Effects of uranium mining discharges on water quality in the Puerco River basin,Arizona and New Mexico

Abstract

From 1967 until 1986, uranium mine dewatering increased dissolved gross alpha, gross beta, uranium and radium activities and dissolved selenium and molybdenum concentrations in the Puerco River as indicated by time trends, areal patterns involving distance from the mines and stream discharge. Additionally, increased dissolved uranium concentrations were identified in groundwater under the Puerco River from where mine discharges entered the river to approximately the Arizona-New Mexico State line about 65 km downstream. Total mass of uranium and gross alpha activity released to the Puerco River by mine dewatering were estimated as 560 Mg (560 × 10⁶ g) and 260 Ci, respectively. In comparison, a uranium mill tailings pond spill on 16 July 1979, released an estimated 1.5 Mg of uranium and 46 Ci of gross alpha activity. Mass balance calculations for alluvial ground water indicate that most of the uranium released did not remain in solution. Sorption of uranium on sediments and uptake of uranium by plants probably removed the uranium from solution.     

Liquid Immiscibility in the Join NaAlSi3O8-CaCO3 to 2.5 GPa and the Origin of Calciocarbonatite Magmas

Field evidence from intrusive and effusive carbonatites supportsthe existence of calciocarbonatite magmas. Published experimentalevidence in the model system Na₂O–CaO–Al₂O₃–SiO₂–CO₂indicated the formation of nearly pure (99%) CaCO₃ immiscibleliquids from a carbonated silicate liquid. This evidence hasbeen used to support interpretations of extremely CaCO₃-richcalciocarbonatite magmas, and immiscible liquids with compositionsof almost pure CaCO₃ in metasomatized mantle peridotite andeclogite. Detailed phase relationships are constructed in themodel system, based on phase fields intersected by the joinNaAlSi₃O₈–CaCO₃ (Ab-CC) at 1.0, 1.5, and 2.5 GPa between1100 and 1500C, and analyzed immiscible liquids. The miscibilitygap between silicate-rich liquid and carbonate-rich liquid intersectedby the join Ab-CC contracts considerably with decreasing pressure:2.5 GPa, between Ab₁₀CC₉₀ (by wt%) and Ab₆₅CC₃₅ above 1310C;1.5 GPa, betweenAb₂₃CC₇₇ and Ab₄₃CC₅₇ above 1285C; 1.0 GPa,not intersected. The liquidus piercing point between calciteand silicates becomes enriched in CaCO₃ with decreasing pressure,from Ab₈₀CC₂₀ at 2.5 GPa to Ab₄₇CC₅₃ at 1.0 GPa. No immiscibleliquid contains more than 80% dissolved CaCO₃, and all containat least 5% Na₂CO₃. A round CaCO₃ phase exhibiting morphologysimilar to that displayed by immiscible liquid globules is determinedto be crystalline calcite under experimental conditions. Thetopology of the phase fields and field boundaries illustratesthe kinds of processes and controls existing in magmatic systems.Calciocarbonatite magmas cannot be produced by equilibrium immiscibilityprocess in the mantle. Carbonated silicate magmas in the crustyield residual calciocarbonatite magmas by fractionation alongthe silicate-calcite field boundary, reached either directlyfrom the silicate liquidus or more commonly via the miscibilitygap. Immiscible carbonaterich magmas when freed from the silicateparent cool down a sleep silicate liquidus until they reacha silicate-carbonate field boundary. There is no experimentalevidence for immiscible calciocarbonatite magmas with > 80%CaCO₃, and calcite lapilli cannot be formed from 99% CaCO₃ magmas.Sovites are surely cumulates. KEY WORDS: carbonatite; join NaAlSi₃O₈–CaCO₃; liquid immiscibility; sovite ^* Corresponding author. Telephone: (818)-395–6239. Fax: (818)-568–0935. e-mail: wjl{at}gpi.caltech.edu     

Isotopic Evolution of the Tonga Arc during Lau Basin Rifting; Evidence from the Volcaniclastic Record

New Pb, Sr and Nd isotope data from volcaniclastic sedimentsrecovered from the Tonga forearc and Lau Basin permit the isotopicevolution of a section of this arc system, next to the modernisland of Ata, to be traced through the backarc basin riftingprocess from 7.0 Ma to the present. The new data suggest thatthe isotopic character of the mantle wedge remains constant,and of Pacific mid-ocean ridge basalt (MORB) character, duringthe early rifting phase. The isotopic evidence supports traceelement data in showing an increase in the sediment contributionto arc petrogenesis about 2–3 m.y. after the start ofLau Basin rifting. Since 0.45 Ma the sediment contribution decreasedto pre-rift values with the initiation of spreading in the adjacentbackarc basin, where the high sediment influence is not seenin the isotopes. The isotopes show a relative increase in thevolcaniclastic compared with pelagic sediment involvement duringrifting. The inferred peak in sediment subduction is probablythe result of a decoupling of the two plates owing to roll-backof the Pacific lithosphere at the time of arc rifting. KEY WORDS: isotopes; Pacific; rifting; subduction; volcanism ^*Corresponding author. Telephone: (508) 289 3437. Fax: (508) 457 2187. e-mail: pclift{at}whoi.edu     

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at     

The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms

Ultramafic xenoliths of garnet lherzolite (?rare spinel), spinellherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxenemegacrysts were collected from Cenozoic basalts in all partsof eastern China. From their modal composition and mineral chemistryall the xenoliths may be placed into three types representing:a fertile or more primitive mantle (garnet lherzolite and spinellherzolite), a refractory or more depleted mantle (spinel harzburgiteand dunite), and inclusions cognate with the host alkali basaltsat mantle pressures (pyroxenite and megacrysts). There are systematicdifferences between the mineral compositions of each type. Spinelshows a wide compositional range and the spinel cr-number [100Cr/(Cr + Al)] is a significant indicator of the xenolithtype. Spinel cr-number and Al₂O₃ of coexisting minerals (spinel,clinopyroxene, and orthopyroxene) are useful as refractory indicatorsfor spinel peridotite in that the cr-number increases and thepercentage of Al₂O₃ decreases with increasing degrees of melting.In garnet peridotite, however, the same functions vary withpressure, not degree of melting. According to P–T estimates,the various xenoliths were derived from a large range of depthsin the upper mantle: spinel peridotite from approximately 11to 22 kb (37–66 km), spinel/garnet lherzolite from 19to 24 kb (62–80 km), and garnet lherzolite from 24 to25 kb (79–83 km). We conclude that the uppermost mantlebeneath eastern China is heterogeneous, with a north-northeastzone of more depleted mantle lying beneath the continental marginand a more primitive mantle occurring towards the continentalinterior.     

In situ acoustic measurements of marine gravel threshold and transport

Measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles. The turbulence in the bottom boundary layer was measured using two electromagnetic current meters, and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material. Visual estimates of bedload were also obtained with an underwater TV camera. The acoustic technique, unlike a conventional bedload sampler, has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected. This has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions. The results of the study show that of the turbulent bursting events which contribute towards the Reynolds stress, only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material. The measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process.     

Comparisons between acoustic measurements and predictions of the bedload transport of marine gravels

Continuous, detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates (q_b) predicted by five bedload transport equations using measurements of the near-bed turbulent current flow. When mean flow data are used in these equations, total q_b estimates are similar to those measured. However, when instantaneous flow data are used, total q_b is over-estimated by approximately one order-of-magnitude. Based on the acoustic measurements, an empirical equation has been obtained that gives accurate estimates of total q_b over a tidal cycle and simulates well the intermittent characteristics of marine bedload transport.     

A REVIEW OF THREE-DIMENSIONAL CHEMICAL STRUCTURE RETRIEVAL SYSTEMS

This paper presents a review of the rapid developments that have taken place over the last few years forthe searching of databases of three-dimensional(3D)molecules.The geometric arrangement of the atomsin a 3D molecule is described by an interatomic distance matrix.This is a form of labelled graph thatcan thus be searched using the subgraph-isomorphism algorithms that are widely used for searchingdatabases of two-dimensional(2D)molecules.Several in-house and commercial systems have beendeveloped for 3D database searching that are based on such techniques.These systems are reviewed andtheir effectiveness demonstrated by examples of their use in the discovery of novel,biologically activemolecules.Current systems represent a molecule by one or a small number of low-energy conformationsand there is hence much interest in the development of representational techniques and searchingalgorithms that account for ihe full set of geometric arrangements that can be adopted by a flexiblemolecule.