Oil-weathering behavior in Arctic environments

Oil-weathering processes in ice-free subarctic and Arctic waters include spreading, evaporation, dissolution, dispersion of whole-oil droplets into the water column, photochemical oxidation, water-in-oil emulsification, microbial degradation, adsorption onto suspended particulate material, ingestion by organisms, sinking, and sedimentation. While many of these processes also are important factors in ice-covered waters, the various forms of sea ice (depending on the active state of ice growth, extent of coverage and/or decay) impart drastic, if not controlling, changes to the rates and relative importance of different oil-weathering mechanisms. Flow-through seawater wave-tank experiments in a cold room at −35°C and studies in the Chukchi Sea in late winter provide data on oil fate and effects for a variety of potential oil spill scenarios in the Arctic. Time-series chemical weathering data are presented for Prudhoe Bay crude oil released under and encapsulated in growing first-year columnar ice through spring breakup.     

Redistriution of Major Elements in the Alteration of Some Basic Lavas during Burial Metamorphism

An Ordovician geanticlinal marine sequence is exposed in theCentral West of New South Wales, Australia. The sequence containingbasic lavas, labile sedimentary, and proclastic rocks, has undergoneextensive adjustment during regional burial metamorphism. Thebasic lavas have a patchy coloration due to the formation ofalternation minerals of the prehnite-pumpellyite facies whichapperar to have formed during the metamorphism. The hererogeneousappearance of the basic lavas reflects a pronouced chemicaland mineralogical departure from the original composition. Twodiverging trends result. One leads to the production of a Ca-enrichedlithology, the other, which by far dominates in area at eachouterop, leads to a ‘spilitic’ lithology. The alterationdomains in many cases cross–cut all primary textural featureand their chemical variation does not correspond to likely variationsin the original lava composition.     

Phase Equilibria in the Metamorphic Rocks of St. Paul Island and Cape North, Nova Scotia

A suite of schists—one from the garnet zone, 19 from thestaurolite zone, 2 from the kyanite isograd, and one from thekyanite zone—were separated into their constituent minerals.Chemical analyses of one chlorite and of 23 sets of coexistingbiotites and garnets were carried out by photometric and titrametricprocedures. Plots of garnet-biotite tie-lines from divariantassemblages on appropriate phase diagrams result in intersectingtie-lines which cannot be ascribed to experimental error. Theoreticalconsiderations argue that at equilibrium, at the same pressureand temperature or at constant pressure and varying temperature,tie-lines of divariant assemblages should not intersect. Possibleexplanations require that diffusion equilibrium of Fe and Mgbe restricted to volumes smaller than that of a hand specimenor that P as well as T varies considerably. Emission spectrographicdeterminations of Fe and Mg in biotite indicate that the Fe/Mgratio varies among biotites little more than a centimeter apart.Such a variation would argue more in favor of a lack of diffusionequilibrium.     

Trace Element and Isotope Geochemistry of the Volcanic Rocks of Bequia, Grenadine Islands, Lesser Antilles Arc: a Study of Subduction Enrichment and Intra-crustal Contamination

Pliocene volcanics on the island of Bequia comprise two interbeddedsuites of basalts and andesites. The isotopically homogeneoussuite (IHS) has a limited range of Sr—Nd—Pb isotopes(⁸⁷Sr/⁸⁶Sr 0.7040–0.7046, ¹⁴³ Nd/¹⁴⁴ Nd 0.5130 and ²⁰⁶Pb/²⁰⁴Pb 19.36–19.51), and mantle-like ¹⁸O values (5.5in clinopyroxene). The isotopically diverse suite (IDS) is characterizedby much wider ranges of radiogenic isotopes (⁸⁷ Sr/⁸⁶Sr 0.7048–0.7077,¹⁴³ Nd/¹⁴⁴ Nd 0.5128–0.5123 and ²⁰⁶ Pb/²⁰⁴ Pb 19.7–20.2),in which all of the Sr and Pb ratios are higher and Nd ratiosare lower than those of the IHS. The IDS is also characterizedby high ¹⁸ O values, up to 7.6 in clinopyroxene. The Sr andPb isotope ratios are too high, and the Nd isotope ratios aretoo low in the IDS for any of these lavas to be derived fromunmodified depleted mantle. Both suites are petrologically very similar and their majorelement compositions and phenocryst contents suggest that theywere formed largely by fractional crystallization of a hydroustholeiitic melt at pressures <3 kbar. The isotopic ratiosand enrichments in large ion lithophile elements (LILE), andto some extent light rare earth elements (LREE), as comparedwith mid-ocean ridge basalts (MORB), of the IHS lavas suggestthat they were derived from a depleted mantle source which hadbeen re-enriched by the addition of 1–4% of a subductioncomponent. This component probably comprised a mixture of dehydrationfluids, and perhaps minor siliceous melts, released from subductingsediments and mafic crust. The extreme isotopic ranges, largeenrichments in incompatible elements, more fractionated LREEpatterns and higher ¹⁸ O values of the IDS lavas are interpretedas resulting from 10–55% assimilation—fractionalcrystallization of sediments, derived from the Guyana Shield,which are present in the arc crust, by IHS type melts. KEY WORDS: trace elements; radiogenic isotopes; arc lavas; Lesser Antilles ^*Corresponding author.     

Crystal-Mush Compaction and the Origin of Pegmatitic Segregation Sheets in a Thick Flood-Basalt Flow in the Mesozoic Hartford Basin, Connecticut

Where the Holyoke flood-basalt flow in the Mesozoic HartfordBasin in Connecticut is thick and contains coarse-grained, horizontalsegregation sheets in its central part, the lower part of theflow is strongly depleted in incompatible elements; where theflow is thin and contains no segregation sheets it is homogeneousthroughout. This chemical variation can be explained only throughcompaction of the partly crystallized basalt. The compositionof the segregation sheets shows that they separated from thebasalt following only 33% crystallization. The segregation sheets,however, are clearly intrusive into the basalt, which must thereforehave already formed a crystal mush with finite strength at thislow degree of crystallinity. The incompatible element concentrationsindicate that the partly crystallized basalt underwent as muchas 28% compaction in the lowest 60 m of the flow. Between 60and 130 m above the base of the flow, the crystal mush becamedilated, and eventually ruptured with formation of the segregationsheets. No segregation sheet has a composition indicating separationafter more than 33% crystallization of the basalt. This is interpretedto indicate that compaction ceased at this stage because ofthe increasing strength of the mush and the increasing densityof the fractionating interstitial liquid KEY WORDS: crystal-mush compaction; segregation shtets; flood basalt; tholeiitie; Connecticut ^*e-mail: philpotts{at}geol.uconn.edu