Henri Bougault

Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

Abstract As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15°N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15°20′N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH4 concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15°05′N, 44°59′W) where serpentinized rocks outcrop. No 3He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15°20′N Transform. High CH4 anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH4 anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH4 concentration is among the highest found along the Mid-Atlantic Ridge Charlou and Donval 1993 , Charlou et al 1997 . CH4 concentrations of 9.9–14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH4 concentrations correspond to only weak 3He anomalies. This CH4-rich plume is also associated with active fault zones that expose peridotite rocks. CH4 output from ultramafic outcrops on the western and eastern intersections of the 15°20′N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH4 anomalies measured in the water column above ultramafic outcrops found between 12°N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge.

Serpentinized peridotites and gabbros in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N

Serpentinized peridotites dredged in the Mid-Atlantic Ridge axial valley at 15°37′N and 16°52′N during theridelente cruise of R.V.Jean Charcot are mantle-derived harzburgites, plastically deformed in low stress/high temperature conditions similar to those inferred for the asthenospheric mantle. This early asthenospheric event was followed by ductile, then brittle, deformation in higher stress and progressively lower temperature conditions. We argue that this deformation occurred during the tectonic uplift of the peridotites throughout the lithosphere of the axial region. The ultramafics studied have mineral compositions similar to those of other mantle-derived Mid-Atlantic Ridge harzburgites. Strongly depleted mineral compositions in samples from the 15°37′N dredge could reflect a high degree of mantle partial melting. Emplacement of peridotites in the seafloor at mid-ocean ridges may not therefore be systematically correlated with low degrees of mantle partial melting. Serpentinized peridotites dredged at 15°37′N were intruded by evolved Na- and Fe-rich apatite and zircon-bearing gabbros. These gabbros were emplaced after the ductile deformation of the peridotites, and prior to their extensive serpentinization. Similar intrusive relationships between extensively fractionated gabbros and mantle peridotites have been observed in samples from the Mid-Atlantic Ridge axial valley south of the Kane transform, in dredged samples from Southwest Indian Ocean fracture zones, and in western Alps ophiolites. We propose that such intrusive relationships are characteristic of magma-starved spreading environments, in which stretching of the axial lithosphere may locally lead to emplacement of peridotites and gabbros in the seafloor.

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

Abstract The outcrops of mantle-derived ultramafic rocks in the 15°N region are the most extensive yet reported for the Mid-Atlantic Ridge. North of the Fifteen Twenty fracture zone, these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall. Ultramafic rocks also crop out extensively south of the Fifteen Twenty fracture zone. Based on dredging and on a morphological analysis of the bathymetric map, we propose that ultramafic outcrops may be common in the crust formed between 14°30′N and 15°50′N during at least the past 2.4 m.y. Moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops. Diving observations also show that the ultramafic rocks are capped, in stratigraphic contact, by a thin layer of basalt. This suggests that these rocks were tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valley. The occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds.

Hydrothermal vents near a mantle hot spot: the Lucky Strike vent field at 37'N on the Mid-Atlantic Ridge

The Lucky Strike hydrothermal field occurs in the summit basin of a large seamount that forms the shallow center of a 65 km long ridge segment near 37°N on the Mid-Atlantic Ridge. The depth and chemistry of the ridge segment are influenced by the Azores hot spot, and this hydrothermal field is the first Atlantic site found on crust that is dominated by a hot spot signature. Multiple hydrothermal vents occur over an area of at least 300 m by 700 m. Vent morphologies range from flanges and chimneys with temperatures of 200–212°C, to black smoker chimneys with temperatures up to 333°C. Cooler fluids from northern vents have higher chlorinities and lower gas volumes, while hotter, southern fluids have chlorinities 20% below seawater with higher gas volumes, suggesting phase separation has influenced their compositions. All gas volumes in fluids are higher than those at TAG and Snake Pit hydrothermal fields. Black smokers exhibit their typical mineralogy, except that barite is a major mineral, particularly at lower-temperature sites, which contrasts with previously investigated Atlantic sites. The fluid chemistry, distribution of the relict sulfide deposits on the seamount summit in the areas investigated using DSV Alvin, and contact relationships between active vent sites and surrounding basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated. Fauna at the Lucky Strike vent sites are dominated by a new species of mussel, and include the first reported sea urchins. The Lucky Strike biological community differs considerably from other vent fauna at the species level and appears to be a new biogeographic province. The Lucky Strike field helps to constrain how variations in the basaltic substrate influence the composition of hydrothermal fluids and solids, because basalt compositions at Lucky Strike are 10–30 times enriched in incompatible elements compared to other Atlantic hydrothermal sites such as TAG, Snake Pit and Broken Spur. The incompatible element

Geochemical morphology of the North Mid-Atlantic Ridge, 10°–24°N: Trace element-isotope complementarity

Abstract The new data presented here from a 10–24°N segment of the North Mid-Atlantic Ridge show that this segment is the most depleted of the 10–70°N ridge section. They also show the existence of: (1) a geochemical gradient from the 14°N anomaly to 17°10′N; (2) a very depleted mantle source (the lowest Sr isotopic ratios found so far in the North Atlantic); and (3) a geochemical limit located at about 17°10′N without any obvious relation with any structural feature. The 15°20′N fracture zone does not show any relationship with respect to this gradient. The basalts located north of 17°10′N have very homogeneous features, which allow their characteristics to be averaged (i.e., 87Sr/86Sr= 0.70238 ± 0.00004, (Nb/Zr)N = 0.28 ± 0.1) and they are defined as normal mid-ocean ridge basalts. The basaltic glasses located south of 17°10′N present a wide spectrum of isotopic compositions and extended rare earth element patterns (from depleted to enriched). Despite this, they have a constant K/Nb of 233 ± 9 (1sM, n = 18) whereas this ratio is 344 ± 29 north of 17°10′N. These observations illustrate the strong coherence of behaviour between K and Nb (Ta) during the petrogenic processes involved in the generation of these mid-ocean ridge basalts and also their fractionation during previous mantle processes. Possible interpretations of mixing processes are discussed and sources at the ridge segment scale are favoured. However, when looking in detail, local heterogeneities are still common and can even be traced back off-axis to 115 my. Placed in the context of the North Atlantic Ridge from 10° to 70°N, the Sr isotopic ratios reveal the Azores superstructure (23–50°N), whereas the trace element ratios (La/Sm-Nb/Zr) trace the second-order structures (33–40°N, 42–48°N) superimposed on the superstructure. This study illustrates the complementarity of information given by certain well chosen trace element ratios on the one hand and by isotopic ratios on the other. Since there is evidence of decoupling between isotopic ratios and/or trace element ratios, it introduces the notion of complementary “chemical memory” as recorded by a given type of trace element ratio or a given type of isotopic ratio

The age and distribution of mantle heterogeneity along the Mid-Atlantic Ridge (31–41°N)

New trace element and isotopic data for basalts from the mid-Atlantic ridge between 31 and 41oN allow a better description of the geochemical gradient south of the Azores triple junction, and the systematics of mantle source heterogeneity. There is a long wavelength enrichment in incompatible trace elements and isotopes associated with the Azores hot spot that extends from the Kurchatov fracture zone near 41oN to the Hayes fracture zone near 33oN. Superimposed on this gradient are local spikes of enrichment, the most prominent being the anomaly near the Oceanographer Fracture Zone (NOFZ). The Oceanographer anomaly spike is reflected modestly in the morphology of the ridge axis, but is not obviously related to a plume. The isotopic data alone are consistent with involvement of subcontinental material, but the samples do not contain the negative Nb‐Ta anomalies which are usually associated with the presence of continental material in the mantle source. Away from the prominent enrichment spikes associated with the Azores and Oceanographer fracture zone, there are systematic relationships in this region between parent=daughter element ratios and isotope ratios. The Pb, Sr and Nd isotope systems all give apparent ages in the range 100 Ma to 300 Ma, with the age increasing with likely parent=daughter fractionation during melting (U=Pb < Rb=Sr < Sm=Nd age). Monte Carlo simulations of an enrichment event in a depleted heterogeneous mantle at 250 Ma produce results that correspond well with the observations for all three isotopic systems. Since this age also corresponds to the pre-opening of the North Atlantic, it raises the possibility that some of the heterogeneity in this region is associated with shallow level mantle heterogeneity resulting from the rifting of Gondwanaland rather than from interaction with mantle plumes. The data may also reflect a mean mixing time for the heterogeneities in the upper mantle source. Sr isotope systematics reveal correlations in a 87 Sr= 86 Sr versus 87 Rb= 86 Sr plot, which are geographically controlled. Data points from 10‐24oN samples and data points from 31‐38oN samples (excluding NOFZ samples) plot on two offset trends of similar slope. Irrespective of the origin of the isotopic variations, these data require end member depleted mantle with distinct isotopic characteristics. Depleted sources with low 87 Rb= 86 Sr (0.005‐0.04) and low (La=Sm)N (<0.5), have 87 Sr= 86 Sr values that vary between 0.70215 and 0.7029. Therefore the depleted mantle source of N-MORB is not a homogeneous reservoir, but shows isotopic variations almost as large as the differences between generic depleted mantle (0.7025) and the enriched Atlantic plumes. Creation of a very heterogeneous depleted mantle in terms of isotopic composition needs to be included as a constraint on models of mantle mixing and convection.

MORB alteration : Rare-earth element/non-rare-earth hygromagmaphile element fractionation

Abstract We have determined the concentrations of rare-earth elements (REE) and non-rare-earth (RE) hygromagmaphile elements (HYG.E): Y, Zr, Nb, Hf, Ta and Th in fresh and altered basaltic samples recovered from the Atlantic Ocean. Whereas the relative abundances of REE and non-RE HYG.E display continuous chondrite-normalized patterns in fresh samples, significant fractionations appear between these elements in altered samples. Seawater alteration results in significant modifications of REE patterns but does not affect Nb/Zr and Ta/Hf ratios. This insensitivity of ( Nb Zr ) N and ( Ta Hf ) N chondrite-normalized ratios makes them more reliable tracers than equivalent REE ratios, i.e. ( La Sm ) N , for studying magmatic processes and deciphering mantle heterogeneities as reflected in oceanic basalts. A quantitative budget of element exchanges during seawater alteration of a basaltic glass is assessed. It demonstrates that Zr, Nb, Hf and Ta remain quantitatively in alteration products whereas Y and the trivalent REE are partially removed; Th seems to be slightly taken up in alteration products. Data on secondary minerals show that REE are not quantitatively retained in smectites whose HYG.E patterns are similar to those of altered basalts. Especially, Ta/Hf ratios in smectites are the same as in the fresh basaltic precursor, and thus could constitute a powerful tool to trace alteration products of the oceanic crust in the sedimentary system. Qualitatively, the fractionations between REE and non-RE HYG.E in altered basalts are considered regarding the recycling of the oceanic crust into the Earth mantle. The results point out that the interactions between a fluid phase (seawater) and the oceanic crust are able to produce a residue enriched in Zr, Hf, Nb and Ta relative to REE. The influence of such processes during subduction is discussed with respect to the genesis of orogenic rocks and ocean island basalts.

The geochemical structure of the South-East Indian Ridge

Isotopic (Sr, Nd, Pb) and trace element data are reported for samples dredged along the South-East Indian Ridge (SEIR). They confirm the existence of a geochemical province under the Indian Ocean which is distinct from the provinces observed under the North Atlantic and Pacific Oceans. This province is characterized by the association of Sr and Nd isotopic enrichment features with a depleted Pb signature and of magmaphile element depletion features (low(La/Sm)N, (Nb/Zr)N, Rb/Sr with high Sm/Nd ratios). On a regional scale, the data emphasize the strong relationship existing between the physical structure of the ridge system and its chemical characteristics. It is suggested that these chemical characteristics are the result of contribution of different mantle sources corresponding to different structures of the ridge system: -“normal ridge” segments, -the ridge segment located on an immediately north of the St. Paul-Amsterdam massif, -off-axis structures located on the structural link between Broken Ridge and Kerguelen. In a model of ocean differentiation from Bulk Earth, it is suggested that SEIR mantle sources are the result of separate episodes of differentiation, all taking place more recently than the differentiation of the Atlantic and Pacific oceans.

Melting relations of a magnesian abyssal tholeiite and the origin of MORBs

Melting relations of a glassy magnesian olivine tholeiite from the FAMOUS area have been studied within the pressure range 1 atm to 15 kbar. From 1 atm to 10 kbar, olivine is the liquidus phase, followed by plagioclase and Ca-rich clinopyroxene. Above 10 kbar, Ca-rich clinopyroxene appears on the liquidus, followed by orthopyroxene and spinel. Near 10 kbar, olivine, orthopyroxene, clinopyroxene, spinel and plagioclase crystallize within 10°C of the liquidus. This indicates that a liquid of this magnesian olivine tholeiite composition could coexist with mantle peridotite at about 10 kbar. This result is in agreement with the geochemistry of Ni; the Ni concentration of the studied sample corresponds to the theoretical concentration in a primary magma [14,15]. These data suggest that at least some magnesian mid-oceanic ridge basalts (MORBs) could be primary melts segregated from the mantle at depths near the transition zone between plagioclase lherzolite and spinel lherzolite (about 10 kbar). Based on this model, the residual mantle after extraction of MORBs should be lherzolite, not harzburgite. High-pressure (7–10 kbar) fractionation models involving olivine, plagioclase and clinopyroxene, which have been proposed by several workers (e.g. [36]) to explain the varieties of MORBs, were re-emphasized based on this melting study. The rare occurrence of clinopyroxene as a phenocryst phase in MORBs is explained by precipitation in a magma chamber at high pressure, or by dissolution of clinopyroxene formed earlier at high pressure.

SrNdPb geochemical morphology between 10° and 17°N on the Mid-Atlantic Ridge: A new MORB isotope signature

Basalts dredged along the Mid-Atlantic Ridge axis between 10°N and 17°N have been studied for their trace element characteristics [1]. To give complementary information on mantle source history and magma genesis, these samples have been analysed for their SrNdPb isotopic compositions. There is a good correlation between the structure of the ridge axis which shows a topographic anomaly centered around 14°N and hygromagmaphile element ratios such as Rb/Sr, (Nb/Zr)N or Sm/Nd as well as isotopic ratios plotted as a function of latitude. The samples coming from the 14°N topographic high show new MORB SrNd isotopic characteristics which pictured in a classical mantle array diagram, put their representative points close to HIMU sources of ocean islands such as St. Helena, Tubuaiand Mangaia. The 14°N mantle source presents geochemical characteristics which indicate mantle differentiation processes and a mantle history that are more distinct than so far envisaged from typical MORB data. Pb data indicates that the 14°N mantle source cannot be the result of binary mixing between a depleted mantle and a HIMU-type source. Rather, the enriched endmember could itself be a mixture of Walvis-like and HIMU-like materials. The geochimical observations presented favour the model of an incipient ridge-centered plume, in agreement with [1].