U-Pb systematics of monazite and xenotime: case studies from the Paleoproterozoic of the Grand Canyon, Arizona

Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain. Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite data from the third sample exhibit dispersion in both the ²⁰⁷Pb/²⁰⁶Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by incomplete dissolution or excess (thorogenic) ²⁰⁶Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the ²⁰⁷Pb/²⁰⁶Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes. Received: 9 June 1996 / Accepted: 18 October 1996     

Isolation of diagenetic overgrowths on quartz sand grains for oxygen isotopic analysis

Diagenetic overgrowths on detrital quartz sand grains in sandstones can be isolated for direct measurement of their oxygen isotope ratios. Interfacial materials between detrital cores and diagenetic overgrowths are dissolved by etching with dilute HF. The weakened interface breaks when grains are agitated ultrasonically. Subsequent particle size separation is an effective means of concentrating the diagenetic component.     

Influence of diet on the distribution of carbon isotopes in animals

The influence of diet on the distribution of carbon isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant carbon isotopic composition.The isotopic composition of the whole body of an animal reflects the isotopic composition of its diet, but the animal is on average enriched in δ¹³C by about 1‰ relative to the diet. In three of the four cases examined, the ¹³C enrichment of the whole body relative to the diet is balanced by a ¹³C depletion of the respired CO₂. The isotopic relationships between the whole bodies of animals and their diets are similar for different species raised on the same diet and for the same species raised on different diets. However, the δ¹³C values of whole bodies of individuals of a species raised on the same diet may differ by up to 2‰. The relationship between the ¹³C/¹²C ratio of a tissue and the ¹³C/¹²C ratio of the diet depends both on the type of tissue and on the nature of the diet. Many of the isotopic relationships among the major biochemical fractions, namely the lipid, carbohydrate and protein fractions, are qualitatively preserved as diet carbon is incorporated into the animal. However, the difference between the δ¹³C values of a biochemical fraction in an animal and in its diet may be as large as 3‰. The δ¹³C values of the biochemical components collagen, chitin and the insoluble organic fraction of shells, all of which are often preserved in fossil material, are related to the isotopic composition of the diet.These results indicate that it will be possible to perform dietary analysis based on the determination of the ¹³C/¹²C ratio of animal carbon. Analysis of the total animal carbon will in most cases provide a better measure of diet than the analysis of individual tissues, biochemical fractions, or biochemical components. The limits of accuracy of this method will generally restrict its application to situations in which the diet is derived from sources with relatively large differences in their δ¹³C values, such as terrestrial vs aquatic organisms or C₃ vs C₄ plants. The method should be applicable to fossil as well as to living material.     

Simulating the dispersal of tephra from the 1991 Pinatubo eruption: Implications for the formation of widespread ash layers

PUFF and HAZMAP, two tephra dispersal models developed for volcanic hazard mitigation, are used to simulate the climatic 1991 eruption of Mt. Pinatubo. PUFF simulations indicate that the majority of ash was advected away from the source at the level of the tropopause (~ 17 km). Several eruptive pulses injected ash and SO₂ gas to higher altitudes (~ 25 km), but these pulses represent only a small fraction (~ 1%) of the total erupted material released during the simulation. Comparison with TOMS images of the SO₂ cloud after 71 and 93 h indicate that the SO₂ gas originated at an altitude of ~ 25 km near the source and descended to an altitude of ~ 22 km as the cloud moved across the Indian Ocean. HAZMAP simulations indicate that the Pinatubo tephra fall deposit in the South China Sea was formed by an eruption cloud with the majority of the ash concentrated at a height of 16–18 km. Results of this study demonstrate that the largest concentration of distal ash was transported at a level significantly below the maximum eruption column height (~ 40 km) and at a level below the calculated height of neutral buoyancy (~ 25 km). Simulations showed that distal ash transport was dominated by atmospheric circulation patterns near the regional tropopause. In contrast, the movement of the SO₂ cloud occurred at higher levels, along slightly different trajectories, and may have resulted from gas/particle segregations that took place during intrusion of the Pinatubo umbrella cloud as it moved away from source.     

The relative stabilities of the reverse and normal polarity states of the earth's magnetic field

Recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states. The main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the Cenozoic. These additional events occur predominantly during reverse polarity time, but it is unlikely that all of them represent true reversal events. Nevertheless the question of the relative stabilities of the polarity states is examined in detail, both for the case when all 57 “events” are included in the reversal chronology and when they are all excluded. It is found that there is not a discernible difference between the stabilities of the two polarity states in either case. Inclusion of these short events does, however, change the structure of the non-stationarity in reversal rate, but still allows a smooth non-stationarity. Only 7 of the 57 short events are pre-38 Ma, but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation. This could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the Paleogene and Late Cretaceous. Overall it would appear that the present geomagnetic polarity timescale for 0–160 Ma is probably a very good representation of the actual history, and that different timescales and additional events now represent only changes in detail.     

A Wireline Piston Core Barrel for Sampling Cohesionless Sand and Gravel Below the Water Table

A coring device has been developed to obtain long and minimally disturbed samples of saturated cohesionless sand and gravel. The coring device, which includes a wireline and piston, was developed specifically for use during hollow-stem auger drilling but it also offers possibilities for cable tool and rotary drilling. The core barrel consists of an inner liner made of inexpensive aluminum or plastic tubing, a piston for core recovery, and an exterior steel housing that protects the liner when the core barrel is driven into the aquifer. The core barrel, which is approximately 1.6m (5.6 feet) long, is advanced ahead of the lead auger by hammering at the surface on drill rods that are attached to the core barrel. After the sampler has been driven 1.5m (5 feet), the drill rods are detached and a wireline is used to hoist the core barrel, with the sample contained in the aluminum or plastic liner, to the surface. A vacuum developed by the piston during the coring operation provides good recovery of both the sediment and aquifer fluids contained in the sediment. In the field the sample tubes can be easily split along their length for on-site inspection or they can be capped with the pore water fluids inside and transported to the laboratory. The cores are 5cm (2 inches) in diameter by 1.5m (5 feet) long. Core acquisition to depths of 35m (115 feet), with a recovery greater than 90 percent, has become routine in University of Waterloo aquifer studies. A large diameter (12.7cm [5 inch]) version has also been used successfully. Nearly continuous sample sequences from sand and gravel aquifers have been obtained for studies of sedimentology, hydraulic conductivity, hydrogeochemistry and microbiology.     

超速电脑X光扫描机

香港山顶地区的明德医院于1990年12月开始使用超速电脑CT 扫描机。这是除美国和日本外在亚太地区首家唯一装备有此种扫描机的医院。Imatron 超速CT 扫描机是目前使用电子射束技术的最先进的诊断成像系统。超速CT 扫描机完全放弃了传统的机械旋转探测器或X 射线管组合,既克服了广泛使用的常规CT 扫描机在扫描速度上的难关,又解决了X 射线管的热散失问题。