><td>0.06</td><td>5.45</td><td>27.94</td><td>0.28</td><td>0.52</td><td>0.06</td><td>28.30</td><td>99.95</td><td>45.07</td><td>19.82</td><td>8.48</td></tr><tr><td>1</td><td>SL99-28A</td><td>45.91</td><td>1.11</td><td>29.53</td><td>6.42</td><td>0.09</td><td>3.03</td><td>0.20</td><td>2.08</td><td>5.11</td><td>0.13</td><td>6.08</td><td>99.69</td><td>0.00</td><td>0.00</td><td>6.08</td></tr><tr><td>1</td><td>SL98-2PeIA</td><td>54.25</td><td>0.73</td><td>20.51</td><td>7.39</td><td>0.23</td><td>2.39</td><td>0.65</td><td>1.38</td><td>3.54</td><td>0.12</td><td>7.38</td><td>98.58</td><td>2.00</td><td>0.88</td><td>6.50</td></tr><tr><td>1</td><td>SL99-30B</td><td>67.83</td><td>0.40</td><td>13.44</td><td>8.35</td><td>0.81</td><td>2.46</td><td>0.25</td><td>0.82</td><td>1.86</td><td>0.13</td><td>3.67</td><td>100.02</td><td>0.00</td><td>0.00</td><td>3.67</td></tr><tr><td>2</td><td>SL99-36D</td><td>33.91</td><td>0.33</td><td>8.65</td><td>2.97</td><td>0.12</td><td>1.52</td><td>28.23</td><td>0.72</td><td>0.15</td><td>0.12</td><td>23.59</td><td>100.32</td><td>47.70</td><td>20.97</td><td>2.62</td></tr><tr><td>2</td><td>SL99-37C</td><td>58.38</td><td>0.75</td><td>20.09</td><td>7.81</td><td>0.09</td><td>2.13</td><td>0.25</td><td>0.94</td><td>3.95</td><td>0.15</td><td>4.84</td><td>99.38</td><td>0.00</td><td>0.00</td><td>4.84</td></tr><tr><td>2</td><td>SL99-38B</td><td>45.55</td><td>0.24</td><td>9.59</td><td>3.65</td><td>0.22</td><td>2.57</td><td>19.28</td><td>0.81</td><td>1.25</td><td>0.12</td><td>17.19</td><td>100.46</td><td>28.10</td><td>12.36</td><td>4.83</td></tr><tr><td>2</td><td>SL99-39B</td><td>12.41</td><td>0.07</td><td>3.36</td><td>3.74</td><td>0.15</td><td>1.44</td><td>43.48</td><td>0.30</td><td>0.22</td><td>0.09</td><td>34.34</td><td>99.59</td><td>78.10</td><td>34.34</td><td>0.00</td></tr><tr><td>2</td><td>SL99-40C</td><td>46.24</td><td>0.53</td><td>14.14</td><td>5.13</td><td>0.08</td><td>3.09</td><td>13.86</td><td>1.08</td><td>1.99</td><td>0.11</td><td>14.02</td><td>100.28</td><td>23.54</td><td>10.35</td><td>3.67</td></tr><tr><td>3</td><td>SL99-10E</td><td>29.65</td><td>0.10</td><td>3.08</td><td>2.30</td><td>0.06</td><td>7.09</td><td>27.25</td><td>0.33</td><td>0.74</td><td>0.12</td><td>28.94</td><td>99.66</td><td>49.10</td><td>21.59</td><td>7.35</td></tr><tr><td>3</td><td>SL99-12A</td><td>20.13</td><td>0.02</td><td>1.67</td><td>0.95</td><td>0.08</td><td>0.82</td><td>42.18</td><td>0.21</td><td>0.33</td><td>0.05</td><td>33.51</td><td>99.95</td><td>74.75</td><td>32.87</td><td>0.64</td></tr><tr><td>3</td><td>SL99-12C</td><td>48.77</td><td>1.12</td><td>23.79</td><td>7.04</td><td>0.06</td><td>3.48</td><td>2.16</td><td>0.48</td><td>6.23</td><td>0.07</td><td>6.58</td><td>99.77</td><td>3.77</td><td>1.66</td><td>4.92</td></tr><tr><td>3</td><td>SL99-13A</td><td>42.05</td><td>0.35</td><td>8.42</td><td>3.60</td><td>0.13</td><td>1.47</td><td>22.44</td><td>0.74</td><td>1.13</td><td>0.08</td><td>19.70</td><td>100.09</td><td>41.62</td><td>18.30</td><td>1.40</td></tr><tr><td>3</td><td>SL99-16A</td><td>48.35</td><td>0.97</td><td>23.29</td><td>8.04</td><td>0.07</td><td>3.16</td><td>3.38</td><td>0.74</td><td>4.73</td><td>0.08</td><td>7.23</td><td>100.06</td><td>5.69</td><td>2.50</td><td>4.73</td></tr><tr><td>4</td><td>SL99-34C</td><td>49.63</td><td>0.75</td><td>18.69</td><td>6.72</td><td>0.14</td><td>2.27</td><td>7.66</td><td>0.55</td><td>3.91</td><td>0.12</td><td>9.47</td><td>99.91</td><td>13.15</td><td>5.78</td><td>3.69</td></tr><tr><td>4</td><td>SL99-41D</td><td>46.44</td><td>1.03</td><td>24.55</td><td>6.77</td><td>0.08</td><td>2.63</td><td>3.77</td><td>1.17</td><td>5.53</td><td>0.09</td><td>7.61</td><td>99.70</td><td>6.59</td><td>2.90</td><td>4.71</td></tr><tr><td>5</td><td>LC99-2C</td><td>38.01</td><td>0.14</td><td>5.56</td><td>2.21</td><td>0.32</td><td>1.15</td><td>28.60</td><td>0.51</td><td>0.97</td><td>0.07</td><td>22.84</td><td>100.37</td><td>49.50</td><td>21.77</td><td>1.07</td></tr></tbody></table>

*Numbers correspond to increasing grade. (1) Fraiteve; (2) Assietta; (3) Albergian; (4) Fineste; (5) Cignana.
<caption>Table 3</caption>

Whole rock trace element analyses (and selected trace element ratios) for the Schistes Lustrés.

<table><thead><tr><th>Element<br>Unit<br>symbol</th><th>Li<br>ppm</th><th>V<br>ppm</th><th>Cr<br>ppm</th><th>Co<br>ppm</th><th>Ni<br>ppm</th><th>Cu<br>ppm</th><th>Zn<br>ppm</th><th>Ga<br>ppm</th><th>Ge<br>ppm</th><th>Rb<br>ppm</th><th>Sr<br>ppm</th><th>Y<br>ppm</th><th>Zr<br>ppm</th><th>Nb<br>ppm</th><th>Sn<br>ppm</th><th>Sb<br>ppm</th><th>Cs<br>ppm</th></tr></thead><tbody><tr><td>Detection<br>limit</td><td>10</td><td>5</td><td>20</td><td>1</td><td>20</td><td>10</td><td>30</td><td>1</td><td>0.5</td><td>1</td><td>2</td><td>0.5</td><td>1</td><td>0.2</td><td>1</td><td>0.2</td><td>0.1</td></tr><tr><th>Analysis<br>method</th><th>TD-<br>ICP</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th></tr><tr><td>SL99-23B</td><td>20</td><td>21</td><td>20</td><td>&lt; 1</td><td>30</td><td>20</td><td>60</td><td>4</td><td>0.9</td><td>12</td><td>1310</td><td>17.9</td><td>21</td><td>2.7</td><td>&lt; 1</td><td>&lt; 0.2</td><td>0.6</td></tr><tr><td>SL99-19A</td><td>100</td><td>174</td><td>140</td><td>23</td><td>90</td><td>100</td><td>100</td><td>29</td><td>2.8</td><td>170</td><td>65</td><td>26.7</td><td>140</td><td>17.6</td><td>4</td><td>0.4</td><td>10.5</td></tr><tr><td>SL99-21A</td><td>&lt; 10</td><td>28</td><td>70</td><td>3</td><td>30</td><td>20</td><td>30</td><td>4</td><td>0.5</td><td>29</td><td>473</td><td>13.6</td><td>55</td><td>2.4</td><td>&lt; 1</td><td>&lt; 0.2</td><td>1.9</td></tr><tr><td>SL99-28A</td><td>120</td><td>262</td><td>210</td><td>17</td><td>100</td><td>120</td><td>130</td><td>37</td><td>2.9</td><td>244</td><td>188</td><td>74.5</td><td>197</td><td>22.8</td><td>7</td><td>0.6</td><td>14.9</td></tr><tr><td>SL98-<br>2PelA</td><td>60</td><td>156</td><td>108</td><td>12</td><td>91</td><td>76</td><td>140</td><td>24</td><td>3.2</td><td>148</td><td>85</td><td>34.6</td><td>179</td><td>24.0</td><td>9</td><td>2.5</td><td>8.1</td></tr><tr><td>SL99-30B</td><td>70</td><td>101</td><td>70</td><td>31</td><td>120</td><td>50</td><td>180</td><td>20</td><td>1.4</td><td>88</td><td>32</td><td>12.0</td><td>62</td><td>6.2</td><td>2</td><td>0.3</td><td>5.2</td></tr><tr><td>SL99-36D</td><td>40</td><td>62</td><td>50</td><td>6</td><td>30</td><td>30</td><td>60</td><td>11</td><td>1.1</td><td>7</td><td>2260</td><td>20.1</td><td>88</td><td>7.3</td><td>2</td><td>0.2</td><td>0.6</td></tr><tr><td>SL99-37C</td><td>70</td><td>182</td><td>160</td><td>6</td><td>50</td><td>20</td><td>120</td><td>34</td><td>3.1</td><td>219</td><td>95</td><td>35.2</td><td>155</td><td>18.3</td><td>4</td><td>3.3</td><td>10.9</td></tr><tr><td>SL99-38B</td><td>60</td><td>93</td><td>90</td><td>13</td><td>60</td><td>60</td><td>70</td><td>15</td><td>1.4</td><td>76</td><td>247</td><td>20.0</td><td>96</td><td>9.0</td><td>2</td><td>0.9</td><td>4.5</td></tr><tr><td>SL99-39B</td><td>30</td><td>24</td><td>20</td><td>&lt; 1</td><td>30</td><td>30</td><td>60</td><td>5</td><td>0.5</td><td>11</td><td>1100</td><td>15.1</td><td>20</td><td>1.8</td><td>&lt; 1</td><td>&lt; 0.2</td><td>0.6</td></tr><tr><td>SL99-40C</td><td>80</td><td>107</td><td>80</td><td>16</td><td>50</td><td>50</td><td>80</td><td>19</td><td>1.3</td><td>109</td><td>415</td><td>20.8</td><td>98</td><td>10.9</td><td>2</td><td>0.6</td><td>6.1</td></tr><tr><td>SL99-10E</td><td>20</td><td>28</td><td>20</td><td>4</td><td>30</td><td>30</td><td>40</td><td>4</td><td>0.6</td><td>30</td><td>517</td><td>12.4</td><td>46</td><td>3.1</td><td>&lt; 1</td><td>0.9</td><td>1.6</td></tr><tr><td>SL99-12A</td><td>10</td><td>11</td><td>&lt; 20</td><td>5</td><td>40</td><td>50</td><td>50</td><td>2</td><td>&lt; 0.5</td><td>15</td><td>322</td><td>13.0</td><td>11</td><td>5.4</td><td>&lt; 1</td><td>&lt; 0.2</td><td>0.8</td></tr><tr><td>SL99-12C</td><td>80</td><td>175</td><td>160</td><td>26</td><td>80</td><td>40</td><td>110</td><td>31</td><td>2.1</td><td>269</td><td>55</td><td>18.2</td><td>184</td><td>20.9</td><td>3</td><td>0.6</td><td>13.8</td></tr><tr><td>SL99-13A</td><td>50</td><td>59</td><td>60</td><td>8</td><td>40</td><td>30</td><td>50</td><td>9</td><td>1.5</td><td>49</td><td>723</td><td>19.0</td><td>80</td><td>7.6</td><td>2</td><td>0.5</td><td>3.6</td></tr><tr><td>SL99-16A</td><td>100</td><td>162</td><td>150</td><td>29</td><td>100</td><td>70</td><td>170</td><td>35</td><td>3.1</td><td>185</td><td>505</td><td>39.8</td><td>197</td><td>23.4</td><td>4</td><td>0.8</td><td>9.2</td></tr><tr><td>SL99-34C</td><td>50</td><td>127</td><td>100</td><td>23</td><td>60</td><td>60</td><td>130</td><td>26</td><td>1.6</td><td>160</td><td>123</td><td>24.1</td><td>132</td><td>16.0</td><td>3</td><td>&lt; 0.2</td><td>9.2</td></tr><tr><td>SL99-41D</td><td>70</td><td>179</td><td>150</td><td>13</td><td>60</td><td>60</td><td>120</td><td>31</td><td>2.0</td><td>250</td><td>98</td><td>25.7</td><td>179</td><td>19.4</td><td>3</td><td>0.6</td><td>12.5</td></tr><tr><td>LC99-2C</td><td>20</td><td>38</td><td>40</td><td>21</td><td>50</td><td>40</td><td>60</td><td>8</td><td>1.1</td><td>41</td><td>413</td><td>15.5</td><td>33</td><td>3.3</td><td>1</td><td>&lt; 0.2</td><td>2.1</td></tr></tbody></table>
<table><caption>Table 3 (continued)</caption><thead><tr><th>Element<br>Unit<br>symbol</th><th>Ba<br>ppm</th><th>La<br>ppm</th><th>Ce<br>ppm</th><th>Pr<br>ppm</th><th>Nd<br>ppm</th><th>Sm<br>ppm</th><th>Eu<br>ppm</th><th>Gd<br>ppm</th><th>Tb<br>ppm</th><th>Dy<br>ppm</th><th>Ho<br>ppm</th><th>Er<br>ppm</th><th>Tm<br>ppm</th><th>Yb<br>ppm</th><th>Lu<br>ppm</th><th>Hf<br>ppm</th><th>Ta<br>ppm</th></tr></thead><tbody><tr><td>Detection<br>limit</td><td>3</td><td>0.05</td><td>0.05</td><td>0.01</td><td>0.05</td><td>0.01</td><td>0.005</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.005</td><td>0.01</td><td>0.002</td><td>0.1</td><td>0.01</td></tr><tr><td>Analysis<br>method</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td><td>FUS-<br>MS</td></tr><tr><td>SL99-23B</td><td>34</td><td>14.90</td><td>25.00</td><td>3.38</td><td>14.00</td><td>3.68</td><td>1.050</td><td>3.97</td><td>0.57</td><td>3.15</td><td>0.57</td><td>1.41</td><td>0.185</td><td>1.11</td><td>0.172</td><td>0.5</td><td>0.21</td></tr><tr><td>SL99-19A</td><td>546</td><td>39.70</td><td>85.10</td><td>8.59</td><td>31.20</td><td>5.04</td><td>0.602</td><td>4.03</td><td>0.67</td><td>4.25</td><td>0.86</td><td>2.66</td><td>0.400</td><td>2.52</td><td>0.380</td><td>3.6</td><td>1.25</td></tr><tr><td>SL99-21A</td><td>58</td><td>9.78</td><td>17.20</td><td>2.33</td><td>9.98</td><td>2.42</td><td>0.632</td><td>2.17</td><td>0.38</td><td>2.31</td><td>0.41</td><td>1.16</td><td>0.154</td><td>0.93</td><td>0.145</td><td>1.3</td><td>0.20</td></tr><tr><td>SL99-28A</td><td>754</td><td>38.40</td><td>103.00</td><td>12.10</td><td>50.10</td><td>14.30</td><td>1.890</td><td>13.70</td><td>2.21</td><td>13.20</td><td>2.57</td><td>6.95</td><td>0.948</td><td>5.69</td><td>0.841</td><td>5.4</td><td>1.74</td></tr><tr><td>SL98-<br>2PeA</td><td>530</td><td>40.80</td><td>86.70</td><td>9.47</td><td>33.90</td><td>6.41</td><td>0.970</td><td>5.26</td><td>0.96</td><td>5.92</td><td>1.29</td><td>3.76</td><td>0.625</td><td>3.98</td><td>0.559</td><td>4.3</td><td>2.11</td></tr><tr><td>SL99-30B</td><td>248</td><td>11.40</td><td>25.60</td><td>3.13</td><td>13.80</td><td>3.48</td><td>0.780</td><td>3.13</td><td>0.48</td><td>2.58</td><td>0.46</td><td>1.29</td><td>0.195</td><td>1.22</td><td>0.191</td><td>1.7</td><td>0.57</td></tr><tr><td>SL99-36D</td><td>13</td><td>27.10</td><td>50.00</td><td>5.84</td><td>23.60</td><td>5.51</td><td>1.820</td><td>5.22</td><td>0.75</td><td>3.92</td><td>0.71</td><td>1.81</td><td>0.258</td><td>1.67</td><td>0.254</td><td>2.4</td><td>0.54</td></tr><tr><td>SL99-37C</td><td>877</td><td>58.00</td><td>113.00</td><td>13.00</td><td>48.90</td><td>7.15</td><td>1.180</td><td>4.61</td><td>0.86</td><td>5.62</td><td>1.28</td><td>4.00</td><td>0.610</td><td>4.09</td><td>0.621</td><td>4.5</td><td>1.36</td></tr><tr><td>SL99-38B</td><td>199</td><td>28.50</td><td>55.40</td><td>6.22</td><td>24.30</td><td>4.89</td><td>1.160</td><td>4.02</td><td>0.63</td><td>3.53</td><td>0.72</td><td>1.96</td><td>0.285</td><td>1.94</td><td>0.301</td><td>2.7</td><td>0.71</td></tr><tr><td>SL99-39B</td><td>27</td><td>10.10</td><td>17.50</td><td>2.43</td><td>11.30</td><td>2.81</td><td>1.030</td><td>2.96</td><td>0.44</td><td>2.37</td><td>0.41</td><td>1.11</td><td>0.149</td><td>0.87</td><td>0.128</td><td>0.5</td><td>0.13</td></tr><tr><td>SL99-40C</td><td>329</td><td>34.50</td><td>63.70</td><td>7.08</td><td>26.30</td><td>4.73</td><td>1.050</td><td>4.15</td><td>0.65</td><td>3.65</td><td>0.76</td><td>2.29</td><td>0.335</td><td>2.19</td><td>0.322</td><td>2.8</td><td>0.80</td></tr><tr><td>SL99-10E</td><td>102</td><td>14.40</td><td>28.80</td><td>3.71</td><td>15.30</td><td>2.91</td><td>0.592</td><td>2.45</td><td>0.37</td><td>2.13</td><td>0.34</td><td>0.95</td><td>0.129</td><td>1.00</td><td>0.162</td><td>1.0</td><td>0.23</td></tr><tr><td>SL99-12A</td><td>50</td><td>8.93</td><td>10.30</td><td>2.15</td><td>9.65</td><td>2.18</td><td>0.486</td><td>2.11</td><td>0.33</td><td>1.91</td><td>0.35</td><td>0.95</td><td>0.132</td><td>0.77</td><td>0.109</td><td>0.2</td><td>0.06</td></tr><tr><td>SL99-12C</td><td>794</td><td>36.00</td><td>70.00</td><td>7.66</td><td>29.10</td><td>4.87</td><td>0.936</td><td>3.12</td><td>0.49</td><td>3.23</td><td>0.63</td><td>2.09</td><td>0.336</td><td>2.56</td><td>0.432</td><td>5.0</td><td>1.46</td></tr><tr><td>SL99-13A</td><td>139</td><td>19.00</td><td>36.70</td><td>4.16</td><td>16.00</td><td>3.04</td><td>0.808</td><td>3.04</td><td>0.50</td><td>3.11</td><td>0.56</td><td>1.68</td><td>0.236</td><td>1.59</td><td>0.231</td><td>2.3</td><td>0.55</td></tr><tr><td>SL99-16A</td><td>427</td><td>60.50</td><td>110.00</td><td>13.90</td><td>51.00</td><td>8.82</td><td>1.840</td><td>6.28</td><td>1.01</td><td>6.36</td><td>1.28</td><td>3.73</td><td>0.543</td><td>3.65</td><td>0.606</td><td>5.2</td><td>1.54</td></tr><tr><td>SL99-34C</td><td>324</td><td>40.10</td><td>83.50</td><td>8.99</td><td>34.50</td><td>6.65</td><td>1.510</td><td>5.32</td><td>0.87</td><td>4.73</td><td>0.89</td><td>2.54</td><td>0.374</td><td>2.34</td><td>0.373</td><td>4.0</td><td>1.16</td></tr><tr><td>SL99-41D</td><td>735</td><td>49.30</td><td>100.00</td><td>11.20</td><td>41.20</td><td>6.17</td><td>1.290</td><td>4.43</td><td>0.76</td><td>4.51</td><td>0.92</td><td>2.95</td><td>0.486</td><td>3.20</td><td>0.497</td><td>5.1</td><td>1.38</td></tr><tr><td>LC99-2C</td><td>99</td><td>16.30</td><td>32.60</td><td>4.06</td><td>16.00</td><td>3.55</td><td>0.821</td><td>3.18</td><td>0.51</td><td>2.74</td><td>0.50</td><td>1.24</td><td>0.171</td><td>1.05</td><td>0.165</td><td>0.9</td><td>0.26</td></tr></tbody></table>
<table><caption>**Table 3 (continued)**</caption><thead><tr><th>Element<br>Unit<br>symbol</th><th>W<br>ppm</th><th>Tl<br>ppm</th><th>Pb<br>ppm</th><th>Bi<br>ppm</th><th>Th<br>ppm</th><th>U<br>ppm</th><th>B<br>ppm</th><th>LOI-CO<sub>2</sub></th><th>CaO excess</th><th>Ba/K<sub>2</sub>O</th><th>Rb/K<sub>2</sub>O</th><th>Cs/K<sub>2</sub>O</th><th>Rb/Cs</th><th>Li/Zr</th></tr></thead><tbody><tr><td>Detection<br>limit</td><td>0.5</td><td>0.05</td><td>5</td><td>0.1</td><td>0.05</td><td>0.01</td><td>2</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Analysis<br>method</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>FUS-<br>MS</th><th>PGNAA</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SL99-23B</td><td>&lt; 0.5</td><td>&lt;0.05</td><td>5</td><td>&lt; 0.1</td><td>1.93</td><td>0.83</td><td>17</td><td>0.98</td><td>-0.10</td><td>147.58</td><td>52.09</td><td>2.60</td><td>20.00</td><td>0.95</td></tr><tr><td>SL99-19A</td><td>2.2</td><td>0.61</td><td>8</td><td>0.1</td><td>12.70</td><td>2.16</td><td>210</td><td>4.17</td><td>0.12</td><td>176.00</td><td>54.80</td><td>3.38</td><td>16.19</td><td>0.71</td></tr><tr><td>SL99-21A</td><td>1.2</td><td>0.05</td><td>5</td><td>&lt; 0.1</td><td>2.43</td><td>2.22</td><td>37</td><td>8.48</td><td>2.68</td><td>112.40</td><td>56.20</td><td>3.68</td><td>15.26</td><td>n.d.</td></tr><tr><td>SL99-28A</td><td>3.1</td><td>1.08</td><td>7</td><td>&lt; 0.1</td><td>19.70</td><td>2.93</td><td>314</td><td>6.08</td><td>0.20</td><td>147.58</td><td>47.76</td><td>2.92</td><td>16.38</td><td>0.61</td></tr><tr><td>SL98-<br>2PelA</td><td>2.1</td><td>0.33</td><td>n.d.</td><td>n.d.</td><td>11.60</td><td>3.84</td><td>166</td><td>6.50</td><td>-0.47</td><td>149.72</td><td>41.81</td><td>2.29</td><td>18.27</td><td>0.34</td></tr><tr><td>SL99-30B</td><td>1.0</td><td>0.43</td><td>11</td><td>0.1</td><td>6.76</td><td>1.16</td><td>93</td><td>3.67</td><td>0.25</td><td>133.39</td><td>47.33</td><td>2.80</td><td>16.92</td><td>1.13</td></tr><tr><td>SL99-36D</td><td>1.1</td><td>0.13</td><td>17</td><td>&lt; 0.1</td><td>5.61</td><td>1.27</td><td>25</td><td>2.62</td><td>1.51</td><td>84.87</td><td>45.70</td><td>3.92</td><td>11.67</td><td>0.45</td></tr><tr><td>SL99-37C</td><td>1.9</td><td>0.90</td><td>6</td><td>&lt; 0.1</td><td>13.80</td><td>2.47</td><td>193</td><td>4.84</td><td>0.25</td><td>221.75</td><td>55.38</td><td>2.76</td><td>20.09</td><td>0.45</td></tr><tr><td>SL99-38B</td><td>1.8</td><td>0.38</td><td>8</td><td>0.1</td><td>7.99</td><td>1.26</td><td>70</td><td>4.83</td><td>3.53</td><td>158.67</td><td>60.60</td><td>3.59</td><td>16.89</td><td>0.63</td></tr><tr><td>SL99-39B</td><td>&lt; 0.5</td><td>0.09</td><td>10</td><td>&lt; 0.1</td><td>1.60</td><td>0.87</td><td>15</td><td>0.00</td><td>-0.28</td><td>125.09</td><td>50.96</td><td>2.78</td><td>18.33</td><td>1.50</td></tr><tr><td>SL99-40C</td><td>1.5</td><td>0.51</td><td>8</td><td>0.1</td><td>8.64</td><td>1.44</td><td>68</td><td>3.67</td><td>0.67</td><td>164.93</td><td>54.64</td><td>3.06</td><td>17.87</td><td>0.82</td></tr><tr><td>SL99-10E</td><td>&lt; 0.5</td><td>0.07</td><td>8</td><td>0.1</td><td>2.81</td><td>1.38</td><td>86</td><td>7.35</td><td>-0.26</td><td>138.41</td><td>40.71</td><td>2.17</td><td>18.75</td><td>0.43</td></tr><tr><td>SL99-12A</td><td>0.5</td><td>0.11</td><td>5</td><td>0.1</td><td>1.16</td><td>0.21</td><td>24</td><td>0.64</td><td>0.29</td><td>153.54</td><td>46.06</td><td>2.46</td><td>18.75</td><td>0.91</td></tr><tr><td>SL99-12C</td><td>1.1</td><td>0.82</td><td>10</td><td>0.1</td><td>16.00</td><td>2.00</td><td>143</td><td>4.92</td><td>0.05</td><td>127.43</td><td>43.17</td><td>2.21</td><td>19.49</td><td>0.43</td></tr><tr><td>SL99-13A</td><td>0.6</td><td>0.18</td><td>14</td><td>0.1</td><td>6.65</td><td>1.31</td><td>81</td><td>1.40</td><td>-0.88</td><td>122.77</td><td>43.28</td><td>3.18</td><td>13.61</td><td>0.63</td></tr><tr><td>SL99-16A</td><td>3.6</td><td>0.56</td><td>19</td><td>0.2</td><td>14.40</td><td>2.37</td><td>135</td><td>4.73</td><td>0.19</td><td>90.22</td><td>39.09</td><td>1.94</td><td>20.11</td><td>0.51</td></tr><tr><td>SL99-34C</td><td>1.8</td><td>0.88</td><td>25</td><td>0.2</td><td>11.90</td><td>2.10</td><td>77</td><td>3.69</td><td>0.29</td><td>82.85</td><td>40.91</td><td>2.35</td><td>17.39</td><td>0.38</td></tr><tr><td>SL99-41D</td><td>1.8</td><td>1.08</td><td>9</td><td>0.1</td><td>14.00</td><td>2.59</td><td>377</td><td>4.71</td><td>0.08</td><td>132.81</td><td>45.17</td><td>2.26</td><td>20.00</td><td>0.39</td></tr><tr><td>LC99-2C</td><td>&lt; 0.5</td><td>0.16</td><td>17</td><td>&lt; 0.1</td><td>2.65</td><td>0.43</td><td>33</td><td>1.07</td><td>0.87</td><td>102.37</td><td>42.39</td><td>2.17</td><td>19.52</td><td>0.61</td></tr></tbody></table>