Introduction

During prograde metamorphism along a subducting slab, the increase in pressure and temperature (P–T) triggers a series of breakdown reactions involving several hydrous minerals1. These reactions release fluids and hydrous melts enriched in mobile chemical species such as Large-Ion Lithophile Elements (LILE), Pb, Sr, and Light Rare Earth Elements (LREE)2,3. The release of these fluids also induces changes in whole-rock geochemical compositions (metasomatism)4,5,6, bulk-rock density (ρ; mass per unit volume of a specific rock)7,8,9, volume of solids + fluids (ΔVr solid + fluid; the change in the volume of a system, accounting for both solids and fluids, along a metamorphic trajectory)10,11, pore fluid pressure or overpressure (Pf, accounting for the thermal expansivity and compressibility effects between fluids and solids)12,13,14, and permeability (κ, the fluid flow through a porous media)15,16,17. Recently, it has been suggested that variations in subduction properties over time (e.g., the transition from free sinking (fast convergence) to mature (low convergence) stages)18 modify physicochemical characteristics in the oceanic crust as the thermal structure of the subduction zone evolves. Yet, the evolution of petrophysical properties and their relation to mass transfer to mantle depths along the plate interface continues to be the subject of intense debate, due to the complex physicochemical changes that determinate the capacity of fluids to migrate under high-pressure and low temperature (HP–LT) conditions19,20,21.

To improve our understanding of this topic, it is essential to address a crucial question: How does the thermal evolution of the subducting slab influence the development of fluid-rich zones, fluid migration pathways, petrophysical properties, and fluid-rock interaction processes (i.e. metasomatism) in subduction zones over time? This interdependence between thermal structure, fluid-rock interaction processes, and petrophysical properties in subduction zones highlights the importance of studying these factors in the rock record of exhumed HP–LT belts. The Acatlán Complex (AC) in southeastern Mexico22 (Fig. 1) is a HP–LT metamorphic complex that records a long-lived cold metamorphic history during the Paleozoic era. Although previous studies have focused on the tectono-metamorphic evolution of the AC (e.g. Ortega-Gutiérrez et al.22 and references therein), a detailed geochemical and petrological characterization of fluid-rock interactions in the AC is still lacking. Consequently, the rock record within the AC may serve as a natural laboratory for studying how its thermal evolution affected the origin, movement, transfer of fluids, petrophysical changes as well as subsequent fluid-rock interactions under HP–LT conditions in this paleo-subduction zone.

Fig. 1
figure 1

Geologic map of the AC. (a) Location of the AC in Mexico. (b) Geological map of the AC modified from Hernández-Uribe et al.23. (c) Geological map of the AC in the regions of Las Minas-Guadalupe-Mimilulco-Ahuatlán. The contacts between each lithodeme were defined by mineral phase isograds. This map was created using the free and open source QGIS v. 3.16.16 (https://qgis.org). (d) Geological cross-section illustrating the lithodemes present in the studied regions.

Full size image

In this report, we present whole-rock analyses of trace elements, Sr–Nd isotopic compositions, classic thermodynamic modeling24 coupled with time-evolving models18, and geochemical modeling to characterize the prograde physicochemical characteristics of HP–LT rocks from the AC. Our results show that the metasomatic geochemical characteristics of the AC were acquired through the interaction between the subducted oceanic crust and fluids released from the dewatering of hydrated mineral phases (such as epidote, chlorite, and potentially lawsonite) during the transition from free sinking to mature subduction stages. Consequently, during these cold stages, variations in ΔVr solid + fluid, ΔVr solid, Pf, and κ in the subducted oceanic crust were triggered by the dewatering reactions. Fluid migration was facilitated by these petrophysical changes, allowing the fluids to migrate parallel to the developed foliation, as recorded in the AC.

Geological setting

The AC, localized in southeastern Mexico (Fig. 1a), has been related to the opening, closure, and subduction of three major Paleozoic oceans: Iapetus, Rheic, and Paleo-Pacific22 (Fig. 1b). It can be divided into three tectonic cycles: (a) a rift/subduction cycle, which occurred during the Ordovician–Silurian period, (b) a pre-orogenic supra crustal deposition and intrusion cycle, which took place during the Devonian-Mississippian period, and (c) a synorogenic supra crustal unit and intrusive cycle, which occurred during the Pennsylvanian-Permian period22. The protoliths of the AC rocks are directly linked with a fragmented ophiolitic complex and its accompanying sedimentary cover25,26 (Fig. 1b).

The peak of the eclogite facies metamorphism is estimated to have occurred at approximately 353 Ma, as determined through Lu–Hf garnet-whole-rock geochronology27, which agrees with some U–Pb zircon ages (341 Ma) from the blueschist rocks28,29. The peak P–T conditions of eclogite-facies metamorphism have been recently estimated at ∼22 kbar, with temperatures ranging between 460 and 675°C23,30. Although the P–T conditions for the blueschist metamorphism have been estimated at 13 kbar and 480 °C, textural evidence indicates that, if lawsonite was stable in the AC, the peak P–T conditions would have been 19 kbar and 505°C23. This suggests that blueschist and eclogite rocks evolved within the same HP–LT unit along a shared subduction trace,  recording distinct peak P–T conditions and exhumation cycles. Finally, the exhumation of the AC concluded in amphibolite-greenschist facies conditions, dated by 316 Ma phengite and amphibole 40Ar-39Ar ages31,32.

Results

In the northeastern part of the AC, the studied HP–LT rocks (blueschists, retrograde eclogites, and garnet-bearing mica-schists) define a NE-SW foliation trend within the Piaxtla Suite (Fig. 1c,d). In the Las Minas-Guadalupe region, blueschists occur as foliated rocks in contact with garnet-bearing mica-schists (Fig. 2a). Blueschists are composed of Na-amphibole, epidote, white mica, garnet, and rutile/titanite (Fig. 2b–d). Na-amphibole and epidote intercalated with white mica form a nemato-lepidoblastic texture (Fig. 2b). Compositional zoning in Na-amphibole exhibits glaucophane in the core, winchite in the intermediate region, and katophorite at the rim23. Interestingly, epidote-rich bands occur intercalated to the nematoblastic textures, and rutile appears as elongated inclusions in Na-amphibole (Fig. 2c). Garnet exhibits poikilitic texture comprised of glaucophane, epidote, titanite/rutile (Fig. 2d). All these observations suggest epidote-glaucophane blueschist metamorphic facies conditions during the prograde stages in the AC.

Fig. 2
figure 2

Structural and textural characteristics of HP–LT rocks from the AC. (a) Blueschist outcrop in the AC. (b) Nematoblastic texture defined by Na-amphibole, epidote, chlorite, and rutile/titanite. (c) Elongated rutile inclusions within Na-amphibole, which are also aligned parallel to the NE-SW foliation (d) Garnet exhibiting a poikiloblastic texture, with inclusions of Na-amphibole, epidote, and titanite/rutile. (e) Massive retrograde eclogites in contact with garnet-bearing mica-schists in the AC. (f) Retrograde eclogite surrounded by segregation structures. (g) Granoblastic texture in a retrograde eclogite, with garnet, clinopyroxene, rutile, epidote/zoisite, and amphibole. (h) In the retrograde eclogite, epidote/zoisite is oriented perpendicular to the NE-SW foliation. (i); Garnet-bearing mica-schist outcrop in the AC. (j) Epidote-rich bands parallel to the NE-SW main foliation in the blueschists. (k) Garnet, plagioclase, and quartz veins cutting through retrograde eclogites. (l) Withe mica, quartz, and plagioclase stromatic structures within retrograde eclogites.

Full size image

In the Mimilulco region (Fig. 1c), mafic rocks, identified as retrograde eclogites, are strain-free with concordant contact with garnet-bearing mica-schist (Fig. 2e) and as blocks within segregation textures of garnet-bearing mica-schist (Fig. 2f). Microscopic observations reveal a primary granoblastic matrix composed of garnet, clinopyroxene, rutile, white mica, epidote/zoisite, and amphibole (Fig. 2g). During exhumation, this mineral assemblage is replaced by retrograde amphibolite-facies characterized by amphibole, plagioclase, titanite, and epidote/zoisite oriented perpendicular to the NE-SW foliation (Fig. 2h). Finally, the most-retrograde stages in the study area are characterized by the stabilization of chlorite and ilmenite.

Garnet-bearing mica-schist extends from the eastern Guadalupe region to the Ahuatlán-Mimilulco region (Fig. 2i). This lithodeme contains thin plagioclase bands intercalated with white micas and, to a lesser extent, thin epidote bands. Its epidote content decreases from west to east in the study region. The foliated texture surrounds garnet porphyroblasts with rim reactions composed of plagioclase. This lithodeme displays the highest degree of ductile deformation, with crenulation in its matrix, mineral phase folding, and boudinage.

Importantly, bands, veins, and segregation structures have been identified in the AC. Firstly, epidote-rich bands are parallel to the NE-SW main foliation in the blueschists (Fig. 2j). The veins, which cut through the eclogitic mafic rocks33, consist mainly of garnet, white mica, and quartz (Fig. 2k), similar to veins characterized by Taetz et al.20. Finally, retrograde eclogites occur within segregation structures composed of white mica, quartz, and plagioclase (Fig. 2l). These structures display stromatic and patch textures, suggesting low degrees of partial melting of the garnet-bearing mica-schist lithodeme28. This marks the most advanced retrograde stage in study area33.

Whole-rock trace elements

Twenty-three samples, consisting of blueschists, retrograde eclogites, and garnet-bearing mica-schists, were selected for a geochemical study to characterize the metasomatic stages of the AC. Their locations and compositions are shown in Figs. 1, 3, Table S1, and Jiménez-Barranco33. Additionally, we compared our geochemical data with a compilation of mafic rocks (n = 1200) associated with HP metamorphism in subduction complex worldwide (WHP–LT; Fig. 3). The major and trace elements, along with their respective references for these HP rocks, are provided in Supplementary Information.

Fig. 3
figure 3

Trace element ratios of the studied HP–LT rocks from the AC. (a,b) Normalized trace element patterns for the blueschists (Group 1) and retrograde eclogites (Group 2) from the AC. Normalization values are based on the MORB average composition from Gale et al.34 (c) Ba/Th vs. K/Th; (d) K vs. Ba/Rb; (e) 1/Pb vs. Ce/Pb; (f) Nb/Zr vs. U/Nb; (g) Pb* vs. Sr*; (h) Pb* vs. Ba/Pb diagrams, illustrating elemental mobilization during dehydration processes in subduction zones. Geochemical reference values include MORB, GLOSS, AOC, and a global compilation of mafic rocks that have underwent HP–LT metamorphism, (details in the Supplementary Information).

Full size image

The mafic HP–LT rocks from the AC exhibit two distinct petrological and trace-elemental characteristics when compared to Mid-Ocean Ridge Basalt34 (MORB; Fig. 3a,b). Group 1 (blueschists) is characterized by the enrichment in LILEs, positive Pb and Sr anomalies, and flat REE patterns (Fig. 3a). Group 2 (retrograde eclogite) exhibits both positive and negative Sr anomalies, enrichment in LILE and LREE, and flat REE patterns (Fig. 3b). We focus on the elemental ratios (Fig. 3c–h), such as Ba/Th, K/Th, Ce/Pb, 1/Pb, and U/Nb, that are known to be mobilized during the dehydration process in subduction zones and can be subsequently transferred to the adjacent rocks2,35,36.

Our results align with the geochemical characteristics observed in WHP–LT rocks that have a record of metasomatic processes along their metamorphic subduction history (Fig. 3c-f). Group 1 shows a positive correlation between Ba/Th and K/Th ratios (20.6–2677 and 566–106,596, respectively; Fig. 3c). Group 2 shows a similar tendency, with Ba/Th ranging from 7.11 to 348 and K/Th from 406 to 12,249 (Fig. 3c). Only 18% and 63% of the samples in Group 1 and Group 2, respectively, have values lower than those in MORBs (Fig. 3c). Garnet-bearing mica-schists show lower Ba/Th and K/Th ratios values compared to those found in MORBs (Fig. 3c).

There is also a negative correlation between K (Group 1: 249–16,762 ppm; Group 2: 166–9045 ppm) and Ba/Rb (Group 1: 3.61–18.0; Group 2: 4.32–17.0), which tends toward the global subducted sediment37 (GLOSS) composition, garnet-bearing mica-schist, and serpentinites (Fig. 3d). Importantly, the mafic HP–LT rocks from the AC have low Ni contents compared to serpentinites (Table S1). In comparison to MORBs, Group 1 and Group 2 rocks contain 33% and 64% of the samples with K depletion and constant Ba/Rb ratios (Fig. 3d).

The 1/Pb vs. Ce/Pb ratios for both Group 1 and 2 are similar those of GLOSS and garnet-bearing mica-schist, exhibiting Pb enrichment (Fig. 3e). Group 1 (81% of the samples) shows increased U/Nb ratios of 0.02 – 0.23, while Group 2 (54% of the samples) shows high Nb/Zr ratios of 0.02–0.14 (Fig. 3f). These geochemical characteristics have affinities with the global composition of altered oceanic crust38 (AOC; Fig. 3f). The Pb* and Sr* enrichment indices (Pb*: 2PbN/ [CeN + PrN] from6; Sr*: 2SrN/ [PrN + NdN]) and Pb vs Ba/Pb diagrams reveal two distinct geochemical characteristics in the HP–LT rocks of the AC (Fig. 3g,h). The first, represented by Group 1, is enriched in Pb*, while the second, represented by Group 2, is enriched in Ba (Fig. 3g,h).

Sr and Nd isotopes

The Sr and Nd isotopic compositions of eleven samples from the AC, including blueschists (Group 1), retrograde eclogites (Group 2), and garnet-bearing mica-schists, are shown in Fig. 4 and Table S2. Our aim is to characterize the isotopic characteristics of the metasomatism in the AC. The Sr and Nd isotope ratios from the AC, along with a group of HP-mafic rocks, mica-schists, MORB, GLOSS, and AOC, as reported by Cohen et al.39, Yáñez et al.40, Plank & Langmuir41, Hauff et al.42, Zheng et al.43, Halama et al.44, Ker et al.45, Keppie et al.46, King et al.47, Wang et al.36, Zhu et al.48, and Muñoz-Montecinos et al.49, are presented as age-corrected initial ratios based on existing metamorphic ages. For MORB, GLOSS, and AOC, which were recently formed, the calculation of age-corrected initial ratios was omitted. The procedure for obtaining the Sr and Nd isotope ratios is described in the Methods section.

Fig. 4
figure 4

The 87Sr/86Sr(t) and εNd(t) of the AC. Age-corrected initial Sr and Nd isotopic compositions of the AC (including existent Sr and Nd isotopic from the AC) compared to HP-mafic rocks, HP-mica-schists, GLOSS, MORB, and AOC (compiled from Cohen et al.39, Yáñez et al.40, Plank & Langmuir41, Hauff et al.42, Zheng et al.43, Halama et al.44, Ker et al.45, Keppie et al.46, King et al.47, Wang et al.36, Zhu et al.48, and Muñoz-Montecinos et al.49. These isotopic ratios are corrected for metamorphic ages to characterize isotopic metasomatism and identify fluid sources responsible for fluid-rock interaction processes in subduction zones.

Full size image

The εNd(350Ma) and 87Sr/86Sr(350Ma) isotopic composition of Group 1 and Group 2 samples from the AC and the compiled HP mafic rocks show comparable 87Sr/86Sr enrichment trends (Fig. 4). Positive εNd(350Ma) values for the blueschists (Group 1) range from + 5.0 to + 8.0 (Fig. 4) and exhibit exceptionally high and variable 87Sr/86Sr(350Ma) ratios (0.7069 – 0.7123), which also are higher than those of Carboniferous seawater75. On the other side, the retrograde eclogites (Group 2), display positive and homogeneous εNd(350Ma) values of + 6.7 to + 6.8 and high 87Sr/86Sr(350Ma) ratios (0.7080 – 0.7084). One retrograde eclogite yielded an exceptionally high 87Sr/86Sr(350Ma) ratio of 0.7175 (Fig. 4). The garnet-bearing mica-schists from the AC exhibit high 87Sr/86Sr(350Ma) ratios values (0.7092–0.7093) and negative εNd(350Ma) (-6.9 to -6.8), which fall within the Sr–Nd isotopic range of GLOSS and mica-schists (Fig. 4).

Petrological modeling: the free sinking to mature stage in subduction zones (t = 15 Myr)

We performed petrological modeling to calculate the mineral assemblage and trace-element compositions of the released fluids (Fig. 5), as well as the petrophysical changes (ΔVr solid + fluid, ΔVr solid, Pf, and κ; Fig. 6) from the subducted oceanic crust to determine the prograde petrophysical history of the AC (see the methods section for further details). Forward modeling was conducted using the Gibbs energy minimization algorithm24 coupled with time-evolving models of the slab top18 (Fig. 4). These time-evolving models documents the time-dependent evolution of the thermal structure in the tectonic history of a subduction zone, where the free sinking to mature subduction stage involves the transition from fast to slow convergence in subduction zones18. For the petrological modeling, we selected two compositions that were considered representative of the initial metamorphic stages of the slab top of the oceanic crust in the AC: average GLOSS37 and MORB34 compositions (Table S3). Additionally, we analyzed two lithologies that represent the peak metamorphic stages of the AC: garnet-bearing mica-schist (AC22-32A) and blueschist (AC22-33) (Table S3). We present results for the model t = 15 Myr, which represents the cold and mature conditions identified through field and petrological observations in the study area (Figs. 1, 2), as this model aligns well with the most recent geothermobarometric data for the AC23,30. Detailed results of the trace-element compositions of the released fluids and petrophysical changes are presented in Table S4 and TableS5.

Fig. 5
figure 5

Mineral assemblages and fluid compositions along the free sinking stage (t = 15 Myr) from18 for sedimentary and mafic composition of the AC. (a,b) Mineral assemblages’ evolution for sedimentary (GLOSS-garnet-bearing mica-schist) and mafic (MORB-blueschist) compositions of the AC. (c,d) MORB-normalized multi-element diagrams of fluids produced from sedimentary and mafic compositions of the AC.

Full size image
Fig. 6
figure 6

Schematic geometry of the slab top showing the petrophysical properties of the subducted oceanic crust from the AC along the free sinking stage (t = 15 Myr) from18. (a) Thermal model showing the P–T evolution of the slab top and slab Moho18. Key parameters were examined along the slab top: (b) the cumulative H2O released (wt.%), (c) the evolution of the volume changes of the solids + fluid (ΔVr solids + fluid), (d) the evolution of the volume changes of the solids (ΔVr solids) alongside the v and κ obtained using the Darcy flow calculation method outlined in Refs.11,12,50.

Full size image

During this stage, the modeled GLOSS involves the dewatering of lawsonite at 1.9 GPa and 477 °C (Fig. 5a). The subsequent stabilization of garnet after this dewatering reaction is observed (Fig. 5a). Along this transition stage (Fig. 6a), dewatering of lawsonite results in an H2O release of 1.0 wt.% (Fig. 6b), which produce that Pf registers a negative change (Table S5). A positive ΔVr solid + fluid of + 2.5% (Fig. 6c), and a negative ΔVr solid of -0.4% are also observed (Fig. 6d). We employed a forward batch fluid equation incorporating residual mineral assemblages from our petrological modeling and their corresponding fluid/mineral partition coefficients to calculate trace element concentrations of the produced fluids from mineral reactions along the free sinking to mature subduction stages (see methodology for further details; Table S4). Our results show that lawsonite dewatering produces fluids enriched in Cs, Ba, Th, La, Ce, and Pb, while being depleted in Rb, U, Sr, Nb, Sm, and Y (Fig. 5b).

The consumption of epidote at 1.9 GPa and 531 ºC (Fig. 5a) realeses 0.4 wt.% H2O (Fig. 6b), increases Pf (Table S5), and results in a ΔVr solid + fluid of -1.0% (Fig. 6c), and ΔVr solid of + 0.1% (Fig. 6d). Such dewatering event generates fluids enriched in Cs, Ba, Th, La, Ce, Pb, and Sr, and depleted in Rb, Nb, Sm, and Y (Fig. 5b). We assume that after the lawsonite and epidote dewatering, the garnet-bearing mica-schist (AC22-32A) may better represent the prograde compositions of the sedimentary part in the AC (Fig. 5a). With this composition, chlorite destabilization starts at 1.9 GPa and 509 °C, with final consumption at 2.1 GPa and 622 °C (Fig. 5a). This mineral consumption produces 1.3 wt.% of H2O and an increase in Pf (Fig. 6b; Table S5). It also produces a progressive positive ΔVr solid + fluid (+ 0.5%) and a negative ΔVr solid (-0.2%) change (Fig. 6c,d). Geochemical modeling indicates that chlorite consumption generates fluids enriched in Cs, Rb, Th, U, La, Ce, Pb, and Sm, while being depleted in Ba, Nb, Sr and Y. (Fig. 5b).

In the mafic portion of the subducted crust (MORB), our results show progressive consumption of lawsonite and chlorite (Fig. 5c), releasing 0.7 wt.% of H2O at 2 GPa and 555 °C (Fig. 6b). This produces a large Pf increase (Table 5S), a positive ΔVr solid + fluid (+ 8.0%), and a negative ΔVr solid (-0.5%) (Fig. 6c,d). These mineral changes cause a decrease in the volume of epidote and amphibole and an increase in clinopyroxene (Fig. 5c). In this case, lawsonite and chlorite generate fluids enriched in Cs, Pb, and Sr, with minor Sm enrichment (Fig. 5d).

During the transition from free sinking to mature stages (Fig. 6a), the blueschist (AC22-33) reflects peak metamorphic compositions in the AC (Fig. 5c). In this model, progressive amphibole breakdown culminates at 2.1 GPa and 618 °C (Fig. 5c), releasing 0.7 wt.% H2O (Fig. 6b), causing a slight change in Pf (Table 5S), and resulting in a positive ΔVr solid + fluid (+ 0.2%) and a negative ΔVr solid (-1.0%) (Fig. 6c,d). Our geochemical modeling shows that amphibole dewatering releases fluids enriched in Cs, Rb, Ba, Th, U, Nb, La, and Pb, and depleted in Sr and Y (Fig. 5d).

Fluid production and permeability

We utilized our thermodynamic results coupled with the Darcy flow calculation method11,12,50 to determine the volume of fluid passing through a cross-sectional area per time, that is, fluid production (v) and κ of the uppermost part of the oceanic crust during the transition from free sinking to mature stage (Fig. 6d; See methods section for further details). The v term considers the length scale of the subducted oceanic crust and 1 Myr for the dewatering evolution. The κ is derived using the equation based on Darcy’s law.

Our analysis reveals that in the sedimentary part, when lawsonite dewatering occurs, v is equal to 1.0 × 10–6 m3/m2 yr and κ is 1.2 × 10−20 m2 (Fig. 6d). When epidote and chlorite are fully consumed, v increases to 5.7 × 10–6 m3/m2 yr and κ rises to 6.8 × 10−20 m2 (Fig. 6d). In the mafic portion (Fig. 6d), once lawsonite and chlorite are consumed v is 3.2 × 10–7 m3/m2 yr and κ is 3.8 × 10−21 m2, while amphibole consumption results in v reaching 2.8 × 10–6 m3/m2 yr and κ reaching 3.4 × 10−20 m2 (Fig. 6d). The total v and κ for the sedimentary oceanic crust amount to 6.7 × 10–6 m3/m2 yr and 8.0 × 10−20 m2, respectively (Fig. 6d). Finally, in the mafic portion, the total v and κ reach 3.2 × 10–6 m3/m2 yr and 3.8 × 10−20 m2, respectively (Fig. 6d).

Discussion

Trace elements and Sr–Nd of the blueschists as tracers to identify fluid sources in subduction zones

The geochemical characteristics of the blueschists (Group 1) from the AC provide key insights into the role of HP–LT fluids in driving physicochemical transformations within the subducted oceanic crust. Blueschists from the AC exhibit a metasomatic geochemical trend, characterized by LILE, Pb and Sr enrichment in comparison with MORB and AOC, as well as a positive correlation between K/Th and Ba/Th, low Ce/Pb and 1/Pb ratios, and high 87Sr/86Sr(350Ma) isotope ratios (Figs. 3, 4). Such geochemical features suggest a complex history likely involving either seafloor alteration or HP–LT fluid-rock interaction during subduction2,36,51. While Bebout2 attributes Ba and Pb enrichment in metabasalts to seafloor alteration, the positive correlation between K/Th and Ba/Th and the low Ce/Pb and 1/Pb ratios observed in the blueschist suggest that HP–LT fluid-rock interaction processes are the cause of these enrichments36. This is further supported by the exceptionally high 87Sr/86Sr(350Ma) ratios observed in Group 1, reaching up to 0.712335 (Fig. 4), which cannot be explained by pre-subduction seawater alteration, as the 87Sr/86Sr ratios of Carboniferous seawater were significantly lower (Fig. 4). Moreover, the lower 87Sr/86Sr values in MORB and AOC (Fig. 4), combined with the absence of serpentinites in this section of the AC (Fig. 1) and their intrinsic low 87Sr/86Sr ratios of serpentinites49, rule out the possibility that the high 87Sr/86Sr(350Ma) values in the blueschists are inherited from the original components (MORB or AOC) or from serpentinite dehydration.

We propose that the metasomatic characteristics of the blueschists from the AC were acquired through interactions between the oceanic crust and two distinct fluid sources: external and in-situ. Regarding the origin of the external fluid source, in the sedimentary portion of the AC, petrological evidence points to the presence of epidote, plagioclase, titanite, and oxide inclusions in garnet porphyroblasts generated by epidote dewatering33. Mica-group minerals, known for their compatibility with LILEs2, along epidote (enriched in LILE, Pb, and Sr)36 and lawsonite (enriched in LREE, Pb, and Sr)52,53 play a key role in this scenario. For instance, the breakdown of mica-group minerals could produce fluids enriched in the K/Th, Ba/Th, and 87Sr/86Sr ratios, while lawsonite and epidote dewatering would release fluids rich in Pb, Sr, LREE, LILE, and 87Sr/86Sr. Our petrological and geochemical results suggest that the breakdown of epidote, alongside chlorite and potentially lawsonite, occurred at 1.9–2.0 GPa and 477–622 °C (Fig. 5a), with a v rate equal to 6.7 × 10–6 m3/m2 yr (Fig. 6d). In general, these dewatering processes generated fluids enriched in Cs, Pb, Sr, and 87Sr/86Sr, while being depleted in Nb, Sm, and Y (Fig. 5b). We propose that these external fluids, enriched in LILEs, and 87Sr/86Sr, likely interacted with the mafic oceanic crust during the transition from free sinking to mature stages, contributing to the elevated Pb and Sr, as well as K/Th, Ba/Th, and 87Sr/86Sr(350Ma) ratios observed in the blueschists from the AC (Figs. 3, 4).

In contrast, in the mafic part from the AC, petrological observations of blueschists reveal aggregates of epidote, phengite, and albite (Fig. 2b), which were likely the result of lawsonite dehydration23,30. Our petrological and geochemical modeling show that lawsonite and chlorite dewatering in the mafic portion occurred at similar pressures to those in the sedimentary portion (1.9–2.0 GPa), but within a temperature range of 477–555 °C (Fig. 5c). Furthermore, these dewatering reactions produced in-situ fluids enriched in Cs, Pb, and Sr, with a minor enrichment in Sm (Fig. 5d), at a v rate of 3.2 × 10–6 m3/m2 yr (Fig. 6d). We interpret that these fluids were not released and, instead, they were re-incorporated into the NE-SW epidote-rich bands, producing the Pb, Sr, and 87Sr/86Sr(350Ma) enrichment and low Ce/Pb and 1/Pb ratios documented in the blueschists from the AC (Figs. 3, 4).

It is also important to consider that slab-derived fluids do not contain sufficient Sm to affect the Nd isotopic systematics of HP–LT rocks54 (Fig. 4; 5b,d), attributed to the low mobility of Middle REEs during dehydration of the subducted oceanic crust5455,56,57. Consequently, the original Nd isotopic signatures in blueschists (Group 1) and retrograde eclogites (Group 2) of the AC have been preserved, resembling those found in MORBs (Fig. 4). These findings corroborate the understanding that Middle REE and Nd isotopes are less susceptible to redistribution during subduction processes36,54.

Petrophysical changes and fluid migration during blueschist to eclogite transition in the Acatlán Complex

Our results reveal that during the transition from blueschist to eclogite conditions in the AC, dehydration reactions in the subducting oceanic crust triggered significant changes in ΔVr solid + fluid, Pf, and ΔVr solid, as well as κ (Fig. 6c,d). For instance, in the sedimentary portion of the AC, dehydration reactions of lawsonite, epidote, and chlorite (1.7–2.1 GPa and 400–620 ºC; Fig. 5a) caused distinct fluctuations in ΔVr solid + fluid, accompanied by variations in Pf, ΔVr solid, and κ (Fig. 6c,d; Table S5). At greater depths (40–60 km) in the mafic oceanic crust, a significant increase in ΔVr solid + fluid (up to 8%) was observed, accompanied by a rise in Pf, a decrease in ΔVr solid of -0.5%, and changes in κ (Fig. 6c,d; Table S5), linking to the dehydration of lawsonite and chlorite during the transition from blueschist to eclogite (Fig. 5c).

Studies of low-velocity layers in subducted crust show that the oceanic crust is overpressurized to depths of 35–40 km in hot subduction zones58 and up to 90 km in cold subduction zones59. Also, under constant overpressure and low κ, fluids produced by the dehydrating oceanic crust may become trapped58, leading to chemical equilibrium with the host rock12. This chemical equilibrium would be relevant in subduction zones, where grain-size reduction results in a decrease in porosity within the subducted oceanic crust58. Yet, if these conditions change, fluids may migrate parallel to the predominant foliation, particularly in zones where the shape-preferred orientation60 define a strong foliation within the subducted crust61. In the AC, the petrophysical changes triggered dehydration-driven fluctuations in overpressure and κ (Fig. 6c,d), suggesting that fluid migration and fluid-rock interaction processes could be facilitated along the NE-SW foliation planes identified in the blueschists (Fig. 1c, 2j). These fluctuations in overpressure and κ are key to understanding how fluid migration and fluid-rock interaction processes occur in cold subduction zones.

Regarding this, transient κ—resulting from temporary fracture opening and sealing—plays a crucial role in facilitating fluid migration through fracture networks, influencing fluid-rock interactions and the geochemical evolution of the subduction interface17. The transient κ values (10−14 to 10−15 m2) observed at Syros Island are consistent with those required to explain fluid migration associated with slow slip events and tectonic tremors17. Studies by Hendriyana and Tsuji62 and Frank et al.63 have estimated κ values as high as 3.7 × 10−12 m2 in warm subduction zones (Nankai and Guerrero), which align with numerical models predicting κ values ranging from 10−11 to 10−14 m264. However, Ganzhorn et al.61 report that blueschists exhibit much lower κ values, between 10−19 m2 to 10−21 m2, while antigorite serpentinites and chlorite schists show κ values lower than 10−21 m2. The κ anisotropy (10–19 to 19–22 m2) in foliated serpentinites has been also proposed to control anisotropic fluid migration along cold subduction zones60. Therefore, such lower κ contributes to deeper overpressurization of the oceanic crust and restricted hydration of the forearc mantle wedge in cold subduction zones60,61.

Our κ estimates are consistent with those obtained from experiments simulating cold subduction zones60,61, suggesting that the AC likely experienced deeper overpressurization and limited hydration of the forearc mantle wedge. Notwithstanding, when Pf exceeds the lithostatic threshold and ΔVr solid + fluid has a positive change (Fig. 6c), fracturing is expected8, triggering the transient state of κ17. Since our findings are based on Darcy-like flow, and transient κ can reach values as high as 10−14 m217, the veins identified in the AC (Fig. 2k) should be a focus for more detailed studies in future research to better understand this phenomenon in the AC.

Concluding remarks

The geochemical analysis of blueschists provides insights into the role of metasomatism produced in the subducted oceanic crust that gave origin to the AC. The blueschists exhibit a distinct metasomatic trend, including a Pb, Sr, and 87Sr/86Sr(350Ma) isotope ratios enrichment, a positive correlation between K/Th and Ba/Th, and low Ce/Pb and 1/Pb ratios, which are indicative of HP–LT fluid-rock interactions in subduction zones rather than solely seafloor alteration. The elevated 87Sr/86Sr(350Ma) ratios observed in the blueschists further support fluid interactions rather than pre-subduction processes, as these values exceed those of Carboniferous seawater and are not consistent with inheritance from either the original components (MORB or AOC) or from serpentinite dehydration. The dewatering of mica-group minerals, epidote, and potentially lawsonite under HP–LT conditions likely contributed to the enrichment in Pb, Sr, LILEs, and 87Sr/86Sr(350Ma) ratios observed in the blueschists. Our findings indicate that the metasomatic characteristics of the blueschists were shaped by interactions between the subducted oceanic crust and both external and in-situ fluids during the transition from free sinking to mature subduction stages in the AC. These external and in-situ fluids were generated from the sedimentary and mafic portions of the AC, respectively. Additionally, the preservation of NE-SW trending epidote-rich bands in the blueschists may reflect fluid migration along foliation planes during this transition, which depends on the ΔVr solid + fluid, Pf, and ΔVr solid, as well as κ evolution of the subducted oceanic crust. Overall, these findings highlight the importance of HP–LT fluid-rock interactions in shaping the geochemical and petrophysical evolution of subducted oceanic crust, offering insights into processes influencing interface seismicity and mantle wedge hydration patterns in cold subduction zones.

Methods

Major and trace elements

Twenty-three samples were selected for a comprehensive geochemical study. These samples include blueschists, retrograde eclogites, and garnet-bearing mica-schists. Their location, field relationships, and composition are depicted in Figs. 1, 2, 3, Table S1, and Jiménez-Barranco33 We have compiled approximately 1200 mafic rocks that are related with HP subduction complexes worldwide, including major and trace whole-rock elements (Table S6).

For geochemical and isotope analyses, 10–20 kg of sample material (blueschists, retrograde eclogites, and garnet-bearing mica-schists) were crushed using a jaw crusher, ground in a disc mill, and finally pulverized with a tungsten carbide mill set. The Agilent 735 ICP-OES was utilized to obtain geochemical data for the major elements, while the Perkin Elmer Elan 9000 was utilized for the trace and REE elements at Activation Laboratories (ActLabs; Ontario, Canada). The geological reference materials employed for the major and trace elements (at ActLabs) are the following: NIST 694, DNC-1, GBW 07113, SY-4, BIR-1a, ZW-C, OREAS 101b, NCS DC86318, BCR-2, USZ 42-2006, REE-1, W-2b. A correlation matrix was prepared using the software Statistica V. 13.

Isotope geochemistry

Eleven samples were selected for the determination of their Rb–Sr and Sm–Nd isotopic ratios (Fig. 4; Table S2). These samples include blueschists, retrograde eclogites, and garnet-bearing mica-schists. Rb, Sr, Sm, and Nd were separated using standard ion-exchange methods at Laboratorio Universitario de Geoquímica Isotópica (LUGIS), Instituto de Geofísica, UNAM. Because Rb causes isobaric interference with Sr on mass 87, and Nd has isobaric interferences with Sm on mases 144, 148, and 150, LUGIS uses different Thermal Ionization Mass Spectrometers (TIMS) to measure these isotopic systems, avoiding potential isobaric interferences caused by contamination from previously analyzed samples. Thus, the Sr and Nd isotopic analyses were performed using a Thermo Scientific Triton Plus mass spectrometer equipped with a thermal ion source, while Rb and Sm isotopic ratios were determined using a Finnigan MAT 262. The Triton Plus is equipped with nine adjustable Faraday collectors and five ion counters, and the MAT 262 has a central cup and seven adjustable Faradays.

All measurements were performed statically. The Rb, Sr, Sm, and Nd samples were loaded as chlorides and analyzed as metal ions on double rhenium filaments. Each run consisted of 30 isotope ratios for Rb and Sm, 60 for Sr, and 70 for Nd. The values (1sd =  ±1σabs) refer to the errors during measurement, expressed in the last two digits; 1 SE(M) = 1σabs /√n. All Sr and Nd isotopic ratios were corrected for mass fractionation through normalization to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The values for the NBS 987 (Sr) standard obtained at LUGIS are 87Sr/86Sr = 0.710256 ± 13 (±1σabs, n = 100), and for the La Jolla (Nd) standard: 143Nd/144Nd = 0.511849 ± 4 (±1σabs, n = 36). The relative uncertainty of 87Rb/86Sr is ±2%, while that of 147Sm/144Nd is ±1.5% (1σ). The relative reproducibility (1σ) of the concentrations of Rb, Sr, Sm, and Nd are ±4.5%, ±1.8%, ±3.2%, and ±2.7%, respectively. The analytical blanks for the samples analyzed in this research were determined to be 0.18 ng Rb, 0.96 ng Sr, 0.73 ng Sm, and 0.72 ng Nd (total procedure blanks).

The age-corrected initial Sr and Nd isotope ratios for a group of HP-mafic rocks and HP-mica-schists from Yáñez et al.40, Halama et al.44, Ker et al.45, Keppie et al.46, King et al.47, Wang et al.36, Zhu et al.48, and Muñoz-Montecinos et al.49 were calculated by using the following equations:

$$^{{{87}}} {\text{Sr}}/^{{{86}}} {\text{Sr}}_{{{\text{initial}}}} =^{{{87}}} {\text{Sr}}/^{{{86}}} {\text{S}}_{{{\text{today}}}} {-}^{{{87}}} {\text{Rb}}/^{{{86}}} {\text{Sr}}({\text{e}}^{{{{\lambda}}}} {-}{ 1}), $$
(1)
$$^{{{143}}} {\text{Nd}}/^{{{144}}} {\text{Nd}}_{{{\text{initial}}}} =^{{{143}}} {\text{Nd}}/^{{{144}}} {\text{Nd}}_{{{\text{today}}}} {-}^{{{147}}} {\text{Sm}}/^{{{144}}} {\text{Nd }}({\text{e}}^{{{\lambda}}} {-}{ 1}), $$
(2)

We have transformed the 143Nd/144Ndinitial to εNdinitial by using the values of 143Nd/144Ndinitial from the Chondritic Uniform Reservoir from Wasserburg et al.65 and the following equation:

$$\varepsilon {\text{Nd}}_{{{\text{initial}}}} = \, \left[ {\left( {^{{{143}}} {\text{Nd}}/^{{{144}}} {\text{Nd}}_{{{\text{initial}}}} } \right)_{{{\text{sample}}}} - \, \left( {^{{{143}}} {\text{Nd}}/^{{{144}}} {\text{Nd}}_{{{\text{initial}}}} } \right)_{{{\text{CHUR}}}} / \, \left( {^{{{143}}} {\text{Nd}}/^{{{144}}} {\text{Nd}}_{{{\text{initial}}}} } \right)_{{{\text{CHUR}}}} } \right] \times {1}0000.$$
(3)

Petrological modeling

Petrological modeling was performed using the GeoPS software (version 3.3.2)24 and the internally consistent thermodynamic data set, ds6266. Two sets of compositions were considered representative of the initial and prograde stages in the uppermost part of the oceanic crust that formed the AC. These compositions include the average GLOSS37 and MORB34 compositions (Table S3) for the initial stages of the AC, and two lithologies from the AC: the garnet-bearing mica-schist (AC22-32A) and the blueschist (AC22-33) compositions (Table S3), representing the prograde stages of this HP–LT metamorphic complex.

We used the XFe3+ ratio in bulk rock for typical N-MORB, established as 0.16 through µ-XANES spectroscopy67, where XFe3+ is defined as Fe3+/(Fe2+  + Fe3+). For the sedimentary (GLOSS and AC22-32A) and mafic (MORB and AC22-33) compositions, the H2O content was fixed at 5.0 and 2.4 wt% (16.4 and 8.1 mol), respectively. These water content values were selected because they are in the middle of the range between the lowest and highest water saturation values found in the subducted oceanic crust7,41.

The compositions were normalized into the CaO–FeO–MgO–Al2O3–Na2O–K2O–SiO2–TiO2–O2–H2O system. We then ran models to obtain mineral assemblages predicted to be stable along the P–T path that represents the transition from the free sinking to mature stages in the slab top18, which overlap with the most recent geothermobarometric data for the AC23,30. The following activity-composition (a–x) relations for solid solution phases were selected for the petrological modeling: clinopyroxene68, amphibole68; garnet69, white mica69, biotite69, chlorite69, orthopyroxene69, ilmenite69; epidote66; and plagioclase70. Pure phases such as quartz/coesite, lawsonite, rutile, titanite, and albite also were included. The accuracy of the calculated phase diagram’s assemblage field boundaries is 0.1 GPa and 50°C at the 2σ level71. Mineral abbreviations used in this work follow the nomenclature of Whitney & Evans72.

Geochemical modeling

The trace-element concentrations in the fluids (Fig. 5b,d; Table S4) were determined through modal fluid calculations employing the following Eq. 73 and compared to the Feineman et al.74:

$${C}_{f}= \frac{{C}_{0}}{{D}_{0}+F\left(1-{D}_{0}\right)},$$
(4)

the \({C}_{f}\) is the concentration of an element in the fluid, while \({C}_{0}\) is the concentration of that element in the source. Moreover, \(F\) is the fluid fraction that is in equilibrium with the solid residuum. The bulk partition coefficient, \({D}_{0}\), can be derived for each element using the following calculation:

$${D}_{0}= {\sum }_{k=0}^{n}{K}_{n}{X}_{n},$$
(5)

where \({K}_{n}\) is the fluid/mineral partition coefficient for a specific element in phase n, and \({X}_{n}\) is the normalized modal proportion of that phase. The \({K}_{n}\) values for clinopyroxene, garnet, rutile, zoisite, mica, lawsonite, and amphibole were sourced from The Geochemical Earth Reference Model (GERM) Partition Coefficients Database (KdD) (https://kdd.earthref.org/KdD/).

Fluid production and permeability

Based on predicted metamorphic fluids along the time-evolving models of the slab top, we determine a fluid production term, v, on the order of 10–6 m3/m2 yr (Fig. 6). This term considers the length scale of the subducted oceanic crust of the thermal model, and it is integrated over the inferred 1 Ma duration of the time-evolving models of the slab top.

Darcy’s Law, which describes fluid flow through a porous media, is used to determine the κ of the oceanic crust using the term v and the following equations11,12,50:

$$Q= kA\left(\text{d}h/\text{d}z\right),$$
(6)

where Q = discharge, A = cross-sectional area, k = hydraulic conductivity, h = hydraulic head, z = length, and dh/dz is the hydraulic gradient driving fluid flow.

The volume of fluid passing through a cross-sectional area per unit of time, that is, the volume flux or Darcy velocity (v) is calculated as follows:

$$v= Q/A.$$
(7)

Based on our thermodynamic results, we have determined a total fluid production for the metasedimentary and mafic portion of the AC (Fig. 6b), and we have obtained the cross-sectional area of the portion of the subducted oceanic crust (Fig. 6a) to obtain a v on the order of 10–6 m3/m2 yr.

By substituting Eq. (7) into Eq. (6) and solving for hydraulic conductivity, we obtain the following equation:

$$k=\frac{v}{d\text{h}/d\text{z}} .$$
(8)

After that, permeability, κ, is related to the volume flux or Darcy velocity (v):

$$ k = k\eta /\rho g,$$
(9)

where \(\rho\) = fluid density, \(\eta\) = dynamic viscosity, and g = gravitational constant. We have used a \(\rho\) of 1230 kg/m3 and a \(\eta\) of 0.0001 Pa.s.