Assessment of fluid Fe isotope fractionation within the TAG mound. In a typical mid-ocean ridge setting, the Fe content and isotopic composition of the hydrothermal fluid is initially set during high-temperature leaching and alteration of basalt in the reaction zone. This process is suggested to enrich the fluid in Fe that is isotopically light (56Fe-depleted) relative to MORB (δ56FeMORB ≈ + 0.1‰)23,25,54−56. Upon entering and migrating through a large sulfide mound such as TAG, the fluid will be progressively modified through mineral precipitation in open spaces as well as reworking of pre-existing hydrothermal precipitates along its path to the seafloor14,57. However, it remains unclear in how far fluid Fe isotope compositions are affected (fractionated) during such interaction24–26. Experimental studies have indicated that chalcopyrite formed in seafloor hydrothermal systems rapidly achieves Fe isotopic equilibrium with the co-existing fluid and hence chalcopyrite δ56Fe values can be used to assess fluid compositions31. Utilizing the fractionation factor provided by ref. 31 (i.e., Δ56Fechalcopyrite−Fe(aq) = 0.09 ± 0.17‰, 2σ), three vein-related chalcopyrite samples from the lower part of the TAG mound (30 to 50 m.b.s.f.) yield equilibrium fluid δ56Fe values of -0.13 to -0.01‰ (± 0.18‰, 2σ; Supplementary Table 1), which are about 0.1 to 0.25‰ lower than MORB values23,56. The range of our inferred δ56Fefluid values overlaps with and extends to slightly higher values relative to recently (1998) measured vent fluids from the Black Smoker Complex (-0.17 to -0.11 ± 0.02‰, 2σ)28. Since anhydrite within the TAG mound dissolves during inactive periods, the preservation of anhydrite in the chalcopyrite-bearing veins indicates that they formed during the current high-temperature hydrothermal cycle that commenced at approximately 100 years ago37,39,58,59. Our inferred and the measured28 fluid datasets can thus be interpreted in conjunction and imply that recent TAG hydrothermal fluids experience small negative shifts in their δ56Fe values (< -0.1‰) during ascent through the mound to the black smoker vent site, which we chiefly attribute to hydrothermal maturation of pyritic sulfides (discussed further below). Following venting, oxidation of Fe2+ and precipitation of Fe-oxyhydroxides within the TAG hydrothermal plume cause a more extensive decrease in the δ56Fe values of dissolved Fe, down to -1.35‰13,32. This final volume of dissolved Fe with low δ56Fe values has been observed to then travel in seawater via currents up to thousands of kilometres away from the TAG site, likely influencing surface planktonic activity in the North Atlantic Ocean13. Our results now allow us to track the isotopic evolution of dissolved Fe throughout the TAG hydrothermal system and reveal an overall and stepwise decrease in δ56Fe values. This finding expands our knowledge on the sequence of (bio-)geochemical processes that contribute to the Fe isotopic signature of hydrothermally sourced Fe8, 24–29,32,54 and will thereby help to further refine identification and quantification of such Fe in the global oceans and in associated sedimentary records8,13,60.
Evolution of sub-seafloor mineralization at TAG constrained by pyrite δ 56 Fe values. Based on the δ56Fefluid values from ref. 28 and those calculated in this study, combined with the Fe isotope fractionation factors of ref. 30 and ref. 34, the range of δ56Fe values of pyrite in equilibrium with recent TAG hydrothermal fluids is expected to be + 0.45 to + 1.32‰ (including the 2σ uncertainty; Fig. 3). Notably, only a small subset of our pyrite data plot within this range (6 out of 47; Fig. 3, Supplementary Table 1). However, unlike chalcopyrite, formation of pyrite in seafloor hydrothermal systems has been proposed to occur via transient precursor mineral species that may impose significant kinetic effects on Fe isotope fractionation26,30,34,61,62. On the basis of a synthesis of available experimental and theoretical data, ref. 34 proposed a two-stage model for the formation and Fe isotopic equilibration of hydrothermal pyrite at high temperature (> 300°C). In this model, a rapid (few days) initial stage of pyrite formation occurs via an aqueous Fe (poly)sulfide precursor phase whose detailed nature (stoichiometry, coordination and magnetic spin) depends on the fluid composition. This short-lived Fe (poly)sulfide precursor obtains Fe isotopic equilibrium with the hydrothermal fluid, and its δ56Fe signature is then transferred without fractionation upon conversion to pyrite. The initial pyrite will thereby be out of Fe isotopic equilibrium with the hydrothermal fluid and will have a δ56Fe value that may be up to ~ 1.5‰ lower than the value expected for pyrite in equilibrium with the fluid (Δ56Fepyrite−Fe(aq) ≈ 0.8–1‰ at 300–450°C)30,34. The δ56Fepyrite value will then gradually increase toward this equilibrium value during a subsequent and much slower stage of pyrite recrystallization and Fe isotopic equilibration with the fluid30,34. Rates of ≳ 1 year to reach pyrite–fluid Fe isotopic equilibrium have been estimated under ideal (experimental) conditions34, although it is not yet clear what timescales apply to the progressive hydrothermal maturation of pyrite commonly observed in natural SMS and VMS systems52,63. Here we attempt to shed further light on this issue and integrate our texturally resolved pyrite Fe isotope dataset into a refined geologic and hydrothermal framework of TAG15,19, 35–39,52.
The lowest δ56Fepyrite values at TAG are found in the massive sulfide mineralization concentrated in the upper parts of the mound (Fig. 3, Table 1). These values are distinctly lower than the estimated δ56Fe values of pyrite in equilibrium with TAG fluids (Fig. 3) and they partly overlap with δ56Fepyrite values reported from seafloor sulfide chimneys elsewhere25,26. The most negative δ56Fepyrite values in our sample suite reach down to -1.27‰ and correspond to porous massive sulfide samples from TAG-4 that exhibit well-developed colloform textures (Fig. 2b). The preservation of such primary depositional features in the massive sulfide and its ‘chimney-like’ pyrite Fe isotope signature are consistent with recent formation at or near the mound surface by growth into open space15. Here, mixing between high-temperature hydrothermal fluid and cold seawater likely led to rapid precipitation of pyrite with kinetically-driven low δ56Fe values (i.e., strong pyrite–fluid disequilibrium), which have not yet been extensively modified by later hydrothermal maturation (Fig. 3).
Exploring this phenomenon in more detail, we find that pyrite samples from the anhydrite veins in the lower part of the TAG mound (Fig. 2c) have low δ56Fe values that overlap the values for massive sulfide (Fig. 3, Table 1). Notably, the majority of these δ56Fepyrite values are lower than the δ56Fe values of co-existing chalcopyrite from the veins. This confirms significant Fe isotopic disequilibrium in this pyrite group, since pyrite should have higher δ56Fe values than those of chalcopyrite if in isotopic equilibrium (Δ56Fepyrite−chalcopyrite ≈ 0.9‰ at 350°C)31. The presence of anhydrite suggests that the veins formed recently ( ≲ 100 years ago37, see above) due to mixing between hydrothermal fluid and entrained seawater within the mound59. Similar to the massive sulfide, the low δ56Fepyrite values of the anhydrite veins can thus be explained by rapid pyrite precipitation followed by only very limited hydrothermal maturation and Fe isotopic equilibration with later fluids (Fig. 3). On the other hand, the coarser-grained pyrite from quartz-pyrite stringer veins in the TAG stockwork (Fig. 2e) have distinctly higher δ56Fe values than those of pyrite from the anhydrite veins (Fig. 3, Table 1). These data are consistent with the deep quartz-pyrite stringer veins having formed due to conductive cooling of the hydrothermal fluid rather than as a result of mixing with seawater, such that the rates of pyrite precipitation were slower and greater Fe isotopic equilibration between pyrite and the fluid could occur26,34,59. Furthermore, crosscutting relationships confirm that the quartz-pyrite stringer veins are older than the anhydrite veins53 and have thus likely been subjected to more extensive hydrothermal maturation, leading to further increase in the δ56Fepyrite values30,34 (Fig. 3).
Pyrite in the sulfide clasts that occur in the different breccia types (Fig. 2d) show δ56Fe values that are overall higher than, but in part overlap the δ56Fepyrite values of the massive sulfide and the anhydrite veins. The values are similar to the δ56Fepyrite values of the quartz-pyrite stringer veins, but are always lower than the estimated δ56Fe values of pyrite in equilibrium with the TAG hydrothermal fluids (Fig. 3, Table 1). Pyrite in these clasts likely have diverse and possibly complex origins that involve combinations of recrystallization and reworking of surficial (massive) and vein-related mineralization as well as in-situ nucleation and growth of new pyrite15. Such heterogeneous pyrite assemblages should initially have δ56Fe values similar to those of the massive and the vein-related pyrite described above, but the δ56Fepyrite values will progressively shift to higher values as a result of variable degrees of hydrothermal maturation during the protracted development of the TAG breccias, thus offering a sensible explanation for the observed data spread in this particular sample group (Fig. 3).
Remarkably, the finely disseminated pyrite preserved within remnant fragments of altered basalt (Fig. 2e) has the highest δ56Fe values observed at TAG, showing only minor overlap with δ56Fepyrite values of the quartz-pyrite stringer veins and the sulfide breccia clasts (Fig. 3, Table 1). Within individual core samples, disseminated pyrite always has distinctly higher δ56Fe values than those of pyrite that texturally overprints the altered basalt clasts (e.g., stringer veins or massive sulfide cement; Fig. 3). High-temperature hydrothermal alteration of the basaltic basement rocks would have commenced during the initial stages of the evolution of TAG and involves chloritization followed by progressive paragonitization and silicification of the basalt, with pyrite forming throughout the alteration sequence38,39. Clasts of such variably altered basalt do not only occur in the TAG stockwork, but have also been incorporated into the mound breccias at depths much shallower than that expected for the top of the basement. The reason for this is poorly understood, but could potentially be related to processes akin to ‘frost jacking and heaving’ during repeated expansion (due to internal anhydrite precipitation) and collapse (anhydrite dissolution) of the TAG mound over several high-temperature hydrothermal cycles15,38. Disseminated pyrite preserved in the basalt clasts may thus represent the most extensively reworked sulfide sample material of this study. Such hydrothermal maturation appears to occur intermittently over tens of thousands of years during which the basaltic basement is progressively altered and assimilated into the TAG breccias15,37−39, imparting a characteristic isotope signature of δ56Fepyrite values in near-equilibrium to equilibrium with the hydrothermal fluids (Fig. 3).
In summary, we interpret the observed range of δ56Fe values for different textural types of pyrite to reflect contrasting modes of formation (fluid–seawater mixing vs. fluid conductive cooling) and variable degrees of progressive hydrothermal maturation during the evolution of the TAG mound and stockwork complex (Fig. 3). In contrast to the idealized rates suggested from experiments ( ≳ 1 year to reach pyrite–fluid equilibrium34), our results suggest that Fe isotopic equilibration during hydrothermal maturation of pyrite occurs over timescales of tens of thousands of years within large and periodically inactive SMS deposits such as TAG, allowing the preservation of the δ56Fepyrite variations that we observe. The observed Fe isotope variations further imply that the W part of the TAG mound (TAG-4) has experienced less extensive hydrothermal maturation than the other parts, consistent with the mineralogical and geochemical asymmetry noted during the original ODP investigation52. Importantly, similar processes can probably explain Fe isotope variations in sulfides from fossil onshore VMS deposits (e.g., immature, low-δ56Fe ‘black ores’ and mature, high-δ56Fe ‘yellow ores’), such as the ones found in the classic Kuroko deposits of Japan33. Our study of TAG therefore concludes that sulfide Fe isotope compositions can provide detailed insight into the nature, longevity and dynamics of hydrothermal processes in SMS deposits and allow us to create a valuable reference framework for future investigation of similar active and fossil hydrothermal systems elsewhere.