Re-interpretation of a mixed petrographic province — The Sintra intrusive complex (Portugal) and related rocks

Sub-alkaline to calc-alkaline granite dominates the high-level plutonic massif of the Serra de Sintra, while two overlapping ring complexes recognised within it consist of four distinct alkaline to sub-alkaline quartz-bearing syenitic units, together with intrusions of a remarkable kaersutite- and (titan?)biotite-bearing theralite heteromorph (Mafraite), contaminated to varying degrees by sialic admixture. Large volumes of sub-volcanic breccia testify to considerable late explosive activity in and near the ring centres. Minor intrusives are approximate compositional counterparts of the plutonic rocks, but more closely resemble lavas forming the extensive “basaltic mantle” of the Lisbon region, where compositions range from olivine basalt, basanite and theralite (luscladite) to subordinate quartz-bearing trachyandesite and trachyte. Undersaturated salic rocks are absent throughout the province. Magmatic differentiation probably played a minimal part in petrogenesis of the Sintra intrusives. The granite is interpreted as a direct product of basement syntexis, the other rocks as products of selective palingenetic reactions between mafraitic magma and crystalline basement (or granite). Evolution of the volcanic and hypabyssal rocks may have been controlled by similar reactions, but evidence favouring differentiation of this suite from sialcontaminated basic parents is almost as strong. There are some points of resemblance between this province and that of the Auvergne region in southern France.     

Circulating subbeam systemsand the physics of pulsar emission

The purpose of this paper is to suggest how detailed single-pulse observations of slow radio pulsars may be utilized to construct an empirical model for their emission. It links the observational synthesis developed in a series of papers by Rankin in the 1980s and 90s to the more recent empirical feedback model of Wright (2003a) by regarding the entire pulsar magnetosphere as a non-steady, non-linear interactive system with a natural built-in delay. It is argued that the enhanced role of the outer gap in such a system indicates an evolutionary link to younger pulsars, in which this region is thought to be highly active, and that pulsar magnetospheres should no longer be seen as being driven by events on the neutron stars polar cap, but as having more in common with planetary magnetospheres and auroral phenomena.Received: 8 May 2003, Published online: 14 November 2003 Correspondence to: Joanna M. Rankin. On leave from: Physics Department, University of Vermont, Burlington, VT 05405, USA     

OSIRIS: AO-assisted integral-field spectroscopy at the Keck Observatory

OSIRIS (OH-Suppressing Infra-Red Integral-field Spectrograph) is a new facility instrument for the Keck Observatory. After seeing first light in February 2005, OSIRIS is currently undergoing commissioning. OSIRIS provides the capability of performing three-dimensional spectroscopy in the near-infrared z, J, H, and K bands at the resolution limit of the Keck II telescope, which is equipped with adaptive optics and a laser guide star. The science case for OSIRIS is summarized, and the instrument and associated data reduction software are described.     

Pn arrivals and lateral variations of Moho geometry beneath the Kaapvaal craton

Pn arrivals from mining-induced earthquakes on the edge of the Witwatersrand basin show that the P wavespeeds in the uppermost mantle are almost constant throughout most of the Kaapvaal craton. The presence of only small wavespeed variations allows the use of a simple method of estimating crustal thicknesses below the stations of the Kaapvaal broad-band network using Pn times that has been compared with results from receiver functions. One thousand three hundred thirty-seven Pn arrivals were used to derive crustal thicknesses at 46 stations on the Kaapvaal craton. The average crustal thicknesses for 19 centrally located stations on each of the northern and southern regions of the craton that yielded well-constrained thicknesses were 50.52±0.88 km and 38.07±0.85 km, respectively. In contrast, the corresponding average thicknesses determined from receiver functions were 43.58±0.57 km and 37.58±0.70 km, respectively. The systematically lower values for receiver functions in the northern part of the Kaapvaal craton that was affected by the Bushveld magmatism at 2.05 Ga, suggest that the receiver functions do not enable the petrological crust mantle boundary to be reliably resolved due to variations in composition and metamorphic grade in a mafic lower crust. The Pn times also suggest pervasive azimuthal anisotropy with maximum wavespeeds of about 8.40 km/s at azimuths of about 15° and 217° in the northern and southern regions of the craton, respectively, and minimum wavespeeds of about 8.25 km/s.     

Atomistic simulation of the structure and elastic properties of pyrite (FeS2) as a function of pressure

Interatomic potential parameters have been derived at simulated temperatures of 0 K and 300 K to model pyrite FeS₂. The predicted pyrite structures are within 1% of those determined experimentally, while the calculated bulk modulus is within 7%. The model is also able to simulate the properties of marcasite, even though no data for this phase were included in the fitting procedure. There is almost no difference in results obtained for pyrite using the two potential sets; however, when used to model FeS₂ marcasite, the potential fitted at 0 K performs better. The potentials have also been used to study the high-pressure behaviour of pyrite up to 44 GPa. The calculated equation of state gives good agreement with experiment and shows that the Fe–S bonds shorten more rapidly that the S–S dimer bonds. The behaviour of marcasite at high pressure is found to be similar to that of pyrite.     

Elucidating hydrological complexity and climate-influenced diagenesis during the early Carboniferous of South Wales

A revised age provided by conodonts from the Gilwern Oolite of the Clydach area of South Wales allows a clearer understanding of the palaeohydrology, palaeoclimatic history and diagenesis of previously correlated oolitic units. Earlier uncertainty over the apparent sub-regional differences in climate during the early Visean (Chadian—Arundian) lowstand is resolved. Previously a humid interval evidenced by prominent palaeo-epikarst capping the Gilwern Oolite along the northern outcrops of the South Wales synclinorium, was not recognized in what were regarded as correlative outcrops in the Chadian Gully Oolite in the southern part of the synclinorium. Conodont dating now shows that the Gilwern Oolite is much older (Courceyan) than the Chadian Gully Oolite, and during the prolonged exposure of the former there was an interval of weathering under a humid climate. This also explains the contrast between the diagenesis seen between the Gilwern and Gully oolites, which are no longer seen as correlatives.