High-temperature Hydrothermal Circulation Research Articles

Characterization of Geochemistry in Hydrothermal Sediments From the Newly Discovered Onnuri Vent Field in the Middle Region of the Central Indian Ridge

The recently discovered Onnuri hydrothermal vent field (OVF) is a typical off-axis ultramafic-hosted vent system, located on the summit of the dome-like ocean core complex (OCC) at a distance of ∼12 km from the ridge axis along the middle region of the Central Indian Ridge (CIR). The plume chemistry with high methane anomaly was consistent with the precursor of hydrothermal activity; however, the fundamental characteristic of the OVF system, such as the hydrothermal circulation process and source of heat, remains poorly understood. Here, we focus on the geochemical features of surface sediments and minerals collected at and around the OVF region in order to better understand this venting system. The results reveal that the OVF sediments are typified by remarkably high concentrations of Fe, Si, Ba, Cu, and Zn, derived from hydrothermal fluid and S and Mg from seawater; depleted C-S isotope compositions; and abundant hydrothermally precipitated minerals (i.e., Fe–Mn hydroxides, sulfide and sulfate minerals, and opal silica). Notably, the occurrence of pure talc and barite bears witness to strong hydrothermal activity in the OVF, and their sulfur and strontium isotope geochemistry agree with extensive mixing of the unmodified seawater with high-temperature fluid derived from the gabbroic rock within the ultramafic-dominated ridge segment. The findings reveal that the OVF is a representative example of an off-axis, high-temperature hydrothermal circulation system, possibly driven by the exothermic serpentinization of exposed peridotites. Given the widespread distribution of OCC with detachment faults, furthermore, the OVF may be the most common type of hydrothermal activity in the CIR, although the paucity of data precludes generalizing this result. This study provides important information contributing to our understanding of the ultramafic-hosted hydrothermal vent system with a non-magmatic heat source along mid-ocean ridges.

Deep high-temperature hydrothermal circulation in a detachment faulting system on the ultra-slow spreading ridge

Coupled magmatic and tectonic activity plays an important role in high-temperature hydrothermal circulation at mid-ocean ridges. The circulation patterns for such systems have been elucidated by microearthquakes and geochemical data over a broad spectrum of spreading rates, but such data have not been generally available for ultra-slow spreading ridges. Here we report new geophysical and fluid geochemical data for high-temperature active hydrothermal venting at Dragon Horn area (49.7°E) on the Southwest Indian Ridge. Twin detachment faults penetrating to the depth of 13 ± 2 km below the seafloor were identified based on the microearthquakes. The geochemical composition of the hydrothermal fluids suggests a long reaction path involving both mafic and ultramafic lithologies. Combined with numerical simulations, our results demonstrate that these hydrothermal fluids could circulate ~ 6 km deeper than the Moho boundary and to much greater depths than those at Trans-Atlantic Geotraverse and Logachev-1 hydrothermal fields on the Mid-Atlantic Ridge.

Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system

Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P -wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ∼2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.

Fossil evidence for serpentinization fluids fueling chemosynthetic assemblages

Among the deep-sea hydrothermal vent sites discovered in the past 30 years, Lost City on the Mid-Atlantic Ridge (MAR) is remarkable both for its alkaline fluids derived from mantle rock serpentinization and the spectacular seafloor carbonate chimneys precipitated from these fluids. Despite high concentrations of reduced chemicals in the fluids, this unique example of a serpentinite-hosted hydrothermal system currently lacks chemosynthetic assemblages dominated by large animals typical of high-temperature vent sites. Here we report abundant specimens of chemosymbiotic mussels, associated with gastropods and chemosymbiotic clams, in approximately 100 kyr old Lost City-like carbonates from the MAR close to the Rainbow site (36 °N). Our finding shows that serpentinization-related fluids, unaffected by high-temperature hydrothermal circulation, can occur on-axis and are able to sustain high-biomass communities. The widespread occurrence of seafloor ultramafic rocks linked to likely long-range dispersion of vent species therefore offers considerably more ecospace for chemosynthetic fauna in the oceans than previously supposed.

Crustal diopsidites from the northern Oman ophiolite: Evidence for hydrothermal circulation through suboceanic Moho

This paper details a new discovery of anorthite-bearing diopsidites (= anorthite diopsidites) from the lowermost crust along Wadi Fizh in the northern Oman ophiolite. The anorthite diopsidites occur as networks within layered gabbros 50 m above the gabbro/peridotite boundary, and are mainly composed of high-Mg diopsidic clinopyroxenes and anorthites with various amounts of uvarovite. They are intermediate in mineral chemical characteristics between the crustal gabbros and the diopsidites, and interpreted as an interaction product between high-temperature Ca-rich hydrothermal fluids and peridotites within the mantle section. The anorthite diopsidites were probably formed by interaction between the fluids involved in diopsidization within the mantle section and the layered gabbro. The fluids responsible for the formation of the anorthite diopsidite were also Ca-rich and carried Cr to form uvarovite from the underlying mantle section. High-temperature hydrothermal circulation plays an important role in the transportation of elements, e.g., Ca and Cr, from the upper mantle to the lower crust across the Moho in the ocean floor.

Deep and high-temperature hydrothermal circulation at mid-ocean ridges

Deep and high-temperature hydrothermal circulation at mid-ocean ridges

Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres

Mafic–ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts. In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology made essentially of pure diopside ( 0.95 < M g M g + F e total < 1 ) whose characteristics do not match a magmatic or mantle origin. Arguments against a magmatic or mantle origin for these diopsidites combine compositional and textural evidence. In spite of their refractory composition, they are strongly depleted in Cr (Cr 2O 3 0.2 wt.%). By the same way other minor elements (Al, Ti, Na…) and rare earth elements have peculiarly low abundances and plot away from magmatic differentiation trends. In a few samples, the paragenetic association includes pure anorthite (An% up to 0.99), minor amounts of forsterite (Fo > 0.95) or traces of andradite. When not deformed, the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and/or carbonate. Diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust. They frequently appear as dykes (former cracks), with a few mm to few tens of cm wide transitional zones, which contain high amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in the Oman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (> 800 °C), which may have leached plagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm the prediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547–575.] who proposed that common anhydrous minerals like pyroxene, plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water. This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations.

Deep and High-temperature Hydrothermal Circulation in the Oman Ophiolite--Petrological and Isotopic Evidence

A systematic search for evidence of high-temperature hydrous alteration within the gabbros of the Samail ophiolite (Oman) shows that most of the gabbros have been affected by successive stages of alteration, starting above 975°C and ending below 500°C. Sr and O isotopic analyses provide constraints on the nature and origin of the fluids associated with these alteration events. Massive gabbros, dykes and veins and their associated minerals depart from mid-ocean ridge basalt (MORB)-source magma signatures (87Sr/86Sr >0·7032 and depleted δ18O 10). A second group of samples, including massive gabbros and high-temperature dykes and veins, is characterized by lower water/rock ratios, in the range of 3–5. These samples display a high-temperature hydrothermal alteration related to seawater ingression. The main fluid channels may be sub-millimetre-sized microcracks with a dominantly vertical attitude, which constitute the recharge hydrothermal system, whereas the dykes and veins represent the discharge part. This model requires that these dykes have been generated by hydration of the crystallizing gabbros via seawater penetration, near the internal wall of the magma chamber.

Recharge flux to ocean-ridge black smoker systems: a geochemical estimate from ODP Hole 504B

We use the Sr-isotopic composition of rock and anhydrite, as a monitor of fluid composition, in DSDP/ODP Hole 504B to calculate the recharge flux to the axial high-temperature hydrothermal circulation. The fluid and rock Sr-isotope profiles are well fit by a tracer transport mass-balance model that approximates fluid–solid exchange by linear kinetics. The calculated time-integrated flux of 1.7±0.2×106 kg m−2 is significantly less than the 5×106 kg m−2 calculated from thermal models that assume all magma is intruded into a high-level magma chamber at the base of the sheeted dykes. Our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in Hole 504B and the intrusion of magma as lenses distributed through the lower oceanic crust. It leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balance.

First report of linnaeite (Co3S4) and millerite (NiS) from active submarine hydrothermal deposits: Rainbow hydrothermal field, Mid-Atlantic Ridge at 3614'N

Linnaeite, millerite and cobalt pentlandite have been discovered in an active seafloor hydrothermal deposit at the Rainbow hydrothermal field at 36° 14'N on the Mid-Atlantic Ridge. All three minerals are exclusively associated with born-ite and chalcopyrite. Textural relationships indicate that chalcopyrite and cobalt pentlandite were the earliest-formed minerals, followed by bornite, and then by linnaeite and millerite. These minerals exhibit a very narrow range of sulfur contents, whereas their contents of Co, Ni, Fe and Cu show a wide variation. The close paragenetic association of linnaeite and millerite with bornite suggests their late precipitation from vent fluids enriched in Co and Ni leached during intense high temperature hydrothermal circulation through ultramafic rocks. The hydrothermal setting of the Rainbow hydrothermal field can be compared with that of the Logatchev hydrothermal field at 14° 45'N on the Mid-Atlantic Ridge, where cobalt pentlandite was first discovered, and with other hydrothermal fields at spreading centers. This comparison indicates that the presence of the Co-Ni-S minerals in present-day submarine massive sulfide deposits is closely linked to ultramafic rocks hosting the hydrothermal field.

Relation between ore-forming hydrothermal systems and extensional deformation in the Solea graben spreading center, Troodos ophiolite, Cyprus

Field relations indicate that high-temperature hydrothermal circulation and accumulation of massive sulfide deposits within the Solea graben of the Troodos ophiolite, Cyprus, followed extreme crustal attenuation. Zones of pervasive, massive epidosite strike parallel to the axis of the Solea graben and to the strike of extensional normal faults. Initial fluid flow, evidenced by preferential epidotization in weakly altered areas surrounding massively altered regions, was focused along joints, microfractures, and (now) low-angle normal-fault zones related to graben formation. Permeability within the sheeted-dike section was enhanced by brittle deformation related to extensional structures as well as through volume reduction inherent in the diabase to epidosite mineralogic phase transformations. Intrusion of high-level gabbros into epidosite zones occurred both before and after significant amagmatic tectonic extension. Structural control on epidotization suggests that intrusion of late stocks into attenuated and highly deformed crust is necessary to drive the vigorous hydrothermal circulation that produced the epidosites and ore bodies of the Solea graben. A similar sequence of events is more likely to occur in the modern oceans along ridge crests with ephemeral magmatism, especially at intermediate- to slow-spreading ridges near transform faults.

Lithium in foraminiferal shells: implications for high-temperature hydrothermal circulation fluxes and oceanic crustal generation rates

The lithium content of planktonic foraminiferal calcite has been determined to evaluate temporal variability of seawater Li concentrations over the past 116 m.y. Mean foraminiferal calcite lithium/calcium in each time interval is no more than 16% greater nor 25% less than the mean Li/Ca of all samples. Li/Ca minima are observed for samples from 50–60 m.y. and 80–90 m.y., with Li/Ca about 25% lower than in adjacent time intervals. At no time during the past 40 m.y does mean Li/Ca appear to be higher than that at present. Subject to the limitations imposed by sample coverage and diagenesis, a similar conclusion holds for the past 116 m.y. Coupled with an oceanic mass balance model for Li, these data suggest that: (1) oceanic Li concentrations and, therefore, high-temperature hydrothermal circulation fluxes during the past 40 m.y. (and perhaps the past 100 m.y.) have not been more than perhaps 30–40% greater than at present for intervals any longer than a million years at most, and (2) these fluxes were not a factor of two higher 100 m.y. ago. By inference, variations in oceanic crustal generation rates over these time periods are similarly limited. Decreases in hydrothermal circulation fluxes and crustal generation rates or fluctuations up to 20% in these rates of a few million years duration are not necessarily ruled out by the Li/Ca data. The lack of variability in Li/Ca over time is not unequivocal evidence that hydrothermal fluxes have not varied because the rates of removal processes may be linked to changes in input fluxes.

The Genesis of the Cornubian Batholith (South-West England): the example of the Carnmenellis Pluton

The origin of the Carnmenellis granite is discussed in the light of new petrographic and chemical data which suggest that the present mineral assemblage comprises refractory, magmatic, subsolidus and hydrothermal phases. The present homogeneity of the pluton results from subsolidus to pervasive hydrothermal reworking involving selective leaching of alkalis and Fe-Mg and an increase in Li, F, B, and Rb, which were introduced, or redistributed, through high temperature hydrothermal circulation. The compositions of the primary minerals were re-equilibrated with respect to these water-rock interactions. Rb-Sr systematics demonstrate that whole rock samples show lower ages than the minerals (260 and 285 Ma, respectively). The perturbation in the former isotope distribution is in agreement with the chemical re-equilibration of the mica with external Rb-rich and Sr-depleted solutions. The distribution of rare-earths reveal that most reside in a few accessory phases, viz. monazite, zircon, and apatite; their near-liquidus fractionation is responsible for the lowering of the bulk REE content compared to pelitic source material and for the present REE pattern of the pluton. Conditions of magma generation by partial melting of cordierite-sillimanite-spinel pelitic gneisses were estimated to be around 800°C and 5 kb with water content in the magma of about 4 wt. per cent. H2O saturation was reached late during the ascent of the magma, making possible the crystallization of muscovite from the residual melt. The highly evolved peraluminous composition of the pluton is not explained by a simple magmatic differentiation. Compared to the Carnmenellis pluton, the Land's End massif of similar mineralogy appears less evolved, either because of less subsolidus reworking or deeper structural emplacement.