Utredning av forhold knyttet til gruveavrenning fra Sulitjelma-feltene: Tålegrenser for ferskvannsfisk, effekter på marint miljø, samt bruksmønster og holdninger til området hos lokalbefolkningen

Pågående overvåkning og tidligere biologiske undersøkelser av gruveavrenning fra Sulitjelma har hatt fokus på øvre del av vassdraget og biologiske forhold i ferskvann. Tilleggsutredningen i 2011 skulle i tillegg til videreutvikling av metoder for fastsettelse av grenseverdier for ferskvannsfisk også dokumentere miljøtilstanden i marint miljø nær tilførselspunktet og gjennomføre en brukerundersøkelse vedrørende arealbruk og holdninger i lokalbefolkningen. Bruk av kjemiske modeller og passive prøvetakere i ferskvann ga resultater som bidrar til økt forståelse av den øvrige vannkjemiens betydning, biotilgjengelige metaller og fisks tålegrenser for belastning. Situasjonen høsten 2011, der høye metallkonsentrasjoner i vannet kan ha gitt effekter på fisk, ble godt beskrevet. Effekten på marint miljø må betraktes som lokal, da kun sedimenter nært tilførselspunktet ble dokumentert påvirket av kobber i et omfang som ga moderat miljøtilstand. Fisk og skalldyr, med unntak av bly i lever av flatfisk, hadde ikke nivåer som overskred kostholdsråd eller omsetningsgrenser for humant konsum. Brukerundersøkelsen avdekket at øvre del av området (Langvatnet) var svært lite brukt, mens bruken tiltok nedover i vassdraget. Brukerne var skeptiske til å spise fisk fra øvre deler av vassdraget, mens dette i mindre grad var tilfellet i nedre deler. Nasjonale myndigheter tilskrives ansvaret for forurensningssituasjonen, og brukerne oppfatter ikke opprydningstiltakene gjennomført til nå som gode nok. Manglende informasjon både om gjennomførte tiltak og dagens miljøsituasjon påpekes av flere brukere

Whole-rock geochemistry of gabbros from the Southwest Indian Ridge: constraints on geochemical fractionations between the upper and lower oceanic crust and magma chamber processes at (very) slow-spreading ridges

Whole-rock major and trace element compositions are presented for a suite of gabbroic samples formed at the Southwest Indian Ridge with the aim of constraining magmatic processes at ultra slow-spreading centres. The gabbros, together with subordinate basalts, dolerites and peridotites were collected from a not, vert, similar700 km2 area around Atlantis Bank, adjacent to the Atlantis II Fracture Zone during cruise JR31 of the RRS James Clark Ross. The large area sampled, the abundance of gabbro, and the recovery of samples representative of all other levels in the oceanic lithosphere, allow an average lower crustal composition to be estimated. The estimated composition is not sufficiently primitive in terms of Mg/Fe or compatible trace element abundances (Ni, Cr) for the bulk crust to be in equilibrium with the mantle. This is probably due to compositional modifications within the mantle during melt extraction, although crystallisation within the crust in an unsampled area, either towards the segment centre or along a flow line, cannot be ruled out. Gabbro compositions show evidence for being mixtures of cumulate crystals and significant proportions of basalt, with the proportion of each end-member dependent on the distribution coefficient of the element in question. This suggests that the concept of ‘trapped melt’ cannot be used to understand the origin of these compositions and consideration of the migration of interstitial liquid within a crystal mush is necessary. The fractionation of incompatible elements between the upper and lower crust correlates with the observed degree of variation in trace element concentrations in basalts from the same spreading segment. This suggests that interstitial liquids are, in part, extracted from the crystal mush and mixed back into subsequently erupted basalts leading to their compositional modification

How partial melts of mafic lower crust affect ascending magmas at oceanic ridges

We present experiments showing that the lower oceanic crust should melt efficiently and quickly when heated by hot ascending magmas. Average plagioclase–olivine and plagioclase–augite pairs from the lower crust at the Southwest Indian Ridge have melt–mineral saturation boundaries at 1,190 and 1,154°C, respectively, and melt rapidly (>0.01 mm/h) at 50°C or more above these temperatures. Melting experiments performed on olivine–plagioclase and augite–plagioclase mineral pairs from actual oceanic lower crustal rock samples and under conditions applicable to a MOR setting (1,220–1,330°C, 1 atm, quartz–fayalite–magnetite oxygen buffer, 0.25–24 h) indicate that the resulting disequilibrium melts are linear mixes of the mineral compositions. The rates of melting are slower than the rate of heat-diffusion into a sample and are approximated as: ξ=2.43×10(−26)×e0.004109×T(degC) (m/s1/2) for augite melted with plagioclase; ξ=3.47×10(−15)×e0.002041×T(degC)for plagioclase melted with augite; and ξ=1.79×10(−21)×e0.0032×T(degC) for plagioclase melted with olivine. Our results indicate that great care must be taken in backward models using basalt chemistry alone to explore mantle-melting processes, assuming only crystallization and fractionation during ascent, as partial melts may mix with intruded hot magma

Lower crustal hydrothermal circulation at slow-spreading ridges: evidence from chlorine in Arctic and South Atlantic basalt glasses and melt inclusions

Hydrothermal circulation at slow-spreading ridges is important for cooling the newly formed lithosphere, but the depth to which it occurs is uncertain. Magmas which stagnate and partially crystallize during their rise from the mantle provide a means to constrain the depth of circulation because assimilation of hydrothermal fluids or hydrothermally altered country rock will raise their chlorine (Cl) contents. Here we present Cl concentrations in combination with chemical thermobarometry data on glassy basaltic rocks and melt inclusions from the Southern Mid-Atlantic Ridge (SMAR; ~ 3 cm year−1 full spreading rate) and the Gakkel Ridge (max. 1.5 cm year−1 full spreading rate) in order to define the depth and extent of chlorine contamination. Basaltic glasses show Cl-contents ranging from ca. 50–430 ppm and ca. 40–700 ppm for the SMAR and Gakkel Ridge, respectively, whereas SMAR melt inclusions contain between 20 and 460 ppm Cl. Compared to elements of similar mantle incompatibility (e.g. K, Nb), Cl-excess (Cl/Nb or Cl/K higher than normal mantle values) of up to 250 ppm in glasses and melt inclusions are found in 75% of the samples from both ridges. Cl-excess is interpreted to indicate assimilation of hydrothermal brines (as opposed to bulk altered rock or seawater) based on the large range of Cl/K ratios in samples showing a limited spread in H2O contents. Resorption and disequilibrium textures of olivine, plagioclase and clinopyroxene phenocrysts and an abundance of xenocrysts and gabbroic fragments in the SMAR lavas suggest multiple generations of crystallization and assimilation of hydrothermally altered rocks that contain these brines. Calculated pressures of last equilibration based on the major element compositions of melts cannot provide reliable estimates of the depths at which this crystallization/assimilation occurred as the assimilation negates the assumption of crystallization under equilibrium conditions implicit in such calculations. Clinopyroxene–melt thermobarometry on rare clinopyroxene phenocrysts present in the SMAR magmas yield lower crustal crystallization/assimilation depths (10–13 km in the segment containing clinopyroxene). The Cl-excesses in SMAR melt inclusions indicate that assimilation occurred before crystallization, while also homogeneous Cl in melts from Gakkel Ridge indicate Cl addition during magma chamber processes. Combined, these observations imply that hydrothermal circulation reaches the lower crust at slow-spreading ridges, and thereby promotes cooling of the lower crust. The generally lower Cl-excess at slow-spreading ridges (compared to fast-spreading ridges) is probably related to them having few if any permanent magma chambers. Magmas therefore do not fractionate as extensively in the crust, providing less heat for assimilation (on average, slow-spreading ridge magmas have higher Mg#), and hydrothermal systems are ephemeral, leading to lower total degrees of crustal alteration and more variation in the amount of Cl contamination. Hydrothermal plumes and vent fields have samples in close vicinity that display Cl-excess, mostly of > 25 ppm, which thus can aid as a guide for the exploration of (active or extinct) hydrothermal vent fields on the axis