Geochemical evolution of melt/peridotite interaction at high pressure in subduction zones
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Keywords: melt rock reaction, garnet peridotite, garnet websterite, slab to mantle mass transfer
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Abstract
Figures
 Figure 1 (a) Geological sketch map of tremolitite layers in retrogressed Grt-peridotite at the contact with migmatitic eclogites. (1) Tremolitite showing sharp contact with the host retrogressed Grt-peridotite. (2) Tr + Phl layer displaying a diffusive contact with retrogressed Grt-peridotite. (b) Cross section of the peridotite host rock contact with sampling location of tremolitites and Chl-peridotites. |  Figure 2 Numerical simulation of melt-rock interaction at peridotite-eclogite interface and reactive percolation of melt through mantle peridotite (a) PM normalised REE patterns of liquids resulting from interaction between mantle peridotite and the initial liquid, assuming high peridotite assimilation (60 % Ol, 20 % Opx, 12 % Cpx, 3 % Grt) and low crystallisation rate (60 % Cpx, 40 % Grt). After 11 interactions, we obtained a pattern that reproduces the LREE/HREE bulk composition of retrogressed Grt-websterite at the mantle-eclogite boundary, i.e. distance 0 in the profile of Figure 1. (b) Evolution of PM normalised REEs in the liquid originated by reactive percolation of reacted melt after Step 1 through the DMM peridotite. The model assumes a high infiltrating melt amount, low assimilation of olivine and high extent of crystallisation. Patterns with coloured squares (except light yellow) are the REE bulk compositions of tremolitites sampled in the Borgo outcrop within the first 30 metres from the migmatitic eclogite-peridotite contact (Figs. 1, S-1). Coloured lines refer to the progressive cells of the model. See the Supplementary Information for details of input parameters and further explanation. |  Figure 3 PM normalised La and Ce/Yb of tremolitite (same symbols as Fig. 2) along the section of Figure 1, compared to the results of the numerical simulation of reactive melt percolation assuming variable crystallisation rate of Opx, Cpx, Grt and Phl. ICR and FCR are Initial and Final Crystallisation Rate. Yellow symbols represent the preferred model. The grey line is the reference DMM. |
| Figure 1 | Figure 2 | Figure 3 |
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Introduction and Petrological Background
The fate of crust-derived melts at warm subduction zones and the transport mechanism of slab-derived components to the supra-subduction mantle is still a matter of debate. Some natural occurrences show that the migration of crust-derived melts into the mantle by porous flow is limited by instant reaction with the peridotites and the consequent production of metasomatic orthopyroxene (±clinopyroxene) and phlogopite hybrid rocks at the slab-mantle interface (e.g., Malaspina et al., 2006
Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006) Polyphase inclusions in garnet-orthopyroxenite (Dabie-Shan, China) as monitors for metasomatism and fluid-related trace element transfer in subduction zone peridotite. Earth and Planetary Science Letters 249, 173–187. https://doi.org/10.1016/j.epsl.2006.07.017
; Vrijmoed et al., 2013Vrijmoed, J.C., Austrheim, H., John, T., Hin, R.C., Corfu, F., Davies, G.R. (2013) Metasomatism in the ultrahigh-pressure Svartberget garnet-peridotite (Western Gneiss Region, Norway): implications for the transport of crust-derived fluids within the mantle. Journal of Petrology 54, 1815–1848. https://doi.org/10.1029/97JB01946
; Endo et al., 2015Endo, S., Mizukami, T., Wallis, S.R., Tamura, A., Arai, S. (2015) Orthopyroxene-rich rocks from the sanbagawa belt (SW Japan): fluid–rock interaction in the forearc slab– mantle wedge Interface. Journal of Petrology 56, 1113–1137. https://doi.org/10.1093/petrology/egv031
). Alternatively, the occurrence of a network of pyroxenite veins in metasomatised mantle xenoliths from arc lavas indicates that metasomatic melts may percolate the mantle by a mechanism of focused flow (e.g., Kepezhinskas et al., 1995Kepezhinskas, P.K., Defant, M.J., Drummond, M.S. (1995) Na metasomatism in the island-arc mantle by slab melt—peridotite interaction: evidence from mantle xenoliths in the North Kamchatka Arc. Journal of Petrology, 36, 1505–1527. https://doi.org/10.1093/oxfordjournals.petrology.a037263
; Arai et al., 2003Arai, S., Ishimaru, S., Okrugin, V.M. (2003) Metasomatized harzburgite xenoliths from Avacha volcano as fragments of mantle wedge of the Kamchatka arc: implication for the metasomatic agent. Island Arc 12, 233–246. https://doi.org/10.1046/j.1440-1738.2003.00392.x
). Melt-peridotite interaction via reactive porous flow has been largely studied in mantle samples from oceanic and subcontinental settings (e.g., Godard et al., 1995Godard, M., Bodinier, J.L., Vasseur, G. (1995) Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes: a chromato- graphic approach. Earth and Planetary Science Letters 133, 449–461. https://doi.org/10.1016/0012-821X(95)00104-K
; Ionov et al., 2005Ionov, D.A., Chanefo, I., Bodinier, J.L. (2005) Origin of Fe-rich lherzolites and wehrlites from Tok, SE Siberia by reactive melt per- colation in refractory mantle peridotites. Contributions to Mineralogy and Petrology 150, 335–353. https://doi.org/10.1007/s00410-005-0026-7
; Borghini et al., 2020Borghini, G., Rampone, E., Zanetti, A., Class, C., Fumagalli, P., Godard, M. (2020) Ligurian pyroxenite-peridotite sequences (Italy) and the role of melt-rock reaction in creating enriched-MORB mantle sources. Chemical Geology 532, 119252. https://doi.org/10.1093/petrology/egv031
). In comparison, we know little about the role of these interactions at high pressure (HP) and relatively low temperature (LT) such as those reached at the slab interface, even in warm subduction zones (e.g., Malaspina et al., 2006Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006) Polyphase inclusions in garnet-orthopyroxenite (Dabie-Shan, China) as monitors for metasomatism and fluid-related trace element transfer in subduction zone peridotite. Earth and Planetary Science Letters 249, 173–187. https://doi.org/10.1016/j.epsl.2006.07.017
; Scambelluri et al., 2006Scambelluri, M., Hermann, J., Morten, L., Rampone, E. (2006) Melt-versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): Clues from trace element and Li abundances. Contributions to Mineralogy and Petrology 151, 372–394. https://doi.org/10.1007/s00410-006-0064-9
; Pellegrino et al., 2020Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
).The Borgo outcrop of the Monte Duria area (Adula-Cima Lunga unit, Central Alps, Italy) is an ideal example of melt-peridotite interaction which occurred under a deformation regime at HP, that enabled the combination of porous and focused flow of eclogite-derived melts into Grt-peridotites (Pellegrino et al., 2020
Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
; mineral abbreviations from Warr, 2021Warr, L.N. (2021) IMA–CNMNC approved mineral symbols. Mineralogical Magazine 85, 291–320. https://doi.org/10.1180/mgm.2021.43
). In the Monte Duria area Grt-peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both eclogites and peridotites record a common HP peak at 2.8 GPa and 750 °C and post-peak high temperature (HT) equilibration at 0.8–1.0 GPa and 850 °C (Tumiati et al., 2018Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S., Malaspina, N. (2018) Granulite-facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): clues from srilankite-and sapphirine-bearing symplectites. Journal of Petrology 59, 115–151. https://doi.org/10.1093/petrology/egy021
). Grt-peridotites show metasomatic pargasitic to edenitic amphibole, dolomite, phlogopite and orthopyroxene after olivine, crystallised after the interaction with crust-derived agents enriched in SiO2, K2O, CO2 and H2O at peak conditions (Tumiati et al., 2018Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S., Malaspina, N. (2018) Granulite-facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): clues from srilankite-and sapphirine-bearing symplectites. Journal of Petrology 59, 115–151. https://doi.org/10.1093/petrology/egy021
; Pellegrino et al., 2020Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
). They also show a “spoon-shape” fractionation in LREE (LaN/NdN = 2.4) related to the LREE enrichment in clinopyroxene and amphibole crystallised in the garnet stability field, interpreted by Pellegrino et al. (2020)Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
as acquired by the interaction with a hydrous melt.At Borgo, migmatised eclogites are in direct contact with retrogressed Grt-peridotites showing a garnet compositional layering, cross cut by a subsequent low pressure (LP) chlorite foliation (Pellegrino et al., 2020
Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
). Eclogite boudins enclosed in migmatites show thin films and interstitial pockets of crystallised melts parallel to the eclogite foliation, indicating that partial melting of eclogites occurred at HP conditions (Pellegrino et al., 2020Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
). Tremolitites occur both at the peridotite/eclogite contact and within the peridotite body and derived from the retrogression of previous Grt-websterites formed after the interaction at HP between eclogite-sourced melts and peridotites (Pellegrino et al., 2020Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
). Both peridotites and tremolitites record a LREE enrichment acquired during the melt/peridotite interaction at HP, decoupled by the selective enrichment in large ion lithophile elements (LILE) acquired by late stage tremolitic hornblende and tremolite crystallisation during a hydration event at LP conditions (Pellegrino et al., 2020Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
). This fluid-assisted event led to the formation of a chlorite foliation post-dating the garnet layering in peridotites, and the retrogression of Grt-websterites into tremolitites (Fig. 1a).Figure 1 (a) Geological sketch map of tremolitite layers in retrogressed Grt-peridotite at the contact with migmatitic eclogites. (1) Tremolitite showing sharp contact with the host retrogressed Grt-peridotite. (2) Tr + Phl layer displaying a diffusive contact with retrogressed Grt-peridotite. (b) Cross section of the peridotite host rock contact with sampling location of tremolitites and Chl-peridotites.
In this work we aim to resolve the role of melt/peridotite interaction at HP conditions in the selective REE transfer from a proxy of subducting slab (eclogitic migmatite) to the overlying mantle (associated retrogressed Grt-peridotite). Percolative reactive flow at decreasing melt/mass and high instantaneous melt/peridotite ratios, combined with moderate extents of fractional crystallisation of the resulting melt (retrogressed Grt-websterites), accounts for an overall REE enrichment and LREE/HREE fractionation observed in the natural example of the Borgo outcrop.
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Field Description and Geochemistry
Tremolitites and Phl-bearing tremolitites occur as bright green layers, a few centimetres up to several tens of cm thick, at the eclogite/peridotite contact and within the peridotite, displaying a boudinage (Fig. 1). The peridotite Grt-layering flows into the necks of the boudins, indicating that the stretching of the tremolitites (previous Grt-websterite) occurred when the peridotites were still in the garnet stability field. A detailed microstructural description is reported in the Supplementary Information (Fig. S-1, Table S-1 and S-2).
We analysed the major and trace element composition of 16 samples (Table S-3) collected along a profile of about 120 m, starting from the eclogite-peridotite contact to the inner part of the peridotite lens (Fig. 1b, Table S-1). As shown in Figure S-2a, the composition of Chl-peridotites displays Mg# (90), high Ni (2097–2518 μg/g), and low Al2O3 (0.77–3.50 wt. %) and CaO (0.62–2.60 wt. %) concentrations. Tremolitites show high Mg# (0.91) and Ni (up to 1390 μg/g) plotting into the field of the ultramafic compositions in the Mg#-Ni variation diagram, with a marked difference with respect to Grt- and Spl-pyroxenites of subcontinental ophiolites (e.g., External Ligurides ophiolites; Montanini et al., 2012
Montanini, A., Tribuzio, R., Thirlwall, M. (2012) Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material. Earth and Planetary Science Letters 351-352, 171–181. https://doi.org/10.1016/j.epsl.2012.07.033
). The Al2O3 concentrations are comparable to mantle values (Table S-3, Fig. S-2b), but they show high SiO2 (up to 57.50 wt. %), high CaO (up to 13.34 wt. %) and Al2O3 versus SiO2/MgO (Fig. S-2b) close to the composition of metasomatic Grt-orthopyroxenites and websterites from Dabie-Shan, which were formed after the interaction of Grt-harzburgites with Si-rich crust-derived melts at UHP (Malaspina et al., 2006Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006) Polyphase inclusions in garnet-orthopyroxenite (Dabie-Shan, China) as monitors for metasomatism and fluid-related trace element transfer in subduction zone peridotite. Earth and Planetary Science Letters 249, 173–187. https://doi.org/10.1016/j.epsl.2006.07.017
).The bulk rock trace element compositions of the analysed peridotites and tremolitites are portrayed in Figure S-3, normalised to the Primitive Mantle (PM). The trace element patterns of the Depleted MORB Mantle (DMM; Salters and Stracke, 2004
Salters, V.J.M., Stracke, A. (2004) Composition of the depleted mantle. Geochemistry Geophysics Geosystems 5. https://doi.org/10.1029/2003GC000597
), of Grt-peridotites from Mt. Duria (same area as Borgo) and the REE patterns of subcontinental Grt-pyroxenites from the External Ligurides are also reported for comparison. Peridotites show REE concentrations close to or slightly lower than the DMM, with fractionated patterns enriched in LREE (LaN/NdN up to 1.07) relative to the MREE and HREE (Fig. S-3a). Tremolitites have REE concentrations up to 4.63 × PM with enrichments in MREE and LREE/HREE two orders of magnitude higher than subcontinental pyroxenites from External Ligurides (Fig. S-3b). Both peridotites close to the contact with the migmatised eclogites, and tremolitites display a negative Eu anomaly, resembling that of eclogite leucosome (dark diamonds in Figs. 2, S-3) produced from a Pl-bearing source. In terms of other trace elements, Chl-peridotites and tremolitites show similar LILE patterns and fluid immobile element (Nb, Zr, Hf) concentrations (Table S-3 and Fig. S-3c,d). Among the fluid mobile elements, the selective enrichment in LILE recorded by the peridotites and tremolitites (Fig. S-3c) resembles that of retrograde tremolitic hornblende, chlorite and tremolite reported by Pellegrino et al. (2020)Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
, indicating that a fluid-mediated metasomatism occurred at LP conditions.top
REE Geochemical Evolution in Grt-websterites and Peridotites at HP
Figure S-4 portrays selected REE (La, Ce, Sm, Yb) and fluid mobile elements (Sr) of peridotites and tremolitites along a 120 m long profile from the migmatised eclogite-peridotite contact (0 m) to the innermost part of the peridotite lens (Fig. 1). Tremolitites show a progressive increase in LREE abundances from the contact up to about 30 m within the adjacent peridotite (Fig. S-4a-c). Among the fluid mobile elements, both tremolitites and peridotites show a progressive depletion in Pb from the contact to 80 m within the peridotite body (Fig. S-3c,d), whereas Sr is almost constant (Fig. S-4e). As demonstrated by Pellegrino et al. (2020)
Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
, the LREE enrichment and fractionation of the peridotites was most likely generated by the infiltration and interaction at HP of the eclogite-derived melt (leading to the emplacement of pristine Grt-websterites), rather than to the LP fluid-assisted metasomatic event that led to the retrogression of Grt-peridotites and websterites into Chl-peridotites and tremolitites, respectively. The reactive percolation of the eclogite-derived melt through the peridotite may be, in turn, responsible for the LREE-HREE fractionation (Ce/Yb), observed in the retrogressed Grt-websterite (now tremolitite) bulk rocks within the first 30 m of peridotite (Figs. 2, S-3; normalisation values are from McDonough and Sun (1995)McDonough, W.F., Sun, S.S. (1995) Composition of the Earth. Chemical Geology 120, 223–253. https://doi.org/10.1016/0009-2541(94)00140-4
.Figure 2 Numerical simulation of melt-rock interaction at peridotite-eclogite interface and reactive percolation of melt through mantle peridotite (a) PM normalised REE patterns of liquids resulting from interaction between mantle peridotite and the initial liquid, assuming high peridotite assimilation (60 % Ol, 20 % Opx, 12 % Cpx, 3 % Grt) and low crystallisation rate (60 % Cpx, 40 % Grt). After 11 interactions, we obtained a pattern that reproduces the LREE/HREE bulk composition of retrogressed Grt-websterite at the mantle-eclogite boundary, i.e. distance 0 in the profile of Figure 1. (b) Evolution of PM normalised REEs in the liquid originated by reactive percolation of reacted melt after Step 1 through the DMM peridotite. The model assumes a high infiltrating melt amount, low assimilation of olivine and high extent of crystallisation. Patterns with coloured squares (except light yellow) are the REE bulk compositions of tremolitites sampled in the Borgo outcrop within the first 30 metres from the migmatitic eclogite-peridotite contact (Figs. 1, S-1). Coloured lines refer to the progressive cells of the model. See the Supplementary Information for details of input parameters and further explanation.
To test this hypothesis, we have numerically simulated the REE gradient applying the Plate Model of Vernieres et al. (1997)
Vernieres, J., Godard, M., 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. https://doi.org/10.1029/97JB01946
using the REE composition of the eclogite leucosome of Pellegrino et al. (2020)Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
as starting melt and the DMM as peridotite matrix. In the first step (Fig. 2a), the crustal melt derived from partial melting of eclogites (black diamonds) reacts with the peridotite (grey line) at the eclogite-peridotite interface. The liquids resulting from increasing interactions (violet to yellow lines of Fig. 2a) are assumed to form veins of websterites (see also Fig. S-5) that are supposed to be the protoliths of tremolitites preserving their original REE composition. After several interactions, the final reacted melt crystallised at the eclogite-peridotite contact likely forms the first websterite DB177, showing a diffusive contact with retrogressed Grt-peridotite (Figs. 1a, 2a). Notably, we obtain a comparable result by using an adakitic melt (adakite2 from Corgne et al., 2018Corgne, A., Schilling, M.E., Grégoire, M., Langlade, J. (2018) Experimental constraints on metasomatism of mantle wedge peridotites by hybridized adakitic melts. Lithos, 308, 213–226. https://doi.org/10.1016/j.lithos.2018.03.006
), as starting melt (see Fig. S-6a). After the interaction at the eclogite-peridotite contact, the reacted melt infiltrates the peridotite producing the REE gradient observed in the tremolitites profile. We assume an initial peridotite porosity of 20 % (Fig. S-6b) that reflects a high peridotite assimilation coupled with the progressive melt consumption through melt-peridotite reaction during the percolation (i.e. a high extent of transient melt crystallisation), simulated by the progressive increase in crystallisation rate of 50 % Opx, 20 % Cpx, 20 % Grt, 10 % Phl (Figs. 2b, S-5). Figure 3 reports the LaN and CeN/YbN resulting from Step 2 calculations (yellow squares) compared with those measured in our tremolitite (coloured squares) along the first 30 m from the eclogite-peridotite contact. Grey scale symbols show the sensitivity of the model to the different extents of initial and final crystallisation rate of the percolating melts in the fractionation of LREE/HREE.Figure 3 PM normalised La and Ce/Yb of tremolitite (same symbols as Fig. 2) along the section of Figure 1, compared to the results of the numerical simulation of reactive melt percolation assuming variable crystallisation rate of Opx, Cpx, Grt and Phl. ICR and FCR are Initial and Final Crystallisation Rate. Yellow symbols represent the preferred model. The grey line is the reference DMM.
The numerical simulation aims to model the effect of interaction between crust-derived melts produced by partial melting of mafic components of the slab with the supra-subduction mantle peridotite at sub-arc depths (3 GPa, 750 °C). This includes a first step of crustal magma stagnation and melt-peridotite reaction at the slab-mantle interface and the following metre-scale percolation of reacted melt within the overlying peridotite that buffers the composition of the infiltrating melt. The comparison between the REE composition of the retrogressed Grt-websterites along the profile and the result of our model suggests that reactive melt infiltration at HP is a plausible mechanism to modify the REE budged of mantle peridotites that lie on top of the subducting crustal slab. Samples from those settings tend to show peculiar LREE “spoon-like” fractionations (e.g., Scambelluri et al., 2006
Scambelluri, M., Hermann, J., Morten, L., Rampone, E. (2006) Melt-versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): Clues from trace element and Li abundances. Contributions to Mineralogy and Petrology 151, 372–394. https://doi.org/10.1007/s00410-006-0064-9
). Moreover, the melt/peridotite interaction and the percolation of slab-derived melts into the overlying mantle may strongly modify the overall REE abundance and LREE/HREE fractionation (e.g., CeN/YbN) of the residual crustal melt within the first 30 m of slab/mantle interface.top
Acknowledgements
We thank C. Prigent and J.C.Vrijimoed for constructive reviews and H.R. Marschall for editorial handling. This work was funded by the Italian Ministry of University and Research (PRIN 2017 - Prot. 2017ZE49E7_005 - The Dynamic Mass Transfer from Slabs to Arcs).
Editor: Horst R. Marschall
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References
Arai, S., Ishimaru, S., Okrugin, V.M. (2003) Metasomatized harzburgite xenoliths from Avacha volcano as fragments of mantle wedge of the Kamchatka arc: implication for the metasomatic agent. Island Arc 12, 233–246. https://doi.org/10.1046/j.1440-1738.2003.00392.x
Show in context Alternatively, the occurrence of a network of pyroxenite veins in metasomatised mantle xenoliths from arc lavas indicates that metasomatic melts may percolate the mantle by a mechanism of focused flow (e.g., Kepezhinskas et al., 1995; Arai et al., 2003).
View in article
Borghini, G., Rampone, E., Zanetti, A., Class, C., Fumagalli, P., Godard, M. (2020) Ligurian pyroxenite-peridotite sequences (Italy) and the role of melt-rock reaction in creating enriched-MORB mantle sources. Chemical Geology 532, 119252. https://doi.org/10.1093/petrology/egv031
Show in context Melt-peridotite interaction via reactive porous flow has been largely studied in mantle samples from oceanic and subcontinental settings (e.g., Godard et al., 1995; Ionov et al., 2005; Borghini et al., 2020).
View in article
Corgne, A., Schilling, M.E., Grégoire, M., Langlade, J. (2018) Experimental constraints on metasomatism of mantle wedge peridotites by hybridized adakitic melts. Lithos, 308, 213–226. https://doi.org/10.1016/j.lithos.2018.03.006
Show in context Notably, we obtain a comparable result by using an adakitic melt (adakite2 from Corgne et al., 2018), as starting melt (see Fig. S-6a).
View in article
Endo, S., Mizukami, T., Wallis, S.R., Tamura, A., Arai, S. (2015) Orthopyroxene-rich rocks from the sanbagawa belt (SW Japan): fluid–rock interaction in the forearc slab– mantle wedge Interface. Journal of Petrology 56, 1113–1137. https://doi.org/10.1093/petrology/egv031
Show in context Some natural occurrences show that the migration of crust-derived melts into the mantle by porous flow is limited by instant reaction with the peridotites and the consequent production of metasomatic orthopyroxene (±clinopyroxene) and phlogopite hybrid rocks at the slab-mantle interface (e.g., Malaspina et al., 2006; Vrijmoed et al., 2013; Endo et al., 2015).
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Godard, M., Bodinier, J.L., Vasseur, G. (1995) Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes: a chromato- graphic approach. Earth and Planetary Science Letters 133, 449–461. https://doi.org/10.1016/0012-821X(95)00104-K
Show in context Melt-peridotite interaction via reactive porous flow has been largely studied in mantle samples from oceanic and subcontinental settings (e.g., Godard et al., 1995; Ionov et al., 2005; Borghini et al., 2020).
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Ionov, D.A., Chanefo, I., Bodinier, J.L. (2005) Origin of Fe-rich lherzolites and wehrlites from Tok, SE Siberia by reactive melt per- colation in refractory mantle peridotites. Contributions to Mineralogy and Petrology 150, 335–353. https://doi.org/10.1007/s00410-005-0026-7
Show in context Melt-peridotite interaction via reactive porous flow has been largely studied in mantle samples from oceanic and subcontinental settings (e.g., Godard et al., 1995; Ionov et al., 2005; Borghini et al., 2020).
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Kepezhinskas, P.K., Defant, M.J., Drummond, M.S. (1995) Na metasomatism in the island-arc mantle by slab melt—peridotite interaction: evidence from mantle xenoliths in the North Kamchatka Arc. Journal of Petrology, 36, 1505–1527. https://doi.org/10.1093/oxfordjournals.petrology.a037263
Show in context Alternatively, the occurrence of a network of pyroxenite veins in metasomatised mantle xenoliths from arc lavas indicates that metasomatic melts may percolate the mantle by a mechanism of focused flow (e.g., Kepezhinskas et al., 1995; Arai et al., 2003).
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Malaspina, N., Hermann, J., Scambelluri, M., Compagnoni, R. (2006) Polyphase inclusions in garnet-orthopyroxenite (Dabie-Shan, China) as monitors for metasomatism and fluid-related trace element transfer in subduction zone peridotite. Earth and Planetary Science Letters 249, 173–187. https://doi.org/10.1016/j.epsl.2006.07.017
Show in context Some natural occurrences show that the migration of crust-derived melts into the mantle by porous flow is limited by instant reaction with the peridotites and the consequent production of metasomatic orthopyroxene (±clinopyroxene) and phlogopite hybrid rocks at the slab-mantle interface (e.g., Malaspina et al., 2006; Vrijmoed et al., 2013; Endo et al., 2015).
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In comparison, we know little about the role of these interactions at high pressure (HP) and relatively low temperature (LT) such as those reached at the slab interface, even in warm subduction zones (e.g., Malaspina et al., 2006; Scambelluri et al., 2006; Pellegrino et al., 2020).
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The Al2O3 concentrations are comparable to mantle values (Table S-3, Fig. S-2b), but they show high SiO2 (up to 57.50 wt. %), high CaO (up to 13.34 wt. %) and Al2O3 versus SiO2/MgO (Fig. S-2b) close to the composition of metasomatic Grt-orthopyroxenites and websterites from Dabie-Shan, which were formed after the interaction of Grt-harzburgites with Si-rich crust-derived melts at UHP (Malaspina et al., 2006).
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McDonough, W.F., Sun, S.S. (1995) Composition of the Earth. Chemical Geology 120, 223–253. https://doi.org/10.1016/0009-2541(94)00140-4
Show in context The reactive percolation of the eclogite-derived melt through the peridotite may be, in turn, responsible for the LREE-HREE fractionation (Ce/Yb), observed in the retrogressed Grt-websterite (now tremolitite) bulk rocks within the first 30 m of peridotite (Figs. 2, S-3; normalisation values are from McDonough and Sun (1995).
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Montanini, A., Tribuzio, R., Thirlwall, M. (2012) Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material. Earth and Planetary Science Letters 351-352, 171–181. https://doi.org/10.1016/j.epsl.2012.07.033
Show in context Tremolitites show high Mg# (0.91) and Ni (up to 1390 μg/g) plotting into the field of the ultramafic compositions in the Mg#-Ni variation diagram, with a marked difference with respect to Grt- and Spl-pyroxenites of subcontinental ophiolites (e.g., External Ligurides ophiolites; Montanini et al., 2012).
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Pellegrino, L., Malaspina, N., Zanchetta, S., Langone, A., Tumiati, S. (2020) High pressure melting of eclogites and metasomatism of garnet peridotites from Monte Duria Area (Central Alps, N Italy): A proxy for melt-rock reaction during subduction. Lithos 358-359, 105391. https://doi.org/10.1016/j.lithos.2020.105391
Show in context In comparison, we know little about the role of these interactions at high pressure (HP) and relatively low temperature (LT) such as those reached at the slab interface, even in warm subduction zones (e.g., Malaspina et al., 2006; Scambelluri et al., 2006; Pellegrino et al., 2020).
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The Borgo outcrop of the Monte Duria area (Adula-Cima Lunga unit, Central Alps, Italy) is an ideal example of melt-peridotite interaction which occurred under a deformation regime at HP, that enabled the combination of porous and focused flow of eclogite-derived melts into Grt-peridotites (Pellegrino et al., 2020; mineral abbreviations from Warr, 2021).
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Grt-peridotites show metasomatic pargasitic to edenitic amphibole, dolomite, phlogopite and orthopyroxene after olivine, crystallised after the interaction with crust-derived agents enriched in SiO2, K2O, CO2 and H2O at peak conditions (Tumiati et al., 2018; Pellegrino et al., 2020).
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They also show a “spoon-shape” fractionation in LREE (LaN/NdN = 2.4) related to the LREE enrichment in clinopyroxene and amphibole crystallised in the garnet stability field, interpreted by Pellegrino et al. (2020) as acquired by the interaction with a hydrous melt.
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At Borgo, migmatised eclogites are in direct contact with retrogressed Grt-peridotites showing a garnet compositional layering, cross cut by a subsequent low pressure (LP) chlorite foliation (Pellegrino et al., 2020).
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Eclogite boudins enclosed in migmatites show thin films and interstitial pockets of crystallised melts parallel to the eclogite foliation, indicating that partial melting of eclogites occurred at HP conditions (Pellegrino et al., 2020).
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Tremolitites occur both at the peridotite/eclogite contact and within the peridotite body and derived from the retrogression of previous Grt-websterites formed after the interaction at HP between eclogite-sourced melts and peridotites (Pellegrino et al., 2020).
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Both peridotites and tremolitites record a LREE enrichment acquired during the melt/peridotite interaction at HP, decoupled by the selective enrichment in large ion lithophile elements (LILE) acquired by late stage tremolitic hornblende and tremolite crystallisation during a hydration event at LP conditions (Pellegrino et al., 2020).
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Among the fluid mobile elements, the selective enrichment in LILE recorded by the peridotites and tremolitites (Fig. S-3c) resembles that of retrograde tremolitic hornblende, chlorite and tremolite reported by Pellegrino et al. (2020), indicating that a fluid-mediated metasomatism occurred at LP conditions.
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As demonstrated by Pellegrino et al. (2020), the LREE enrichment and fractionation of the peridotites was most likely generated by the infiltration and interaction at HP of the eclogite-derived melt (leading to the emplacement of pristine Grt-websterites), rather than to the LP fluid-assisted metasomatic event that led to the retrogression of Grt-peridotites and websterites into Chl-peridotites and tremolitites, respectively.
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To test this hypothesis, we have numerically simulated the REE gradient applying the Plate Model of Vernieres et al. (1997) using the REE composition of the eclogite leucosome of Pellegrino et al. (2020) as starting melt and the DMM as peridotite matrix.
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Salters, V.J.M., Stracke, A. (2004) Composition of the depleted mantle. Geochemistry Geophysics Geosystems 5. https://doi.org/10.1029/2003GC000597
Show in context The trace element patterns of the Depleted MORB Mantle (DMM; Salters and Stracke, 2004), of Grt-peridotites from Mt.
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Scambelluri, M., Hermann, J., Morten, L., Rampone, E. (2006) Melt-versus fluid-induced metasomatism in spinel to garnet wedge peridotites (Ulten Zone, Eastern Italian Alps): Clues from trace element and Li abundances. Contributions to Mineralogy and Petrology 151, 372–394. https://doi.org/10.1007/s00410-006-0064-9
Show in context In comparison, we know little about the role of these interactions at high pressure (HP) and relatively low temperature (LT) such as those reached at the slab interface, even in warm subduction zones (e.g., Malaspina et al., 2006; Scambelluri et al., 2006; Pellegrino et al., 2020).
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Samples from those settings tend to show peculiar LREE “spoon-like” fractionations (e.g., Scambelluri et al., 2006).
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Tumiati, S., Zanchetta, S., Pellegrino, L., Ferrario, C., Casartelli, S., Malaspina, N. (2018) Granulite-facies overprint in garnet peridotites and kyanite eclogites of Monte Duria (Central Alps, Italy): clues from srilankite-and sapphirine-bearing symplectites. Journal of Petrology 59, 115–151. https://doi.org/10.1093/petrology/egy021
Show in context Both eclogites and peridotites record a common HP peak at 2.8 GPa and 750 °C and post-peak high temperature (HT) equilibration at 0.8–1.0 GPa and 850 °C (Tumiati et al., 2018).
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Grt-peridotites show metasomatic pargasitic to edenitic amphibole, dolomite, phlogopite and orthopyroxene after olivine, crystallised after the interaction with crust-derived agents enriched in SiO2, K2O, CO2 and H2O at peak conditions (Tumiati et al., 2018; Pellegrino et al., 2020).
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Vernieres, J., Godard, M., 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. https://doi.org/10.1029/97JB01946
Show in context To test this hypothesis, we have numerically simulated the REE gradient applying the Plate Model of Vernieres et al. (1997) using the REE composition of the eclogite leucosome of Pellegrino et al. (2020) as starting melt and the DMM as peridotite matrix.
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Vrijmoed, J.C., Austrheim, H., John, T., Hin, R.C., Corfu, F., Davies, G.R. (2013) Metasomatism in the ultrahigh-pressure Svartberget garnet-peridotite (Western Gneiss Region, Norway): implications for the transport of crust-derived fluids within the mantle. Journal of Petrology 54, 1815–1848. https://doi.org/10.1029/97JB01946
Show in context Some natural occurrences show that the migration of crust-derived melts into the mantle by porous flow is limited by instant reaction with the peridotites and the consequent production of metasomatic orthopyroxene (±clinopyroxene) and phlogopite hybrid rocks at the slab-mantle interface (e.g., Malaspina et al., 2006; Vrijmoed et al., 2013; Endo et al., 2015).
View in article
Warr, L.N. (2021) IMA–CNMNC approved mineral symbols. Mineralogical Magazine 85, 291–320. https://doi.org/10.1180/mgm.2021.43
Show in context The Borgo outcrop of the Monte Duria area (Adula-Cima Lunga unit, Central Alps, Italy) is an ideal example of melt-peridotite interaction which occurred under a deformation regime at HP, that enabled the combination of porous and focused flow of eclogite-derived melts into Grt-peridotites (Pellegrino et al., 2020; mineral abbreviations from Warr, 2021).
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Supplementary Information
The Supplementary Information includes:
Download Table S-1 (xlsx)
Download Table S-2 (xlsx)
Download Table S-3 (xlsx)
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Figures
Figure 1 (a) Geological sketch map of tremolitite layers in retrogressed Grt-peridotite at the contact with migmatitic eclogites. (1) Tremolitite showing sharp contact with the host retrogressed Grt-peridotite. (2) Tr + Phl layer displaying a diffusive contact with retrogressed Grt-peridotite. (b) Cross section of the peridotite host rock contact with sampling location of tremolitites and Chl-peridotites.
Figure 2 Numerical simulation of melt-rock interaction at peridotite-eclogite interface and reactive percolation of melt through mantle peridotite (a) PM normalised REE patterns of liquids resulting from interaction between mantle peridotite and the initial liquid, assuming high peridotite assimilation (60 % Ol, 20 % Opx, 12 % Cpx, 3 % Grt) and low crystallisation rate (60 % Cpx, 40 % Grt). After 11 interactions, we obtained a pattern that reproduces the LREE/HREE bulk composition of retrogressed Grt-websterite at the mantle-eclogite boundary, i.e. distance 0 in the profile of Figure 1. (b) Evolution of PM normalised REEs in the liquid originated by reactive percolation of reacted melt after Step 1 through the DMM peridotite. The model assumes a high infiltrating melt amount, low assimilation of olivine and high extent of crystallisation. Patterns with coloured squares (except light yellow) are the REE bulk compositions of tremolitites sampled in the Borgo outcrop within the first 30 metres from the migmatitic eclogite-peridotite contact (Figs. 1, S-1). Coloured lines refer to the progressive cells of the model. See the Supplementary Information for details of input parameters and further explanation.
Figure 3 PM normalised La and Ce/Yb of tremolitite (same symbols as Fig. 2) along the section of Figure 1, compared to the results of the numerical simulation of reactive melt percolation assuming variable crystallisation rate of Opx, Cpx, Grt and Phl. ICR and FCR are Initial and Final Crystallisation Rate. Yellow symbols represent the preferred model. The grey line is the reference DMM.