Progressive lawsonite eclogitization of the oceanic crust: Implications for deep mass transfer in subduction zones Open Access

Mass and elemental transfer take place in convergent margins on Earth. For example, global water cycling occurs via the subduction of the metamorphosed oceanic lithosphere to the deep mantle (Schmidt and Poli, 1998). During metamorphism of the subducting lithosphere, the slab top experiences a series of metamorphic dehydration reactions with depth, resulting in aqueous fluid and/or hydrous melt release at fore- and subarc depths (Hernández-Uribe et al., 2020a, 2020b). Lawsonite and phengite may remain stable beyond subarc depths, conveying large amounts of water to the mantle (Pawley, 1994; Hernández-Uribe and Palin, 2019). Lawsonite is particularly important for mass cycling on Earth because it can host ~11.5 wt% of H₂O in its structure (as crystal-bound OH−) as well as non-trivial amounts of Sr, Pb, Th, U, rare earth elements (REE), and Sr-Pb isotope signatures; furthermore, lawsonite stability extends to ~300 km depth (Pawley, 1994; Tsujimori et al., 2006a; Hara et al., 2018; Fornash et al., 2019; Whitney et al., 2020). Thus, lawsonite can be a great mass carrier to the deep Earth and its breakdown is crucial for deep fluid release and trace-element cycling over a substantial depth range within a subduction zone.

Lawsonite equilibria at (ultra)high-pressure ([U]HP) conditions remain paradoxical in spite of being crucial for subduction-zone processes: petrological experiments and petrological models indicate that lawsonite is common at pressure and temperature (P-T) conditions relevant to subduction zones, however lawsonite-bearing (U)HP assemblages are rare on Earth (Zack et al., 2004; Clarke et al., 2006; Whitney and Davis, 2006; Tsujimori and Ernst, 2014). Explanations for the lack of lawsonite eclogites in the rock record are related to both formation and exhumation issues. In the former, some works have suggested that lawsonite eclogites form only in cold subduction zones (Tsujimori et al., 2006a; Hernández-Uribe and Palin, 2019), whereas in the latter, scarcity of lawsonite eclogites in the rock record has been explained by the necessity of cold, wet, and/or rapid exhumation (Zack et al., 2004; Clarke et al., 2006; Tsujimori et al., 2006a, 2006b; Whitney and Davis, 2006; Wei and Clarke, 2011).

In this contribution, we use petrological modeling to evaluate the viability of lawsonite-eclogite formation during subduction of the uppermost oceanic crust in a typical subduction zone, from subduction initiation to its adulthood. We find that infant subduction zones do not stabilize lawsonite-bearing assemblages; by contrast, lawsonite assemblages dominate the upper oceanic crust during mature stages of subduction. These results provide a simple but elegant answer for the lawsonite paradox and have key implications for the deep water and elemental recycling on Earth.

To explore the formation of lawsonite-bearing (U)HP assemblages during subduction, we considered different compositions representative of the uppermost mafic portion of the oceanic crust: a normal mid-ocean-ridge basalt (N-MORB), an oceanic island basalt (OIB), and a gabbro. Phase modeling was performed using Theriak-Domino software (de Capitani and Petrakakis, 2010) and a-x models for constraining phase equilibria in metabasic rock types (see the Supplemental Material¹ for details). Metamorphism was modeled along the P-T paths defined by the reference model from Holt and Condit (2021), which provides time-evolving slab-top P-T paths recording the evolving thermal structure of a subduction zone. Such modeled paths are representative of a typical subduction zone given that they are consistent with the rock record and with other thermomechanical models (cf. Holt and Condit, 2021). Calculations were performed for rocks entering the trench at different stages of subduction: (1) at ~5.7 m.y. (subduction infancy), (2) at ~11.9 m.y. (free-sinking stage), (3) at ~32.7 m.y. (mature subduction), and (4) at ~49.6 m.y. (adult subduction).

The metamorphosed mafic portion of the oceanic crust does not stabilize lawsonite during the subduction infancy (Fig. 1). During the free-sinking stage, the N-MORB and OIB stabilize lawsonite-bearing assemblages at the onset of the modeled P-T path, but lawsonite is subsequently consumed in the middle part of the path at ~1.73−1.75 GPa and ~493−506 °C. The gabbroic lower oceanic crust does not stabilize lawsonite during this stage (Fig. 1). As the subduction zone matures and cools down with time, a major part of the slab-top P-T path evolves through the lawsonite stability field in the N-MORB and OIB, yet lawsonite still breaks down at ~2.65−2.67 GPa and ~648−667 °C; by contrast, in the gabbro, lawsonite is not stable in the earlier part of the modeled P-T path but stabilizes at ~2.39 GPa and ~695 °C (Fig. 1). Finally, in the adult subduction stage, the N-MORB and OIB form and preserve lawsonite-bearing assemblages through the entire P-T path, whereas the gabbro stabilizes lawsonite in the latter portion of the P-T path (Fig. 1). Our current predictions for the gabbro may differ if: (1) gabbro is located close to the slab Moho where paths are colder and enhance lawsonite stability; and (2) the dehydration of the slab mantle contributes with fluid (Spandler and Pirard, 2013), expanding lawsonite stability.

Figure 2 shows the detailed phase-equilibria evolution for the N-MORB. At subduction infancy, the metamorphosed MORB is dominated by epidote-bearing greenschist- and amphibolite-facies assemblages. During the free-sinking stage of the slab, the N-MORB stabilizes as much as ~23 vol% of lawsonite at blueschist facies; when lawsonite is consumed, the assemblage is replaced by epidote- and garnet-bearing blueschist- and eclogite-facies assemblages. Lawsonite consumption is translated to a loss of ~2.1 wt% of H₂O stored in the solids. In a mature subduction zone, the N-MORB crust forms as much as ~31 vol% of lawsonite; this mineral is consumed during the gradual transition between blueschist- and eclogite-facies assemblages. Lawsonite and amphibole consumption in this transition results in a loss of ~4.4 wt% of H₂O. Finally, in the last modeled P-T path, the N-MORB stabilizes ~9−31 vol% of lawsonite during subduction. Here, lawsonite stability expands to UHP conditions, within the coesite stability field. At the end of the modeled path, the UHP crust conveys ~1 wt% of H₂O to the deep mantle.

We also explored how different pre- or syn-subduction geochemical processes affect lawsonite eclogitization of the slab top in mature subduction zones. For this, temperature-composition (T-X) diagrams were calculated to explore the mineralogical changes that may occur between an N-MORB and (1) the altered mafic oceanic crust, (2) the global composition of the subducting sediments, (3) a mantle-wedge serpentinite, and (4) an extreme case of Ca-Al metasomatism (see the Supplemental Material). Phase diagrams were calculated at 2.50 GPa, a pressure representative of the maximum depth of recovery of HP rocks and a representative pressure of subarc depths (Agard et al., 2009).

The temperature limit of lawsonite stability is not affected by seafloor alteration and is similar to the one calculated for the subducting sediments as well as for the Ca-Al metasomatite—in all these cases, lawsonite breaks down at ~625−635 °C at 2.50 GPa (Fig. 3). By contrast, hybridization between the altered crust and the serpentinized mantle wedge inhibits lawsonite stability in rocks with >50% hybridization (Fig. 3).

The proportion of lawsonite, however, strongly varies depending on the whole-rock composition, mainly by changes in the bulk Ca/Al ratio (Table S1 in the Supplemental Material). For instance, the amount of lawsonite slightly increases with alteration (by ~5 vol%; Fig. 3B). Intense hybridization between the altered crust and the mantle wedge serpentinites results in the decrease of lawsonite proportion until its eventual consumption (Fig. 3B). Hybridization between the mafic crust and the sediments decreases lawsonite proportion as the composition departs from MORB; for example, at 625 °C, lawsonite proportions decreases from ~15 vol% in the MORB to ~5 vol% in the subducting sediments (Fig. 3B). Finally, as sediments are affected by different degrees of Ca-Al metasomatism, the amount of lawsonite increases significantly, by as much as ~60 vol% in extreme scenarios (Fig. 3B).

We find that pre- or syn-subduction geochemical processes affecting the slab top control the transformation of the crust to lawsonite-bearing assemblages. For example, lawsonite (U)HP assemblages are favored by seafloor alteration and Ca-Al metasomatism. By contrast, lawsonite stability is significantly inhibited by high degrees of hybridization (>50%) of the mafic crust with the serpentinized mantle wedge (Fig. 3) in cases where no external high-Ca/Al metasomatic fluids occur. This may imply that tectonic mélanges (i.e., slab-mantle interface analogues) that experience significant hybridization are prone to form lawsonite-free (U)HP lithologies, plausibly accounting for the lack of lawsonite in some tectonic mélanges.

The observed lawsonite-stability systematics has key implications for the petrological imprint on Earth. Exhumation during early stages of subduction would result in the recovery of HP metabasites dominated by epidote-bearing assemblages (e.g., metamorphic soles; Fig. 2A). Early exhumation of the upper oceanic crust during the free-sinking stage would result in the recovery of lawsonite-bearing blueschist assemblages (Fig. 2B), whereas exhumation later in the path would result in the recovery of epidote-bearing blueschists and eclogites (Fig. 2B). Only with aging of the subduction zone would exhumation likely result in the recovery of lawsonite-bearing eclogites. Importantly, at the maximum depth of recovery (~2.5 GPa; Agard et al., 2009) and where most lawsonite-eclogite P-T conditions are observed (Tsujimori et al., 2006a; Wei and Clarke, 2011; Tsujimori and Ernst, 2014; Whitney et al., 2020), only oceanic crust that subducted late in the subduction-zone lifetime evolves through such P-T conditions (Figs. 1 and 3).

Based on the above, a simple but elegant answer for the lawsonite paradox is that the formation of lawsonite eclogite is favored in mature subduction zones (Figs. 1 and 2). The preservation of lawsonite in this scenario would still necessitate particular exhumation conditions in order to preserve lawsonite during exhumation (Zack et al., 2004; Clarke et al., 2006; Tsujimori et al., 2006a, 2006b; Whitney and Davis, 2006; Wei and Clarke, 2011). Our conclusion may remain valid for warmer- or colder-than-average subduction-zone conditions—at the onset of subduction initiation, high- and very high-T/P paths would preclude lawsonite stabilization. Finally, we note that our calculations represent a maximum of the amount of lawsonite stable in slab-top lithologies given that fluid fractionation was not considered.

Overall, our thermodynamic calculations agree with the rock record. For example, blueschist with pseudomorphs after lawsonite from Syros (Greece) are interpreted to represent former lawsonite that did not survive prograde heating (Schumacher et al., 2008; Hamelin et al., 2018). Furthermore, evidence from the rock record (Agard et al., 2018) shows that exhumation of HP and low-temperature rocks from coherent terranes happens later in the subduction history compared to high-temperature rocks, which exhume early in nascent subduction zones, in agreement with our models. Importantly, lawsonite eclogite is absent in terranes that exhumed during early stages of subduction, whereas terranes that exhumated late in the lifetime of a subduction zone do show lawsonite eclogites (Agard et al., 2009, and references therein).

The dominance of lawsonite-free (U)HP metabasites (and the systematic scarcity of lawsonite [U]HP metabasites) in the subduction-related rock record indirectly suggests that exhumation may be favored during early and warm stages of subduction, where epidote is more prevalent than lawsonite (Figs. 1 and 2). Overall, this seems to agree with other studies suggesting an exhumation bias in the rock record toward hotter conditions (van Keken et al., 2018). Alternatively, the lack of lawsonite in (U)HP metabasites in the rock record could also suggest that subduction zones do not tend to reach full maturation to promote the formation of lawsonite eclogites.

Intense Ca-Al metasomatism and different styles of seafloor alteration are important for stabilizing significant amounts of lawsonite (Fig. 3). While such lithologies may be minor compared to the overall mafic portion in the oceanic crust, their contribution to the water- and mass-transfer cycle in subduction zones is significant (Martin et al., 2014; Vitale Brovarone and Beyssac, 2014; Hernández-Uribe et al., 2020a). We, however, find that regardless of the degree of alteration and/or metasomatism, the P-T range where lawsonite is stable at subarc depths is not significantly affected (i.e., not expanded to higher temperatures; Fig. 3). Thus, we argue that the role of metasomatism may be more limited than previously thought. For example, if a rock experienced Ca-Al metasomatism near to or at peak conditions (e.g., 2.5 GPa; Fig. 3) during the infancy of the subduction zone, lawsonite would be absent in the phase assemblage (Fig. 3; Fig. S1 in the Supplemental Material). Similarly, if a Ca-Al metasomatite (e.g., epidosite) subducted early in the lifetime of a subduction zone or prior to full maturation, the mineral assemblage would be dominated by epidote-bearing assemblages as the P-T path evolves outside the lawsonite stability field, regardless of Ca-Al metasomatism and/or seafloor alteration. Ca-Al metasomatism, however, can increase the capacity for element transfer of the subducting crust by increasing lawsonite proportion (Fig. 3; Fig. S1).

Accepted models for mass cycling on Earth rely on lawsonite as one of the main carriers of H₂O and trace elements to the deep mantle (Martin et al., 2014; Vitale Brovarone and Beyssac, 2014; Fornash et al., 2019; Whitney et al., 2020). Our models presented here challenge—to some extent—such a conception. We show that lawsonite breakdown and lawsonite-driven fluid release occurs at different depths within the forearc region depending on the lifetime stage of the subduction zone; yet lawsonite remains stable in the subducting crust in mature subduction zones. Therefore, we argue that the potential of lawsonite in the mafic crust to carry mass deep into mantle is limited by the aging of the subduction zone. Lawsonite-driven mass transfer is key for long-term global cycling in subduction zones that reach full maturation (>30 m.y.), whereas in nascent and infant subduction zones, lawsonite-driven cycling is negligible. In these scenarios, other phases such as amphibole, phengite, and/or epidote (Hacker, 2008; Hernández-Uribe et al., 2020a) drive elemental cycling.

We thank C.B. Condit and A.F. Holt for providing the slab-top paths used in this work. This work was funded, in part, by a grant from the Japan Society for the Promotion of Science Kakenhi JP21H01174 to Tsujimori. Reviews by C. Wei and two anonymous reviewers greatly improved the final version of this work and are gratefully acknowledged. M. Norman is thanked for editorial handling.