Marguerite Godard

Orogenic, ophiolitic and abyssal peridotites

Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earths upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

Relationships between geochemistry and structure beneath a palaeo-spreading centre: a study of the mantle section in the Oman ophiolite

The Oman ophiolite exposes a large and well-preserved mantle section beneath a palaeo-spreading centre. The mantle section is mainly composed of extremely refractory harzburgites with relatively homogeneous modal and major element compositions. Nevertheless, our trace element data exhibit variations connected with the main mantle structures, which allow us to define three geochemical and structural domains. The main harzburgitic mantle section, mainly constituted of strongly refractory harzburgites characterised by chondrite-normalised REE patterns that are steadily depleted from HREE to LREE. These rocks are interpreted as mantle residues after s 15% melt extraction. Their REE signature can be explained by melt transport associated with partial melting. The diapir areas (mainly the Maqsad diapir), defined by plunging lineations. They are constituted of harzburgites with roughly the same modal composition as the main mantle section but distinct, concave-upward REE patterns. The regions of most active upwelling (characterised by sub-vertical lineations) are further distinguished by higher Al2O3/CaO ratios and TiO2 contents. This character is ascribed to focused partial melt upwelling. The diapirs are interpreted as local instabilities in upwelling mantle, possibly triggered by feedback mechanisms between deformation and melt percolation. The Maqsad diapir is topped by a thick, dunitic, mantle^crust transition zone (MTZ) that displays the same trace-element signature as the diapir. However, the dunites are distinguished by low Mg# values and Ni contents. Together with structural evidence, this allows us to interpret the MTZ dunites as diapir harzburgites that were strongly modified by olivine-forming melt^rock reactions at high melt/ rock ratios. The MTZ is thought to act as a major collecting zone for mantle melts. The cpx-harzburgites from the base of the mantle section. These rocks are distinguished by high clinopyroxene contents (s 5%), low AL2O3/CaO and ‘spoon-shaped’ REE patterns. They were individualised from the rest of the harzburgite mantle section by a cpxforming melt^rock reaction at decreasing malt mass. This reaction probably occurred at near-solidus conditions along the lithosphere^asthenosphere boundary. The formation of these three domains may be integrated in a geodynamic scenario involving the reactivation of an oceanic lithosphere, a process that would be related to the ridge propagator identified in the Oman ophiolite. fl 2000 Elsevier Science B.V. All rights reserved.

A plate model for the simulation of trace element fractionation during partial melting and magma transport in the Earth's upper mantle

We propose a new plate model for the simulation of trace element transfer during magmatic and metasomatic processes taking place in the Earths upper mantle. As in previously published plate models, porous flow is accounted for by propagation of fluid batches through macrovolumes of mantle rocks. Being released from spatiotemporal constraints, the plate model allows much more freedom than the one-dimensional porous-flow models for the simulation of fluid-rock interactions. Hence this approach may account for a wide range of mantle processes, including melt extraction during compaction of molten peridotites, porous flow associated with Chromatographic effects, or fluid-rock reactions occurring upon melt infiltration at the base of the conductive mantle. The applications presented in this study show several results consistent with published trace element data for mantle rocks and basaltic volcanism. In particular, the proposed models may provide simple explanations for (1) the ultra-rare-earth-element-depleted composition of peridotites and interstitial melts residual after mid-ocean ridge basalt extraction, (2) the negative correlation between light rare earth element / heavy rare earth element (LREE/HREE) ratio and refractory character of peridotites, as observed in several suites of mantle rocks, and (3) the origin of ultra-LREE-enriched meta-somatic fluids infiltrated in the lithospheric mantle.

Effects of mineralogical reactions on trace element redistributions in mantle rocks during percolation processes: A chromatographic approach

Mantle rock studies provide evidence of interaction with upwelling magmas. In erogenic lherzolites, one of the most conspicuous effects of these interactions is the development of harzburgite and dunite bands. Recent studies have suggested that these bands were formed at the expense of the host lherzolites by melt-rock reactions associated with magma percolation. In order to better understand the geochemical effects associated with percolation-reaction processes, we propose a numerical model of melt infiltration that takes into account modal variations in time and space resulting from melt-rock reactions. Melt volume variations are considered by means of porosity variations, and a local equilibrium approach is used for trace element exchange between melt and minerals. The transport of trace elements by the interstitial melt is described by a mass balance equation while the modal variations are constrained by the mineralogical trends observed in refractory peridotites massifs. The model is applied to RBE, Cr and Ni in percolated peridotites affected by an olivine-forming reaction, with the aim of reproducing the evolution of these elements in refractory peridotites from the Ronda massif. Our modelling can explain the negative correlation between the LREE/HREE ratio and the HREE content and between Cr and Ni in the Ronda refractory peridotites. Our results validate the hypothesis that, in the Ronda, the bands of refractory peridotites represent porous-flow channels formed by olivine-forming melt-rock reaction, at increasing melt volume. Because similar geochemical features are observed in ophiolitic peridotites and in mantle xenoliths, it is likely that melt-rock reactions associated with magmatic infiltration are widespread and represent important mantle processes.

Leg 209 summary : processes in a 20-km-thick conductiver boundary layer beneath the Mid-Atlantic Ridge, 14°-16°N

This paper provides a summary of postcruise scientific results from Ocean Drilling Program (ODP) Leg 209 available to date, building upon shipboard observations and syntheses summarized in the Leg 209 Initial Results volume. During Leg 209, 19 holes were drilled at 8 sites along the Mid-Atlantic Ridge from 14°43´ to 15°44´N, mainly in residual mantle peridotite intruded by gabbroic rocks, in order to understand the tectonic and structural processes responsible for formation of oceanic lithosphere with abundant residual peridotite exposed on the seafloor coupled with a relatively low proportion of volcanic rocks. Based on proportions of recovered lithologies, the entire area may be underlain by mantle peridotite with ~20%–40% gabbroic intrusions and impregnations. Impregnated peridotites with olivine + two pyroxenes + plagioclase + spinel that apparently formed in equilibrium probably record crystallization from primitive mid-ocean-ridge basalt at pressures of 0.5–0.6 GPa. Metamorphic equilibria record isobaric cooling to ~1100°C at this pressure. Thus, the conductively cooled thermal boundary layer beneath the Mid-Atlantic Ridge in this region is >15 km thick. Combined crystallization and reaction with residual peridotite formed a series of impregnated peridotites recording increasing Na content at nearly constant Mg#; this process could explain some of the variation in fractionation-corrected Na (e.g., Na = 8.0) observed in mid-ocean-ridge basalts. Clinopyroxene textures and compositions record such impregnation processes, and they are particularly well documented for Site 1274. Other Leg 209 gabbroic rocks formed from extensive crystallization of highly evolved melts, indicating that a substantial proportion of melt entering the thermal boundary layer crystallizes entirely beneath the seafloor, with no volcanic equivalent. Alteration of peridotites occurred over a range of temperatures and is the result of three distinct processes: rock-dominated serpentinization with formation of brucite in olivine-rich lithologies, fluid-dominated serpentinization with formation of magnetite and no brucite, and fluid-dominated talc alteration with addition of SiO2 as well as H2O and oxygen. The latter two processes also exhibit detectable trace element metasomatism that is distinct in its character from the igneous impregnation described in the previous paragraph. Microstructures show that most residual peridotites were not ductilely deformed at temperatures less than ~1200°C. Structural and paleomagnetic data require tectonic rotations of relatively undeformed blocks; some rotations probably exceeded 60° around nearly horizontal axes parallel to the rift axis. Rotations occurred along several generations of high-temperature mylonitic shear zones extending deeper than 15 km depth and numerous faults at lower temperature. Early formed shear zones and faults were passively rotated around later features; such a process could have produced low-angle fault surfaces without slip on low-angle faults. This region provides end-member examples of processes that are common at many or most slow-spreading ridges. Osmium isotope ratios indicate an ancient history of depletion for residual peridotites from the 14°–16°N region along the Mid-Atlantic Ridge. Though depleted Os isotope ratios in peridotite have been reported elsewhere along the global ridge system, the values from this region are among the most depleted. In general, Os isotope ratios from mid-ocean-ridge basalts are systematically more radiogenic than Os isotope ratios from ridge peridotite samples, suggesting a polygenetic heterogeneous source for mid-ocean-ridge basalts. Geochemical studies of zircons from Leg 209 gabbroic rocks and impregnated peridotites, together with other ridge and arc-related zircons, indicate that ridge zircons have systematically lower fractionation-corrected U and Th concentrations compared to arc zircons. This observation provides a tool for interpreting the tectonic provenance of ancient detrital zircons and indicates an arclike provenance for Hadean detrital zircons. Geobiological studies and aerobiological studies were also undertaken during Leg 209. The geobiological work found no measurable microbial enhancement of olivine dissolution rate, possibly because the samples from Leg 209 were sterile. The aerobiological study determined that dust from North Africa, collected from the derrick of the JOIDES Resolution during Leg 209, contains a variety of abundant microorganisms.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Ion irradiation of carbonaceous interstellar analogues - Effects of cosmic rays on the 3.4 μm interstellar absorption band

Context. A 3.4 μm absorption band (around 2900 cm-1), assigned to aliphatic C-H stretching modes of hydrogenated amorphous carbons (a-C:H), is widely observed in the diffuse interstellar medium, but disappears or is modified in dense clouds. This spectral difference between different phases of the interstellar medium reflects the processing of dust in different environments. Cosmic ray bombardment is one of the interstellar processes that make carbonaceous dust evolve. Aims. We investigate the effects of cosmic rays on the interstellar 3.4 μm absorption band carriers. Methods. Samples of carbonaceous interstellar analogues (a-C:H and soot) were irradiated at room temperature by swift ions with energy in the MeV range (from 0.2 to 160 MeV). The dehydrogenation and chemical bonding modifications that occurred during irradiation were studied with IR spectroscopy. Results. For all samples and all ions/energies used, we observed a decrease of the aliphatic C-H absorption bands intensity with the ion fluence. This evolution agrees with a model that describes the hydrogen loss as caused by the molecular recombination of two free H atoms created by the breaking of C-H bonds by the impinging ions. The corresponding destruction cross section and asymptotic hydrogen content are obtained for each experiment and their behaviour over a large range of ion stopping powers are inferred. Using elemental abundances and energy distributions of galactic cosmic rays, we investigated the implications of these results in different astrophysical environments. The results are compared to the processing by UV photons and H atoms in different regions of the interstellar medium. Conclusions. The destruction of aliphatic C-H bonds by cosmic rays occurs in characteristic times of a few 108 years, and it appears that even at longer time scales, cosmic rays alone cannot explain the observed disappearance of this spectral signature in dense regions. In diffuse interstellar medium, the formation by atomic hydrogen prevails over the destruction by UV photons (destruction by cosmic rays is negligible in these regions). Only the cosmic rays can penetrate into dense clouds and process the corresponding dust. However, they are not efficient enough to completely dehydrogenate the 3.4 μm carriers during the cloud lifetime. This interstellar component should be destroyed in interfaces between diffuse and dense interstellar regions where photons still penetrate but hydrogen is in molecular form.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid‐Atlantic Ridge 36°14′N)

Rainbow is a dome-shaped massif at the 36°14′N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (∼350°C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220°C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at ∼160–260°C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

Photoluminescence of hydrogenated amorphous carbons - Wavelength-dependent yield and implications for the extended red emission

Context. Hydrogenated amorphous carbons (a-C:H or HAC) have proved to be excellent analogs of interstellar dust observed in galaxies diffuse interstellar medium (DISM) through infrared vibrational absorption bands (3.4 μm, 6.8 μm, and 7.2 μm bands). They exhibit photoluminescence (PL) after excitation by UV-visible photons, and are possible carriers for the extended red emission (ERE), a broad red emission band observed in various interstellar environments. Aims. As many candidate materials/molecules can photoluminesce in the visible, along with the carrier abundance, the PL efficiency represents one of the strongest constraints set by such ERE observations. We wish to precisely characterize the PL behavior of a-C:H as a family of materials. Methods. The a-C:H samples are produced in the form of films deposited on substrates by plasma-enhanced chemical vapor deposition. The produced films were analyzed in transmission by UV-visible and IR spectroscopy, and the wavelength dependent PL spectra were recorded. The intrinsic absolute quantum yield η was then rigorously calculated taking self-absorption of the PL by the film and interfaces effects into account. Results. A wide range of different laboratory synthesized a-C:H were analyzed. Their PL properties are dependent on the optical gap E04 :w henE04 decreases from 4.3 eV to 2.8 eV, the a-C:H vary from highly (η ∼ 1%) yellow photoluminescent soft materials to hard materials that emit a wider PL band in the red spectral range, with a lower efficiency (η ∼ 0.01−0.1%). For any given a-C:H, the PL characteristics (central wavelength, band width and efficiency) are found to be essentially constant over the explored excitation range (λexc > 250 nm). We compared the characteristics of the produced interstellar dust analog to the constraints imposed by the ERE observations. Conclusions. As for ERE observations, PL efficiencies and band widths of a-C:H are both correlated to the PL central wavelengths. The excitation responsible for the a-C:H emission is efficient over a wide spectral range that matches the ERE excitation. The present a-C:H encounter difficulties for the diffuse ISM ERE observations (η ≥ 10%) in simultaneously satisfying the high quantum yield criteria and PL spectral characteristics. We still need to investigate the role of a small number of residual oxygen atoms in the laboratory-produced a-C:H network in quenching the PL yield, as well as to consider the interstellar temperature effect for our analogs.