Carbon-rich chondritic clast PV1 from the Plainview H-chondrite regolith breccia: Formation from H3 chondrite material by possible cometary impact

Chondritic clast PV1 from the Plainview H-chondrite regolith breccia is a subrounded, 5-mm-diameter unequilibrated chondritic fragment that contains 13 wt% C occurring mainly within irregularly shaped 30-400-μm-size opaque patches. The clast formed from H3 chondrite material as indicated by the mean apparent chondrule diameter (310 μm vs. ∼300 μm in H3 chondrites), the mean Mg-normalized refractory lithophile abundance ratio (1.00 ± 0.09×H), the previously determined O-isotopic composition (Δ¹⁷O = 0.66‰ vs. 0.68 ± 0.04‰ in H3 chondrites and 0.73 ± 0.09‰ in H4-6 chondrites), the heterogeneous olivine compositions in grain cores (with a minimum range of Fa1-19), and the presence of glass in some chondrules. Although the clast lacks the fine-grained, ferroan silicate matrix material present in type 3 ordinary chondrites, PV1 contains objects that appear to be recrystallized clumps of matrix material. Similarly, the apparent dearth of radial pyroxene and cryptocrystalline chondrules in PV1 is accounted for by the presence of some recrystallized fragments of these chondrule textural types. All of the chondrules in PV1 are interfused indicating that temperatures must have briefly reached ∼1100°C (the approximate solidus temperature of H-chondrite silicate). The most likely source of this heating was by an impact. Some metal was lost during impact heating as indicated by the moderately low abundance of metallic Fe-Ni in PV1 (∼14 wt%) compared to that in mean H chondrites (∼18 wt%). The carbon enrichment of the clast may have resulted from a second impact event, one involving a cometary projectile, possibly a Jupiter-family comet. As the clast cooled, it experienced hydrothermal alteration at low water/rock ratios as evidenced by the thick rims of ferroan olivine around low-FeO olivine cores. The C-rich chondritic clast was later incorporated into the H-chondrite parent-body regolith and extensively fractured and faulted.     

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.     

Depositional processes and particle characteristics on fans in the Southern Alps, New Zealand

Characteristics of surface particles on four fans dominated by different depositional processes are investigated in the sedimentary ranges of the Aoraki–Mount Cook area, Southern Alps, New Zealand. Statistical testing shows that on three of the fans all indices of particle size and roundness differ significantly according to the depositional process: snow avalanches produce the largest and most angular particles, streamflows the smallest and most rounded, with debris flows in between. On the fourth fan affected by all of these depositional processes, particle size and roundness indicate that snow avalanches are presently dominant with streamflow playing a secondary role. The results also show that all indices of particle size are strongly correlated with each other and suggest that the principal (a-) or intermediate (b-) axis length may provide a satisfactory substitute for more complex indices of size. Indices of particle shape generally do not differ significantly between the fans, reflecting both the strong control exerted on particle shape by source area lithology and structure and the geologic similarity of the four basins. Changes in particle size and roundness occur with down-fan distance on the snow avalanche, debris flow, and hybrid fans but not on the streamflow fan.     

Tourist Carrying Capacity Measures: Crowding Syndrome in the Caribbean

International tourism is increasing at an unprecedented rate. Understanding the variety of national and local impacts of this increase is of importance to a growing number of governments. Butler's resort cycle model (1974, 1980, 1991) provides for several long‐term possibilities as to the relationship between crowding and growth. McElroy, de Albuquerque, and Dioguardi (1993) focus on one of those possibilities. Specifically, using their penetration ratio, they predict that as tourist crowding continues for a group of Caribbean islands, the appeal of these islands decreases in the eyes of potential tourists and that, as time increases, the growth rate of the affected islands, actually decreases. Our article indicates that such a simple, straight‐line relationship between increased crowding and a decrease in the rate of change may not be inevitable; indeed, diseconomies of scale may be avoided. The use of a curvilinear regression function reveals how both positive and negative scale economies existed in the Caribbean during the years 1992 through 1996.     

Morphometric controls and geomorphic responses on fans in the Southern Alps,New Zealand

Morphometric variables associated with 41 debris‐?ow and 18 ?uvial fans and their basins in the Southern Alps of New Zealand are examined. The results show statistically signi?cant differences in the area, maximum elevation, relief and ruggedness (Melton's R) of the basin and the area, gradient, and apex and toe elevations of the fan between debris‐?ow and ?uvial sites. Concavity of the fan longitudinal pro?le also differs between the two fan types, although this could not be tested statistically. Most of these morphometric differences re?ect differences in processes and environmental controls on them. Discriminant analysis indicates that basin area and fan gradient best differentiate the two fan types by process. Moderately strong correlations exist, on both debris‐?ow and ?uvial fans, between basin area or Melton's R and fan area. Correlations between basin area or Melton's R and fan gradient are generally weaker. The results of this study also indicate that on debris‐?ow‐prone fans the fan gradient and basin Melton's R have lower thresholds which overlap little with upper thresholds associated with basins where only stream?ow reaches the fan. These thresholds may therefore have value in preliminary morphometric assessments of debris‐?ow hazard on fans in the Southern Alps. Copyright © 2004 John Wiley & Sons, Ltd.     

Commingling and mixing of S-type peraluminous, ultrapotassic and basaltic magmas in the Cayconi volcanic field, Cordillera de Carabaya, SE Peru

The Cayconi district of the Cordillera de Carabaya, SE Peru, exposes a remnant of an upper Oligocene–Lower Miocene (22.2–24.4 Ma) volcanic field, comprising a diverse assemblage of S-type silicic and calc-alkaline basaltic to andesitic flows, members of the Picotani Group of the Central Andean Inner Arc. Basaltic flows containing olivine, plagioclase, clinopyroxene, ilmenite and glass, and glassy rhyolitic agglutinates with phenocrystic quartz, cordierite, plagioclase, sanidine, ilmenite and apatite, respectively exhibit mineralogical and geochemical features characteristic of medium-K mafic and Lachlan S-type silicic lavas. Cordierite-bearing dacitic agglomerates and lavas, however, are characterized by dispersed, melanocratic micro-enclaves and phenocrysts set in a fine-grained quartzo-feldspathic matrix. They contain a bimodal mica population, comprising phlogopite and biotite, as well as complexly zoned, sieve-textured plagioclase grains, sector-zoned cordierite, sanidine, quartz, irregular patches of replaced olivine, clinopyroxene and orthopyroxene and accessory phases including zircon, monazite, ilmenite and chromite. The coexistence of minerals not in mutual equilibrium and the growth/dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing. Trachytic-textured andesite flows interlayered with olivine+plagioclase–glomerophyric, calc-alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite, Ca-rich sanidine, ilmenite and aluminous spinel. The mineralogical and mineral chemical relationships in both the dacites and the trachytic-textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts, not represented by exposed rock types, and both the S-type silicic and calc-alkaline mafic magmas. Such mixing relationships are commonly observed in the Oligo-Miocene rocks of the Cordillera de Carabaya, suggesting that the S-type rocks in this area and, by extension, elsewhere derive their unusually high K₂O, Ba, Sr, Cr and Ni concentrations from commingling and mixing with diverse, mantle-derived potassic mafic magmas.     

Area selection for diamond exploration using deep-probing electromagnetic surveying

Previously proposed methods of area selection for diamond-prospective regions have predominantly relied on till geochemistry, airborne geophysics, and/or an appraisal of tectonic setting. Herein we suggest that a novel, deep-probing geophysical technique—electromagnetic studies using the natural-source magnetotelluric (MT) method—can contribute to such an activity. Essentially, diamondiferous regions must have (1) old lithosphere, (2) thick lithosphere, and (3) lithosphere that contains high concentrations of carbon. Deep-probing MT studies are able to address all three of these. The second and the third of these can be accomplished independently using MT, but for the first the geometries produced from modelling the MT observations must be interpreted with appropriate interaction with geologists, geochemists and other geophysicists. Examples are given from the Slave and Superior cratons in North America, with a brief mention of an area of the Rae craton, and general speculations about possible diamondiferous regions.