Niels Jöns

Magnetite in seafloor serpentinite—Some like it hot

Serpentinization of mantle peridotite generates molecular hydrogen that can be exploited by microorganisms to gain metabolic energy; however, the mechanisms that control hydrogen generation and magnetite formation during serpentinization remain poorly understood. We have examined partly to completely serpentinized peridotites recovered during the Ocean Drilling Program and find a remarkable variation in the abundance of magnetite. Some completely serpentinized peridotites have as much as 6.15 wt% magnetite, whereas others are nearly magnetite free (

The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context: implications for the amalgamation of the Gondwana supercontinent

We examined the petrological characteristics of the Graphite group and Androyan group in Southern Madagascar, south of the prominent Ranotsara shear zone, and we performed U-Pb SHRIMP dating on zircon and U-Th-total Pb dating on monazite. Widespread high-temperature metamorphism is evidenced by Spl–Qtz assemblages occurring over ca . 75,000 km 2 in the whole Androyan group. The occurrence of symplectites consisting of Crd + Kfs + Qtz + Opx or Crd + Kfs + Q333tz + Bt, which are interpreted as pseudomorphs after osumilite, is restricted to a smaller area of about 250 km 2 . Furthermore, in some pelites Spr + Qtz + Sil or Opx + Sil + Qtz formed the peak-metamorphic assemblage, which broke down to Crd ± Spl. Orthopyroxene in metapelites is aluminous with Al 2 O 3 = 9–10 wt%. Peak-metamorphic conditions of T = 950–1000 °C and P = 8–11 kbar are followed by decompression at high temperatures, as shown by the formation of Crd + Opx 2 (Opx with 6–8 wt% Al 2 O 3 ) symplectites from Grt–Qtz–Opx 1 (8–9 wt% Al 2 O 3 ). The pressure decrease is furthermore constrained by Spr–Crd symplectites in SiO 2 -undersaturated metapelites, and extensive formation of late-stage cordierite in the whole Androyan group. During subsequent cooling, cordierite broke down to form And + Qtz + Carbonate/Chl. Throughout the Androyan group, ages of 560–530 Ma have been obtained from monazite (M 2 metamorphism). Samples which do not contain ultrahigh-temperature assemblages provide evidence for an earlier metamorphic event at 650–600 Ma (M 1 ) in monazite cores. Zircon generally shows both metamorphic ages. Therefore, the deduced clockwise P-T evolution of the UH T metamorphism is interpreted to correspond to the M 2 stage, which affected the whole Androyan group. P-T conditions of the older M 1 metamorphism are generally unrecognisable. High temperature metamorphic conditions during M 2 are likely caused by intense charnockite emplacement. The near-isothermal decompression points to subsequent rapid exhumation of the formerly overthickened crust during the M 2 metamorphism at 560–530 Ma. We interpret this metamorphic stage to reflect the assembly of the Gondwana supercontinent, most likely related to the collision of the Tanzania Craton with the Azania microcontinent subsequent to closure of the Mozambique Ocean.

From Closure of the Mozambique Ocean to Gondwana Breakup : New Evidence from Geochronological Data of the Vohibory Terrane, Southwest Madagascar

A multi‐thermochronometer study of basement rocks, using combined Pb‐Pb zircon, U‐Th‐total Pb monazite, U‐Pb SHRIMP titanite, 40Ar/39Ar biotite, and apatite fission track data, was performed to derive a detailed cooling history for the Vohibory terrane, southwest Madagascar. The main metamorphism at ca. 640–610 Ma is interpreted to represent the closure of the Mozambique Ocean and the subsequent accretion of the Vohibory arc terrane to the active continental margin of Azania (proto‐Madagascar). Contemporaneous with the assumed main Gondwana‐forming collision between Azania/India and the Congo craton at ca. 535 Ma, slow cooling (8.1°–3.6°C/m.yr.) indicates extrusion of the Vohibory block. This central part of the East African orogen reached thermal equilibrium between 500 and 350 Ma. During late Carboniferous/Triassic rifting between East Africa and Madagascar, the Vohibory terrane was the favored starting point for crustal extension, causing basement rock cooling (up to 5.3°C/m.yr.) and heating (up to 1.6°C/m.yr.). The Jurassic passive margin evolution was accompanied by rift locus migration to the west of the Vohibory block. The resulting rift geometry and associated sedimentation caused flexural isostatic response and inversion of the Vohibory part of the late Carboniferous/Triassic rift.

Focused radiogenic heating of middle crust caused ultrahigh temperatures in southern Madagascar

Internal heating can cause melting, metamorphism, and crustal weakening in convergent orogens. This study evaluates the role of radiogenic heat production (RHP) in a Neoproterozoic ultrahigh-temperature metamorphic (UHTM) terrane exposed in southern Madagascar. Monazite and zircon geochronology indicates that the Paleoproterozoic Androyen and Anosyen domains (i) collided with the oceanic Vohibory Arc at ~630 Ma, (ii) became incorporated into the Gondwanan collisional orogen by ~580 Ma, and (iii) were exhumed during crustal thinning at 525–510 Ma. Ti-in-quartz and Zr-in-rutile thermometry reveals that UHTM occurred over >20,000 km^2, mostly within the Anosyen domain. Assuming that U, Th, and K contents of samples from the field area are representative of the middle to lower crust during orogenesis, RHP was high enough—locally >5 μW/m^3—to cause regional UHTM in <60 Myr. We conclude that, due in large part to the stability and insolubility of monazite at high crustal temperatures, RHP was the principal heat source responsible for UHTM, obviating the need to evoke external heat sources. Focused RHP probably thermally weakened portions of the middle crust, gravitationally destabilizing the orogen and facilitating thinning via lateral extrusion of hot crustal sections.

Metasomatism Within the Ocean Crust

From ridge to trench, the ocean crust undergoes extensive chemical exchange with seawater, which is critical in setting the chemical and isotopic composition of the oceans and their rocky foundation. Although the overall exchange fluxes are great, the first-order metasomatic changes of crustal rocks are generally minor (usually <10% relative change in major element concentrations). Drastic fluid-induced metasomatic mass transfers are limited to areas of very high fluid flux such as hydrothermal upflow zones. Epidotization, chloritization, and serizitization are common in these upflow zones, and they often feature replacive sulfide mineralization, forming significant metal accumulations below hydrothermal vent areas. Diffusional metasomatism is subordinate in layered (gabbroic-doleritic-basaltic) crust, because the chemical potential differences between the different lithologies are minor. In heterogeneous crust (mixed mafic-ultramafic lithologies), however, diffusional mass transfers between basaltic lithologies and peridotite are very common. These processes include rodingitization of gabbroic dikes in the lithospheric mantle and steatitization of serpentinites in contact to gabbroic intrusions. Drivers of these metasomatic changes are strong across-contact differences in the activities of major solutes in the intergranular fluids. Most of these processes take place under greenschist-facies conditions, where the differences in silica and proton activities in the fluids are most pronounced. Simple geochemical reaction path models provide a powerful tool for investigating these processes. Because the oceanic crust is hydrologically active throughout much of its lifetime, the diffusional metasomatic zones are commonly also affected by fluid flow, so that a clear distinction between fluid-induced and lithology-driven metasomatism is not always possible. Heterogeneous crust is common along slow and ultraslow spreading ridges, were much of the extension is accommodated by faulting (normal faults and detachment faults). Mafic-ultramafic contacts hydrate to greater extents and at higher temperatures than uniform mafic or ultramafic masses of rock. Hence, these lithologic contacts turn mechanically weak at great lithopheric depth and are prone to capture much of the strain during exhumation and uplift of oceanic core complexes. Metasomatism therefore plays a critical role in setting rheological properties of oceanic lithosphere along slow oceanic spreading centers, which – by length – comprise half of the global mid-ocean ridge system.

U‐Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid‐Atlantic Ridge

New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected 206Pb/238U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1–20 µm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected 206Pb/238U dates for the inclusion-rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206Pb/238U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges.

Fluid circulation and carbonate vein precipitation in the footwall of an oceanic core complex, Ocean Drilling Program Site 175, Mid‐Atlantic Ridge

Carbonate veins recovered from the mafic/ultramafic footwall of an oceanic detachment fault on the Mid-Atlantic Ridge record multiple episodes of fluid movement through the detachment and secondary faults. High-temperature (∼75–175°C) calcite veins with elevated REE contents and strong positive Eu-anomalies record the mixing of up-welling hydrothermal fluids with infiltrating seawater. Carbonate precipitation is most prominent in olivine-rich troctolite, which also display a much higher degree of greenschist and sub-greenschist alteration relative to gabbro and diabase. Low-temperature calcite and aragonite veins likely precipitated from oxidizing seawater that infiltrated the detachment fault and/or within secondary faults late or post footwall denudation. Oxygen and carbon isotopes lie on a mixing line between seawater and Logatchev-like hydrothermal fluids, but precipitation temperatures are cooler than would be expected for isenthalpic mixing, suggesting conductive cooling during upward flow. There is no depth dependence of vein precipitation temperature, indicating effective cooling of the footwall via seawater infiltration through fault zones. One sample contains textural evidence of low-temperature, seawater-signature veins being cut by high-temperature, hydrothermal-signature veins. This indicates temporal variability in the fluid mixing, possibly caused by deformation-induced porosity changes or dike intrusion. The strong correlation between carbonate precipitation and olivine-rich troctolites suggests that the presence of unaltered olivine is a key requirement for carbonate precipitation from seawater and hydrothermal fluids. Our results also suggest that calcite-talc alteration of troctolites may be a more efficient CO2 trap than serpentinized peridotite.

Rare earth element evolution and migration in plagiogranites: a record preserved in epidote and allanite of the Troodos ophiolite

Plagiogranites from the Troodos ophiolite in Cyprus are occasionally epidotised, either partially or completely. Epidotisation phenomena include replacement of pre-existing minerals and filling of miarolitic cavities. In addition to epidote, miarolites in one plagiogranite body (located near the village of Spilia) contain coexisting ferriallanite-(Ce) and allanite-(Y). Textural and geochemical evidence indicates that late-stage REE-enriched granitic melt facilitated crystallisation of magmatic ferriallanite-(Ce). High REE contents persisted after fluid exsolution, causing crystallisation of allanite-(Y) from hydrothermal fluids in the miarolites. The REE pattern of the hydrothermal allanite-(Y) is characterised by LREE and Eu depletion, similar to the parent plagiogranitic magma. As allanite had sequestered most of the REE in the fluid, epidote took over as the principle hydrothermal mineral. Epidote in Troodos plagiogranites records a fluid evolutionary trend beginning with REE-rich–Eu-depleted similar to allanite-(Y) and gradually transforming into the REE-depleted–Eu-enriched pattern prevalent throughout ‘conventional’ sub-seafloor fluids. A comparison of allanite-bearing and allanite-absent plagiogranites from the same locality suggests that REE-bearing fluids migrated from the plagiogranites. Similar fluid evolution trends observed in diabase-hosted epidote, located adjacent to a large plagiogranite body, suggest influx of plagiogranite-derived REE-bearing fluids. Epidotisation in oceanic settings is usually considered to be the result of alteration by high fluxes of seawater-derived hydrothermal fluids. Although epidotisation by magmatic fluids has been suggested to occur in plagiogranites, our study shows that this autometasomatic process is the dominant mechanism by which epidosites form in plagiogranites. Furthermore, epidotisation of diabase has been attributed solely to seawater-derived fluids, but we show that it is possible for diabase-hosted epidosites to form by migration of plagiogranite-derived fluids.