Mutual replacement reactions in alkali feldspars I: microtextures and mechanisms

Intracrystal microtextures formed by a process of mutual replacement in alkali feldspars record fluid–rock reactions that have affected large volumes of the Earth’s crust. Regular, ≤1 μm-scale ‘strain-controlled’ perthitic microtextures coarsen, by up to 10³, by a dissolution–reprecipitation process, producing microporous patch or vein perthites on scales >100 μm. We have developed earlier studies of such reactions in alkali feldspar cm-scale primocrysts in layered syenites from the Klokken intrusion, South Greenland. We present new hyperspectral CL, SEM images, and laser ICPMS analytical data, and discuss the mechanism of such replacement reactions. The feldspars grew as homogeneous sodic sanidines which unmixed and ordered by volume diffusion during cooling into the microcline field at ~450°C, giving regular, fully coherent ‘braid’ cryptoperthite. At ≤450°C the crystals reacted with a circulating post-magmatic aqueous fluid. The braid perthite behaved as a single reactant ‘phase’ which was replaced by two product phases, incoherent subgrains of low albite and microcline, with micropores at their boundaries. The driving force for the reactions was coherency strain energy, which was greater than the surface energy in the subgrain mosaic. The external euhedral crystal shapes and bulk major element composition of the primocrysts were unchanged but they became largely pseudomorphs composed of subgrains usually with the ‘pericline’ and ‘adularia’ habits (dominant {110} and subordinate {010} morphology) characteristic of low T growth. The subgrains have an epitactic relationship with parent braid perthite. Individual subgrains show oscillatory zoning in CL intensity, mainly at blue wavelengths, which correlates with tetrahedral Ti. Regular zoning is sometimes truncated by irregular, discordant surfaces suggesting dissolution, followed by resumption of growth giving regular zoning. Zones can be traced through touching subgrains, of both albite and microcline, for distances up to ~500 μm. At ≤340°C, the microcline subgrains underwent a third stage of unmixing to give straight lamellar film perthites with periodicities of ~1 μm, which with further cooling became semicoherent by the development of spaced misfit dislocations. Sub-grain growth occurred in fluid films that advanced through the elastically strained braid perthite crystals, which dissolved irreversibly. Braid perthite was more soluble than the strain-free subgrain mosaics which precipitated from the supersaturated solution. Some volumes of braid texture have sharp surfaces that suggest rapid dissolution along planes with low surface energies. Others have complex, diffuse boundaries that indicate a phase of coherent lamellar straightening by volume diffusion in response to strain relief close to a slowly advancing interface. Nucleation of strain-free subgrains was the overall rate-limiting step. To minimise surface energy subgrains grew with low energy morphologies and coarsened by grain growth, in fluid films whose trace element load (reflected in the oscillatory zoning) was dictated by the competitive advance of subgrains over a range of a few tens of mm. The cross-cutting dissolution surfaces suggest influxes of fresh fluid. Removal of feldspar to give 2 vol% porosity would require a feldspar:fluid ratio of ~1:26 (by wt). The late reversion to strain-controlled exsolution in microcline subgrains is consistent with loss of fluid above 340°C following depressurization of the intrusion. A second paper (Part II) describes trace element partitioning between the albite and microcline subgrains, and discusses the potential of trace elements as a low-T geothermometer. This paper and the Part II are dedicated in memory of J.V. Smith and W.L. Brown, both of whom died in 2007, in acknowledgement of their unrivalled contributions to the study of the feldspar minerals over more than half a century.     

Infrared and Raman spectroscopy studies of the α–β phase transition in cristobalite

Infrared and Raman spectra of cristobalite are presented as a function of temperature through the phase transition. The modes are assigned and the assignments compared to those of earlier workers. The compatibility of modes at the G-point of the a-phase with the X and G-points of the ß-phase is given. In the transition region of ca. 500–550 K, smooth changes in intensity, frequency and linewidths are seen in many modes, indicative of coexistence of a- and ß-forms.     

The age of orogenesis in the Nambucca Slate Belt: A K‐Ar study of low‐grade regional metamorphic rocks

K‐Ar ages of biotite and hornblende from undeformed granodiorite plutons and of slaty and phyllitic rocks, ranging from prehnite‐pumpellyite metagreywacke to greenschist fades, have been determined in an attempt to define the age of orogenesis in the eastern part of the Nambucca Slate Belt. The plutons have K‐Ar ages of 226–227 m.y. (biotite) and 228–231 m.y. (hornblende) that provide a younger age limit for deformation. The lower grade metamorphic rocks yield a range of ages including some comparable with the depositional age of the rocks as indicated by fossils. Rocks of pumpellyite‐actinolite and greenschist facies give a more coherent group of ages which suggest orogenesis at about 250–255 m.y. Specimens of these latter rocks that have been affected by a later structural episode than that during which slaty cleavage formed, yield slightly older ages, which may result from the inclusion of minor amounts of environmental excess ⁴⁰Ar.

Support for the 250–255 m.y. age comes from previously determined radiometric ages from the western part of the Slate Belt, although the presence of granitic bodies perhaps as old as 289 m.y., some closely associated with high‐grade regional metamorphic rocks, may indicate the presence of additional earlier orogenic movements in this region.     

A final summary of measurements of cosmogenic AI26 in stone meteorites

A summary is given of the final results of measurements of cosmogenic Al²⁶ activity in a total of 50 stone meteorites, mainly chondrites, carried out using coincidence gamma-spectrometry.     

Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada

Ancient crustal rocks provide the only direct evidence for the processes and products of early Earth differentiation. SHRIMP zircon U-Th-Pb dating has identified, amongst the Acasta gneisses of the western Slave Province, Canada, two metatonalites and a metagranodiorite that have igneous ages of 4002 ± 4, 4012 ± 6 and 4031 ± 3 Ga respectively. These are the first identified Priscoan terrestrial rocks. A record of metamorphic events at ∼3.75, ∼3.6 and ∼1.7 Ga also is preserved. These discoveries approximately double, to ∼40 km², the area over which ∼4.0 Ga gneisses are known to occur. A single older zircon core in one sample suggests that rocks as old as 4.06 Ga might yet be found in the region. As early as 4.03 Ga, terrestrial differentiation was already producing tonalitic magmas, probably by partial melting of pre-existing, less differentiated crust. Received: 28 February 1997 / Accepted: 9 July 1998     

Bayesian modelling the retreat of the Irish Sea Ice Stream

We present an 8000‐year history spanning 650 km of ice margin retreat for the largest marine‐terminating ice stream draining the former British–Irish Ice Sheet. Bayesian modelling of the geochronological data shows the ISIS expanded 34.0–25.3 ka, accelerating into the Celtic Sea to reach maximum limits 25.3–24.5 ka before a collapse with rapid marginal retreat to the northern Irish Sea Basin (ISB). This retreat was rapid and driven by climatic warming, sea‐level rise, mega‐tidal amplitudes and reactivation of meridional circulation in the North Atlantic. The retreat, though rapid, is uneven, with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry. Initially, wide calving margins and adverse slopes encouraged rapid retreat (～550 m a^?1) that slowed (～100 m a^?1) at the topographic constriction and bathymetric high between southern Ireland and Wales before rates increased (～200 m a^?1) across adverse bed slopes and wider and deeper basin configuration in the northern ISB. These data point to the importance of the ice bed slope and lateral extent in predicting the longer‐term (>1000 a) patterns and rates of ice‐marginal retreat during phases of rapid collapse, which has implications for the modelling of projected rapid retreat of present‐day ice streams. Copyright © 2013 John Wiley & Sons, Ltd.     

Mississippian coral assemblages from the Khenifra area,Central Morocco: Biostratigraphy,biofacies, palaeoecology and palaeobiogeography

Analysis of Mississippian coral assemblages from the Khenifra region of Central Morocco together with data from foraminiferal/algal microfossils has established new age dating of 5 localities within the Azrou–Khenifra Basin: Souk El Had and Sidi Lamine, where corals occur mainly in biostromes protected by oolitic shoals, Tabainout, where corals have been recorded in different environments related to microbial mounds, Alhajra Almatkouba, where corals occur in biostromal reworked beds and Tiouinine, where corals occur in a well structured, fringing reef. This study demonstrates the presence of richer more diverse coral assemblages than previously recorded, in a variety of environmental settings. These coral assemblages strengthen correlations with the Adarouch area in the NE part of the Azrou–Khenifra Basin. It is emphasised that in the upper Viséan there are close similarities with rugose coral assemblages in other parts of the Western Palaeotethys including North Africa, SW Spain and NW Europe, and that all belong to the same biogeographic province.