Stable Isotope Evidence for the Role of Diffusion, Infiltration, and Local Structure on Contact Metamorphism of Calc-Silicate Rocks at Noth Peak, Utah

The effects of infiltration, diffusion, and local structureon the contact metamorphism of the Cambrian Weeks Formation,a calcareous argillite located in western Utah, were examinedusing stable isotope systematics and mineral mass-balance considerations.The emplacement of the Jurassic Notch Peak granitic stock resultedin metamorphism characterized with increasing proximity to theintrusion by phlogopite, diopside, and wollastonite isograds.The isograds are generally concentric around the stock, withdiopside and wollastonite isograds {small tilde}600 and 400m from the contact, respectively, but are deflected near a pre-intrusionfault where the wollastonite isograd is 1. 5 km from the contact.Where the isograds are concentric, the wollastonite isogradmarks an isotopic front with whole-rock ¹⁸O values approachingthe 9.5% value of the stock. In contrast, the ¹⁸O values inthe unmetamorphosed to diopside-grade rocks range from 16.3to 20.2%. Near the fault the isotopic front extends throughthe diopside zone, suggesting that the fault was a major conduitfor magmatic water. Water—rock ratios for the diopside- and wollastonite-gradesamples determined from mineralogic mass balance are nearlyone order of magnitude larger than ‘one-box’ closed-systemratios determined from shifts in stable isotope ratios. Chromatographicmodels for isotopic exchange and propagation of isograds showthat one-dimensional infiltration of magmatic water throughpore spaces would lead to an isotopic front at 50 m from thestock and the wollastonite isograd would be only 8 km from thestock. These distances are significantly smaller than observed.It is suggested that most of the magmatic water flowed throughfractures or thin permeable layers, with the extent of isotopicexchange between the magmatic water in these conduits and theadjacent rocks being controlled by the extent of reaction progressin the rocks. Considerations of CO₂—H₂O interdiffusivitiesin fluids indicate that removal of CO₂ from the reaction frontstoward the intrusion or the fault was sufficiently rapid tocontrol the geometry of the isograds.     

Cosmogenic radionuclide dating indicates ice‐sheet configuration during MIS 2 on Nordaustlandet,Svalbard

Hormes, A., Akçar, N. & Kubik, P. W. 2011: Cosmogenic radionuclide dating indicates ice‐sheet configuration during MIS 2 on Nordaustlandet, Svalbard. Boreas, 10.1111/j.1502‐3885.2011.00215.x. ISSN 0300‐9483.0300‐9843 Glacial geological field surveys, aerial image interpretation and cosmogenic radionuclide (CRN) dating allowed us to reconstruct the ice‐sheet configuration on Nordaustlandet, the northernmost island of the European sector on the margin of the Arctic Ocean. The timing of deglaciation was investigated by determining the ²⁶Al and ¹⁰Be ages of glacially scoured bedrock, weathered periglacial blockfields and glacial erratic boulders. Only ¹⁰Be ages were useful for our interpretations, because of unresolved analytical problems with ²⁶Al. Fjords and lowlands on Nordaustlandet yielded Late Weichselian ¹⁰Be ages, indicating that actively erosive ice streams scoured the coastal fjord bathymetry during marine isotope stage (MIS) 2. In Murchisonfjorden, ground‐truthed air‐photograph interpretation and ¹⁰Be ages of boulders indicated a cold‐based glacier ice cover during MIS 2 on higher plateaus. ¹⁰Be ages and lithological studies of erratic boulders on higher and interior plateaus of Prins Oscars Land (>200–230 m a.s.l.) suggest that the Mid‐Weichselian glaciation (MIS 4) might have been more extensive than that during MIS 2.     

Magnetostratigraphy

Magnetostratigraphy involves the study of changes in the Earth's magnetic field that are recorded in sediments and rocks. These include short-term fluctuations (secular variation), polarity transitions and magnetic reversal sequences. All have important applications in the Earth sciences. They can be used to calculate the rates of sea-floor spreading and sediment deposition rates, and for time-correlation of rock sequences, especially when no other means are available.     

Paving the way for conservation?

Limestone pavement is a unique part of Britain's physical landscape. Along with other forms of naturally weathered rock, it is under threat from exploitation for the horticultural market. Valued for its delicate and sculptured form, limestone pavement has been frequently used as decorative stone in rockeries and other landscaped ground. If this unique landscape is to be protected, its exploitation must stop. Yet it must be remembered that the use of rock in gardens and landscaped ground has an important role in raising the public profile of geology within urban areas. We have a simple message: the use of limestone pavement is wrong and must stop, but the use of other forms of stone in urban gardens is beneficial for the Earth sciences.