Whole rock analytical techniques and glas analytical techniques of silicic magmas from the South East Rift, Manus Basin

There has been much recent interest in the origin of silicic magmas at spreading centres away from any possible influence of continental crust. Here we present major and trace element data for 29 glasses (and 55 whole-rocks) sampled from a 40 km segment of the South East Rift in the Manus Basin that span the full compositional continuum from basalt to rhyolite (50-75 wt % SiO2). The glass data are accompanied by Sr-Nd-Pb, O and U-Th-Ra isotope data for selected samples. These overlap the ranges for published data from this part of the Manus Basin. Limited increases in Cl/K ratios with increasing SiO2, La-SiO2 and Yb-SiO2 relationships, and the oxygen isotope data rule out models in which the more silicic lavas result from partial melting of altered oceanic crust or altered oceanic gabbros. Rather, the data form a coherent array that is suggestive of closed-system fractional crystallization and this is well simulated by MELTS models run at 0.2 GPa and QFM (quartz-fayalite-magnetite buffer) with 1 wt % H2O, using a parental magma chosen from the basaltic glasses. Although some assimilation of altered oceanic crust or gabbro cannot be completely ruled out, there is no evidence that this plays an important role in the origin of the silicic lavas. The U-series disequilibria are dominated by 238U and 226Ra excesses that limit the timescale of differentiation to less than a few millennia. Overall, the data point to rapid evolution in relatively small magma lenses located near the base of thick oceanic crust; we speculate that this was coupled with relatively low rates of basaltic recharge. A similar model may be applicable to the generation of silicic magmas elsewhere in the ocean basins.

Chemical composition of Fe-Mn nodules and crusts from the North Atlantic

In terms of physical and genetic features studied nodules are assigned to the sedimentational manganese-iron type (Canary Basin) and to the iron-containing type (Guyana Basin). They are enriched in Fe, Co and Pb and depleted in Mn, Ni, Cu, and Zn. Cores of the nodules from the Canary Basin consist primarily of peridotite, gabbro, dolerite, and metamorphic rock. Great predominance of peridotite and gabbro indicates that blocks of ocean crust underwent considerable displacement, bringing the third and fourth layers of a crust to the surface.

Lithology, mineralogy and geochemistry of ODP Hole 118-735B and 176-735B respectively

Ocean Drilling Program (ODP) Hole 735B, located on Atlantis Bank on the Southwest Indian Ridge, penetrated 1508 meters below seafloor with an average recovery of 87%, providing a nearly continuous sample of a significant part of oceanic Layer 3. Based on variations in texture and mineralogy, 12 major lithologic units are recognized in the section, ranging from 39.5 to 354 m thick. The principal lithologies include troctolite, troctolitic gabbro, olivine gabbro and microgabbro, gabbro, gabbronorite and Fe-Ti oxide gabbro, gabbronorite, and microgabbro. Highly deformed mylonites, cataclasites, and amphibole gneisses are locally present, as are small quantities of pyroxenite, anorthositic gabbro, and trondhjemite. Downhole variations in mineral composition, particularly for olivine and clinopyroxene, show a number of cyclic variations. Plagioclase compositions show the widest variations and correspond to different degrees of deformation and alteration as well as primary processes. Downhole chemical variations correspond reasonably well with variations in mineral compositions. Iron and titanium mainly reflect the presence of Fe-Ti oxide gabbros but show some cyclical variations in the lower part of the core where oxide gabbros are sparse. CaO is highly variable but shows a small but consistent increase downhole. MgO is more uniform than CaO and shows a very small downward increase. Sulfur and CO2 contents are generally low, but S shows significant enrichment in lithologic Unit IV, which consists of Fe-Ti oxide gabbro, reflecting the presence of sulfide minerals in the sequence. The lithologic, mineralogical, and geochemical data provided here will allow detailed comparisons with ophiolite sections as well as sections of in situ ocean crust drilled in the future.

Chemical and mineral composition of rocks from the Philippine Sea

The book is devoted to geology of the Philippine Sea floor. This region is studied most extensively among other marginal seas of the Pacific Ocean. Rocks of the sedimentary and basalt layers within this sea have been studied during five legs of D/S Glomar Challenger. International geological expedition on board R/V Dmitry Mendeleev carried out according to the Project ''Ophiolites of Continents and Comparable Rocks of the Ocean Floor''obtained unique collection of rocks from the second and third layers of the ocean crust in the Philippine Sea. The book provides detailed petrographic and geochemical description of igneous and sedimentary formations from the Philippine Sea and compares them with rocks of the continental ophiolite association. An analysis of structure and history of the ocean crust formation in the region is based on all known geological information. The main periods of tectonic movement activation and nature of their manifestations within the sea are shown.

Sulphur mineralogy and geochemistry of serpentinites and gabbros from the Atlantis Massif

In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150-200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high d34Ssulfide values and low d34Ssulfate values, which reflect a multistage history primarily controlled by seawater-gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity.

Geochemical composition of minerals obtained from ODP Sites 180-1109, 180-1117 and 180-1118

During the last 8 m.y. the Papuan Peninsula region of Papua New Guinea has been affected by extension which opened the Woodlark Basin. The present-day spreading tip is located at the foot of the Moresby Seamount, a crustal block whose northern flank is an active low-angle normal fault related to this extension. During Ocean Drilling Program Leg 180 (7 June-11 August 1998), 11 sites (1108-1118) were drilled along a north-south-trending transect across the Woodlark Basin just ahead of the spreading tip. Four of these sites (1118, 1109, 1114, and 1117) reached the crystalline basement, which is composed of diabase and gabbro. Sites 1118 and 1109, located on the Woodlark Rise, belong to the hanging wall block, and Sites 1114 and 1117, located on the crest of the Moresby Seamount, belong to the footwall block and the fault zone itself. Most of the basalt, diabase, and gabbro that were recovered show a well-preserved magmatic texture. The diabase, which is the most abundant rock type, has a coarse-grained ophitic texture composed of poikilitic clinopyroxene including radiating, locally skeletal plagioclase laths with interstitial iron oxide grains. Secondary mineralogy consists of chlorite, zeolite, calcite, albite, and quartz. The gabbro shows a medium-grained granular texture. The magmatic mineralogy consists of euhedral laths of plagioclase and anhedral interstitial clinopyroxene. Secondary mineralogy consists of a magnesio to actinolitic hornblende, chlorite, clinozoisite, zeolite, quartz, and calcite. The retrograde metamorphic evolution of both gabbro and diabase occurred under low amphibolite to subgreenschist facies conditions associated mainly with brittle deformation and the development of a local low-temperature shear zone. This shows no evidence for high thermal gradient in the crust during the continental rifting.

Argon, uranium, thorium and lead composition and ages of ODP Leg 180 sites

During Ocean Drilling Program (ODP) Leg 180, 11 sites were drilled in the vicinity of the Moresby Seamount to study processes associated with the transition from continental rifting to seafloor spreading in the Woodlark Basin. This paper presents thermochronologic (40Ar/39Ar, 238U/206Pb, and fission track) results from igneous rocks recovered during ODP Leg 180 that help constrain the latest Cretaceous to present-day tectonic development of the Woodlark Basin. Igneous rocks recovered (primarily from Sites 1109, 1114, 1117, and 1118) consist of predominantly diabase and metadiabase, with minor basalt and gabbro. Zircon ion microprobe analyses gave a 238U/206Pb age of 66.4 ± 1.5 Ma, interpreted to date crystallization of the diabase. 40Ar/39Ar plagioclase apparent ages vary considerably according to the degree to which the diabase was altered subsequent to crystallization. The least altered sample (from Site 1109) yielded a plagioclase isochron age of 58.9 ± 5.8 Ma, interpreted to represent cooling following intrusion. The most altered sample (from Site 1117) yielded an isochron age of 31.0 ± 0.9 Ma, interpreted to represent a maximum age for the timing of subsequent hydrothermal alteration. The diabase has not been thermally affected by Miocene-Pliocene rift-related events, supporting our inference that these rocks have remained at shallow and cool levels in the crust (i.e., upper plate) since they were partially reset as a result of middle Oligocene hydrothermal alteration. These results suggest that crustal extension in the vicinity of the Moresby Seamount, immediately west of the active seafloor spreading tip, is being accommodated by normal faulting within latest Cretaceous to early Paleocene oceanic crust. Felsic clasts provide additional evidence for middle Miocene and Pliocene magmatic events in the region. Two rhyolitic clasts (from Sites 1110 and 1111) gave zircon 238U/206Pb ages of 15.7 ± 0.4 Ma and provide evidence for Miocene volcanism in the region. 40Ar/39Ar total fusion ages on single grains of K-feldspar from these clasts yielded younger apparent ages of 12.5 ± 0.2 and 14.4 ± 0.6 Ma due to variable sericitization of K-feldspar phenocrysts. 238U/206Pb zircon, 40Ar/39Ar K-feldspar and biotite total fusion, and apatite fission track analysis of a microgranite clast (from Site 1108) provide evidence for the existence of a rapidly cooled 3.0 to 1.8 Ma granitic protolith. The clast may have been transported longitudinally from the west (e.g., from the D'Entrecasteaux Islands). Alternatively, it may have been derived from a more proximal, but presently unknown, source in the vicinity of the Moresby Seamount.

(Table 2) Chlorine geochemistry and mineralogy of altered oceanic crust samples

Bulk chlorine concentrations and chlorine stable isotope compositions were determined for hydrothermally altered basalt (extrusive lavas and sheeted dikes) and gabbro samples (n = 50) from seven DSDP/ODP/IODP drill sites. These altered oceanic crust (AOC) samples span a range of crustal ages, tectonic settings, alteration type, and crustal depth. Bulk chlorine concentrations range from < 0.01 wt.% to 0.09 wt.%. In general, higher chlorine concentrations coincide with an increase in temperature of alteration and amphibole content. d37Cl values of whole rock AOC samples range from -1.4 to +1.8 per mil. High d37Cl values (>=~0.5 per mil) are associated with areas of higher amphibole content. This observation is consistent with theoretical calculations that estimate amphibole should be enriched in 37Cl compared to co-existing fluid. Negative to near zero d37Cl values are found in areas dominated by clay minerals. Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. AOC is a major Cl host in the subducting oceanic lithospheric slab. Here we show that bulk chlorine concentrations are ~3 times higher than previous estimates resulting in a greater contribution of Cl to the mantle.

Trace element composition of IODP Site 304-U1309 and 305-U1309 (Table A2)

IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30°N on the Mid-Atlantic Ridge (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on 228 samples representative of the different lithologies sampled at this location. Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the section; they have depleted MORB compositions similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt-rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (> 70% modal olivine) and troctolites having high Mg# (82-89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb 0.06-0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60-86 and low trace element contents (Yb 0.125-2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (< 50), low Ni (~65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere. Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30°N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere.

Concrete deterioration due to sulfide-bearing aggregates

Dans la région de Trois-Rivières (Québec, Canada), plus de 1 000 bâtiments résidentiels et commerciaux montrent de graves problèmes de détérioration du béton. Les problèmes de détérioration sont liés à l’oxydation des sulfures de fer incorporés dans le granulat utilisé pour la confection du béton. Ce projet de doctorat vise à mieux comprendre les mécanismes responsables de la détérioration de béton incorporant des granulats contenant des sulfures de fer, et ce afin de développer une méthodologie pour évaluer efficacement la réactivité potentielle de ce type de granulats. Un examen pétrographique détaillé de carottes de béton extraites de fondations résidentielles montrant différents degré d’endommagement a été réalisé. Le granulat problématique contenant des sulfures de fer a été identifié comme un gabbro à hypersthène incorporant différentes proportions (selon les différentes localisations dans les deux carrières d’origine) de pyrrhotite, pyrite, chalcopyrite et pentlandite. Les produits de réaction secondaires observés dans les échantillons dégradés comprennent des formes minérales de "rouille", gypse, ettringite et thaumasite. Ces observations ont permis de déterminer qu’en présence d’eau et d’oxygène, la pyrrhotite s’oxyde pour former des oxyhydroxides de fer et de l’acide sulfurique qui provoquent une attaque aux sulfates dans le béton. Tout d’abord, la fiabilité de l’approche chimique proposée dans la norme européenne NF EN 12 620, qui consiste à mesurer la teneur en soufre total (ST,% en masse) dans le granulat pour détecter la présence (ou non) de sulfures de fer, a été évaluée de façon critique. Environ 50% (21/43) des granulats testés, représentant une variété de types de roches/lithologies, a montré une ST > 0,10%, montrant qu’une proportion importante de types de roches ne contient pas une quantité notable de sulfure, qui, pour la plupart d’entre eux, sont susceptibles d’être inoffensifs dans le béton. Ces types de roches/granulats nécessiteraient toutefois d’autres tests pour identifier la présence potentielle de pyrrhotite compte tenu de la limite de ST de 0,10 % proposée dans les normes européennes. Basé sur une revue exhaustive de la littérature et de nombreuses analyses de laboratoire, un test accéléré d’expansion sur barres de mortier divisé en deux phases a ensuite été développé pour reproduire, en laboratoire, les mécanismes de détérioration observés à Trois-Rivières. Le test consiste en un conditionnement de 90 jours à 80°C/80% RH, avec 2 cycles de mouillage de trois heures chacun, par semaine, dans une solution d’hypochlorite de sodium (eau de javel) à 6% (Phase I), suivi d’une période pouvant atteindre 90 jours de conditionnement à 4°C/100 % HR (Phase II). Les granulats ayant un potentiel d’oxydation ont présenté une expansion de 0,10 % au cours de la Phase I, tandis que la formation potentielle de thaumasite est détectée par le regain rapide de l’expansion suivi par la destruction des échantillons durant la Phase II. Un test de consommation d’oxygène a également été modifié à partir d’un test de Drainage Minier Acide, afin d’évaluer quantitativement le potentiel d’oxydation des sulfures de fer incorporés dans les granulats à béton. Cette technique mesure le taux de consommation d’oxygène dans la partie supérieure d’un cylindre fermé contenant une couche de matériau compacté afin de déterminer son potentiel d’oxydation. Des paramètres optimisés pour évaluer le potentiel d’oxydation des granulats comprennent une taille de particule inférieure à 150 μm, saturation à 40 %, un rapport de 10 cm d’épaisseur de granulat par 10 cm de dégagement et trois heures d’essai à 22ᵒC. Les résultats obtenus montrent que le test est capable de discriminer les granulats contenant des sulfures de fer des granulats de contrôle (sans sulfures de fer) avec un seuil limite fixé à 5% d’oxygène consommé. Finalement, un protocole d’évaluation capable d’estimer les effets néfastes potentiels des granulats à béton incorporant des sulfures de fer a été proposé. Le protocole est divisé en 3 grandes phases: (1) mesure de la teneur en soufre total, (2) évaluation de la consommation d’oxygène, et (3) un test accéléré d’expansion sur barres de mortier. Des limites provisoires sont proposées pour chaque phase du protocole, qui doivent être encore validées par la mise à l’essai d’un plus large éventail de granulats.

Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed "gabbro glacier'' model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. Weestimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000-10506 degrees C to 6006 degrees C are yielded within about 10-30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges.