L.P. Field

Magmatic history of Dabbahu, a composite volcano in the Afar Rift, Ethiopia

Dabbahu is a composite volcano at the north end of the active Manda-Hararo segment of the Afar Rift in northern Ethiopia. We present 93 new whole-rock analyses, mineral analyses from 65 samples, and 9 new 40Ar39Ar dates for rocks ranging in composition from mildly alkaline basalt through trachyandesite to peralkaline rhyolite (comendite and pantellerite) erupted from Dabbahu. These data, supplemented by a new geological map, are used to provide insights into the evolution of the volcano. We show that Dabbahu has been active for over 67 k.y., but an apparent hiatus occurred between the eruption of comendite (29 ka) and pantellerite (ca. 8 ka) lavas. Mineral data for olivine, clinopyroxene, plagioclase, and alkali feldspar show a remarkably extensive range of solid solution across the rock suite consistent with protracted fractionation from basalt to rhyolite. The parental basalt is compositionally similar to recent rift-related basalts in the Manda-Hararo rift, with low initial H2O contents (<1 wt%). Closed-system fractionation increased H2O contents of residual liquids sufficiently for some rhyolites to erupt both explosively and effusively. The diverse magma types were erupted from a relatively closely spaced network of vents and fissures. Field evidence indicates that magmas were not erupted in a simple fractionation sequence. Some mixing occurred between magmas of less-evolved compositions and more-evolved compositions shortly prior to, or during, eruption. The differentiation of basalt to rhyolite must have occurred on time scales that were relatively short compared to the lifetime of the volcano, probably due to the small volumes of basalt intruded into the crust and consequently enhanced cooling and crystallization rates. A network of stacked sills or closely spaced dikes in the shallow to midcrust represents the most plausible configuration of the subvolcanic plumbing system. Input of new magma batches into such a system may serve as a key eruption trigger at Dabbahu.

Unusual morphologies and the occurrence of pseudomorphs after ikaite (CaCO3·6H2O) in fast growing, hyperalkaline speleothems

Abstract Unusual speleothems, associated with hyperalkaline (pH > 12) groundwaters have formed within a shallow, abandoned railway tunnel at Peak Dale, Derbyshire, UK. The hyperalkaline groundwaters are produced by the leaching of a thin layer (<2 m) of old lime-kiln waste on the soil-bedrock surface above the tunnel by rainwater. This results in a different reaction and chemical process to that more commonly associated with the formation of calcium carbonate speleothems from Ca-HCO3-type groundwaters and degassing of CO2. Stalagmites within the Peak Dale tunnel have grown rapidly (averaging 33 mm y-1), following the closure of the tunnel 70 years ago. They have an unusual morphology comprising a central sub-horizontallylaminated column of micro- to nano-crystalline calcium carbonate encompassed by an outer sub-vertical assymetric ripple-laminated layer. The stalagmites are composed largely of secondary calcite forming pseudomorphs (<1 mm) that we believe to be predominantly after the ‘cold climate’ calcium carbonate polymorph, ikaite (calcium carbonate hexahydrate: CaCO3 · 6H2O), with minor volumes of small (<5 μm) pseudomorphs after vaterite. The tunnel has a near constant temperature of 8-9°C, which is slightly above the previously published crystallization temperatures for ikaite (<6°C). Analysis of a stalagmite actively growing at the time of sampling, and preserved immediately within a dry nitrogen cryogenic vessel, indicates that following crystallization of ikaite, decomposition to calcite occurs rapidly, if not instantaneously. We believe this is the first occurrence of this calcium carbonate polymorph observed within speleothems.

Retention of chlorine-36 by a cementitious backfill

Abstract Radial diffusion experiments have been carried out to assess the migration of 36Cl, as chloride, through a cementitious backfill material. Further experiments in the presence of cellulose degradation products were performed to assess the effect of organic ligands on the extent and rate of chloride diffusion. Results show that breakthrough of 36Cl is dependent on chloride concentration: as the carrier concentration increases, both breakthrough time and the quantity retained by the cement matrix decreases. Experiments in the presence of cellulose degradation products also show a decrease in time to initial breakthrough. However, uptake at various carrier concentrations in the presence of organic ligands converges at 45% of the initial concentration as equilibrium is reached. The results are consistent with organic ligands blocking sites on the cement that would otherwise be available for chloride binding, though further work is required to confirm that this is the case. Post-experimental digital autoradiographs of the cement cylinders, and elemental mapping showed evidence of increased 36Cl activity associated with black ash-like particles in the matrix, believed to correspond to partially hydrated glassy calcium-silicate-sulfate-rich clinker.

The solubility of nickel and its migration through the cementitious backfill of a geological disposal facility for nuclear waste

This work describes the solubility of nickel under the alkaline conditions anticipated in the near field of a cementitious repository for intermediate level nuclear waste. The measured solubility of Ni in 95%-saturated Ca(OH)2 solution is similar to values obtained in water equilibrated with a bespoke cementitious backfill material, on the order of 5×10(-7)M. Solubility in 0.02M NaOH is one order of magnitude lower. For all solutions, the solubility limiting phase is Ni(OH)2; powder X-ray diffraction and scanning transmission electron microscopy indicate that differences in crystallinity are the likely cause of the lower solubility observed in NaOH. The presence of cellulose degradation products causes an increase in the solubility of Ni by approximately one order of magnitude. The organic compounds significantly increase the rate of Ni transport under advective conditions and show measurable diffusive transport through intact monoliths of the cementitious backfill material.

Recognizing anthropogenic modification of the subsurface in the geological record

Humankind, in its technological development, is increasingly utilizing both mineral resources from Earths interior and developing the rock mass as a resource in itself. In this paper we review the types of anthropogenic intrusion, at different depth ranges, that can modify the physical structure and chemistry of the subsurface. Using examples from across the world, but with emphasis on the UK, and physical models of the induced modifications, we predict what kind of subsurface signatures a geologist of the future might recognize as anthropogenic, including boreholes, tunnels and caverns, waste and resource storage facilities, mineral workings and military test traces. The potential of these anthropogenic signatures to be discriminated from natural analogues is discussed against known or modelled processes of deterioration and transformation over geological timescales of millennia or longer.