# From mantle peridotites to hybrid troctolites: Textural and chemical evolution during melt-rock interaction history (Mt. Maggiore, Corsica, France)

Valentin Basch, Elisabetta Rampone, Laura Crispini, Carlotta Ferrando, Benoit Ildefonse, Marguerite Godard

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## To cite this version:

Valentin Basch, Elisabetta Rampone, Laura Crispini, Carlotta Ferrando, Benoit Ildefonse, et al. From mantle peridotites to hybrid troctolites: Textural and chemical evolution during melt-rock interaction history (Mt. Maggiore, Corsica, France). Lithos, 2018, 323, pp.4-23. 10.1016/j.lithos.2018.02.025. hal-01975126

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From mantle peridotites to hybrid troctolites:
textural and chemical evolution during melt-rock interaction history
(Mt.Maggiore, Corsica, France)

Basch, Valentin<fnref n="1" />; Rampone, Elisabetta<fnref n="1" />; Crispini, Laura<fnref n="1" />; Ferrando, Carlotta<fnref n="2" />; Ildefonse, Benoit<fnref n="2" />; Godard, Marguerite<fnref n="2" />

http://dx.doi.org/10.1016/j.lithos.2018.02.025

Corresponding Author:

Basch Valentin

Dipartimento di Scienze della Terra, dell'Ambiente e della Vita (DISTAV)

Università degli Studi di Genova

Corso Europa 26

I-16132 Genova (Italy)

Email: valentin.basch@gmail.com

<footnote n="1">¹DISTAV, University of Genova, Corso Europa 26, I-16132 Genova, Italy</footnote>

<footnote n="2">²Géosciences Montpellier, University of Montpellier, CNRS, Montpellier, France.</footnote>
32 **Abstract**

Recent studies investigate the replacement formation of hybrid troctolites from mantle peridotites after multiple stages of melt-rock reaction. However, these studies are not conducted in a field-controlled geological setting displaying the clear evolution from the protolith to the end-product of the reactions. The Mt.Maggiore peridotitic body exposes a clear evolution from spinel lherzolite to plagioclase-bearing lithotypes (plagioclase peridotites, olivine-rich troctolites and troctolites) during two distinct episodes of melt-rock interaction. In the spinel facies, the reactive percolation of a LREE-depleted melt leads to the dissolution of mantle pyroxenes and the growth of olivine crystals, forming replacement spinel dunites. The progressive evolution from spinel lherzolite to harzburgite to replacement dunite is accompanied by a change of olivine Crystallographic Preferred Orientation (CPO), from axial-[100] in the lherzolite to axial-[010] olivine CPO in the dunites, indicative of deformation in presence of melt. The initial percolating melt composition is consistent with single melt increments after 6% partial melting of a depleted mantle source. Reactive melt percolation leads to a progressive enrichment in the melt M-HREE absolute concentrations, while preserving its LREE depletion, consistent with the enriched analyzed HREE composition of olivine in the spinel dunite.

In the shallower plagioclase facies, the melts modified by reactive melt percolation impregnate the spinel-facies lithotypes, leading to the dissolution of olivine and crystallization of plagioclase and orthopyroxene in the peridotites. This impregnation stage is also observed in the spinel dunites, forming hybrid olivine-rich troctolites and troctolites. The dissolution-precipitation reactions forming hybrid troctolites cause a progressive textural evolution of the olivine matrix, with the disruption of deformed coarse grains into undeformed small rounded grains. This textural evolution is not accompanied by clear changes in the olivine CPO, indicating low instantaneous melt/rock ratios during the impregnation process. Olivine, plagioclase and clinopyroxene REE compositions analyzed in troctolite fit a process of impregnation with a progressive closure of the porosity (at decreasing melt mass), leading to the crystallization of trapped melt and REE enrichments during the last crystallization increments.

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62 **Keywords: Alpine ophiolites; Melt-rock interaction; Reactive porous flow; Melt impregnation; Olivine texture; Olivine trace elements.**
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## 1. Introduction

Many studies have highlighted the importance of melt-rock interactions in developing structural and geochemical heterogeneities in peridotites at different mantle depths (e.g., Dijkstra et al., 2001, 2003; Lissenberg and Dick, 2008; Soustelle et al., 2009, 2010, 2014; Collier and Kelemen, 2010; Higgie and Tommasi, 2012, 2014; Tursack and Liang, 2012; Saper and Liang, 2014; Dygert et al., 2016; Paquet et al., 2016; Renna et al., 2016). Specifically, melt-rock reactions have been invoked as important rock-forming processes acting in the upwelling mantle in extensional settings. In such geodynamic environments, replacive spinel harzburgites and dunites have been inferred as the product of open-system reactive melt percolation, driven by pyroxene dissolution and olivine crystallization at deep (spinel-facies) mantle levels (e.g., Kelemen et al., 1995a, b, 2000, 2007; Dick et al., 2008, 2010; Piccardo et al., 2007; Rampone et al., 2008; Rampone and Borghini, 2008; Lambart et al., 2009; Liang et al., 2011; Pirard et al., 2013; Dygert et al., 2016), whereas plagioclase-rich peridotites, ubiquitously found in ophiolitic and oceanic settings, are widely thought to result from melt impregnation at shallower (plagioclase-facies) mantle depths (e.g., Dijkstra et al., 2001, 2003; Borghini et al., 2007; Rampone and Borghini, 2008; Tursack and Liang, 2012; Saper and Liang, 2014).

Melt-rock interactions have also been increasingly invoked as key processes in the formation and compositional evolution of the oceanic crust, from several lines of evidence: (1) the study of chemical zoning and melt inclusions in lava phenocrysts (Laubier et al., 2014; Coumans et al., 2016), (2) peculiar compositional trends observed in mid-ocean ridge basalts (MORBs), not consistent with a process of pure fractional crystallization (Collier and Kelemen, 2010; Van den Bleeken, 2011; Paquet et al., 2016), (3) olivine-rich troctolites found enclosed in the lowermost oceanic crust, thought to represent the “hybrid” reactive product of the interaction between dunites from the crust-mantle boundary and percolating MORB melts, dissolving olivine and crystallizing interstitial plagioclase and clinopyroxene (Lissenberg and Dick, 2008; Suhr et al., 2008; Drouin et al., 2009, 2010; Renna and Tribuzio, 2011; Sanfilippo and Tribuzio, 2013; Higgie and Tommasi, 2012; Sanfilippo et al., 2013, 2014, 2015, 2016; Rampone et al., 2016). These latter studies have thus inferred the possible incorporation of mantle slivers into the lower oceanic crust at slow-spreading ridges through multiple melt-rock interaction episodes leading to the replacive formation of dunites, olivine-rich troctolites and troctolites from mantle peridotites. However, the mantle origin of the dunitic matrix and the resulting hybrid origin of troctolites (i.e., by dunite melt impregnation
rather than by fractional crystallization of a primitive melt) is difficult to demonstrate without documenting the evolution from mantle peridotite protolith to the hybrid end-product.

The Alpine-Apennine ophiolites are predominantly constituted by mantle peridotites and represent analogues of the oceanic lithosphere formed at ocean/continent transition and slow- to ultra-slow spreading settings (Rampone et al., 1997, 2004, 2008; Rampone and Piccardo, 2000; Müntener and Piccardo, 2003; Müntener et al., 2004; Piccardo et al., 2004; Borghini et al., 2007; Manatschal and Müntener, 2009). Previous studies have documented that many Alpine-Apennine peridotites record various stages of melt-rock interaction, which occurred at different lithospheric levels during the extensional mantle exhumation (e.g. Rampone and Borghini, 2008; Piccardo and Guarnieri, 2010).

The Mt.Maggiore (Alpine Corsica) peridotitic body displays lithological variations clearly related to two episodes of melt-rock interaction: a first reactive melt percolation event at spinel-facies, leading to the replacement of harzburgites and dunites, followed by melt impregnation of the spinel peridotites and dunites at plagioclase-facies, crystallizing plagioclase ± orthopyroxene, and dissolving olivine (Jackson and Ohnenstetter, 1981; Rampone et al., 1997, 2008; Müntener and Piccardo, 2003; Piccardo and Guarnieri, 2010). Previous studies report a first spinel-facies reactive melt percolation in the spinel peridotites, leading to the replacement of dunites (Rampone et al., 1997, 2008; Müntener and Piccardo, 2003; Piccardo and Guarnieri, 2010). It is followed by a melt impregnation of the spinel peridotites and dunites at plagioclase-facies, crystallizing plagioclase ± orthopyroxene, and dissolving olivine. In the field, the primary association between spinel- and plagioclase-peridotites, dunites, and olivine-rich troctolites is preserved, and the replacement of plagioclase peridotites and troctolites from the spinel-facies lithotypes during the melt impregnation stage is therefore clearly recognizable. The Mt.Maggiore peridotitic body is thus an ideal case study to investigate the structural and geochemical evolution of a mantle peridotite protolith during the multiple melt-rock interaction stages leading to the replacement of hybrid troctolites.

In this paper, we present new field mapping, coupled with Electron Backscattered Diffraction (EBSD) measurements, and mineral major and trace elements analyses (by Electron Probe Micro-Analyses (EPMA) and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), respectively) of Mt.Maggiore spinel and plagioclase peridotites, spinel dunites, and troctolites, which aim at documenting and discussing the textural and mineral chemical changes during formation of hybrid troctolites. We focus specifically on combined textural observations (EBSD mapping) and trace element
geochemistry of olivine. Olivine is the major constituent in peridotites and troctolites, and is extensively involved in the melt-rock reactions described in the oceanic lithosphere. Although the behaviour of many trace elements in olivine is still poorly understood, recent studies outlined the potential of olivine minor and trace elements in the investigation of mantle processes, from melting to melt-rock interactions (e.g., Drouin et al., 2009; De Hoog et al., 2010; Foley et al., 2013; Lissenberg et al., 2013; Sanfilippo et al., 2014; D'Errico et al., 2016; Rampone et al., 2016).

Here we show that the trace element composition of olivine, coupled to EBSD analyses, on a field-controlled sample set, constitute a powerful approach to investigate melt-rock reaction processes leading to the origin of hybrid troctolites. Major outcomes of this work are the demonstrated and quantified textural evolution of the olivine matrix during the replacive formation of hybrid troctolites, and the evidence for microstructural and geochemical inheritance of olivine in the troctolites from both dunite-forming and troctolite-forming melt-rock interaction stages.

## 2. Structural and petrologic background

The Corsican mafic-ultramafic bodies are part of the Alpine-Apennine ophiolites, thought to represent the lithosphere remnants of the narrow Jurassic Ligurian Tethys basin (400-500 km) opened by passive lithosphere extension leading to slow- to ultra-slow spreading oceanization (Rampone & Piccardo, 2000; Manatschal & Müntener, 2009). The Mt. Maggiore peridotitic massif has been interpreted as the deepest part of the reconstructed “Schistes Lustrés” ophiolitic sequence, exposed in the eastern Alpine Corsica (Fig. 1) (Jackson & Ohnenstetter, 1981). Although it lacks any crustal cover, it preserves clear field relationships between the mantle peridotites and associated pyroxenite and gabbroic lithotypes. Previous studies showed that the peridotitic body records a multi-stage lithospheric exhumation history, through various episodes of melt-rock interaction from deep (spinel-facies) to shallower levels (plagioclase-facies) (Rampone et al., 1997, 2008; Müntener & Piccardo, 2003; Piccardo & Guarnieri, 2010).

A first event of olivine-saturated reactive melt percolation leads to the dissolution of mantle clinopyroxene and orthopyroxene, and crystallization of olivine at spinel facies conditions. This melt-rock interaction is recorded in the spinel lherzolites and harzburgites by the development of olivine embayments partly replacing mantle orthopyroxene and clinopyroxene. Extensive reactive melt percolation is observed with the replacive formation
of spinel dunite pods and the partial to complete dissolution of pyroxenite layers primarily associated with the spinel peridotites. In the clinopyroxene-poor spinel lherzolites and harzburgites, the trace element compositions of clinopyroxenes are relatively homogeneous and LREE depleted, consistent with residual mantle after low-degrees (5-8%) of fractional melting (Rampone et al., 1997, 2008).

In the field, the Mt.Maggiore spinel peridotites grade to plagioclase-bearing peridotites showing microstructural characteristics indicative of a plagioclase- (± opx) crystallizing, olivine-dissolving melt impregnation (Rampone et al., 1997, 2008; Müntener & Piccardo, 2003; Piccardo & Guarnieri, 2010). Highly impregnated plagioclase peridotites often display plagioclase-rich gabbronoritic veinlets, crystallized after segregation of the percolating melts. Rock-forming minerals in the gabbronoritic veinlets also show LREE depleted patterns, indicating the depleted signature of the impregnating melts, which thus do not correspond to an aggregated MORB (Rampone et al., 2008).

Together, the reported melt-rock interaction processes suggest the open-system reactive percolation of an olivine-saturated depleted melt at spinel facies levels followed by impregnation of the peridotites by migrating melts at shallower, colder lithospheric depths. Rampone et al. (2008) described the progressive modification of the percolating melt composition during the pyroxene-dissolving, olivine-crystallizing reactive melt percolation. They inferred that melt-rock interaction occurring at spinel-facies leaded to a progressive decrease in the olivine saturation and increase in pyroxene and plagioclase saturation. Together with a change in lithospheric depth during upwards melt percolation (from spinel-facies to plagioclase-facies), the compositional variations of the melt enabled an evolution in the type of melt-rock interaction from an olivine-crystallizing, pyroxene-dissolving reactive melt percolation at spinel-facies to a plagioclase ± orthopyroxene ± clinopyroxene melt impregnation at plagioclase-facies. Both spinel and plagioclase peridotites were intruded by later gabbroic dykes, ranging from olivine gabbros to diorites, showing a clear MORB-type affinity (Piccardo and Guarnieri, 2010).

### 3. Geology of the studied area

We performed a detailed mapping of the Monte Maggiore peridotitic body and merged our lithological and structural data with the already published data of Jackson and Ohnenstetter (1981) in Fig. 1. The Mt.Maggiore peridotitic massif is mainly composed of granular spinel and plagioclase peridotites, showing in places a weak foliation with a
preferential elongation of mantle pyroxenes (Fig. 1). The spinel peridotites show variations in modal compositions, ranging from olivine-poor lherzolites (60% olivine) to olivine-rich harzburgites (85% olivine) (Table 1). The mantle peridotites are primarily associated to decimetre-size sub-vertical pyroxenite layers, showing a constant NW-SE orientation. In places, these pyroxenite layers are partially dissolved. Similar sub-vertical NW-SE orientations are reported for the foliation observed in the spinel peridotites. In places, the spinel peridotites progressively grade to metre-size spinel dunitic bodies, through a diffuse to sharp contact (Fig. 2a,b). The occurrence of aligned spinel trails within the dunite, indicative of the former presence of a pyroxenite layer, in which pyroxenes were completely dissolved (Fig. 2b), further evidences the replacive nature of the spinel dunites.

The spinel peridotites grade to plagioclase-bearing peridotites, showing an enrichment in interstitial plagioclase and orthopyroxene. The most plagioclase-rich samples (up to 30% interstitial minerals) show a steeply dipping (50-70°S) modal layering of plagioclase enrichment, oriented ESE-WNW (Figs. 1, 3), and associated gabbronoritic veinlets (Fig. 2c). The plagioclase peridotites are associated to bodies of olivine-rich troctolites and troctolites, previously described as “cumulates” (Rampone et al., 1997; Müntener et al., 2003; Piccardo and Guarnieri, 2010). Based on new field observations, we infer that these troctolitic bodies correspond to the plagioclase-facies melt impregnation of the spinel dunites. At the impregnation front, i.e. the transition from preserved spinel-bearing lithotypes to impregnated plagioclase-bearing lithotypes, the spinel peridotite and associated dunite pods grade to replacive plagioclase peridotite and troctolitic bodies, respectively. These troctolitic bodies have variable modal compositions from 70 to 90% of granular olivine and up to 30% of interstitial plagioclase (Fig. 2d,e). The most impregnated samples show a ESE-WNW layering of plagioclase enrichment and formation of gabbronoritic veinlets, similar to what is observed in the plagioclase peridotite (Figs. 1, 3). All spinel- and plagioclase-bearing lithotypes and structures are crosscut by steeply dipping gabbroic dikes (60-80°S), trending E-W to ESE-WNW (Figs. 1, 3).

At the scale of the massif, zones that preserve the primary evolution of mantle peridotites and associated lithologies are separated by low-grade alteration zones rich in serpentine veins (reported as *Serpentine veinlets-bearing peridotite* in Fig. 3). These veinlets often show a common orientation and thus form bands of altered peridotites, surrounding lenses of fresh granular peridotites (Fig. 3). Based on the field occurrences and lithological associations, we selected a preserved area bearing primary associations from spinel- to
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plagioclase-bearing lithotypes (black box in Fig. 1) to perform detailed mapping (Fig. 3) and
sampling.

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**4. Sampling and Analytical Methods**

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In the selected area (Figs. 1, 3), we collected samples along traverses showing
primary associations of spinel peridotite and spinel dunite, grading to plagioclase peridotite,
olivine-rich troctolite and troctolite. Table 1 reports the modal composition of 22 samples,
namely 3 spinel peridotites, 3 spinel dunites, 6 plagioclase peridotites, 4 olivine-rich
troctolites (>75% modal olivine) and 6 troctolites (<75% modal olivine). We performed
structural EBSD mapping of all samples, and mineral major (EPMA) and trace element (LA-
ICP-MS) chemical analyses of 7 samples, namely 1 spinel peridotite, 1 spinel dunite, 1
plagioclase peridotite, 1 olivine-rich troctolite and 3 troctolites. Detailed methodologies for
EBSD, major and trace elements analyses can be found in Supplementary Material.

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**5. Petrography**

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In granular spinel peridotites, olivine, orthopyroxene, clinopyroxene, and spinel are
coarse-grained, up to centimetre-size. Olivines and pyroxenes (orthopyroxene +
clinopyroxene) are deformed, and display kink bands and undulatory extinctions,
respectively. Clinopyroxenes and orthopyroxenes both show thin exsolution lamellae of the
complementary pyroxene. Spinel is mostly coarse-grained in the peridotite assemblage but it
is also found at the rim of mantle orthopyroxene porphyroclasts, as symplectitic intergrowths
of orthopyroxene + spinel ± clinopyroxene, which have been related to cooling and
incorporation of the peridotites at lithospheric temperatures (970-1100°C) (Rampone et al.,
2008). Within the analyzed samples, spinel peridotites show a strong variation of modal
composition of olivine, from 58% to 78% by vol. (Table 1). Melt-rock reaction
microstructures, i.e. the development of undeformed olivine embayments replacing exsolved
orthopyroxene and clinopyroxene, are extensively observed in spinel peridotites with the
highest modal olivine contents (“reacted spinel peridotite” in this study), as previously
documented by Rampone et al. (2008) and Piccardo and Guarnieri (2010).

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Spinel dunites associated to spinel peridotites are mostly characterized by a very
coarse-grained texture of olivines (up to 3 centimetres in size, Fig.5a), that display clear kink
bands and an irregular shape. In places, coarse-grained rounded spinels (up to millimetre-
size) form aligned trails within the dunite.

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Spinel-bearing, plagioclase peridotites preserve locally the microstructural evidence of the first reactive melt percolation: (1) deformation and exsolution in olivine and pyroxenes, respectively; (2) olivine embayments partially replacing mantle pyroxenes and [orthopyroxene + spinel] exsolutions. Further microstructures are indicative of melt impregnation of the spinel-bearing peridotitic matrix. Interstitial plagioclase and orthopyroxene crystallize at the expense of olivine and exsolved clinopyroxene. The most impregnated peridotites are characterized by an enrichment up to 25% of modal interstitial minerals (plagioclase + orthopyroxene ± clinopyroxene) (Table 1). In these samples, where melt focuses to form gabbronoritic veinlets (Fig. 2c), the melt crystallizes both idiomorphic orthopyroxene and clinopyroxene at the contact between the gabbronoritic veinlet and the host peridotite, indicating that saturation of clinopyroxene is reached. These newly formed magmatic minerals are free of exsolution and deformation, and can be recognized from the mantle pyroxene relicts.

Within the replace troctolitic bodies associated with plagioclase peridotites (Fig. 2d), the texture of the olivine matrix varies with the modal abundance of interstitial minerals. Within olivine-rich troctolites (>75% modal olivine, Figs. 4a,b, 5b), olivines are coarse-grained (up to centimetre-size), show irregular contacts and are often deformed with the occurrence of kink bands (Fig. 5). Interstitial plagioclase is found at triple grain junctions (Fig. 4a) and along olivine grain boundaries (Figs. 4b, 5c,d). Within the most impregnated troctolites (<75% modal olivine, Figs. 4c,d, 5d), some olivines show smaller grain size (millimetre-size) and are undeformed (Fig. 5d). However, troctolites still preserve coarse deformed grains of olivine (Figs. 4d and 5d) in the most dunitic areas. The plagioclase occurs as poikilitic crystals enclosing rounded small grains of olivine (Fig. 4d), and it is associated in places to poikilitic clinopyroxene.

**6. Textural analysis**

Microstructural observations from spinel dunite to olivine-rich troctolite to troctolite suggest a progressive variation of the texture of the olivine matrix with increasing modal composition of interstitial phases. In order to quantify the textural evolution observed in these samples, we used the EBSD maps, from which we obtain four textural parameters for each sample: (1) the number of grains, (2) the grain area (µm²), (3) the grain aspect ratio (long axis/short axis of a grain), and (4) the grain shape factor (Perimeter / Equivalent Perimeter of circle with the same area) (Mainprice et al., 2014; Methodology details in Supplementary
Material). The aspect ratio is indicative of the crystal elongation, and the shape factor is indicative of the tortuosity of a the grain boundary. The textural parameters calculated for each sample are reported in Table 1, and are plotted in Figure 6 against modal olivine contents. At decreasing modal olivine (from 99% vol. in the spinel dunite to 70% vol. in the troctolite) during the transition from spinel dunite to olivine-rich troctolite to troctolite, we observe a global trend of increasing number of grains (from 12 to 59), coupled to decreasing grain area (from 38mm² to 6mm²), aspect ratio (from 1.85 to 1.43) and shape factor (from 2.49 to 1.77) (Fig. 6). This is consistent with the observed microstructural evolution from large irregular olivines in the spinel dunites to increasingly rounded small olivines in the most impregnated troctolites (Figs. 4 and 5). Similar textural evolution was described by Boudier (1991) in the Oman ophiolite mantle-crust transition zone, to explain the replacement of wehrlites at the expense of dunites.

## 7. Crystallographic Preferred Orientation of olivine

The Mt. Maggiore peridotites, dunites, olivine-rich troctolites and troctolites are characterized by a very coarse centimetric grain size, therefore the low number of grains does not enable the quantification of the strength of Crystallographic Preferred Orientations (CPO) by the J-index, defined as the volume-averaged integral of the squared orientation densities, sensitive to peaks in the orientation distribution function (i.e. single grains when only a few olivine crystals are analyzed; Bunge, 1982; Ben Ismail and Mainprice, 1998). Olivine CPO are thus used as qualitative indicators of first-order structural variations between the analyzed lithologies. The interstitial minerals are mainly poikilitic and a single crystal can cover almost the entire thin section, it is thus impossible to measure any CPO for plagioclase, orthopyroxene and clinopyroxene crystallized from the impregnating melt. Representative olivine CPO and 2-10° misorientation rotation axes for each described lithotype (spinel peridotite, reacted spinel peridotite, spinel dunite, plagioclase peridotite, olivine-rich troctolite, troctolite) are shown in Figure 7.

Olivine in both unreacted spinel peridotite (Fig. 7a) and plagioclase peridotite (Fig. 7d) are characterized by an axial-[100] CPO, with [100] axis showing the strongest preferred orientation in the foliation plane, [010] axis maximum oriented normal to the foliation plane, and [001] maximum within the foliation plane. The dominant low-angle misorientations within olivines are accommodated by both [001] and [010] rotation axes, with a maximum on [001] (Fig. 7). This indicates that plastic deformation of olivine crystals was related to
dislocation creep with activation of (010)[100] slip system, the easiest at high temperatures conditions (1100-1200°C), and (001)[100], another commonly observed slip system activated at high temperature in olivine (Ben Ismail and Mainprice, 1998; Tommasi et al., 2000, Karato et al., 2008; Drouin et al., 2010; Higgie & Tommasi, 2012). A joint activation of the two described (010)[100] and (001)[100] slip systems in olivine is a common explanation to account for the CPO in natural peridotites deformed under asthenospheric conditions (Tommasi et al., 2000).

Olivines in the reacted spinel peridotite (Fig. 7b), spinel dunite (Fig. 7c), olivine-rich troctolite (Fig. 7e) and troctolite (Fig. 7f) are characterized by an axial-[010] CPO, with the strongest axis orientation being [010] normal to the foliation. Axial-[010] CPO have been previously described in impregnated peridotites (Ben Ismail et al., 2001; Le Roux et al., 2008; Tommasi et al., 2008), hybrid olivine-rich gabbroic rocks (Higgie and Tommasi, 2012, 2014), and experiments of melt segregation during deformation (Holtzman et al., 2003). The development of such patterns has been explained by: (1) activation of dominant (010)[100], together with (010)[001] slip systems (Tommasi et al., 2000; Mainprice et al., 2005); (2) 3D deformation involving stretching in multiple directions in the foliation plane (Tommasi et al., 1999); (3) static recrystallization (Tommasi et al., 2008); and (4) deformation in presence of melt (Holtzman et al., 2003; Le Roux et al., 2008; Kaczmarek and Tommasi, 2011) (see discussion below). The low-angle misorientations (2-10°) of olivine in these lithotypes are mainly accommodated by [010] rotation axis (Fig. 7). From spinel dunite to olivine-rich troctolite and troctolite, we observe a randomization of the orientation of the [100] axis. Furthermore, from olivine-rich troctolite to troctolite, the misorientation along [010] decreases in concentration, suggesting either an annealing with progressive impregnation or the preferential corrosion of olivine grains along [010] subgrain boundaries (see discussion below).

## 8. Major and trace elements mineral compositions

Major and trace elements compositions of minerals in the studied spinel peridotite, plagioclase peridotite, spinel dunite, olivine-rich troctolite, and troctolites are reported in Table S2-5 of Supplementary Material. The standard deviations of the mineral trace elements analyses are reported in Table S6 of Supplementary Material. Overall, our data are consistent with mineral compositions reported in previous studies on the Mt.Maggiore peridotitic body
(Rampone et al., 1997, 2008; Müntener and Piccardo, 2003; Piccardo and Guarnieri, 2010), except for the trace element abundances of olivine, which constitute an entirely new data set.

Olivines from spinel peridotites, plagioclase peridotites, spinel dunites, olivine-rich troctolites and troctolites show homogeneous major elements compositions at high Forsterite (Fo = 89.4-90.2) and Ni (2514-3064 ppm) contents (Table S2). However, variations in trace elements concentrations are observed between the described lithotypes. Olivines in spinel peridotites show low contents of Ti (10-15 ppm), Y (0.01-0.02 ppm) and Zr (0.002-0.003 ppm), whereas higher contents in these elements (e.g. Ti = 41-94 ppm) at constant Fo values are observed in olivines from dunite, olivine-rich troctolite and troctolite (Fig. 8). Similar variations in trace elements (REE, Ti, Zr, Hf) at constant Fo content, ruling out a simple fractional crystallization process, were documented in olivines of Erro-Tobbio olivine-rich troctolites (Rampone et al., 2016), and interpreted as the result of melt-rock reaction processes. Sanfilippo et al. (2014) reported enrichments in Ti/Y and Zr/Y in olivines of Internal Ligurides and Pineto olivine-rich troctolites and troctolites, and they also ascribed such variations to the reaction between the olivine matrix and a modified melt. Olivine REE patterns show a significant variability between the analyzed lithotypes (Figs. 9a,b, 10a,b). Olivines in spinel peridotites show low HREE abundances (YbN = 0.05-0.095 times C1), and high HREE fractionation (DyN/YbN = 0.05-0.08), comparable to olivines from the Gakkel Ridge lherzolites reported by D'Errico et al. (2016). Olivines in the plagioclase peridotite show higher REE absolute concentrations (YbN = 0.18-0.25 times C1) at constant HREE fractionation (DyN/YbN = 0.05-0.07). In the spinel dunite, olivines show higher REE abundances relative to olivines in peridotites (YbN = 0.3-0.38 times C1). In terms of HREE, they resemble olivines in olivine-rich troctolites from Mid-Atlantic Ridge (Drouin et al., 2009) and Erro-Tobbio (Rampone et al., 2016), but they display lower MREE/HREE fractionation (DyN/YbN = 0.26-0.43). Olivines from olivine-rich troctolite and troctolites have similar REE composition. They also show enriched HREE absolute concentrations (YbN = 0.26-0.59), but at stronger HREE fractionation (DyN/YbN = 0.10-0.19).

Clinopyroxenes in the spinel peridotite, plagioclase peridotite and troctolite show similar Mg-value interval (Mg# = 88.7-91.4) to clinopyroxenes analyzed in the same lithotypes by Rampone et al. (1997, 2008) (Table S3). Variations are observed in Ti and Al concentrations between lithotypes, with low Ti (TiO2 = 0.20-0.29 wt%) and high Al (Al2O3 = 5.71-6.46 wt%) concentrations in spinel peridotites, and higher Ti (TiO2 = 0.23-0.62 wt%) and lower Al (Al2O3 = 3.39-6.02 wt%) concentrations in plagioclase peridotite. As documented by Rampone et al. (1997, 2008), clinopyroxene porphyroclasts in the plagioclase
peridotites show a high compositional variability from core to rim. Cores preserve the low-Ti, high-Al signature, similar to the composition of clinopyroxenes porphyroclasts in spinel peridotites, whereas the rims show higher Ti, and lower Al contents. A similar zoning is observed in interstitial and poikilitic clinopyroxene from the troctolite, from relatively low-Ti (TiO₂ = 0.29-0.41 wt%), and high-Al (Al₂O₃ = 5.2-5.7 wt%) cores to high-Ti (TiO₂ = 0.44-0.91 wt%), and low-Al (Al₂O₃ = 3.83-4.94 wt%) rims. In both spinel and plagioclase peridotites, clinopyroxenes show strong LREE depletion (Ce/N/Yb_N = 0.018-0.044), and flat MREE-HREE patterns (Figs. 9c, 10c) comparable to clinopyroxene compositions reported in depleted lherzolites from the Gakkel Ridge (D'Errico et al., 2016). Higher HREE absolute compositions have been measured in clinopyroxene cores from plagioclase peridotites (Yb_N = 7-12 times C1). Clinopyroxenes in troctolites also exhibit significant LREE depletion (Ce_N/Yb_N = 0.015-0.043). In all plagioclase-bearing lithotypes (peridotite and troctolite) the rims of clinopyroxene show an enrichment in Ti, Sc, V, Y and MREE-HREE (Yb_N = 12-15 times C1), the development of convex-upward REE patterns and Eu negative anomalies, as previously documented by Rampone et al. (2008) and Piccardo and Guarnieri (2010).

Plagioclase in the plagioclase peridotite, olivine-rich troctolite and troctolite show high Anorthite contents (An = 85.1-88.1) and very low Sr abundances (15.5-24.6 ppm), as previously documented by Rampone et al. (1997, 2008) and Piccardo and Guarnieri (2010) (Table S4). In all studied samples, REE patterns of plagioclase are characterized by significant and similar LREE depletion (Ce_N/Sm_N = 0.19-0.49). However, variations in absolute REE concentrations are observed between the different lithotypes. The lowest abundances are observed in plagioclase of plagioclase peridotites (e.g. Sm_N = 0.2-0.4 times C1), whereas they progressively increase in troctolites (Sm_N = 0.4-1.2 times C1) and olivine-rich troctolites (Sm_N = 0.7-1.5 times C1).

Orthopyroxene is only found in the spinel and plagioclase peridotites. It shows homogeneous Mg-values (Mg# = 89.2-90.3) in both lithotypes (Table S5). As observed for clinopyroxene, the REE absolute concentrations of orthopyroxenes in the spinel peridotite (Yb_N = 1.9-2.5 times C1, Ce_N/Yb_N = 0.002-0.006) are comparable to those of orthopyroxenes in the Gakkel Ridge depleted lherzolites (D'Errico et al., 2016) (Figs. 9f, 10f). In the plagioclase peridotite, orthopyroxenes show higher absolute REE concentrations (Yb_N = 2.3-5 times C1) at similar REE fractionation (Ce_N/Yb_N = 0.002-0.004). They are consistent with orthopyroxene compositions documented by the previous study of Rampone et al. (2008).

# 9. Discussion
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**9.1 Origin of replacive dunites by spinel-facies reactive porous flow**

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As reported in previous studies, the Mt. Maggiore peridotitic body records an olivine-
crystallizing, pyroxene-dissolving reactive melt percolation stage at spinel facies conditions
(Rampone et al., 1997, 2008; Müntener and Piccardo, 2003; Piccardo and Guarnieri, 2010).
This melt-rock interaction is documented by the development of olivine embayments
replacing mantle pyroxenes, and the partial dissolution of pyroxenite layers commonly
associated with the peridotites. The reacted lithotypes show a high modal variability, from
depleted spinel lherzolites to pyroxene-poor harzburgites until the formation of dunite bodies
in the most extensively reacted peridotites. The dunitic bodies are characterized by irregular
to straight contacts with the spinel peridotite (Fig. 2a) and show in places spinel trails that are
indicative of the former presence of a pyroxenite layer, completely dissolved during
extensive reactive porous flow (Fig. 2b). Despite the high modal variability during evolution
from spinel lherzolite to reacted harzburgite to spinel dunite, Rampone et al. (2008) reported
rather constant bulk-rock Mg-values (Mg# = 89.8-90.5), and olivines accordingly define a
very narrow range of Fo<sub>89.4-90.2</sub> content variation. Constant bulk Mg-values at increasing
modal olivine follow reactive porous flow trends (Rampone et al., 2008) and are indicative of
a buffering of the percolating melt at Mg# ≈ 75 (i.e. in equilibrium with olivine Fo<sub>90</sub>, Collier
and Kelemen, 2010) during reactive porous flow. All lines of evidence point to a replacive
origin of the dunites in the Mt.Maggiore peridotitic body. In the following, we discuss the
structural and geochemical variability of the olivine matrix accompanying the observed
evolution from spinel lherzolites to spinel harzburgites to spinel dunites.

9.1.1 Structural evolution during dunitization

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The replacive formation of the dunitic bodies is driven by an olivine-crystallizing,
pyroxene-dissolving reactive porous flow in the spinel lherzolites. The modal olivine
enrichment from spinel lherzolites to reacted spinel peridotites to spinel dunites is observed
with the growth of the pre-existing olivines at the expense of ortho- and clinopyroxene,
through the development of olivine embayments corroding mantle pyroxenes. The growth of
olivine is confirmed by the observed olivine texture in the spinel dunites, namely very coarse
grains of olivine (up to 3cm in size, Fig. 6b), showing irregular shapes (Fig. 6d).

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During the evolution from spinel lherzolite to reacted spinel peridotite to spinel dunite
(Fig. 7a,b,c), the olivine CPO progressively varies from an axial-[100] CPO in the spinel
lherzolite to an axial-[010] CPO in the spinel dunite. Axial-[100] olivine CPO is largely
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described in natural peridotites (Tommasi et al., 2000; Soustelle et al., 2009, 2010) as the
result of dislocation creep and the joint activation of (010)[100] and (001)[100], the two
easiest slip systems in olivine at asthenospheric conditions (Tommasi et al., 2000).

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The analyzed axial-[010] CPO in the spinel dunite is consistent with previously reported olivine CPO in replacive dunites from French Polynesia xenoliths (Tommasi et al., 2004). As already mentioned (see section “Crystallographic Preferred Orientation of olivine”), the development of such patterns has been explained by several processes and conditions (pressure, strain, water content; Karato et al., 2008; Jung et al., 2009). An axial-[010] CPO of olivine can be formed by the joint activation of dominant (010)[100], together with (010)[001] slip system (Tommasi et al., 2000; Mainprice et al., 2005), commonly activated at high pressures (>3 GPa; Jung and Karato, 2001; Jung et al., 2009). However, the Mt.Maggiore peridotites display microstructural evidence that the reactive melt percolation forming replacive dunites occurred at spinel facies (Rampone et al., 2008; Piccardo and Guarnieri, 2010), therefore, the pressure conditions are too low for activation of [001] glide in olivine (Jung and Karato, 2001; Mainprice et al., 2005; Jung et al., 2009). Olivines in the Mt.Maggiore dunites are deformed, irregular and very coarse-grained, thus the development of axial-[010] CPO is not likely to be related to static recrystallization, as proposed by Tommasi et al. (2008). Field and microstructural evidence support the formation of dunites after extensive pyroxene-dissolving, olivine-crystallizing melt-rock interaction. Accordingly, we infer that the CPO evolution observed in olivine from spinel lherzolite (axial-[100]) to spinel dunite (axial-[010]) is due to deformation in presence of melt, as was described in melt-bearing experiments of deformation (Holtzman et al., 2003) and in the Lherz refertilized peridotites (Le Roux et al., 2008). Dunitic zones correspond to the most reacted lithotype, thus to the strongest accumulation of melts over time. However, the preservation of a clear olivine CPO implies that the instantaneous melt/rock ratio remained, during the whole process of reactive porous flow, below the critical value allowing for loss of cohesion of the solid matrix (20-40%; Rosenberg and Handy, 2005). The reacted spinel harzburgite (Fig. 7b) is characterized by a transitional CPO of olivine, between the spinel lherzolite (Fig. 7a) and the spinel dunite (Fig. 7c), with a decreasing concentration of the [100] axis and increasing concentration of [010]. This is indicative of a progressive change of the CPO, as a function of the cumulated melt fraction and strain (Higgie and Tommasi, 2012).

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A change in the rotation axes accommodating the low-angle misorientations (<10°) within olivine crystals is observed during the dunitization process. Olivines in spinel lherzolites and spinel dunites show low-angle misorientation accommodated by both [001]

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and [010] rotation axes, with major accommodation by [001] axis for olivines in spinel lherzolites and major accommodation by [010] axis for olivines in spinel dunites. This variation can be ascribed to a change in the preferential slip system activation of olivine, from (010)[100] to (001)[100] in spinel peridotites and spinel dunites, respectively (Tommasi et al., 2000).

9.1.2 *Chemical changes of percolating melt: REE modelling*

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No significant major and minor elements compositional variability is observed in olivines from spinel peridotites and spinel dunites (Fo = 89.4-90.2, Ni = 2500-3000ppm). On the other hand, olivines in the spinel dunites are characterized by much higher incompatible trace elements contents (e.g. Ti, Zr, Y, Fig. 8) with respect to olivines in the spinel peridotites. Also, they show higher absolute REE concentrations (YbN = 0.3-0.38 times C1) coupled to lower MREE/HREE fractionation (DYN/YbN = 0.26-0.43) than olivines in spinel peridotites (YbN = 0.05-0.095 times C1; DYN/YbN = 0.05-0.08) (Figs. 9a,b, 10a,b). In terms of such trace elements, olivines in the spinel dunites more closely resemble olivines in the troctolites, although bearing a different MREE/HREE fractionation (Fig. 9b).

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To better understand the trace element variability related to the described process of reactive melt percolation at spinel facies, we performed a geochemical modelling using Plate Model numerical simulations (Vernières et al., 1997). This model reproduces a rock-dominated reactive melt percolation, during which the composition of the melt is buffered by the solid matrix, varying porosity and modal composition along the percolation column (Fig. 11). The selected host rock forming the mantle column is the depleted spinel lherzolite MM2/12 from Rampone et al. (2008), whose modal composition (Ol:Opx:Cpx = 0.74:0.20:0.06) is similar to spinel peridotites analysed in this study (Table S7). The spinel peridotites from Mt.Maggiore were previously interpreted as residual peridotites (Rampone et al., 1997, 2008; Piccardo and Guarnieri, 2010) after 6% fractional melting, following the equation of Johnson et al. (1990) and Warren (2016). Rampone et al. (1997, 2008), using simple AFC modeling, inferred that melts percolating the spinel and plagioclase peridotites (after the partial melting event) had also a LREE-depleted chemical signature. They hypothesized that such depleted signature could either represent a primary characteristic of the melt, or it was acquired during the reactive melt percolation through the depleted spinel lherzolite (Rampone et al., 2008).
We performed the Plate Model numerical simulations (Vernières et al., 1997) with different, variably LREE-depleted, initial melts. We noticed that an N-MORB starting composition (Workman and Hart, 2005) although reproducing the MREE/HREE fractionation and absolute concentrations of olivine in the spinel dunite, does not explain the strong LREE-depletion of plagioclase and clinopyroxene analyzed in the plagioclase peridotite and troctolite (Fig. S1). This observation, together with the evidence of preserved LREE-depleted compositions of reacted spinel peridotites (Rampone et al., 2008), corroborate the inference that the percolating melts had likely a primary LREE-depleted signature. Accordingly, we selected an initial melt composition corresponding to a depleted single melt increment obtained after 6% fractional melting of a slightly depleted mantle source, consistent with previous assumptions (Rampone et al., 1997, 2008; Piccardo and Guarnieri, 2010) (Fig. 11a, Table S7). In order to model the reactive percolation leading to the replacive formation of the dunites, we assumed the following reaction:

$$0.6 \text{ opx}_1 + 0.4 \text{ cpx}_1 + \text{melt}_1 = \text{olivine}_2 + \text{melt}_2$$

in which mantle pyroxenes ($M_a$ = 60% opx + 40% cpx) are dissolved by the incoming melt ($melt_1$), while crystallizing olivine ($M_c$ = 100% olivine), at global [$M_a/M_c$] = 1.08. The partition coefficients used for olivine, orthopyroxene and clinopyroxene, after Warren (2016), are reported in Table S7. The computed initial REE trace element compositions of clinopyroxene and orthopyroxene in the host depleted lherzolite fit well the analyzed mineral compositions (Fig. 11a), whereas discrepancies are observed between computed and analyzed olivine REE compositions, both in terms of absolute concentrations and fractionation (Fig. 11a). We infer that the lower trace element abundances and stronger fractionation observed in the analyzed olivine in spinel peridotite is the result of subsolidus reequilibration between clinopyroxene and olivine, as described by Sun and Liang (2014). Calculated olivine-clinopyroxene REE partitioning (Sun and Liang, 2012, 2013a,b, 2014) yield low equilibrium temperature of 1000-1100°C (Fig. S2), consistent with the low equilibration temperature (970-1100°C) calculated by different geothermometers (Ca in opx, Sc and V opx-cpx partitioning; see Rampone et al., 2008) for the spinel facies equilibration in non-reacted peridotites.

The reactive melt percolation in the spinel lherzolite column leads to the progressive replacement of mantle pyroxenes by olivine, and thus to the formation of a dunite (Fig. 11). During progressive reaction, the porosity in the rock column increases from 1% in the starting lherzolite to 3% in the replacive dunite, as expected in a melt-rock reaction process characterized by [$M_a/M_c$] > 1. The modified melt composition, i.e. the melt in equilibrium
with the spinel dunite (Fig. 11b, Table S7), shows global REE enrichment at constant LREE
fractionation, and olivine in equilibrium with the modified melt (YbN = 0.35 times C1,
DyN/YbN = 0.31, Fig. 11b) fit well the analyzed HREE patterns of olivines in the spinel
dunite (Fig. 11b), both in terms of absolute concentration (YbN = 0.3-0.38 times C1) and
fractionation (DyN/YbN = 0.26-0.43). We thus infer that the enriched REE compositions
measured in olivines from the spinel dunite are acquired by equilibration of the olivine matrix
with the percolating melt, modified after reactive porous flow in the spinel peridotites.

Olivines from the South Lanzo replacive dunites documented by Sanfilippo et al. (2014, 2017) lower absolute trace element contents (YbN = 0.21-0.31 times C1; Ti = 26-33 ppm) than olivines in spinel dunites Mt.Maggiore (this study; YbN = 0.3-0.38 times C1). However, Sanfilippo et al. (2014, 2017) interpreted these elongated replacive dunite bodies, more than decametre-size, as dunite channels allowing focussed flow of a MORB-type melt. They considered the lack of a “reactive signature” of incompatible trace elements in olivines of Lanzo replacive dunites as reflecting equilibration with melts migrating through them by channelized flow, at high melt/rock ratios (Kelemen et al., 1995b; Spiegelman and Kelemen, 2003). In the Mt.Maggiore peridotites, the studied dunites are found as metre-size dunitic pods, and not as the elongated bodies commonly ascribed to channelized flow at high melt/rock ratios (Kelemen et al., 1995a). We infer that dunite formation involved moderate melt/rock ratios (consistent with the preservation of clear olivine CPOs described above), which did not allow the formation of high-porosity dunite channels. The REE-enriched patterns analyzed in olivines from the Mt.Maggiore spinel dunites (Fig. 11b) thus represent the preserved “reactive signature” of incompatible elements, as a consequence of melt/rock ratios involved in the reactive porous flow lower than those required to form high-porosity dunite channels (Kelemen et al., 1995b; Sanfilippo et al., 2017).

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## 9.2 Origin of hybrid troctolites by plagioclase-facies melt impregnation

At Mt.Maggiore, spinel peridotites (including clinopyroxene-poor spinel lherzolites, reacted harzburgites and dunites) grade to plagioclase-bearing lithotypes across the impregnation front observed at kilometre-scale in the field (Figs. 1, 3) (Rampone et al., 2008; Piccardo and Guarnieri, 2010). The plagioclase peridotites show microstructures indicative of an olivine-dissolving, [plagioclase + orthopyroxene]-crystallizing melt-rock interaction. In the studied area, the plagioclase peridotites are in contact with metre-size troctolitic bodies showing variable modal composition, from 70% to 90% of olivine (Fig. 5, Table 1) and up to
30% of interstitial minerals (plagioclase ± clinopyroxene). These olivine-rich troctolites and troctolites show similar microstructures to the plagioclase peridotites, indicative of an olivine-dissolving, plagioclase-crystallizing melt-rock interaction. Olivine-rich troctolites and troctolites also show large variations in trace element (Yb, Ti, Y, Zr) contents and HREE fractionation of olivine (Figs. 8, 9, 10), at almost constant Fo = 89.4-90.2, difficult to reconcile with a simple “cumulate” origin (Rampone et al., 1997; Piccardo and Guarnieri, 2010). Field, microstructural and geochemical evidence thus support a replacement origin of the troctolitic bodies, likely related to the impregnation of pristine spinel dunites.

In the following, we discuss the textural and microstructural evolution of the olivine matrix during impregnation of the spinel dunite and replacement formation of olivine-rich troctolite and troctolite. We then discuss the depleted and SiO₂-rich chemical signature of impregnating melts and provide a REE modelling of the melt impregnation process, to account for the observed mineral compositions in the troctolites.

### 9.2.1 *Structural evolution during impregnation*

Textural variations related to dissolution-precipitation reactions and replacement formation of olivine-rich troctolites have been extensively described in oceanic settings (Drouin et al., 2010; Ferrando et al., this volume), ophiolites (Sanfilippo et al., 2013; Rampone et al., 2016) and melt-rock interaction experimental studies (Van den Bleeken et al., 2010; Saper and Liang, 2014; Francomme et al., this volume). The described hybrid troctolites are characterized by rounded grains of olivines, enclosed in poikilitic minerals crystallized from the percolating melt. Similar crystallographic orientations of neighbouring rounded olivine grains suggest that they formerly belonged to a single olivine grain, corroded and disrupted by the reactive melt (Suhr et al., 2008; Drouin et al., 2010). In this study, the field-controlled geological setting enables the direct observation of the evolution of textures from the dunitic protolith to the troctolitic end-product during progressive impregnation process (i.e., at decreasing modal contents of olivine). Olivines in the dunite are deformed, irregular and very coarse grained, up to 3 centimetres in size (Figs. 4, 5, 6). Olivine corrosion in the dunite starts at triple grain junctions (Fig. 4a) and evolves following grain boundaries (Fig. 4b), while crystallizing interstitial plagioclase. In the most impregnated samples, the extensive corrosion of olivine leads to the disruption of large irregular olivines into several smaller and rounded olivine grains (Fig. 4c,d) that are enclosed in the poikilitic plagioclase. Disrupted olivines often show less deformation than the former dunitic olivine (Fig. 5),
suggesting a preferential corrosion along subgrain boundaries, as previously inferred by Suhr et al. (2008) and Drouin et al. (2010) in the Mid-Atlantic Ridge olivine-rich troctolites.

The quantification of the textural evolution of the olivine matrix from spinel dunite to troctolite reveals a correlation between the modal composition of olivine and the calculated mean grain number, grain area, aspect ratio and shape factor (Fig. 6). During progressive impregnation of the dunite (i.e. decreasing modal olivine), we observe an increase in grain number (from 12 to 59), accompanied with a decrease in grain area (from $38\text{mm}^2$ to $6\text{mm}^2$), aspect ratio (from 1.85 to 1.43) and shape factor (from 2.49 to 1.77) of olivine grains (Fig. 6). This evolution is indicative of the progressive corrosion and disruption of the dunitic matrix by the reactive melt (Fig. 5) (Drouin et al., 2010). The end-product of the impregnation process is a troctolite, composed of up to 30% of interstitial minerals, characterized by undeformed small rounded olivine grains enclosed in poikilitic plagioclase and clinopyroxene (Figs. 4, 5, 6). The decreasing concentration of the [010] olivine low-angle misorientation from dunite to olivine-rich troctolite to troctolite (Fig. 7c,e,f) is consistent with the described process of preferential corrosion along subgrain boundaries, and the decrease in grain deformation during reaction and disruption of a coarse deformed olivine into several rounded smaller olivines (Suhr et al., 2008; Drouin et al., 2010). Our EBSD analyses demonstrate the whole textural evolution, from the mantle protolith to the troctolite end-product, previously inferred to explain the origin of olivine-rich troctolites in oceanic and ophiolitic settings (Suhr et al., 2008; Drouin et al., 2010; Sanfilippo et al., 2013; Rampone et al., 2016). Magmatic cumulate textures in olivine-rich troctolites and troctolites can thus develop as a consequence of dissolution-precipitation reactions in a mantle-derived protolith.

No major change of olivine CPO is observed during impregnation and replacement of plagioclase peridotites and hybrid troctolites. Olivines in the plagioclase peridotite show an axial-[100] CPO, and olivines in the troctolite show a axial-[010], similar to the spinel lherzolite and spinel dunite protolith, respectively (Fig. 7). The preservation of the protolith CPO of olivine in plagioclase peridotite and troctolite, suggests low instantaneous melt-rock ratios involved in the impregnation process (Higgie and Tommasi, 2012). Our results differ from the olivine CPO randomization described during the replacement formation of olivine-rich troctolites from the Atlantis Massif (Drouin et al., 2010), suggesting melt/rock ratios allowing the loss of cohesion of the solid matrix. Overall, this indicates that there is no universal CPO signature of melt-rock interactions originating hybrid troctolites (Drouin et al., 2010), since the modification or preservation of the protolith CPO of olivine entirely depends on the physical parameters (melt/rock ratio, temperature, pressure, strain,
water content; Karato et al., 2008; Jung et al., 2009) involved in each melt-rock reaction stage.

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### 9.2.2 Chemical signature of impregnating melts

Despite being part of the same impregnation event crystallizing plagioclase + orthopyroxene in the plagioclase peridotite, no interstitial orthopyroxene is found in the troctolites. Numerous studies describe the impregnation of peridotites by a silica-saturated melt, leading to the crystallization of plagioclase + orthopyroxene (Dijkstra et al., 2003; Piccardo et al., 2007; Rampone and Borghini, 2008; Rampone et al., 2008; Van den Bleeken, 2011; Dygert et al., 2016). They suggest that the impregnating melt acquired its silica-rich, orthopyroxene-saturated signature during a previous pyroxene-dissolving, olivine-crystallizing reactive porous flow in the spinel facies (Rampone et al., 2008). Kelemen et al. (1995b) and Dijkstra et al. (2003) showed that melt fractions produced by shallow mantle melting underneath mid-ocean ridges have more SiO₂-rich compositions than high-pressure mantle melts, and are thus closer to orthopyroxene saturation (Dijkstra et al., 2003). The reaction of these melts with the host harzburgitic wall-rock, dissolving pyroxenes and crystallizing olivine, will rapidly drive the melt to orthopyroxene-saturation (Kelemen et al., 1995b; Dijkstra et al., 2003). The absence of interstitial orthopyroxene crystallization in the troctolites suggests that the orthopyroxene saturation of impregnating melts observed in the plagioclase peridotite is not only related to the spinel-facies reactive melt percolation, but also to shallower dissolution-precipitation reactions involving pyroxene corrosion during impregnation. Microstructures characterizing the impregnation stage indicate the corrosion of olivine + mantle pyroxenes in the plagioclase peridotites (Rampone et al., 2008; Piccardo and Guarnieri, 2010), whereas only olivine is being corroded during impregnation of the spinel dunite. Therefore, we infer that the difference in the melt-rock interaction occurring in peridotites and dunites (peridotite-dissolution vs olivine-dissolution, respectively) lead to local variations in the melt compositions, resulting in orthopyroxene saturation in the melt percolating the peridotites (Dijkstra et al., 2003).

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Previous studies of the Mt. Maggiore plagioclase peridotites and troctolites interpreted the impregnating melts to be previously modified by the spinel facies reactive porous flow (Rampone et al., 1997, 2008). They reported geochemical compositions of interstitial minerals (clinopyroxene and plagioclase) in equilibrium with a LREE depleted, M-HREE-enriched melt (Rampone et al., 1997, 2008; Piccardo and Guarnieri, 2010). The M-HREE
enrichment was thought to be acquired during pyroxene-dissolving, olivine-crystallizing reactive melt percolation (AFC modelling, Rampone et al., 2008). Our Plate Model numerical simulation of the dunitization stage (see discussion above) confirms the M-HREE-enriched signature of the melt modified after the spinel facies melt-rock interaction.

In order to explain the REE compositional variations documented in interstitial minerals in the troctolite (Fig. 9c,e), as well as the observed change in HREE fractionation between olivines in the dunite ($Dy_N/Yb_N$ = 0.26-0.43) and in the troctolite ($Dy_N/Yb_N$ = 0.10-0.19) (Fig. 9b), we performed a second step of Plate Model simulation of the impregnation process in the spinel dunites (Fig. 12a). The selected host rock and input melt composition are the output of the first stage of reactive melt percolation, namely a spinel dunite (100% olivine, 3% porosity, $Yb_N$ = 0.35 times C1, $Dy_N/Yb_N$ = 0.31) impregnated by a LREE-depleted, M-HREE-enriched melt, modified during the spinel-facies reactive percolation (Table S7). We assumed the following reaction:

$$olivine_2 + melt_2 = 0.9 plag + 0.1 cpx + melt_3$$

in which olivine ($M_a$ = 100% olivine) is dissolved by the impregnating melt ($melt_2$), while crystallizing interstitial plagioclase and clinopyroxene ($M_c$ = 90% plagio + 10% cpx), at global [$M_a/M_c$ = 0.96]. Partition coefficients used for olivine and clinopyroxene were the same as those used for the spinel facies reactive percolation stage (Table S7). For plagioclase, we used a selected compilation of partition coefficients from Bedard (2001); Aigner-Torres et al. (2007) and Laubier et al. (2014) (Table S7).

Plate Model results show that the impregnation of the spinel dunite progressively leads to the dissolution of olivine and to the crystallization of plagioclase and clinopyroxene, thus causing the replacement formation of a hybrid troctolite composed of Ol:Plag:Cpx = 0.73:0.25:0.02 (Fig. 12b), similar to what is observed in the field (Table 1). The porosity decreases during impregnation, from 3% in the spinel dunite to 2% in the troctolite, as expected from a process characterized by a [$M_a/M_c$] <1. The modified melt composition, i.e. in equilibrium with the replacement troctolite, preserves its LREE depletion and is slightly less enriched in M-HREE (up to 28 times C1, Table S7) with respect to the input melt (Fig. 12b). A remarkable feature is the development of a negative Eu anomaly in the modified melt composition after crystallization of plagioclase.

Plagioclase in equilibrium with the modified melt fit well the analyzed compositions of plagioclase cores in the troctolites, both in terms of LREE fractionation and absolute REE concentrations (Fig. 12b). It is remarkable, however, that plagioclase REE contents display large variations at constant LREE fractionation both between different lithotypes and within
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single grains (e.g. cores of large crystals vs. thin interstitial rims). Specifically, the
progressive decrease in interstitial plagioclase modal content from troctolite to olivine-rich
730
731 troctolite is correlated with an enrichment in plagioclase trace elements absolute
732 concentrations (SmN up to 1.5 times C1) (Figs. 9, 10). As for plagioclase, computed
733 clinopyroxene in equilibrium with the modified melt fit well the analyzed compositions of
734 clinopyroxene cores in the troctolite. However, we observed significant REE enrichments at
735 the rims of large clinopyroxene crystals, as well as in thin interstitial grains (Figs. 9, 10),
736 similar to what was described in previous papers (Rampone et al., 1997, 2008; Piccardo and
737 Guarnieri, 2010). In these studies, within-grain variations in interstitial minerals (plagioclase
738 and clinopyroxene) were interpreted as the effect of late-stage crystallization of trapped melt
739 fractions.

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Computed olivine in equilibrium with the troctolite-forming melt shows lower HREE
741 absolute concentrations and MREE/HREE fractionation (YbN = 0.29 times C1, DyN/YbN =
742 0.3) than olivines analyzed in the troctolite (YbN = 0.26-0.59 times C1, DyN/YbN = 0.10-0.19)
743 (Fig. 12b). This discrepancy cannot be explained by a subsolidus reequilibration between
744 clinopyroxene and olivine (Sun and Liang, 2014), since most of the troctolites are composed
745 exclusively of olivine + interstitial plagioclase (Fig. 5; Table 1). Moreover, subsolidus
746 reequilibration with clinopyroxene would lower HREE absolute contents of olivine (Sun and
747 Liang, 2014), as observed in spinel peridotite (see discussion above), whereas HREE contents
748 of olivine in the troctolites are higher than the computed olivine in equilibrium with the
749 impregnating melt (Fig. 12b). This is further supported by high equilibration temperatures
750 (1150°C and 1250°C) computed using olivine-clinopyroxene HREE partitioning in troctolite
751 (Sun and Liang, 2012, 2013a,b, 2014) (Fig. S2), consistent with previous geothermometric
752 estimates of the impregnation stage reported by Rampone et al. (2008).

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In order to explain the REE enrichments in interstitial plagioclase and clinopyroxene
754 as well as the stronger MREE/HREE fractionation in olivine of troctolites, we have modelled
755 further crystallization steps at decreasing melt mass, resulting in the closure of the porosity
756 (Fig. 13). The [M_a/M_c] ratio varies along the reaction process from 0.72 to 0.27 for the final
757 crystallization step. During the progressive crystallization and decrease in porosity (from 2%
758 to 0.47%), the melt composition evolves towards a fractionated HREE-enriched composition.
759 Olivines in equilibrium with this HREE-enriched melt (Yb_N = 0.54 times C1; Dy_N/Yb_N = 0.2)
760 fit the analyzed MREE-HREE concentrations of olivine in the troctolites (Yb_N = 0.26-0.59
761 times C1, Dy_N/Yb_N = 0.10-0.19) (Fig. 13a). Lower MREE absolute concentrations analysed
762 in some olivine grains (see the grey field in Figure 13a) could reflect subsolidus

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reequilibration between clinopyroxene and olivine (Sun and Liang, 2012, 2013a,b, 2014) (Fig. S2), decreasing olivine absolute REE concentrations and DyN/YbN fractionation (Sun and Liang, 2014).

During the last increments of crystallization (from 0.47% to 0.13% porosity), the melt shows a global REE enrichment, while preserving its LREE fractionation (Fig. 13b,c). Plagioclase and clinopyroxene in equilibrium with the progressively REE-enriching melt cover the range of variation analyzed in crystal cores and rims of the plagioclase peridotite, olivine-rich troctolite and troctolite, both in terms of absolute REE concentrations and LREE fractionation (Fig. 13b,c). The modelled process of impregnation at decreasing melt mass (from 2% to 0.13% porosity) is consistent with the process of trapped melt crystallization previously proposed by Rampone et al. (1997, 2008) and Piccardo and Guarnieri (2010) to explain the trace elements (Ti, Zr, Y, REE) enrichments analyzed at the rims of large clinopyroxene crystals (Figs. 9, 10). The correlation between the plagioclase absolute REE concentrations and modal content of olivine (REE enrichments at higher modal content of olivine) is indicative of an increased contribution of trapped melt crystallization in olivine-rich zones, and thus of lower instantaneous melt/rock ratios during their impregnation.

## Summary and concluding remarks

The Mt. Maggiore peridotitic body is an ideal case study to investigate the field-controlled evolution from spinel lherzolite to spinel dunite to hybrid troctolite (Fig. 14), after two distinct melt-rock interaction processes. The first melt-rock interaction is an olivine-crystallizing pyroxene-dissolving reactive percolation of a LREE-depleted melt, ultimately resulting in the formation of replacive dunite at spinel facies. This stage led to: (1) olivine growth at the expense of mantle pyroxenes, and development of very coarse and irregular texture of the olivine matrix in the spinel dunite, (2) structural changes of olivine CPO from axial-[100] in the spinel lherzolite to axial-[010] in the replacive dunite, (3) REE enrichment in the olivine matrix during progressive dunitization.

At shallower plagioclase facies, the melt impregnation of the spinel lherzolites and dunites caused the replacive formation of plagioclase peridotites and hybrid troctolites, respectively. This olivine-dissolving, [plagioclase + clinopyroxene]-crystallizing melt-rock interaction is documented in the troctolites by: (1) partial dissolution and disruption of the dunitic olivine matrix by low instantaneous melt/rock ratios, leading to a textural evolution towards small rounded undeformed olivine grains enclosed in poikilitic plagioclase, (2)
796

preservation of the olivine CPO from the spinel-facies protoliths, (3) crystallization of LREE-depleted, HREE-enriched interstitial clinopyroxene and plagioclase from the melt modified after reactive melt percolation, and change of the MREE-HREE fractionation in the olivine matrix, and (4) REE enrichments in clinopyroxene rims and interstitial plagioclase during late-stage trapped melt crystallization.

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The field-controlled study of the evolution from spinel lherzolite protolith to the troctolite end-product points to the possible CPO and geochemical inheritance from the protolith during processes of melt-rock interaction (depending on the involved melt-rock ratio). As a result, the structural and geochemical features analyzed in hybrid lithotypes are representative not only of the last event of melt-rock interaction, but of the whole evolution from mantle protolith to hybrid end-product. Olivine is the only mineral present in all lithotypes during the evolution from mantle lherzolites to replacive dunites to hybrid troctolites, therefore trace elements analyses in olivine are a perfect tool to investigate these melt-rock interaction processes and a good understanding of trace elements behaviour in olivine is of upmost importance.

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## Acknowledgements

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We would like to thank Paolo Campanella and Alessandra Gavoglio, as well as Christophe Nevado and Doriane Delmas for the realisation of the thin section and the high-quality polishing. We also thank Fabrice Barou for assistance with the EBSD analyses, Andrea Risplendente for assistance with the EPMA analyses, and Vincent Faucheux for assistance with field work. This project has been supported by the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007–2013/ under REA-Grant Agreement No. 608001, ‘ABYSS’.

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820 FIGURE CAPTIONS

821

822 **Figure 1:** Geological map of the lithological variations and associated structures of the Mt.Maggiore peridotites (Corsica). This structural map merges new data measured on the field with the data of Jackson and Ohnenstetter (1981). The black square represents the area studied into more detail, where the sampling has been done (see Fig. 3 for more detail).

826

827 **Figure 2:** Representative lithological associations of the Monte Maggiore peridotitic body. A: Plagioclase peridotite grading to plagioclase-poor dunite; B: Spinel dunite showing spinel-rich zones; C: Strongly impregnated plagioclase peridotite, where the percolating melt segregated into gabbronoritic veinlets; D: Plagioclase peridotite grading to ol-rich troctolite; E: Troctolite.

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832 **Figure 3:** A: Representative sketch of the measured structures and lithological associations of the Mt.Maggiore peridotites in the sampling area; B: Structural cross-section along the A-A' line (indicated in the representative sketch); C: Three-dimensional lithological variations of the preserved spinel- and plagioclase-peridotite lenses and associated mantle structures.

836

837 **Figure 4:** Microstructural evolution with increasing impregnation of a spinel dunite. A: Ol-rich troctolite, plagioclase crystallized at the triple grain junction, and partly corroded pre-existing olivine; B: Ol-rich troctolite, plagioclase corroded olivine along grain boundaries; C: Troctolite, interstitial to poikilitic plagioclase surrounds smaller olivine grains; D: Troctolite, poikilitic plagioclase includes rounded olivines. In the top left corner, an olivine is almost completely disrupted into two different rounded grains. The dashed red lines highlight a kink band in a coarse deformed olivine.

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844 **Figure 5:** EBSD phase (left column) and misorientation (right column) maps showing the textural evolution of the olivine matrix with ongoing impregnation (decreasing modal olivine from A to D). A: Spinel dunite M15-14, 99% olivine; B: Ol-rich troctolite M15-9, 85% olivine; C: Troctolite M15-8A, 73% olivine; D: Troctolite M15-8B, 73% olivine. The 0-10° scale indicates misorientation from the average orientation of a grain.

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850 **Figure 6:** Evolution of textural parameters of olivine plotted against the modal composition of olivine in samples from spinel dunite to ol-rich troctolite and troctolite. A: Number of grains; B: Grain area; C: Aspect Ratio; D: Shape Factor.

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854 **Figure 7:** Representative olivine Crystallographic Preferred Orientation and 2-10 degrees misorientation Inverse Pole Figures of the described lithotypes from Mt.Maggiore peridotitic body. A:

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Spinel lherzolite M15/7B; B: Reacted spinel harzburgite M15/13; C: Spinel dunite M15/05; D: Plagioclase peridotite M15/12B; E: Olivine-rich troctolite M15/1; F: Troctolite M15/8A.

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**Figure 8:** Forsterite (mol%) vs selected trace elements in olivine. A: Yb normalized to C1-chondrite, normalizing values after Sun and McDonough (1989); B: Ti (ppm); C: Y (ppm); D: Zr (ppm). Compositional fields after Rampone et al. (2016); (1) Peridotites, (2) Troctolites, (3) Olivine gabbros; and single analyses from Sanfilippo et al. (2014).

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**Figure 9:** C1-normalized REE composition of rock-forming minerals. A: Olivine in spinel and plagioclase peridotite; B: Olivine in spinel dunite, ol-rich troctolite and troctolite; C: Clinopyroxene in spinel and plagioclase peridotite; D: Clinopyroxene in troctolite; E: Plagioclase; F: Orthopyroxene. C1-chondrite normalization values after Sun and McDonough (1989). The compositional fields represent Gakkel Ridge abyssal lherzolites from D'Errico et al. (2016), Mid-Atlantic Ridge olivine-rich troctolites from Drouin et al. (2009), Erro Tobbio ophiolitic troctolites from Rampone et al. (2016), and Mt. Maggiore plagioclase peridotites and troctolites from Rampone et al. (1997, 2008). The solid red line is the average of the residual lherzolite field after Warren (2016). For clinopyroxene, a distinction is made between cores (full symbol) and rims (empty symbols) of crystals.

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**Figure 10:** Primitive Mantle-normalized trace elements compositions of the rock-forming minerals. A: Olivine in spinel and plagioclase peridotite; B: Olivine in spinel dunite, ol-rich troctolite and troctolite; C: Clinopyroxene in spinel and plagioclase peridotite; D: Clinopyroxene in troctolite; E: Plagioclase; F: Orthopyroxene. Primitive Mantle normalization values are after Sun and McDonough (1989). The compositional fields are similar to Fig. 9. For Clinopyroxene, a distinction is made between cores (full symbol) and rims (empty symbols) of crystals.

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**Figure 11:** Evolution of chondrite-normalized REE abundances and modal compositions computed by numerical simulation (Plate model, Vernières et al., 1997) during a process of reactive melt percolation at spinel facies. Detail of input parameters is given in the text. C1-chondrite normalization values are after Sun and McDonough (1989). A: initial stage, spinel lherzolite; B: dunite formed after reactive melt percolation.

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**Figure 12:** Evolution of chondrite-normalized REE abundances and modal compositions computed by numerical simulation (Plate model, Vernières et al., 1997) during a process of impregnation at plagioclase facies. Detail of input parameters is given in the text. C1-chondrite normalization values after Sun and McDonough (1989). A: initial stage, spinel dunite; B: troctolite formed after melt impregnation.

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**Figure 13:** Evolution of chondrite-normalized REE abundances of rock-forming minerals by numerical simulation (Plate Model, Vernières et al., 1997) during last steps of impregnation at decreasing melt mass and closure of porosity. Detail of input parameters is given in the text. C1-chondrite normalization values after Sun and McDonough (1989). A: olivine; B: Clinopyroxene; C: Plagioclase.

**Figure 14:** Representative sketch of the textural evolution from spinel lherzolite to troctolite during multi-stage melt-rock interaction history.

## REFERENCES

Aigner-Torres, M., Blundy, J., Ulmer, P. and Pettke, T., 2007. Laser Ablation ICPMS study of trace element partitioning between plagioclase and basaltic melts: an experimental approach. Contributions to Mineralogy and Petrology, 153, 647-667, doi: 10.1007/s00410-006-0168-2.

Bédard, J.H., 2001. Parental magmas of the Nain Plutonic Suite anorthosites and mafic cumulates: a trace element modelling approach. Contributions to Mineralogy and Petrology, 141, 747-771, doi: 10.1007/s004100100268.

Ben Ismail, W., Mainprice, D., 1998. An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy. Tectonophysics, 296, 145-157, doi: 10.1016/S0040-1951(98)00141-3.

Ben Ismail, W., Barruol, G. and Mainprice, D., 2001. The Kaapvaal craton seismic anisotropy: petrophysical analyses of upper mantle kimberlite nodules. Geophysical Research Letters, 28, 2497-2500, doi: 10.1029/2000GL012419.

Borghini, G., Rampone, E., Crispini, L., De Ferrari, R. and Godard, M., 2007. Origin and emplacement of ultramafic-mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy), Lithos, 94, 210-229, doi: 10.1016/j.lithos.2006.06.014.

Boudier, F., 1991. Olivine xenocrysts in picritic magmas: an experimental and microstructural study. Contributions to Mineralogy and Petrology, 109, 114-123.

Bunge, H. J., 1982. Texture analysis in material sciences. Butterworths, London

Collier, M.L., Kelemen, P.B., 2010. The case for reactive crystallization at Mid-Ocean Ridges. Journal of Petrology, 51, 1913-1940, doi: 10.1093/petrology/egq043.

Coumans, J.P., Stix, J., Clague, D.A., Minarik, W.G. and Layne, G.D., 2016. Melt-rock interaction near the Moho: Evidence from crystal cargo in lavas from near-ridge seamounts, Geochimica et Cosmochimica Acta, 191, 139-164, doi: 10.1016/j.gca.2016.07.017.
936

D'Errico, M.E., Warren, J.M. and Godard, M., 2016. Evidence for chemically heterogeneous
Arctic mantle beneath the Gakkel Ridge. *Geochimica et Cosmochimica Acta*, 174, 291-312, doi:
10.1016/j.gca.2015.11.017.

937

938

939

De Hoog, J.C.M., Gall, L. and Cornell, D.H., 2010. Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry. *Chemical Geology*, 270, 196-215, doi: 10.1016/j.chemgeo.2009.11.017.

940

941

942

943

Dick, H.J.B., Tivey, M.A. and Tucholke, B.E., 2008. Plutonic foundation of a slow-spreading ridge segment: Oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W. *Geochemistry, Geophysics, Geosystems*, 9, Q05014, doi: 10.1029/2007GC001645.

944

945

946

947

Dick, H.J.B., Lissenberg, C.J. and Warren, J.M., 2010. Mantle melting, melt transport, and delivery beneath a Slow-Spreading Ridge: The Paleo-MAR from 23°15′N to 23°45′N. *Journal of Petrology* 51, 425–467, doi: 10.1093/petrology/egp088.

948

949

950

951

Dijkstra, A.H., Drury, M.R. and Vissers, R.L.M., 2001. Structural petrology of plagioclase peridotites in the West Othris Mountains (Greece): Melt impregnation in mantle lithosphere. *Journal of Petrology*, 42, 5-24, doi: 10.1093/petrology/42.1.5.

952

953

954

955

Dijkstra, A.H., Barth, M.G., Drury, M.R., Mason, P.R.D. and Vissers, R.L.M., 2003. Diffuse porous melt flow and melt-rock reaction in the mantle lithosphere at a slow-spreading ridge: A structural petrology and LA-ICP-MS study of the Othris Peridotite Massif (Greece). *Geochemistry, Geophysics, Geosystems*, 4, 8613, doi: 10.1029/2001GC000278.

956

957

958

959

960

Drouin, M., Godard, M., Ildefonse, B., Bruguier, O. and Garrido, C.J., 2009. Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge (IODP Hole U1309D, 30°N). *Chemical Geology*, 264, 71-88, doi: 10.1016/j.chemgeo.2009.02.013.

961

962

963

964

965

Drouin, M., B. Ildefonse, and M. Godard, 2010. A microstructural imprint of melt impregnation in slow spreading lithosphere: Olivine-rich troctolites from the Atlantis Massif, Mid-Atlantic Ridge, 30°N, IODP Hole U1309D, *Geochemistry, Geophysics and Geosystems*, 11, Q06003, doi:10.1029/2009GC002995.

966

967

968

969

970

Dygert, N., Liang, Y. and Kelemen, P.B., 2016. Formation of plagioclase lherzolite and associated Dunite-Harzburgite-Lherzolite sequences by multiple episodes of melt percolation and melt-rock reaction: an example from the Trinity ophiolite, California, USA. *Journal of Petrology*, 57, 815-838, doi: 10.1093/petrology/egw018.

971

972

973

974

975

Ferrando, C., Godard, M., Ildefonse, B. and Rampone, E., this issue. Melt transport and mantle assimilation at Atlantis Massif (IODP Site U1309): evidence from chemical profiles along olivine crystallographic axes. Lithos, this volume.

976

977

978

979

Foley, S.F., Prelevic, D., Rehfeldt, T. and Jacob, D.E., 2013. Minor and trace elements in olivines as probes into early igneous and mantle melting processes. *Earth and Planetary Science Letters*, 363, 181-191, doi: 10.1016/j.epsl.2012.11.025.

980

981

982

983
984

985

986

987

988

989

990

991

992

993

994

995

996

997

998

999

1000

1001

1002

1003

1004

1005

1006

1007

1008

1009

1010

1011

1012

1013

1014

1015

1016

1017

1018

1019

1020

1021

1022

1023

1024

1025

1026

1027

1028

1029

1030

1031

Francomme, J.E., Fumagalli, J., Borghini, G. and Capitani, G.C., this issue. Role of pressure and melt composition on olivine-rich troctolite formation through reactive dissolution and crystallization: an experimental study at 0.5 and 0.7 GPa. Lithos, this volume

Günther, D. and Heinrich, C., 1999. Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier. Journal of Analytical Atomic Spectrometry, 14, 1363-1368, doi: 10.1039/A901648A.

Hielscher, R., and Schaeben, H., 2008. A novel pole figure inversion method: specification of the MTEX algorithm. Journal of Applied Crystallography, 41, 1024-1037, doi: 10.1107/S0021889808030112.

Higgie, K. and Tommasi, A., 2012. Feedbacks between deformation and melt distribution in the crust-mantle transition zone of the Oman ophiolite. Earth and Planetary Science Letters, 359-360, 61-72, doi: 10.1016/j.epsl.2012.10.003.

Higgie, K. and Tommasi, A., 2014. Deformation in a partially molten mantle: Constraints from plagioclase lherzolites from Lanzo, western Alps. Tectonophysics, 615-616, 167-181, doi: 10.1016/j.tecto.2014.01.007.

Holtzman, B.K., Kohlstedt, D.L., Zimmerman, M.E., Heidelbach, F., Hiraga, T. and Hustoft, J., 2003. Melt segregation and strain partitioning: Implications for seismic anisotropy and mantle flow. Science, 301, 1227-1230, doi: 10.1126/science.1087132.

Jackson, M.D., Ohnenstetter, M., 1981. Peridotite and Gabbroic structures in the Monte Maggiore Massif, Alpine Corsica. Journal of Geology, 89, 703-719, doi: 0022-1376/81/8906-003.

Jochum, K. P., Nohl, U., Herwig, K., Lammel, E., Stoll, B. and Hofmann, A. W., 2005. GeoReM: A New Geochemical Database for Reference Materials and Isotopic Standards. Geostandards and Geoanalytical Research, 29, 333-338. doi: 10.1111/j.1751-908X.2005.tb00904.x.

Johnson, K.T.M., Dick, H.J.B. and Shimizu, N., 1990. Melting in the oceanic upper mantle: An Ion Microprobe study of diopsides in abyssal peridotites. Journal of Geophysical research, 95, 2661-2678, doi: 10.1029/JB095iB03p02661.

Jung, H., Karato, S., 2001. Water-induced fabric transitions in olivine. Science, 293, 1460-1463, doi: 10.1126/science.1062235.

Jung, H., Mo, W. and Green, H.W., 2009. Upper mantle seismic anisotropy resulting from pressure-induced slip transition in olivine. Nature Geoscience, 2, 73-77, doi: 10.1038/NGEO389.

Kaczmarek, M.A., Tommasi, A., 2011. Anatomy of an extensional shear zone in the mantle, Lanzo massif, Italy. Geochemistry, Geophysics, Geosystems, 12, Q0AG06, doi: 10.1029/2011GC003627.

Karato, S., Jung, H., Katayama, I. and Skemer, P., 2008. Geodynamic significance of seismic anisotropy of the Upper Mantle: new insights from laboratory studies. Annual Review Earth Planetary Science, 36, 59-95, doi: 10.1107/S0021889808030112.
1032

1033

1034

1035

1036

1037

1038

1039

1040

1041

1042

1043

1044

1045

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1047

1048

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1050

1051

1052

1053

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1055

1056

1057

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1060

1061

1062

1063

1064

1065

1066

1067

1068

1069

1070

1071

1072

1073

1074

1075

1076

1077

1078

1079

Kelemen, P.B., Shimizu, N. and Salters, V.J.M., 1995a. Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels. Nature, 375, 747-753, doi: 10.1038/375747a0.

Kelemen, P.B., Whitehead, J.A., Aharonov, E. and Jordahl, K.A., 1995b. Experiments on flow focusing in soluble porous media, with applications to melt extraction from the mantle. Journal of Geophysical Research, 100, 475-496, doi: 10.1029/94JB02544.

Kelemen, P.B., Braun, M. and Hirth, G., 2000. Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites. Geochemistry, Geophysics, Geosystems, 1, doi: 10.1029/1999GC000012.

Kelemen, P.B., Kikawa, E., Miller, D.J. and Shipboard Scientific Party, 2007. 1. Leg 209 Summary: Processes in a 20-km-thick conductive boundary layer beneath the Mid-Atlantic Ridge, 14°-16°N. Proceedings of the Ocean Drilling Program, Scientific Results Volume 209: College Station TX (Ocean Drilling Program), 1-33, doi: 10.2973/odp.proc.sr.209.001.2007.

Lambart, S., Laporte, D. and Schiano, P., 2009. An experimental study of focused magma transport and basalt-peridotite interactions beneath mid-ocean ridges: implications for the generation of primitive MORB compositions. Contributions to Mineralogy and Petrology, 157, 429-451, doi: 10.1007/s00410-008-0344-7.

Laubier, M., Grove, T.L. and Langmuir, C.H., 2014. Trace element mineral/melt partitioning for basaltic and basaltic andesitic melts: An experimental and laser ICP-MS study with application to the oxidation state of mantle source regions. Earth and Planetary Science Letters, 392, 265-278, doi: 10.1016/j.epsl.2014.01.053.

Laukert, G., Von der Handt, A., Hellebrand, E., Snow, J.E., Hoppe, P. and Klügel, A., 2014. High-pressure reactive stagnation recorded in abyssal pyroxenites from the ultraslow-spreading Lena Trough, Arctic Ocean. Journal of Petrology, 55, 427-458, doi: 10.1093/petrology/egt073.

Le Roux, V., Tommasi, A. and Vauchez, A., 2008. Feedback between melt percolation and deformation in an exhumed lithosphere-asthenosphere boundary. Earth and Planetary Science Letters, 274, 401-413, doi: 10.1016/j.epsl.2008.07.053.

Liang, Y., Schiemenz, A., Hesse, M.A. and Parmentier, E.M., 2011. Waves, channels, and the preservation of chemical heterogeneities during melt migration in the mantle. Geophysical Research Letters, 38, L20308, doi: 10.1029/2011GL049034.

Lissenberg, J.C., Dick, H.J.B., 2008. Melt-rock reaction in the lower oceanic crust and its implications for the genesis of mid-ocean ridge basalt. Earth and Planetary Science Letters, 271, 311-325, doi: 10.1016/j.epsl.2008.04.023.

Lissenberg, C.J., MacLeod, C.J., Howard, K.A. and Godard, M., 2013. Pervasive reactive melt migration through fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean). Earth and Planetary Science Letters, 361, 436-447, doi: 10.1016/j.epsl.2012.11.012.
1080

1081

1082

1083

1084

1085

1086

1087

1088

1089

1090

1091

1092

1093

1094

1095

1096

1097

1098

1099

1100

1101

1102

1103

1104

1105

1106

1107

1108

1109

1110

1111

1112

1113

1114

1115

1116

1117

1118

1119

1120

1121

1122

1123

1124

1125

1126

Mainprice, D., Tommasi, A., Couvy, H., Cordier, P. and Frost, D.J., 2005. Pressure sensitivity of olivine slip systems and seismic anisotropy of Earth's upper mantle. Nature, 433, 731-733, doi: 10.1038/nature03266.

Mainprice, D., Bachmann, F., Hielscher, R., Schaeben, H., 2014. Descriptive tools for the analysis of texture projects with large datasets using MTEX: strength, symmetry and components. Geological Society of London, Special Publications, 409, doi: 10.1144/SP1409.1148.

Manatschal, G. and Müntener, O., 2009. A type sequence across an ancient magma-poor ocean-continent transition: the example of the western Alpine Tethys ophiolites. Tectonophysics, 473, 4-19, doi: 10.1016/j.tecto.2008.07.021.

Müntener, O. and Piccardo, G.B., 2003. Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basin. In: Dilek, Y., Robinson, P.T. (Eds.), Ophiolites in Earth History: Geological Society of London Special Publication, 218, 69-89, doi: 0305-8719/03.

Müntener, O., Pettke, T., Desmurs, L., Meier, M. and Schaltegger, U., 2004. Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust-mantle relationships. Earth and Planetary Science Letters, 221, 293-308, doi: 10.1016/S0012-821X(04)00073-1.

Paquet, M., Cannat, M., Brunelli, D., Hamelin, C. and Humler, E., 2016. Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61 to 67°E). Geochemistry, Geophysics, Geosystems, 17, 4605-4640, doi: 10.1002/2016GC006385.

Pearce, N. J. G., Perkins, W. T., Westgate, J. a., Gorton, M. P., Jackson, S. E., Neal, C. R. and Chenery, S. P., 1997. A Compilation of New and Published Major and Trace Element Data for NIST SRM 610 and NIST SRM 612 Glass Reference Materials. Geostandards and Geoanalytical Research, 21, 115-144, doi: 10.1111/j.1751-908X.1997.tb00538.x.

Piccardo, G.B., Müntener, O., Zanetti, A., Romairone, A., Bruzzone, S., Poggi, E. and Spagnolo, A., 2004. The Lanzo south peridotite: Melt/peridotite interaction in the mantle lithosphere of the Jurassic Ligurian Tethys. Ofioliti, 29, 37-62, doi: 10.4454/ofioliti.v29i1.205.

Piccardo, G.B. and Vissers, R.L.M., 2007. The pre-oceanic evolution of the Erro-Tobbio peridotite (Voltri Massif - Ligurian Alps, Italy). Journal of Geodynamics, 43, 417-449, doi: 10.1016/j.jog.2006.11.001.

Piccardo, G.B., Zanetti, A. and Müntener, O., 2007. Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence. Lithos, 94, 181-209, doi: 10.1016/j.lithos.2006.07.002.

Piccardo, G.B., Guarnieri, L., 2010. The Monte Maggiore peridotite (Corsica, France): a case study of mantle evolution in the Ligurian Tethys. Geological Society of London, 337, 7-45, doi: 10.1144/SP337.2 0305-8719/10.
1127

1128

1129

1130

1131

1132

1133

1134

1135

1136

1137

1138

1139

1140

1141

1142

1143

1144

1145

1146

1147

1148

1149

1150

1151

1152

1153

1154

1155

1156

1157

1158

1159

1160

1161

1162

1163

1164

1165

1166

1167

1168

1169

1170

1171

1172

1173

1174

Pirard, C., Hermann, J. and O'Neill, H.ST.C., 2013. Petrology and geochemistry of the Crust-Mantle boundary in a nascent arc, Massif du Sud ophiolite, New Caledonia, SW Pacific. Journal of Petrology, 54, 1759-1792, doi: 10.1093/petrology/egt030.

Rampone, E., Piccardo, G.B., Vannucci, R. and Bottazzi, P., 1997. Chemistry and origin of trapped melts in ophiolitic peridotites. Geochimica et Cosmochimica Acta, 21, 4557-4569, doi: 0016-7037/97.

Rampone, E. and Piccardo, G.B., 2000. The ophiolite-oceanic lithosphere analogue: new insights from the Northern Apennines (Italy). In: Dilek Y, Moores EM, Elthon D, Nicolas A (eds) Ophiolites and oceanic crust: new insights from field studies and the oceanic drilling program. Geological Society of America Special Paper, Boulder, 349, 21-34, doi: 10.1130/0-8137-2349-3.21.

Rampone, E., Borghini, G., 2008. Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): Insights on magmatic processes in extending lithospheric mantle. European Journal of Mineralogy, 20, 573-585, doi: 10.1127/0935-1221/2008/0020-1807.

Rampone, E., Romairone, A. and Hofmann, A.W., 2004. Contrasting bulk and mineral chemistry in depleted mantle peridotites: evidence for reactive porous flow. Earth and Planetary Science Letters, 2018, 491-506, doi: 10.1016/S0012-821X(03)00679-4.

Rampone, E., Piccardo, G.B. and Hofmann, A.W., 2008. Multi-stage melt-rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence. Contributions to Mineralogy and Petrology, 156, 453-475. doi: 10.1007/s00410-008-0296-y.

Rampone, E., Borghini, G., Godard, M., Ildefonse, B., Crispini, L. and Fumagalli, P., 2016. Melt/rock reaction at oceanic peridotite/gabbro transition as revealed by trace element chemistry of olivine. Geochimica et Cosmochimica Acta, 190, 309-331, doi: 10.1016/j.gca.2016.06.029.

Renna, M.R., Tribuzio, R., 2011. Olivine-rich troctolites from Ligurian ophiolites (Italy): Evidence for impregnation of replacive mantle conduits by MORB-type melts. Journal of Petrology, 52, 1763-1790, doi: 10.1093/petrology/egr029.

Renna, M.R., Tribuzio, R. and Ottolini, L., 2016. New perspectives on the origin of olivine-rich troctolites and associated harrisites from the Ligurian ophiolites (Italy). Journal of the Geological Society, 174, doi: 10.1144/jgs2015-135.

Rosenberg, C.L. and Handy, M.R., 2005. Experimental deformation of partially melted granite revisited: implications for the continental crust. Journal of Metamorphic Geology, 23, 19-28, doi: 10.1111/j.1525-1314.2005.00555.x.

Sanfilippo, A. and Tribuzio, R., 2012. Building of the deepest crust at a fossil slow-spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites). Contributions to Mineralogy and Petrology, 165, 705-721, doi: 10.1007/s00410-012-0831-8.

Sanfilippo, A., Dick, H.J.B. and Ohara, Y., 2013. Melt-rock reaction in the Mantle: Mantle troctolites from the Parece Vela ancient back-arc spreading center. Journal of Petrology, 54, 861-885, doi: 10.1093/petrology/egs089.
1175

1176

1177

1178

1179

1180

1181

1182

1183

1184

1185

1186

1187

1188

1189

1190

1191

1192

1193

1194

1195

1196

1197

1198

1199

1200

1201

1202

1203

1204

1205

1206

1207

1208

1209

1210

1211

1212

1213

1214

1215

1216

1217

1218

1219

1220

1221

1222

Sanfilippo, A., Tribuzio, R. and Tiepolo, M., 2014. Mantle-crust interactions in the oceanic lithosphere: Constraints from minor and trace elements in olivine. Geochimica et Cosmochimica Acta, 141, 423-439, doi: 10.1016/j.gca.2014.06.012.

Sanfilippo, A., Morishita, T., Kumagai, H., Nakamura, K., Okino, K., Hara, K., Tamura, A. and Arai, S., 2015. Hybrid troctolites from Mid-Ocean Ridges: Inherited mantle in the lower crust, Lithos, 232, 124-130, doi: 10.1016/j.lithos.2015.06.025

Sanfilippo, A., Dick, H.J.B., Ohara, Y. and Tiepolo, M., 2016. New insights on the origin of troctolites from the breakaway area of the Godzilla Megamullion (Parece Vela back-arc basin): The role of melt-mantle interaction on the composition of the lower crust. Island arc, 25, 220-234, doi: 10.1111/iar.12137.

Sanfilippo, A., Tribuzio, R., Ottolini, L. and Hamada, M., 2017. Water, lithium and trace element compositions of olivine from Lanzo South replace mantle dunites (Western Alps): New constraints on melt migration processes at cold thermal regimes. Geochimica et Cosmochimica Acta, 214, 51-72, doi: 10.1016/j.gca.2017.07.034.

Saper, L., Liang, Y., 2014. Formation of plagioclase-bearing peridotite and plagioclase-bearing wehrlite and gabbro suite through reactive crystallization: an experimental study. Contributions to Mineralogy and Petrology, 167, 985, doi: 10.1007/s00410-014-0985-7.

Seyler, M., Lorand, J.P., Dick, H.J.B. and Drouin, M., 2007. Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15°20′N: ODP Hole 1274A. Contributions to Mineralogy and Petrology, 153, 303, doi: 10.1007/s00410-006-0148-6.

Soustelle, V., Tommasi, A., Bodinier, J.L., Garrido, C.J. and Vauchez, A., 2009. Deformation and reactive melt transport in the mantle lithosphere above a large-scale partial melting domain: the Ronda peridotite massif, Southern Spain. Journal of Petrology, 50, 1235-1266, doi: 10.1093/petrology/egp032.

Soustelle, V., Tommasi, A., Demouchy, S. and Ionov, D.A., 2010. Deformation and fluid-rock interaction in the supra-subduction mantle: microstructures and water contents in peridotite xenoliths from the Avacha volcano, Kamchatka. Journal of Petrology, 51, 363-394, doi: 10.1093/petrology/egp085.

Soustelle, V., Walte, N.P., Geeth, M.A., Manthilake, M. and Frost, D.J., 2014. Melt migration and melt-rock reactions in the deforming Earth's upper mantle: Experiments at high pressure and temperature. Geology, 42, 83-86, doi: 10.1130/G34889.1.

Spiegelman, M. and Kelemen, P.B., 2003. Extreme chemical variability as a consequence of channelized melt transport. Geochemistry, Geophysics, Geosystems, 4, 1055-1072, doi: 10.1029/2002GC000336.

Suhr, G., Hellebrand, E., Johnson, K. and Brunelli, D., 2008. Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D. Geochemistry, Geophysics, Geosystems, 9, Q10007, doi: 10.1029/2008GC002012.
1223

1224

1225

1226

1227

1228

1229

1230

1231

1232

1233

1234

1235

1236

1237

1238

1239

1240

1241

1242

1243

1244

1245

1246

1247

1248

1249

1250

1251

1252

1253

1254

1255

1256

1257

1258

1259

1260

1261

1262

1263

1264

1265

1266

1267

1268

1269

1270

Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society of London, Special Publications, 42, 313-345, doi: 10.1144/GSL.SP.1989.042.01.19.

Sun, C., Liang, Y., 2012. Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water and temperature. Contributions to Mineralogy and Petrology, 163, 807-823, doi: 10.1007/s00410-011-0700-x.

Sun, C., Liang, Y., 2013a. The importance of crystal chemistry on REE partitioning between mantle minerals (garnet, clinopyroxene, orthopyroxene, and olivine) and basaltic melts. Chemical Geology, 358, 23-36, doi: 10.1016/j.chemgeo.2013.08.045.

Sun, C., Liang, Y., 2013b. Distribution of REE and HFSE between low-Ca pyroxene and lunar picritic glass melts around multiple saturation points. Geochimica et Cosmochimica Acta, 119, 340-358, doi: 10.1016/j.gca.2013.05.036.

Sun, C., Liang, Y., 2014. An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites. Chemical Geology, 372, 80-91, doi: 10.1016/j.chemgeo.2014.02.014.

Tommasi, A., Tikoff, B. and Vauchez, A., 1999. Upper mantle tectonics: three-dimensional deformation, olivine crystallographic fabrics and seismic properties. Earth and Planetary Science Letters, 168, 173-186, doi: 10.1016/S0012-821X(99)00046-1.

Tommasi, A., Mainprice, D., Canova, G., Chastel, Y., 2000. Viscoplastic self-consistent and equilibrium-based modeling of olivine lattice preferred orientations: Implications for the upper mantle seismic anisotropy. Journal of Geophysical Research, 105, 7893-7908, doi: 10.1029/1999JB900411.

Tommasi, A., Godard, M., Coromina, G., Dautria, J.M. and Barsczus, H., 2004. Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: A petrological and microstructural study of mantle xenoliths from French Polynesia. Earth and Planetary Science Letters, 227, 539-556, doi: 10.1016/j.epsl.2004.09.019.

Tommasi, A., Vauchez, A. and Ionov, D.A., 2008. Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia). Earth and Planetary Science Letters, 272, 65-77, doi: 10.1016/j.epsl.2008.04.020.

Tursack, E., Liang, Y., 2012. A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations. Contributions to Mineralogy and Petrology, 163, 861-876, doi: 10.1007/s00410-011-0703-7.

Van Achterberg E., Griffin W.L. and Stiefenhofer J., 2001. Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes. Contribution to Mineralogy and Petrology, 141, 397-414, doi: 10.1007/s004100000236.

Van den Bleeken, G., Müntener, O., Ulmer, P., 2011. Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper
1271 mantle. Contribution to Mineralogy and Petrology, 161, 921-945, doi: 10.1007/s00410-010-0572-5.

1273

1274 Vernières, L., Godard, M. and Bodinier, J.L., 1997. A plate model for the simulation of trace element fractionation during partial melting and magma transport in the Earth's upper mantle. Journal of Geophysical Research, 102, 24771-24784, doi: 10.1029/97JB1946.

1277

1278 Warren, J.M., 2016. Global variations in Abyssal Peridotite Compositions. Lithos, 248-251, 193-219, doi: 10.1016/j.lithos.2015.12.023.

1280

1281 Workman, R.K. and Hart, S.R., 2005. Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters, 231, 53-72, doi: 10.1016/j.epsl.2004.12.005.
<figure><img src="image_3.png" /></figure>

Figure 3

<caption>Figure 3</caption>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>

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

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

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

<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /><figcaption>Figure 3</figcaption></figure>

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

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

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

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

<figure><img src="image_15.png" /><figcaption>D</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure 5: Granite M15/14</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Figure 9</figcaption></figure>

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

<figure><img src="image_22.png" /></figure>

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

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

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

<figure><img src="image_26.png" /></figure>
<figure><img src="image_27.png" /></figure>
<caption>Figure 11</caption>

Reactive melt percolation - Spinel facies

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>
<caption>Figure 12</caption>

Melt impregnation - Plagioclase facies

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>
<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>
<figure><img src="image_39.png" /><figcaption>Figure 14. ① Spinel peridotite</figcaption></figure>

② Spinel harzburgite

<figure><img src="image_40.png" /></figure>

③ Spinel dunite

<figure><img src="image_41.png" /></figure>

④ Olivine-rich troctolite

<figure><img src="image_42.png" /></figure>

⑤ Troctolite

<figure><img src="image_43.png" /></figure>

Reactive porous flow

Impregnation
<table><caption>**Table 1**
Click here to download Table: Table 1 Analyzed samples.docx</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Lithotype</th><th colspan="5">Modal composition</th><th colspan="4">Textural parameters</th></tr><tr><th>Olivine</th><th>Plagio</th><th>Cpx</th><th>Opx</th><th>Spinel</th><th>N° Grains</th><th>Grain area (μm²)</th><th>Aspect Ratio</th><th>Shape Factor</th></tr></thead><tbody><tr><td>M15/7</td><td>Sp. lherzolite</td><td>58</td><td>0</td><td>11</td><td>25</td><td>6</td><td>63</td><td>3.81E+06</td><td>1.64</td><td>2.46</td></tr><tr><td>M15/7B</td><td>Sp. lherzolite</td><td>62</td><td>0</td><td>13</td><td>23</td><td>2</td><td>44</td><td>8.04E+06</td><td>1.64</td><td>2.64</td></tr><tr><td>M15/13</td><td>Sp. harzburgite</td><td>78</td><td>0</td><td>2</td><td>20</td><td>0</td><td>40</td><td>8.47E+06</td><td>1.73</td><td>2.35</td></tr><tr><td>M15/15</td><td>Plag. lherzolite</td><td>74</td><td>0</td><td>10</td><td>15</td><td>1</td><td>42</td><td>9.71E+06</td><td>1.67</td><td>1.82</td></tr><tr><td>M15/12B</td><td>Plag. harzburgite</td><td>65</td><td>8</td><td>8</td><td>18</td><td>1</td><td>40</td><td>7.44E+06</td><td>1.69</td><td>2.29</td></tr><tr><td>M15/20A</td><td>Plag. harzburgite</td><td>58</td><td>17</td><td>1</td><td>24</td><td>0</td><td>28</td><td>6.21E+06</td><td>1.67</td><td>2.65</td></tr><tr><td>M15/18A</td><td>Plag. harzburgite</td><td>50</td><td>25</td><td>1</td><td>24</td><td>0</td><td>33</td><td>6.82E+06</td><td>1.52</td><td>2.04</td></tr><tr><td>M15/19</td><td>Plag. harzburgite</td><td>50</td><td>18</td><td>7</td><td>25</td><td>0</td><td>21</td><td>5.82E+06</td><td>1.56</td><td>2.19</td></tr><tr><td>M15/19B</td><td>Plag. harzburgite</td><td>48</td><td>15</td><td>6</td><td>31</td><td>0</td><td>37</td><td>5.73E+06</td><td>1.83</td><td>2.28</td></tr><tr><td>M15/3</td><td>Spinel dunite</td><td>97</td><td>0</td><td>0</td><td>0</td><td>3</td><td>12</td><td>3.81E+07</td><td>1.71</td><td>2.49</td></tr><tr><td>M15/5</td><td>Spinel dunite</td><td>99</td><td>0</td><td>0</td><td>0</td><td>1</td><td>51</td><td>6.54E+06</td><td>1.67</td><td>2.22</td></tr><tr><td>M15/14</td><td>Spinel dunite</td><td>99</td><td>0</td><td>0</td><td>0</td><td>1</td><td>24</td><td>1.91E+07</td><td>1.85</td><td>2.25</td></tr><tr><td>M15/9</td><td>Ol-rich troctolite</td><td>85</td><td>15</td><td>0</td><td>0</td><td>0</td><td>37</td><td>1.13E+07</td><td>1.59</td><td>2.24</td></tr><tr><td>M15/8A2</td><td>Ol-rich troctolite</td><td>82</td><td>17</td><td>0</td><td>0</td><td>1</td><td>39</td><td>1.13E+07</td><td>1.76</td><td>2.07</td></tr><tr><td>M15/1</td><td>Ol-rich troctolite</td><td>80</td><td>20</td><td>0</td><td>0</td><td>0</td><td>55</td><td>6.36E+06</td><td>1.71</td><td>1.89</td></tr><tr><td>M15/11</td><td>Ol-rich troctolite</td><td>78</td><td>20</td><td>0</td><td>0</td><td>2</td><td>51</td><td>7.58E+06</td><td>1.63</td><td>1.92</td></tr><tr><td>M15/8A3*</td><td>Troctolite</td><td>75</td><td>14</td><td>10</td><td>0</td><td>1</td><td>41</td><td>9.14E+06</td><td>1.43</td><td>1.92</td></tr><tr><td>M15/10A**</td><td>Troctolite</td><td>74</td><td>18</td><td>2</td><td>5</td><td>1</td><td>82</td><td>4.50E+06</td><td>1.54</td><td>1.86</td></tr><tr><td>M15/8B</td><td>Troctolite</td><td>73</td><td>25</td><td>1</td><td>0</td><td>1</td><td>44</td><td>9.25E+06</td><td>1.63</td><td>1.77</td></tr><tr><td>M15/8A</td><td>Troctolite</td><td>73</td><td>25</td><td>1</td><td>0</td><td>1</td><td>36</td><td>1.15E+07</td><td>1.54</td><td>1.86</td></tr><tr><td>M15/6</td><td>Troctolite</td><td>71</td><td>23</td><td>1</td><td>0</td><td>5</td><td>32</td><td>1.20E+07</td><td>1.71</td><td>1.91</td></tr><tr><td>M15/10B**</td><td>Troctolite</td><td>70</td><td>19</td><td>3</td><td>7</td><td>1</td><td>59</td><td>6.01E+06</td><td>1.59</td><td>2.11</td></tr></tbody></table>

<caption>**Table 1: Studied samples, modal compositions and mean textural parameters of the olivine matrix.**</caption>

<footnote>*clinopyroxene-bearing troctolite</footnote>

<footnote>**sample at the contact between plagioclase peridotite and troctolite</footnote>
Background dataset Figure S1

Click here to download Background dataset for online publication only: Figure S1 MORB melt.pdf
Background dataset Table S7

Click here to download Background dataset for online publication only: Table S7 Model parameters.xlsx