Melt transport mechanisms have an important impact on the structure and composition of percolated host rocks and migrating melts. Melt migration is usually assumed to occur by diffuse percolation along grain boundaries or by focused migration in veins and dikes (e.g. Sparks and Parmentier, 1991; Kelemen et al., 1997). However, transmission electron microscope (TEM) studies revealed the occurrence of melt pockets along dislocations and subgrain boundary networks or microcracks in single mineral grains, suggesting that melts can migrate within deformed crystals (Drury and Van Roermund, 1989; De Kloe, 2001). Diffusion between crystals and melts is more efficient than solid–solid diffusion (e.g. Costa et al., 2008, 2020; Zhang and Cherniak, 2010). Intracrystalline melt transport is thus expected to affect chemical re-equilibration at the grain scale, leading to fast re-equilibration between the percolated matrix and migrating melts (Drury and Van Roermund, 1989). In turn, diffusive re-equilibration can lead to significant mineral trace element enrichments when associated with reactive melt transport (e.g. Sanfilippo et al., 2014; Rampone et al., 2016; Basch et al., 2018; Ferrando et al., 2020).

Recent studies demonstrated that melt–rock interactions and reactive porous flow are key processes in the global geochemical budget of the oceanic lithosphere and in the formation of the lower oceanic crust. During melt percolation within the mantle, reactive dissolution–precipitation processes between melt and mineral matrix can lead to significant chemical modification of migrating melts (Collier and Kelemen, 2010; Saper and Liang, 2014; Rampone et al., 2016; Basch et al., 2019a; Ferrando et al., 2021). Additionally, harzburgite–dunite mantle slivers can be incorporated at the base of the oceanic crust and transformed, by melt impregnation, into “hybrid” (i.e. mantle-derived) olivine-rich gabbroic rocks (Suhr et al., 2008; Drouin et al., 2009; Higgie and Tommasi, 2012; Sanfilippo et al., 2014; Basch et al., 2018, 2019b; Ferrando et al., 2018, 2020).

Reactive melt percolation has been extensively documented in oceanic environments characterized by a thick thermal lithosphere and thermal boundary layer, namely slow- and ultra-slow-spreading ridges. In these cold spreading environments, melts are produced at depth and need to percolate through a thick mantle column before reaching crystallization depths (e.g. Hebert and Montési, 2010; Sleep and Warren, 2014; Basch et al., 2019a, b; Rampone et al., 2020). These large-scale percolation processes enhance the reactivity of the melt towards the host mantle rock (e.g. Liang, 2003; Morgan and Liang, 2005) and lead to extensive impregnation of the mantle section (Sleep and Warren, 2014; Rampone et al., 2020). Accordingly, slow-spreading environments are ideal settings to study processes of melt migration and their implication in the composition of the oceanic lithosphere. The Alpine–Apennine ophiolites are key analogues of lithosphere formed in ultra-slow-spreading ridges and hyperextended passive margin settings (e.g. Manatschal and Müntener, 2009; Rampone et al., 2014, 2020; Piccardo, 2016; Rampone and Sanfilippo, 2021). These ophiolites are predominantly constituted of mantle peridotites, which record various stages of melt–rock interaction that occurred at different mantle depths (e.g. Rampone and Borghini, 2008; Piccardo and Guarnieri, 2010; Basch et al., 2019a; Rampone et al., 2020).

In this contribution, we investigate the trace element compositions of olivine in olivine-rich troctolites primarily associated with the Erro-Tobbio ophiolitic mantle peridotites (Voltri Massif, Ligurian Alps, Italy; Fig. 1). Basch et al. (2019b) inferred that these olivine-rich troctolites have a multi-stage replacive origin, involving reactive percolation and interaction of MORB-type (mid-ocean ridge basalt) melt with mantle dunite. Within the replacive troctolite, the olivine matrix is characterized by two generations of olivine, namely (i) coarse deformed olivines (olivine1) and (ii) fine-grained undeformed olivines (olivine2). Rampone et al. (2016) documented a decoupling between olivine texture and its trace element composition, with substantial trace element enrichments in olivine1, representing relicts of the pre-existing mantle dunite matrix. In this paper, we show that these trace element enrichments result from a process of intracrystalline melt migration within deformed olivine1 crystals, as evidenced by the occurrence of oriented polyphase solid inclusions.

The Erro-Tobbio ultramafic body pertains to the Alpine–Apennine ophiolites (Fig. 1; see Basch et al., 2019b). These ophiolites represent the lithospheric remnants of the narrow oceanic basin of the Jurassic Ligurian Tethys, which opened by passive extension and breakup of the Europe–Adria continental lithosphere, followed by slow-spreading oceanization (e.g. Manatschal and Müntener, 2009; Rampone et al., 2020).

The Erro-Tobbio peridotites preserve microstructures and geochemical compositions indicative of a composite melt–rock interaction history related to the exhumation of this mantle section from spinel to plagioclase facies depths to shallow oceanic environments (Rampone et al., 2004, 2005, 2016, 2020; Borghini and Rampone, 2007; Borghini et al., 2007; Piccardo and Vissers, 2007; Rampone and Borghini, 2008; Basch et al., 2019b). The plagioclase-bearing impregnated mantle peridotites are primarily associated with a hectometre-size body of olivine-rich troctolites and crosscutting gabbroic dikes (Borghini et al., 2007; Borghini and Rampone, 2007; Rampone and Borghini, 2008; Rampone et al., 2016; Basch et al., 2019b), interpreted as early magmatic episodes within the thinned lithospheric mantle in ocean–continent transition settings (e.g. Manatschal and Müntener, 2009; Basch et al., 2019b).

Within the Erro-Tobbio troctolitic body, clear structural relationships provide a good field control on melt migration and intrusion processes (see Basch et al., 2019b). Namely, the troctolitic body is formed of a host troctolite (Troctolite A) crosscut by a second generation of troctolitic metre-size bodies (Troctolite B). The host troctolite has been interpreted to be of replacive origin, i.e. formed by melt–rock interaction processes during reactive melt percolation within the lithospheric mantle. Subsequent focused melt percolation and melt intrusions formed the Troctolite B and gabbroic dikes, respectively (see Fig. 19 in Basch et al., 2019b). The replacive formation of the host Troctolite A involved the impregnation of a deformed dunitic matrix, i.e. partial dissolution of olivine and concomitant crystallization of interstitial plagioclase and clinopyroxene (ol1 ± sp1 + melt1 = ol2 + pl2 ± cpx2+ melt2). Partial dissolution of the pre-existing dunite was accompanied by extensive recrystallization of the olivine matrix. This is clearly evidenced by electron backscatter diffraction (EBSD) analyses indicating two generations of olivine in the replacive troctolite (Fig. 2a; Basch et al., 2019b), namely (i) coarse corroded deformed olivine, representing relicts of the pre-existing mantle dunite matrix (olivine1; Fig. 2a, b), and (ii) chadacrystic fine-grained euhedral undeformed olivine, a product of the melt–rock interaction process (olivine2; Fig. 2a, c). Notably, the latter fine-grained olivine is not necessarily crystallized from the melt involved in the dissolution–precipitation process leading to the formation of the replacive Troctolite A; rather, they result from the dismembering of coarse-grained olivine1 during their partial dissolution (see Fig. 15 in Basch et al., 2019b, for more detail on the evolution of olivine textures and associated crystallographic preferred orientations during progressive melt–rock interaction).

In the Erro-Tobbio troctolitic body, the extensive dissolution–precipitation processes that formed the host troctolite led to the progressive modification of the residual melt composition, as modelled by Basch et al. (2019b) and Rampone et al. (2016) for major and trace element compositions, respectively. Rampone et al. (2016) modelled the impact of the reactive crystallization process on the melt composition and demonstrated that the replacive formation of the troctolite was associated with strong trace element enrichments in the residual melt. Remarkably, Rampone et al. (2016) reported an apparent decoupling between texture and trace element chemical signature of olivine, documenting enriched trace element compositions in the coarse-grained deformed olivines (olivine1) rather than in the fine-grained euhedral olivines (olivine2). In this contribution, we aim to unravel the origin of the decoupling between olivine texture and trace element composition and assess its relationship to grain-scale melt migration processes.

Because of the replacive character of the troctolites, the composition of the pre-existing matrix of mantle olivine1 has been partially re-equilibrated towards an equilibrium composition with the percolating melt; as a result, olivines forming the matrix show major element and compatible to moderately incompatible element compositions fitting intermediately between the compositions of olivine in the peridotites and gabbros (Rampone et al., 2016). Concomitantly, dissolution–precipitation led to specific enrichments in highly incompatible trace elements in the residual melt and re-equilibrated minerals (see Rampone et al., 2016).

In order to compare the trace element enrichments in the olivine of Troctolites A with the compositions of precursor mantle crystals and magmatic olivine in gabbros, we included in this work the trace element compositions of olivines in spinel and plagioclase lherzolites, dunites, olivine-rich troctolites, and gabbroic dikes. The list of studied samples and the whole olivine trace element dataset are reported in Supplement Tables S1 and S2, respectively. Further information about the major and trace element mineral composition of studied samples are found in Rampone et al. (2016) and Basch et al. (2019b). Laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) trace element analyses of olivine have been combined with field emission scanning electron microscope (FE-SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) analyses of micron-size elongated inclusions occurring within deformed olivine1. Detailed methodologies are given in the Supplement.

Within the Erro-Tobbio replacive troctolite (Troctolite A), the pre-existing matrix of olivine1 is characterized by coarse-grained, up to centimetre-size, olivine crystals (Fig. 2). These olivines show intense deformation with the occurrence of numerous parallel subgrain boundaries (Fig. 2a, b, c). Lobate contacts against interstitial plagioclase and clinopyroxene (Fig. 2b, c) testify to the partial dissolution of olivine during the impregnation of the pre-existing dunite and the formation of the replacive troctolite (Basch et al., 2019b).

Microstructural investigation (FE-SEM, EBSD) of the deformed olivine crystals finds evidence of the presence of numerous parallel 10 to 50 µm long inclusions (Fig. 2d). These inclusions show a preferred orientation parallel to high-temperature subgrain boundaries of the host olivine (Fig. 2d). They show straight, planar shapes (Figs. 2d, 3a) and occur either at subgrain boundaries or parallel to olivine kink band and subgrain boundaries (Fig. 2d). Additionally, detailed EBSD misorientation maps of single olivine grains show that the inclusions are parallel to crystal dislocations of the host olivine (Fig. 3a). Late serpentine veinlets crosscut the olivine crystal and its inclusions.

SEM-EDS (EDS signifies energy-dispersive X-ray spectroscopy) and EBSD analyses revealed that the planar inclusions are solid and polyphase, mainly formed of amphibole, clinopyroxene and chromite (Figs. 3b, c, S1), with some pore space at the tip of the inclusion (Fig. 3b). The different mineral phases within the inclusions show different crystallographic orientations and do not show any systematic crystallographic orientation with respect to the host olivine (Fig. 3c). Within single inclusions, each phase shows a single orientation (Fig. 3c), suggesting that all portions of a given phase belong to a single interstitial crystal. FE-SEM backscattered electron (BSE) imaging and TEM high-angle annular dark-field (HAADF) images of single inclusions highlighted that they vary in thickness along the inclusion (Fig. 3b; Fig. 4a). Moreover, the walls of the inclusions and the contacts between the mineral phases forming the inclusions are irregular (Figs. 3b, 4a).

FE-SEM EDS and TEM EDX chemical analyses of the mineral phases forming the inclusions (Figs. 3d, 4b, S1) revealed that clinopyroxene has a Mg-rich diopsidic composition (Mg# = 87.3 mol %–92.8 mol %; Al2O3 = 2.33 wt %–3.77 wt %; TiO2=0.38–0.93 wt %; Supplement S3) and amphiboles are pargasites (Mg# =  86.6 mol %–89.7 mol %; Al2O3=10.13 wt %–13.13 wt %; Na2O = 3.50 wt %–4.60 wt %; TiO2= 0.28 wt %–0.58 wt %; Supplement Table S3). It is noteworthy that the pargasitic amphibole is associated with submicron rutile grains (Fig. 4c).

In this section, we document the trace element composition of olivine within all the studied lithologies, with a special focus on the textural complexity in the Troctolite A (olivine1 vs. olivine2). Olivine in situ trace element analyses were performed using a 102 µm laser ablation spot size. These analyses document the trace element composition of olivine crystals as “bulk analyses”, including the olivine-hosted inclusions documented in the previous section. Although a small spot size (10–20 µm) would amplify the contribution of the inclusion within the bulk olivine analyses, the determination of the low trace element concentrations of olivine required a large 102 µm spot size.

Olivines in spinel and plagioclase lherzolites show homogeneous compositions in terms of moderately incompatible elements (Ni, Mn, Zn, Co), consistent with the compositions reported in Alpine–Apennine impregnated lherzolite by Sanfilippo et al. (2014) and Rampone et al. (2016). They are characterized by high Ni (2815–3571 ppm; Fig. 5a) and relatively low Mn (1087–1167 ppm; Fig. 5b), Zn (29–61 ppm; Fig. 5c) and Co (141–158 ppm; Fig. 5d) concentrations. Olivines in olivine gabbros define a wide compositional range, well correlated with their Forsterite content. They exhibit a positive correlation between Forsterite content (81.3 mol %–89.2 mol %) and Ni concentrations (973–2356 ppm; Fig. 5a) and a negative correlation with Mn (1329–2198 ppm; Fig. 5b), Zn (56–112 ppm; Fig. 5c) and Co (132–198 ppm; Fig. 5d). All olivines (olivine1+ olivine2) in the olivine-rich troctolites display Ni (1627–2689 ppm; Fig. 5a), Mn (1340–1855 ppm; Fig. 5b), Zn (35–74 ppm; Fig. 5c) and Co (122–173 ppm; Fig. 5d) compositions that are intermediate between olivines in the olivine gabbros and in the spinel and plagioclase lherzolites.

Highly incompatible elements in olivine do not follow a linear trend of correlation with the Forsterite content between all lithotypes (Fig. 6). Olivines in spinel and plagioclase lherzolites show low HREE (heavy rare earth element) abundances (YbN=0.015–0.303; Fig. 6a; N= normalized to C1 chondrite based on Sun and McDonough, 1989) and low Y (0.008–0.08 ppm; Fig. 6b), Ti (11–61 ppm; Fig. 6c) and Zr concentrations (0.004–0.016 ppm; Fig. 6d), similar to the mantle olivine trace element concentrations reported by Demouchy and Alard (2021). Olivines in the olivine gabbros display modest variations of incompatible trace element abundances despite their significant range of Forsterite content (81.3 mol %–89.2 mol %). They show low HREE abundances (YbN=0.09–0.73; Fig. 6a) and low Y (0.018–0.128 ppm; Fig. 6b), Ti (43–172 ppm; Fig. 6c) and Zr concentrations (0.001–0.229 ppm; Fig. 6d), consistent with olivine trace element compositions reported in oceanic MORB settings (Ferrando et al., 2018) and in the Erro-Tobbio olivine gabbros (Rampone et al., 2016). On the other hand, olivines in olivine-rich troctolites and associated dunites show significant trace element variations at relatively constant Forsterite contents (Fo = 87.3 mol %–89.2 mol %). A clear correlation is observed between the olivine texture and trace element composition, with the strongest enrichments in HREE (YbN=0.23–0.91; Fig. 6a, e), Ti (51–349 ppm; Fig. 6b), Y (0.019–0.329 ppm; Fig. 6c) and Zr concentrations (0.001–0.694 ppm; Fig. 6d, e) documented in the most corroded and deformed olivine1. Fine-grained granular olivine2 also shows variable HREE (YbN=0.23–0.67; Fig. 6a, e), Ti (63–224 ppm; Fig. 6b), Y (0.023-0.196 ppm; Fig. 6c) and Zr (0.001–0.381; Fig. 6d) concentrations, although they show weaker trace element enrichments compared to the olivine1 (Fig. 6). Core-rim analyses, as well as geochemical profiles of up to five analyses within the largest olivine1 crystals, did not provide evidence of any systematic chemical zoning within olivine1 and olivine2.

The formation of the Erro-Tobbio olivine-rich troctolites involved extensive dissolution–precipitation reactions between a pre-existing dunitic matrix and a percolating MORB-type melt. This reactive melt migration process led to progressive trace element enrichments in the residual melt. Unexpectedly, this trace element enrichment is documented in coarse deformed olivines texturally corresponding to the pre-existing dunitic matrix. Interestingly, trace element enrichments in deformed olivine crystals are systematically correlated with the occurrence of 10 to 50 µm long elongated polyphase inclusions oriented parallel to the host olivine subgrain boundaries. These inclusions are mainly formed of amphibole, clinopyroxene and chromite and show irregular contacts with the host crystal and between the phases forming the inclusion. We interpret these inclusions as testifying to a process of intracrystalline melt migration, during which melts evolve to become residuals after the reactive crystallization of the olivine-rich troctolites percolated within the most deformed olivine crystals along subgrain boundaries and iso-oriented crystal dislocations. We infer that the presence of intracrystalline melt greatly enhanced the diffusive re-equilibration of the percolated crystals, thus acquiring the enriched character expected in neoformed olivine crystals. This grain-scale melt migration process has strong geochemical implications and can lead to the efficient re-equilibration of a percolated rock.