Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ13C and δ18O values in most concretion bodies range from −10·0‰ to −17·3‰ and +0·3‰ to −0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack-lining brown fibrous calcite records pore fluid re-oxygenation during a depositional hiatus, followed by increasing Fe content and δ13C related to bacterial methanogenesis. Brown colouration results from an included gel-like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi-rigid 'proto-concretions' formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack-lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ13C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ18O range for petrographically identical cement (−1·3‰ to −15·6‰) is difficult to explain

⁴⁰Ar/³⁹Ar dating of hydrothermal activity, biota and gold mineralization in the Rhynie hot-spring system, Aberdeenshire, Scotland

This study presents a new high-precision <sup>40</sup>Ar/<sup>39</sup>Ar age for the Devonian hot-spring system at Rhynie. Hydrothermal K-feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock, date the hydrothermal activity, the fossilised biota, and syn – K-feldspar gold mineralization at 403.9 ± 2.1 Ma (2σ). Oxygen isotope data for the parent fluid (−4‰ to 2‰) show that the K-feldspar was precipitated from a dominantly meteoric fluid, which mixed with magmatic fluids from a degassing magma chamber. The <sup>40</sup>Ar/<sup>39</sup>Ar age (403.9 ± 2.1 Ma [2σ]) when recalculated (407.1 ± 2.2 Ma [2σ]) with respect to the astronomically tuned age for Fish Canyon sanidine (28.201 ± 0.023 Ma [1σ]), also provides a robust marker for the polygonalis-emsiensis Spore Assemblage Biozone within the Pragian-?earliest Emsian. Furthermore, the age identifies the Devonian pull-apart volcano-sedimentary basins of the British and Irish Caledonides (and their root zones), as specific targets for future gold exploration

A Devonian auriferous hot spring system, Rhynie, Scotland

The Early Devonian Rhynie hot spring system is the oldest known and is of the low sulphidation type. It extends for at least 1.5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments, plant-bearing sinters and andesitic lavas. The age of sedimentation and hydrothermal activity has been determined by palynological (Pragian) and radio-metric (396 ± 12 Ma) techniques. The outlier is a half graben with a complex stepped western margin. The Devonian rocks show intense hydrothermal alteration along the fault zone. The main alteration minerals are quartz, K-feldspar, calcite, hematite and illitic and chloritic clays. Multiple chert veining and brecciation are widely developed, and geyserite and vent material are also present. Pyrite occurs in veins and all alteration facies. Sinters and altered rocks contain high concentrations of Au, As, Sb, Hg, W and Mo. Gold occurs in arsenian pyrite and as sub-micron particles in oxidized rocks. The fluid(s) responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water. However, incursions of high temperature (300–440°C) magmatic fluids occurred with δD–65‰ and δ18O around +8.5‰. δ34S (pyrite) and initial 87Sr/86Sr ratios (vein calcite) lie mainly within the ranges +3.4‰ to +8.5‰ and 0.71138 to 0.71402 respectively. These data indicate that late Proterozoic Dalradian metasediments are a likely source for S and Sr but other sources are possible. δ13C values for caliche and vein calcite imply derivation of carbon from non-organic sources. The Rhynie cherts were deposited from a low salinity fluid of probable meteoric origin (δ18Ochert+ 13.1‰ to +16.5‰) which had interacted with the basement rocks and sediments (high Xe/Ar, Br/Cl and I/Cl ratios). Plant-bearing chert yielded an 40Ar/39Ar ratio (292.1± 0.6) significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere

Falklands: Facts and fiction

A reply to McDowall, R.M. (2005) Falkland Islands biogeography: converging trajectories in the South Atlantic Ocean. Journal of Biogeography, 32, 49–62. A recent contribution (McDowall, 2005) analysed the biogeography of the Falkland Islands, an archipelago situated in the south‐western Atlantic, known in Spanish as Islas Malvinas. After reviewing the literature, McDowall (2005) concluded that the biotic and geological connections of the Falklands conflict with each other because the biota shows apparent relationships with Patagonia, whereas the geology suggests a historical relationship with South Africa. He considered that these results indicate that Croizat's dictum that ‘earth and life evolve together’‘does not have general application in the way that some believe it has’ (McDowall, 2005, p. 59), thus ruling out vicariance as an appropriate explanation for the evolution of the Falklands’ biota.Fil: Morrone, Juan José. Universidad Nacional Autónoma de México; MéxicoFil: Posadas, Paula Elena. Museo Paleontológico Egidio Feruglio; Argentin