Showing papers by "Chris Harris published in 2001"

Origins of Large Volume Rhyolitic Volcanism in the Antarctic Peninsula and Patagonia by Crustal Melting

Abstract: Voluminous rhyolitic volcanism along the palaeo-Pacific margin of Gondwana was marked by three principal episodes of magmatism. The first of these ( V1) is essentially coincident with the main episode of Karoo–Ferrar magmatism at ∼184 Ma. A younger ( V2) episode occurred at ∼168 Ma, and a third episode ( V3) occurred in the interval 157–153 Ma. We evaluate the origin of V1 and V2 rhyolites from the Antarctic Peninsula using major and trace element and isotopic (Sr, Nd, O) data. An isotopically uniform (87Sr/86Sri ∼0·707; eNdi ∼ −3) andesite–dacite magma was generated as a result of anatexis of ‘Grenvillian age’ hydrous mafic lower crust, linked to earlier, arc-related underplating. The lower-crustal partial melts would have mixed with fractionated components of the mafic underplate, followed by subsequent storage and homogenization. Early Jurassic ( V1) rocks of the southern Antarctic Peninsula are interpreted as melts of upper-crustal paragneiss, which have mixed with the isotopically uniform magma in upper-crustal magma chambers. The V2 rhyolites are the result of assimilation–fractional crystallization of the isotopically uniform magma. This occurred in upper-crustal magma chambers involving assimilants with similar isotopic composition to that of the magma. A continental margin setting was crucial in developing hydrous, readily fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the Discovery–Shona–Bouvet group of plumes, thought to be responsible for the Karoo magmatic province. The progression (old to young) of volcanism from NE to SW in Patagonia and south to north in the Antarctic Peninsula is consistent with migration away from the mantle plumes towards the proto-Pacific margin of Gondwana during rifting and break-up.

Crustal Contamination and Fluid–Rock Interaction during the Formation of the Platreef, Northern Limb of the Bushveld Complex, South Africa

Abstract: The Platreef is the main platinum group element (PGE)-bearing and contains some of the most important magmatic ore deposits yet discovered. The intrusion covers an area of facies of the northern limb of the Bushveld complex, but unlike the Merensky Reef of the eastern and western limbs, it is in direct roughly 65 000 km (e.g. Tankard et al., 1982) and lies almost entirely within the bounds of the Transvaal Basin contact with the country rock. Mineral separate O values for samples from the Upper Zone and Main Zone of the northern limb (Fig. 1). The layered igneous rocks (the Rustenburg Layered Suite, SACS, 1980) can be divided into eastern indicate crystallization from a well-mixed, already contaminated, magma having a O value of 7·5‰. Pyroxenes from the pyroxenites and western limbs of approximately the same size, and a smaller northern limb. The northern limb differs in facies of the Platreef at Sandsloot Mine have O values that are up to 2·4‰ higher than pyroxenes from the Upper and Main several important aspects from the more well-known eastern and western limbs (e.g. Van der Merwe, 1976; Zones. These differences can be explained by additional assimilation of up to 18% dolomite, which is in contact with the intrusion at Buchanan et al., 1981; Cawthorn et al., 1985; Eales & Cawthorn, 1996). First, the layered rocks show a this locality. Samples from the Platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted pronounced transgression across the floor rocks from a with fluids during slow cooling. Quartz veins with granophyric level above the Magaliesberg Quartzite Formation south margins have O values between 10·1 and 12·2‰, which of the town of Potgietersrus, across the whole Pretoria suggest that the fluid that passed through the cooling Platreef had Group, and the banded iron formations and dolomites a O value of 7–9‰. These data, together with hydrogen isotope of the older Chuniespoort Group, onto Archaean granitic data from minerals and whole-rock samples ( D of biotite −60 rocks (Fig. 1). Second, the igneous layering shows an to −88‰) of the Platreef suggest interaction with magmatic fluid apparent transgressive relationship with respect to the at low water/rock ratios. Interaction of this fluid with the calcsilicate contact, such that ultramafic rocks of the Lower Zone footwall rocks lowered their O values. abut against the contact south of Potgietersrus, whereas to the north, rocks forming the base of the intrusion are progressively higher in the stratigraphic succession of the

Comparison of the main sequence of reflexive saccades and the quick phases of optokinetic nystagmus.

Abstract: BACKGROUND/AIMS Abnormalities in the saccadic main sequence are an important finding and may indicate pathology of the ocular motor periphery or central neurological disorders. In young or uncooperative patients it can be difficult eliciting a sufficient number of saccades to measure the main sequence. It is often assumed that the quick phases of optokinetic nystagmus (OKN) are identical to saccades. If this were the case, it would be feasible to use OKN, an involuntary response that is easily evoked, as a simple way of eliciting many saccades. The aim of this study was to determine whether reflexive saccades and the quick phases of OKN are indeed identical, and whether OKN quick phases could have a clinical role in identifying patients with slow saccades. METHODS OKN and reflexive saccades were recorded from 10 healthy adults using an infrared limbus eye tracker and bitemporal DC electro-oculography simultaneously. OKN was stimulated by rotating a full field patterned curtain around the subject at 10–50°/s. Reflexive saccades were elicited to red LED targets at 5–20° eccentricity. RESULTS OKN quick phases tended to have a longer duration compared to saccades, but these differences were not significant. OKN quick phases had a slightly lower peak velocity compared to saccades, which was statistically significant (p CONCLUSION The main sequence for duration is the same for reflexive saccades and OKN quick phases. The main sequence for peak velocity is slightly faster for reflexive saccades than OKN quick phases, but the difference is unlikely to be of clinical significance. As an illustration of the potential of this technique, the authors demonstrate that OKN quick phases show similar slowness to saccades in a child with brainstem pathology caused by Gaucher disease type III. It is concluded that recording OKN may be a simple clinical means for approximating the main sequence.

Familial vestibulocerebellar disorder maps to chromosome 13q31- q33: a new nystagmus locus

Abstract: Purpose: To determine a gene locus for a family with a dominantly inherited vestibulocerebellar disorder characterised by early onset, but not congenital nystagmus. Design: Observational and experimental study. Methods: We carried out a phenotypic study of a unique four generation family with nystagmus. We performed genetic linkage studies including a genome wide search. Results: Affected family members developed vestibulocerebellar type nystagmus in the first two years of life. A higher incidence of strabismus was noted in affected members. Haplotype construction and analysis of recombination events linked the disorder to a locus (NYS4) on chromosome 13q31-q33 with a lod score of 6.322 at θ=0 for D13S159 and narrowed the region to a 13.8 cM region between markers D13S1300 and D13S158. Conclusions: This study suggests that the early onset acquired nystagmus seen in this family is caused by a single gene defect. Identification of the gene may hold the key to understanding pathways for early eye stabilisation and strabismus.

Diagnosing children presenting with asymmetric pendular nystagmus

Abstract: Horizontal asymmetric nystagmus usually occurs in one of three situations: secondary to an intracranial lesion, with monocular visual loss, or as part of the triad that constitutes the diagnosis of spasmus nutans (asymmetric nystagmus, abnormal head posture, head shake). Clinical records of 277 children, presenting with congenital nystagmus over an 8-year period were reviewed. Nystagmus was asymmetric in 24 of 277 cases. Seven of these patients were diagnosed with spasmus nutans. This is a rare condition that is only diagnosed retrospectively based on the absence of any abnormal neuroimaging or electrophysiological findings. Twelve of 24 patients had intracranial pathology and all had abnormal visual evoked potentials (VEPs). Five patients were diagnosed with congenital sensory defect nystagmus including one with albinism, three with congenital cone dysfunction, and one with cone-rod dystrophy. This paper stresses that although neuroimaging is necessary in all patients presenting with asymmetric nystagmus, such nystagmus can also occur with retinal disease or albinism and indicates the importance of non-invasive VEP/ERG testing in all forms of nystagmus.

Audiometric abnormalities in children with Gaucher disease type 3.

Abstract: Exogenous enzyme replacement therapy achieves satisfactory biomedical correction in Gaucher type 1 disease and may halt or reverse neurological progression in type 3, while it does not appear to influence the outcome in type 2. In view of the therapeutic possibilities, early detection and monitoring of type 3 Gaucher disease, as well as evaluation of the effectiveness of enzyme therapy on neuronopathic involvement is necessary. The objective of this study was to evaluate the extent of brainstem disease in children with proven Gaucher type 3, by means of an audiological test battery. We studied 9 patients with Gaucher type 3 disease. The tests included baseline audiometric tests, as well as auditory brainstem evoked responses (ABR), acoustic reflexes and medial olivo-cochlear suppression by contralateral noise tests, that involve overlapping but not identical efferent and afferent pathways and brainstem structures. We found a constellation of abnormalities including bilaterally raised acoustic reflexes, poor medial olivo-cochlear suppression, and very poor brainstem evoked potentials. These abnormalities could be due to a single lesion in the dorsomedial brainstem, or to multiple lesions, and further study is needed to clarify this issue. Combined audiological tests may provide information on the severity of the neurological involvement and should therefore be part of a standard assessment for the diagnosis as well as for long term neurological monitoring of Gaucher type 3 patients.

Pleistocene Dolomite from the Namibian Shelf: High 87Sr/86Sr and 18O Values Indicate an Evaporative, Mixed-Water Origin

Abstract: Dolomites from Upper Pleistocene sandstone and diatomaceous mud recovered from a coast-parallel, 4-km-wide trough on the Namibian shelf between Hottentot Bay and Spencer Bay have 87Sr/86Sr ratios between 0.70946 and 0.71309. Marine mollusc shells associated with the dolomites have a modern seawater Sr isotope ratio of 0.70915. The greater-than-seawater Sr ratios indicate unequivocally that groundwaters were involved in dolomite formation. Groundwaters, containing radiogenic Sr derived from weathering of Precambrian basement rocks, were focused into local troughs, mixed with seawater, and evaporated in the arid climate. The 13CPDB values of -4.0 to -2.4o/oo indicate uptake of bicarbonate derived in part from microbial degradation of organic matter, and 18OPDB values of 1.4 to 6.4o/oo suggest that the dolomite cement precipitated from a variably evaporated groundwater-seawater mixture. Dolomite cementation of the basal bioclastic sand and gravel occurred during emergence when sea level was below the trough sill depth of 65 m, most likely between 500 and 130 ka. Overlying calcareous and diatomaceous Eemian mud and sand are dolomitic only in the deepest, central trough area and contain 26 to 45 wt % disseminated dolomite. The 13CPDB values of -0.3 to -0.5o/oo and 18OPDB values of 5.7 to 6.4o/oo suggest that the disseminated dolomite formed from an evaporated groundwater-seawater mixture largely by replacement of biogenic carbonate when the shelf was subaerially exposed between 75 and 11 ka. Holocene transgressive shelly gravels and muddy sands contain only reworked Pleistocene dolomite, but Holocene dolomite may occur in coastal salt pans onshore.

Spontaneous reversal of nystagmus in the dark

Abstract: Aim—To report five children with horizontal jerk nystagmus in whom eye movement recordings in the dark revealed a spontaneous reversal in the direction of the nystagmus beat. Three patients were blind in one eye and were diagnosed as having a manifest latent nystagmus (MLN), and two patients had strabismus and congenital nystagmus (CN). Methods—Eye movements were recorded using DC electro-oculography with simultaneous video recording, including infrared recording in total darkness. Results—Four patients had decelerating velocity slow phase jerk nystagmus when recorded under natural lighting conditions; the fifth case had accelerating velocity and linear slow phase jerk nystagmus. Under absolute darkness, nystagmus reversed in direction of beat with a mixture of linear and decelerating velocity slow phase waveforms. One child with unilateral anophthalmos could wilfully reverse the beat direction of his nystagmus by trying to look with his blind eye in the light and dark. Conclusions—These observations support the theory that LN/MLN beat direction is determined by the “presumed” viewing eye and may be consciously controlled. The spontaneous reversal of beat direction in the dark suggests eye dominance is predetermined. Eye movement recordings identified mixed nystagmus waveforms indicating that CN (accelerating velocity slow phases) and LN/MLN (linear/ decelerating velocity slow phases) coexist in these subjects. (Br J Ophthalmol 2001;85:428‐431)

The effects of early Cambrian metamorphism in western Dronning Maud Land, East Antarctica: a carbon and oxygen isotope study of fluid-rock interaction in the Sverdrupfjella Group

Abstract: Abstract The Sverdrupfjella Group in western Dronning Maud Land is a 1200 to 900 Ma orogenic belt that experienced a thermal overprint in the early Cambrian. Evidence for distinct episodes of fluid-rock interaction is found in calc-silicate rocks, veins and retrograde mineral assemblages. A sequence of altered but undeformed basalts exhibit extreme 18O depletion (δ18O as low as −1.8%) apparently due to interaction with meteoric water during regional (possibly early Cambrian) metamorphism. In the central Kirwanveggen, metasomatic calc-silicates, and retrograde mineral assemblages are associated with late high-strain zones of probable Cambrian age. The former have δ13C values which overlap those of massive metacarbonate units 150 km to the NE, and imply regional scale movement of fluids. The latter record 18O depletion relative to unretrogressed equivalent rocks and suggest interaction with externally derived fluids.