Naturally precipitating vaterite (μ-CaCO3) spheres: unusual carbonates formed in an extreme environment

Vaterite, a rare hexagonal CaCO₃ polymorph, was identified in precipitates forming at a supraglacial sulfur spring, in Borup Fiord Pass, northern Ellesmere Island, Canadian High Arctic. Vaterite occurs in a precipitate mound along with calcite, gypsum, and native sulfur. The unusual conditions of the site, including an extremely cold climate, supersaturated alkaline waters, and the presence of gypsum, mimics conditions used to grow vaterite in laboratory experiments. Stable isotope data suggest that vaterite may preferentially form during colder periods of the year. Vaterite found at the site is characterized by 2- to 10-μm rounded to spherical shaped particles (comprising smaller 0.5- to 2-μm spheres) as both individuals and in chainlike structures. The spherical habit of vaterite resembles carbonate structures that have been interpreted to be organic; however, δ¹³C values are indicative of an inorganic origin. The thick permafrost, and the extreme cold and dry environment make this site a good terrestrial analog for carbonate precipitates that might be expected at potential deposits associated with water discharge on other planetary bodies.     

Analysis of Re, Au, Pd, Pt and Rh in NIST Glass Certified Reference Materials and Natural Basalt Glasses by Laser Ablation ICP-MS

A new technique for the in situ analysis of Re, Au, Pd, Pt and Rh in natural basalt glass by laser ablation (LA)-ICP-MS is described. The method involves external calibration against NIST SRM 612/613 or 614/615 glass certified reference materials, internal standardisation using Ca, and ablation with a 200 μm wide beam spot and a pulsed laser repetition rate of 50 Hz. Under these conditions, sensitivities for Re, Au, Pd, Pt and Rh analyte ions are ˜ 5000 to 100,000 cps/μg g^-1. This is sufficient to make measurements precise to ˜ 10% at the 2-10 μg g^-1 level, which is well within the range of concentrations expected in many basalts. For LA-ICP-MS calibration and a demonstration of the accuracy of the technique, concentrations of Re, Au, Pd, Pt and Rh in the NIST SRM 610/611 (˜ 1 to 50 μg g^-1), 612/613 (˜ 1 to 7 μg g^-1), 614/615 (˜ 0.2 to 2 μg g^-1) and 616/617 (˜ 0.004 to 2 μg g^-1) glasses were determined by solution-nebulisation (SN)-ICP-MS. Using the 612/613 or 614/615 glasses as calibration standards, LA-ICP-MS measurements of these elements in the other NIST glasses fell within ˜ 15% of those determined by SN-ICP-MS. Replicate LA-ICP-MS analyses of the 612/613 and 614/615 glasses indicate that, apart from certain anomalous domains, the glasses are homogeneous for Re, Au, Pd, Pt and Rh to better than 3.5%. Two LA-ICP-MS analyses of natural, island-arc basalt glasses exhibit large fractionations of Re, Au and Pd relative to Pt and Rh, compared to the relative abundances in the primitive mantle.     

Phase-array decomposition of a seismic section

A seismic re fraction/wide-angle reflection profile is analysed for the presence of correlated events ('phases'). The correlation problem is formulated in terms of temporally, spatially and frequency-local complex covariances. For robustness, the method concentrates on phase rather than amplitude information. This allows a computationally efficient algorithm that can make allowance for signal correlation length and can model curved wavefronts. A statistical test based on residual phase misfit across the analysed subarray is used to assess the probability that a detected event represents a real correlated signal.
With our chosen analysis parameters and confidence level (over 99.9 per cent). 1222 events were detected in the data. Using simple techniques based on 1-D earth models, detected events are associated with a small number of particular wave types. In this way, we have succeeded in classifying almost 95 per cent of the detected events. Those that remain describe those components of the data that are inconsistent with our simple ray paths in the 1-D assumption and with our prescribed tolerance. These include reverberations, near-surface guided waves and reflected waves from strongly laterally inhomogeneous structures. According to our modelling, about 25 per cent of the detected events are consistent with simple P -wave reflected energy, and these are to a very large extent (over 85 per cent) distinct from all the other wave-type models we have used. A direct mapping of the detected events into the offset-depth domain reveals dear internal and external consistencies among the detections for the various wave types. Estimated earth structure is consistent with models from previous analyses based on much larger data sets.
We have thus succeeded in extracting correlated events from the data and decomposing these, approximately but meaningfully, into distinct classes (ray paths)     

Petrogenesis of lavas from the AMAR Valley and Narrowgate region of the FAMOUS Valley, 36°–37°N on the Mid-Atlantic Ridge

Basaltic lavas from the AMAR Valley and the Narrowgate region of the FAMOUS Valley on the Mid-Atlantic Ridge (36° to 37°?N) range in texture from aphyric to highly plagioclase phyric (>25% large plagioclase phenocrysts). Based on ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, most of these lavas can be subdivided into two distinct, isotopically homogeneous, groups: Group I has lower ⁸⁷Sr/⁸⁶Sr (0.70288±1) and higher ¹⁴³Nd/¹⁴⁴Nd (0.51312±1) ratios; Group II has higher ⁸⁷Sr/⁸⁶Sr (0.70296±1) and lower ¹⁴³Nd/¹⁴⁴Nd (0.51309±2) ratios. Most Group II lavas are aphyric, whereas Group I lavas are primarily plagioclase phyric. Lavas from both groups show a wide range in incompatible element abundance ratios (e.g., Zr/Nb =6–29; (La/Sm)_n=0.6–1.7). Aphyric lavas have relatively constant Sc (40±1.5?ppm) abundances and CaO/Al₂O₃ ratios (0.80±0.02). Group I lavas are confined primarily to the AMAR rift valley floor whereas Group II lavas are found along the east and west marginal highs. We interpret the isotopic differences between the two groups as reflecting a temporal change in the upwelling mantle beneath this region of the Mid-Atlantic Ridge which is south of the Azore Islands. For each group, a petrogenetic model consistent with the geochemical data is multi-stage decompression melting of an initially enriched, homogeneous, mantle source region. If the early derived, incompatible-element enriched, melt increments are not always pooled with subsequent increments, the erupted magma batches may have the major element characteristics of melts derived by 10 to 20% melting, but with incompatible element abundance ratios reflecting the change from an enriched to depleted source during the incremental melting process. In this process an initially homogeneous source can generate primary magmas with the required range in incompatible element abundance ratios shown by each group. The nearly constant CaO/Al₂O₃ ratios and Sc contents of the aphyric lavas with decreasing Mg?? reflects subsequent polybaric fractionation of clinopyroxene, plagioclase and olivine over the pressure interval 8–6?kbar (24–18?km), followed by rapid transport to the surface and eruption. There is no geochemical evidence for a crustal magma chamber beneath this section of the Mid-Atlantic Ridge.     

The normal grain growth behaviour of nominally pure calcitic aggregates

The normal grain growth behaviour of four different, but all nominally pure, calcite powders (99%+ analytic grade calcite, 99.7% chalk, 99.97% crushed Iceland Spar, 99.95%+ chelometric grade calcite) has been investigated as a function of temperature (550, 600, 650, 700 °C) and confining pressure (100, 190 MPa) under both “dry” and hydrostatic (P _fluid = P _total) conditions. The initial particle size of both the analytic grade and chelometric grade calcite was about 5 μm, and that of the chalk was about 3 μm, while the experiments on the Iceland Spar were conducted on powders of three different initial particle sizes (3.4, 7.5, 38.5 μm). On each material, at each pressure/temperature condition 6 to 15 experiments, equally spaced in log time from 15 minutes to 50 days, were conducted. Under dry conditions all four materials recrystallized to aggregates which contained less than 2% porosity and which had a grain size of between 4 and 20 μm (depending on the initial particle size). Subsequently the aggregates coarsened by normal grain growth, with the kinetics of the growth process being controlled by the rate at which the grain boundaries could drag the residual pores with them as they migrated. Under nominally identical conditions both the mechanism and rates of pore drag differed greatly for the different materials, implying that this process is highly sensitive to trace solute impurity concentrations. This sensitivity renders the task of providing a systematic account of dry calcite grain growth kinetics highly problematic. Under hydrostatic conditions all the powders followed the same normal grain growth kinetics in which the growth process was rate-controlled by diffusion through the pore fluid on the grain boundaries. An activation enthalpy of 162.6 kJ mol⁻¹ and an activation volume of 34.35 cm³ mol⁻¹ was obtained for this process. Received: 23 May 1996 / Accepted: 8 July 1997     

Structure of the Mount Raymond transverse zone at the southern end of the Wyoming salient, Sevier fold-thrust belt, Utah

The Mount Raymond transverse zone (MRTZ) forms the east-west-trending boundary between the Wyoming salient of the Sevier fold-thrust belt and the Uinta/Cottonwood arch in north-central Utah. Major faults in the zone dip 40° to 45° north. Our structural analysis indicates that the MRTZ contains both contractional and extensional structures. The contractional structures (thrusts and related folds) initially formed as part of a southeast-verging, northeast-trending thrust system. This system gradually curved and merged to the north with the east-verging Absaroka thrust system in the apex of the Wyoming salient. The contrast in trend between structures in the MRTZ and those in the apex of the Wyoming salient reflects the initial curvature of the salient. This curve formed because the stratigraphic sequence involved in thrusting thinned gradually southward, toward the axis of a proto-Uinta arch. The present east-west trend of the MRTZ developed during Laramide uplift of the Unita/Cottonwood arch, an event which tilted the faults into their present position. Cenozoic crustal extension subsequently reactivated segments of the MRTZ. In sum, the evolution of the MRTZ illustrates how post-thrusting processes can affect the map-view geometry of thrust belts to create transverse zones.