Haematite natural crystals: non-linear initial susceptibility at low temperature

Several works have reported that haematite has non-linear initial susceptibility at room temperature, like pyrrhotite or titanomagnetite, but there is no explanation for the observed behaviours yet. This study sets out to determine which physical property (grain size, foreign cations content and domain walls displacements) controls the initial susceptibility. The performed measurements include microprobe analysis to determine magnetic phases different to haematite; initial susceptibility (300 K); hysteresis loops, SIRM and backfield curves at 77 and 300 K to calculate magnetic parameters and minor loops at 77 K, to analyse initial susceptibility and magnetization behaviours below Morin transition. The magnetic moment study at low temperature is completed with measurements of zero field cooled-field cooled and AC susceptibility in a range from 5 to 300 K. The minor loops show that the non-linearity of initial susceptibility is closely related to Barkhausen jumps. Because of initial magnetic susceptibility is controlled by domain structure it is difficult to establish a mathematical model to separate magnetic subfabrics in haematite-bearing rocks.

Geochemical composition of sulfide and oxide minerals from ODP Sites 193-1188 and 193-1189, PACMANUS field

Ocean Drilling Program (ODP) Leg 193 recovered core from the active PACMANUS hydrothermal field (eastern Manus Basin, Papua New Guinea) that provided an excellent opportunity to study mineralization related to a seafloor hydrothermal system hosted by felsic volcanic rocks. The purpose of this work is to provide a data set of mineral chemistry of the sulfide-oxide mineralization and associated gold occurrence in samples drilled at Sites 1188 and 1189. PACMANUS consists of five active vent sites, namely Rogers Ruins, Roman Ruins, Satanic Mills, Tsukushi, and Snowcap. In this work two sites were studied: Snowcap and Roman Ruins. Snowcap is situated in a water depth of 1670 meters below sea level [mbsl], covers a knoll of dacite-rhyodacite lava, and is characterized by low-temperature diffuse venting. Roman Ruin lies in a water depth of 1693-1710 mbsl, is 150 m across, and contains numerous large, active and inactive, columnar chimneys. Sulfide mineralogy at the Roman Ruins site is dominated by pyrite with lesser amounts of chalcopyrite, sphalerite, pyrrhotite, marcasite, and galena. Sulfide minerals are relatively rare at Snow Cap. These are dominated by pyrite with minor chalcopyrite and sphalerite and traces of pyrrhotite. Native gold has been found in a single sample from Hole 1189B (Roman Ruins). Oxide minerals are represented by Ti magnetite, magnetite, ilmenite, hercynite (Fe spinel), and less abundant Al-Mg rich chromite (average = 10.6 wt% Al2O3 and 5.8 wt% MgO), Fe-Ti oxides, and a single occurrence of pyrophanite (Mn Ti O3). Oxide mineralization is more developed at Snowcap, whereas sulfide minerals are more extensive and show better development at Roman Ruins. The mineralogy was obtained mainly by a detailed optical microscopy study. Oxide mineral identifications were confirmed by X-ray diffraction, and mineral chemistry was determined by electron probe microanalyses.

Magnetic properties of maar lake sediments from Otago, New Zealand

Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits at the base of the cored sedimentary sequence yielded two new 40Ar/39Ar dates of 24.51±0.24 Ma and 23.38±0.24 Ma (2sigma). The younger date agrees within error with a previously published 40Ar/39Ar date of 23.17±0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have overlapped with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic concentration and lamina thickness indicate a decrease in erosional input and increase in diatom productivity throughout the depositional period, suggesting a long-term (tens of thousands of years) climatic change in New Zealand in the latest Oligocene/earliest Miocene.

(Table 1) Sulfur contents and isotope composition in rocks and minerals from DSDP/ODP Hole 504B

DSDP Hole 504B is the only hole in oceanic crust to penetrate through the volcanic section and into hydrothermally altered sheeted dikes. We have carried out petrologic and sulfur isotopic analyses of sulfide and sulfate minerals and whole rocks from the core in order to place constraints on the geochemistry of sulfur during hydrothermal alteration of ocean crust. The nearly 600 m-thick pillow section has lost sulfur to seawater and has net d34S = -1.8 per mil due to degassing of SO2 during crystallization and subsequent low temperature interaction with seawater. Hydrothermally altered rocks in the 200 m-thick transition zone are enriched in S and 34S (4300 ppm and +3.0 +/-1.2 per mil, respectively), whereas the more than 500 m of sheeted dikes contain 720 ppm S with d34S = +0.6 +/-1.4 per mil. These data are consistent with the presence of predominantly basaltic sulfur in hydrothermal fluids deep in the crust: following precipitation of anhydrite during seawater recharge, small amounts of seawater sulfate were reduced at temperatures >250°C through conversion of igneous pyrrhotite to secondary pyrite and minor oxidation of ferrous iron in the crust. The S- and 34S-enrichments of the transition zone are the results of seawater sulfate reduction and sulfide deposition during subsurface mixing between upwelling hot (up to 350°C) hydrothermal fluids and seawater. Seawater sulfate was probably reduced through oxidation of ferrous iron in hydrothermal fluids and in the transition zone rocks. Alteration of the upper crust resulted in loss of basaltic sulfur to seawater, fixation of minor seawater sulfur in the crust and redistribution of magmatic sulfur within the crust. This caused net increases in sulfur content and d34S of the upper 1.8 km of the oceanic crust.

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.

Geologia, petrologia, geocronologia e mineralizações sulfetadas do complexo ézio, província mineral de carajás, Brasil

Dissertação (mestrado)—Universidade de Brasília, Instituto de Geociências, Pós-Graduação em Geologia, 2015.

Tracing acidification induced by Deccan Phase 2 volcanism

The Deccan Volcanic Province (DVP) was built up by three major phases of eruptions; the most voluminous of which, the Deccan Phase 2, encompassed the Cretaceous–Palaeogene (KT) boundary. Deccan eruptions have been implicated as a contributor to the end-Cretaceous mass extinction, however, mechanism by which volcanic activity affected biota remains poorly understood. We applied a combination of rock magnetic techniques scanning electron microscopy to characterize mineral assemblages of three sections of intertrappean lacustrine sediments from the north-western Maharashtra Deccan Volcanic Provinces. Our results indicate that in sediments deposited during the early stages of the Deccan Phase 2, the Daïwal River and Dhapewada sequences, iron-bearing mineral association is dominated by detrital iron oxides (magnetite and hematite) sourced from the weathering of the surrounding basaltic bedrocks, with minor contribution form authigenic iron sulphides (framboidal pyrite, pyrrhotite and/or greigite). The sediments deposited during the final stages of Phase 2 (the Podgawan sequence) differ significantly in their characteristics. In particular, the Podgawan sediments have 1) very low magnetic susceptibility values, but higher terrigenous fraction (clays and shales) content; 2) more complex assemblage of magnetic minerals, 3) ubiquitous presence of Fe–Ca–Ce vanadates; and 4) unusual lithological variations in the middle part of the section (represented by a charcoal-rich level that is capped by a red clay layer containing fossilized bacterial colonies). We suggest that these unusual characteristics reflect increased acidity in the region during the deposition of the Podgawan sequence, likely due to cumulative effects of volcanic aerosols released during the Deccan Phase 2 eruptions. The combination of these features may be used to recognize episodes of increased acidity in the geological record. Our results also contribute to understanding of local vs. global effects of the Deccan volcanism.