Radioactive Disequilibrium Research Articles

Direct alpha-recoil as a process to generate U-234/U-238 disequilibrium in groundwater

ABSTRACTWe discuss quantitatively the capability of direct α-recoil to create observable radioactive disequilibrium (U-234/U-238 activity ratio > 1) in flowing groundwater. Coupled equations for radioactive decay chain are formulated for the solid U source and for the corresponding groundwater receiving the recoiling U-234 atoms. U-rich fracture coating material and the water flowing in the fracture are discussed in detail.The novel modelling approach worked well and provides a useful tool for more detailed site-specific studies with more detailed input data. The approach appears feasible because the equations are based on straightforward mass balance considerations.α-recoil -induced enrichment of U-234 in groundwater is a slow process: the increase of U-234/U-238 activity ratio to notable values above unity will take hundreds to thousands of years. Strong groundwater flow prevents any local recoil-induced U-234 enrichment in the water.

Development of U-Th Radioactive Disequilibrium Analyses for Carbonate Minerals and its Applications to Methane Seeps and Fault Fracture Zones

Development of U-Th Radioactive Disequilibrium Analyses for Carbonate Minerals and its Applications to Methane Seeps and Fault Fracture Zones

Radionuclides in the Mulde River System after the August‐2002 Flood

AbstractRadionuclides were investigated in sediments from the rivers Freiberger Mulde, Zwickauer Mulde and Vereinigte Mulde in Saxony/Germany as well as in areas potentially affected by the uranium mining and milling in Saxony in order to study the consequences of the August‐2002 flood with respect to mobilization and transport of heavy‐metal containing sediments. Sampling points were chosen to allow comparison of the results with those of earlier investigations performed about one decade ago. In addition, samples of high‐flood sediments in urban areas were investigated. Radionuclide concentrations in sediments from the Freiberger Mulde reveal the geogenic background. The Zwickauer Mulde is still strongly affected by the former uranium mining and milling activities, but 238U and 226Ra activity concentrations are significantly lower than one decade ago. The same holds true for the Vereinigte Mulde, the activity concentrations being, however, lower due to dilution of sediments during long‐range transport. To which degree the decreases are due to the interim remediation work or due to the flood event of 2002 cannot be decided. Definitive influences of the flood, i.e. mobilization and transport of materials from mine dumps and tailing ponds can be seen for some of the areas affected by uranium mining and milling. 238U/226Ra and 226Ra/210Pb radioactive disequilibria allow distinguishing the different types of materials polluting the sediments and their transport pathways. The radiological relevance of the results is discussed.

234Th: 238U disequilibrium as an indicator of sediment resuspension in Thermaikos Gulf, northwestern Aegean Sea

234Th: 238U disequilibria have been used extensively as tracers of particle dynamics in marine environments. 234Th ( t 1/2=24.1 days) can be used as “proxy” for particle reactive pollutants, due to their similar rapid rate of scavenging onto particles and subsequent removal from the water column, to the sediments. Radioactive disequilibrium can be exploited to determine the rates and time-scales of processes occurring over days to months; in this instance the residence times of dissolved and particulate species with the benthic nepheloid layer (BNL). Three sampling cruises were undertaken in Thermaikos Gulf (NW Aegean Sea) during contrasting periods, to examine the impact of natural and anthropogenic activity on sediment resuspension. September and October 2001 represented background and trawling periods, respectively; January 2002 represented a mixed period, of trawling and storms. Dissolved 234Th is scavenged actively at the BNL, in the presence of suspended particulate material (SPM), with a mean residence time of 16 days. There is a weak inverse correlation between dissolved 234Th residence time and SPM concentration in the BNL, with the shortest residence times occurring during October 2001. No relationship was observed between particulate 234Th activities and SPM concentrations, indicating that particles are rapidly removed from the system, either by sinking or advection. The mean particulate 234Th residence time is 5 days.

Minimum speed limit for ocean ridge magmatism from 210Pb–226Ra–230Th disequilibria

Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges-where the heat budget controls crustal structure, hydrothermal activity and a vibrant biosphere-the tempo of magmatic inputs in these regions remains poorly understood. Such timescales can be assessed, however, with natural radioactive-decay-chain nuclides, because chemical disruption to secular equilibrium systems initiates parent-daughter disequilibria, which re-equilibrate by the shorter half-life in a pair. Here we use 210Pb-226Ra-230Th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the Earth's mantle can be transported, accumulated and erupted in a few decades. This implies that magmatic conditions can fluctuate rapidly at ridge volcanoes. 210Pb deficits of up to 15 per cent relative to 226Ra occur in normal mid-ocean ridge basalts, with the largest deficits in the most magnesium-rich lavas. The 22-year half-life of 210Pb requires very recent fractionation of these two uranium-series nuclides. Relationships between 210Pb-deficits, (226Ra/230Th) activity ratios and compatible trace-element ratios preclude crustal-magma differentiation or daughter-isotope degassing as the main causes for the signal. A mantle-melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly. The likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied.

U isotope ratios as tracers of groundwater inputs into surface waters: Example of the Upper Rhine hydrosystem

Major elements, trace elements and Sr and U isotopes were analyzed in the dissolved load of water samples collected in the main reservoirs of the Rhine graben, i.e. the Rhine River, its main tributaries and the shallow and deep ground waters in several different aquifers. Our results confirm that the variations of major elements, trace elements (especially Sr and Rb) and Sr isotope ratios of the water samples are mainly correlated to the lithological variations of the watershed. The atmospheric inputs can be considered as a minor cause for the chemical variability of the surface waters in the Rhine hydrosystem, except for a few chemical elements in the mountain streams. Human activity, in particular the pollution by the exploitation of the Alsace potash mines, mainly affects the Na and Cl concentrations of the Rhine and the south alluvial groundwaters but remains negligible for the other water samples. The specific study of the Lauter stream located in the lower Triassic sandstones (Buntsandstein sequences) demonstrates the importance of groundwater influences onto the chemical composition of river waters. Up to 50% of the total dissolved load (TDS) of this stream is supplied by deep waters; this contribution significantly modifies the chemical composition of the stream, especially the 238U– 234U radioactive disequilibrium, which, in turn, can be used as a specific tracer of deep water inputs into surface waters and could be useful to distinguish in the dissolved load of rivers, surface weathering fluxes from deeper ones.

226Ra or 226Ra/Ba dating of Holocene volcanic rocks: application to Mt. Etna and Merapi volcanoes

This paper shows how 226Ra– 230Th disequilibria can be used to date Holocene volcanic rocks from some well selected volcanoes. A systematic study of these disequilibria on historical or well-dated volcanic samples is indeed first required to test the applicability of this method. Two examples are described here to illustrate its potential. In the case of Mt. Etna, the good correlation observed between ( 226Ra) 0 activities at the time of eruption and Th contents in lava flows from the last two millennia [M. Condomines, J.C. Tanguy, V. Michaud, Magma dynamics at Mt. Etna: constraints from U–Th–Ra–Pb radioactive disequilibria and Sr isotopes in historical lavas, Earth Planet. Sci. Lett. 132 (1995) 25–41] is used to infer the ages of several newly analysed lava flows. The calculated ages are in good agreement with those deduced from the archaeomagnetic curve describing the variation of the geomagnetic field direction in southern Italy [J.C. Tanguy, I. Bucur, J.F.C. Thompson, Geomagnetic secular variation in Sicily and revised ages of historic lavas from Mt. Etna, Nature 318 (1985) 453–455, J.C. Tanguy, M. Le Goff, V. Chillemi, A. Paiotti, C. Principe, S. La Delfa, G. Patane, Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires, C. R. Acad. Sci. Paris 329 (1999) 557–564, J.C. Tanguy, M. Le Goff, C. Principe, S. Arrighi, V. Chillemi, A. Paiotti, S. La Delfa, G. Patane, Archaeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits. Earth Planet. Sci. Lett. 211 (2003) 111–124]. We also present a whole set of new U-series data on historical, recent, and older samples from Merapi (Indonesia), and show that the ( 226Ra)/Ba ratio has probably maintained a quasi-steady state value during at least the past four millennia, and can be used to infer the ( 226Ra) 0/Ba ratio of old volcanics at the time of eruption, and thus their ages. Comparison with 14C ages available on three samples [R. Gertisser, J. Keller, Temporal variations in magma composition at Merapi volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity, J. Volcanol. Geotherm. Res. 123 (2003) 1–23] shows an excellent agreement. These dating methods, based on the post-eruptive decrease of 226Ra excesses can be confidently used to date young rocks on both volcanoes, an important step to infer their recent eruptive history and magmatic evolution. It also opens the possibility to extend the geomagnetic field variation curve back into the past few millennia. The promising results obtained in this work should encourage new systematic U-series studies to test the applicability of such methods to other permanently active volcanoes showing 226Ra excesses.

Radioactivity distribution in some Permian sediments from the Iratı́ and Corumbataı́ Formations of the Paraná Basin, Southeastern Brazil

In the city of Limeira, southeastern Brazil, an important exposure of Permian sediments of the Paraná basin was revealed by an open pit mine that exploits limestone for production of soil correction compounds and raw materials for the ceramic industry. The radioactivity of these sediments was investigated in some detail and the results provided a general view of the vertical distributions of uranium, thorium and potassium concentrations and of the element ratios U/K, U/Th and Th/K. In general, the concentrations of the main natural radioactive elements are low, with uranium being enriched in some limestone and shale levels. In addition the results showed that the 238U series is in radioactive disequilibrium in many of the analyzed samples. Although the origin of the observed disequilibrium has not been investigated, the results suggest that it is due to weathering processes and water interaction with the rock matrix.

Steady-state 226Ra/ 230Th disequilibrium in mantle minerals: Implications for melt transport rates in island arcs

Measurements of the concentrations of the 238U decay series isotopes ( 234U, 230Th, 226Ra) have been used to estimate the rates at which magma is generated and transported in the mantle. The usual assumption is that solid mantle minerals are in radioactive equilibrium prior to melting. However, if one or more of the nuclides in the chain is strongly concentrated by a minor mineral, and if the diffusivity of that nuclide is large enough, steady-state radioactive disequilibrium can result in the solid phase. It can be inferred from available data that radium is strongly concentrated in minor hydroxyl-bearing mantle minerals (phlogopite and amphibole) relative to Th, and Ra diffusion in clinopyroxene is fast relative to the typical grain diameter at ca. 1100°C. Consequently, we show with simple analytical models that a steady-state Ra deficiency in clinopyroxene (cpx), accompanied by a complementary steady-state Ra excess in neighboring phlogopite (phlog) or amphibole (amph), is likely to be the normal situation in hydrous mantle peridotite with average clinopyroxene grain radii of ca. 1 mm. The steady state ( 226Ra/ 230Th) (parentheses indicating activity ratio) in the hydrous mineral is limited roughly by the mass ratio with clinopyroxene (i.e. cpx/phlog or cpx/amph) and could be as high as 10–100. The exceptionally high ( 226Ra/ 230Th) of some island arc lavas could therefore be a result of preferential contribution of phlogopite or amphibole during partial melting of hydrous mantle. This effect may ease time constraints for source-to-surface melt migration at island arcs. Incipient melting of hydrous minerals from channel walls during melt transport and/or late-stage incorporation of phlogopite or amphibole into arc magmas may also contribute to generating high ( 226Ra/ 230Th). Steady-state ( 226Ra/ 230Th) disequilibrium due to diffusive loss of 226Ra from clinopyroxene is also important for melt/solid and fluid/solid partitioning, and must be incorporated into models relating isotopic disequilibrium to melt and fluid transport rates. Diffusive effects could be important for other U-series nuclides in some circumstances.

Disequilibria in the disintegration series of U and Th and chemical parameters in thermal spring waters from the Tatun volcanic area (Taiwan)

SummaryThe activity concentrations of238U,234U,230Th,226Ra,232Th, and228Th in thermal spring waters in the Tatun volcanic area were determined. Parameters including acidity, Cl-and {SO42-concentrations in spring waters at the sampling sites have been investigated to allow interpretation of the migration of the radionuclides, and to elucidate the influence of these parameters on the variations of radionuclide contents. Radioactive disequilibria were found in uranium and thorium series in thermal spring waters. The contents of uranium and thorium decreased with increasing pH. The ratios of230Th/234U,226Ra/230Th and228Th/232Th show significant disequilibria. The226Ra/230Th ratio (0.60–34.8) decreased with the Cl-or {SO42-concentration. All228Th/232Th ratios (1.01–9.49) deviated from unity due to the co-precipitation of228Ra with barium and lead sulfate.

Merapi Volcano, Central Java, Indonesia: A case study of radionuclide behavior in volcanic gases and its implications for magma dynamics at andesitic volcanoes

For more than 20 years, volcanic gases have been regularly collected at Merapi Volcano, Indonesia, and have been subsequently analyzed for their (210Pb), (210Bi), and (210Po) activities and SO2 contents. These new results show the high volatility of the three radionuclides in andesitic gases, although their emanation coefficients (0.94%, 3.5%, and ≤53% for 210Pb, 210Bi, and 210Po, respectively) are significantly lower than those observed at basaltic volcanoes. This emphasizes the major role of magma temperature on the degassing of these metals, which are mainly transported in volcanic gases as Pb‐chloride compounds and as Bi‐ and Po‐metallic species. Radioactive disequilibria between 210Pb, 210Bi, and 210Po in the gas phase are characteristic of degassing processes and gas paths within the edifice. Gases released at both Gendol and Woro fumarolic fields are of magmatic origin, but their radionuclide content is strongly altered by secondary processes (chemical reactions with surrounding brines and deposition of sublimates in the ground). High‐temperature gases collected in the summit gas plume are of pure primary magmatic origin and are likely directly tapped in the degassing reservoir. Gases arising from the growing domes are strongly depleted in the most volatile isotopes and gas species. This suggests that magma mostly degasses at depth in open system, explaining why the volcano usually undergoes no explosive eruptions. The gas flux at Merapi is sustained by a deep magma supply which is twice the extrusion rate of lava. Unerupted degassed magma thus progressively accumulates beneath the summit, yielding an overpressure which can trigger dome collapse without precursor.

China: Anhui Kena New Materials — nanoparticulate TiO 2

Hydrothermal circulation and subsequent eruption of seawater at mid-ocean ridges and back-arc basins has great potential to modulate deep ocean biogeochemistry, acting as both a source and a sink for many trace elements and their isotopes. The influence of hydrothermal vents as a source of iron and manganese has been demonstrated in all ocean basins. However, the long-range impact of scavenging by hydrothermal particles has yet to be documented in detail. We use dissolved and particulate measurements of long-lived radiogenic (230Th, 231Pa) and primordial (232Th) radionuclides to investigate the nature and geographic scale of scavenging processes occurring within a hydrothermal plume in the Southeast Pacific Ocean sampled during the GEOTRACES GP16 section. Due to their radioactive disequilibrium with respect to production by their parent uranium isotopes, 230Th and 231Pa provide unique insights into the rates of scavenging. We find strong deficits in 230Th and 231Pa coincident with elevated particulate Mn and Fe(OH)3, indicating that trace metal scavenging is widespread and likely the result of the strong affinity of trace metals for nanoparticulate metal surface sites. The chemical composition of the particulate material is closely linked to the scavenging intensity of 230Th and 231Pa, expressed as distribution coefficients between solid and solution. A comparison of 230Th and 231Pa inventories with mantle-derived 3He as well as a mass balance of 230Th and 231Pa suggests continuous scavenging removal over the course of the entire 4000 km transect. Unlike the two radiogenic isotopes, 232Th is enriched above what would be expected from assuming identical scavenging behavior to 230Th, indicating a hydrothermal source of colloidal, unreactive 232Th.

Radioactive disequilibrium in the different rock types in Wadi Wizr, the Eastern Desert of Egypt

Remote sensing and radiometric survey techniques in the area between Qusier and Mersa Alam show variation of content of natural radioactivity in Wadi Wizr area, Eastern Desert of Egypt Gamma-ray, and laser fluorimetry were applied to measure the natural radioactivity content in representative content in representative samples from Wadi Wizr. The concentration of 226Ra ranged from 36 to 661 Bq kg −1, while the radioactivity ratio between 226Ra/ 214Pb and 214Pb/ 214Bi ranged from 0.7 to 1.4. The activity ratio 238U/ 226Ra% was found to be in the range 6.4%–72.2%. A state of disequilibrium between the two radionuclides in the uranium series is clear. The trend of disequilibrium was obvious in all the investigated sites and correlating both radionuclides, 238U/ 226Ra demonstrated the site variation influence. Samples were also analyzed by XRD to reveal their mineral composition.

Release of uranium from rock matrix—a record of glacial meltwater intrusions?

Uranium release observed in a rock matrix around water-carrying fractures was studied using U-series disequilibrium (USD) modelling and mass balance calculations. Several release scenarios were tested, with specific attention to the glacial aspects. The release appears to have occurred in two or three violent episodes during the last 300 ky. A release after the last glaciation can be excluded on mass flow grounds. Continuous release for more than 300 ky can be excluded on radioactive disequilibrium grounds. Repeated inflows of oxic glacial meltwater seem to have triggered the release episodes.

Mineral-Melt Partitioning of Uranium, Thorium and Their Daughters

Research Article| January 01, 2003 Mineral-Melt Partitioning of Uranium, Thorium and Their Daughters Jonathan Blundy; Jonathan Blundy CETSEI, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, United Kingdom Search for other works by this author on: GSW Google Scholar Bernard Wood Bernard Wood CETSEI, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, United Kingdom Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2003) 52 (1): 59–123. https://doi.org/10.2113/0520059 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jonathan Blundy, Bernard Wood; Mineral-Melt Partitioning of Uranium, Thorium and Their Daughters. Reviews in Mineralogy and Geochemistry 2003;; 52 (1): 59–123. doi: https://doi.org/10.2113/0520059 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search The uranium and thorium decay series (hereafter “U-series”) include the nuclides of ten elements, all of which can be found at trace levels in rocks and minerals. The relatively short half-lives of the U-series nuclides give them considerable potential to decipher a wide variety of natural processes. The common observation of secular radioactive disequilibrium between parent and daughter nuclides provides a time dimension that is not possible with the more commonly used trace elements. However, just like conventional trace elements, the behavior of U-series elements depends on their partitioning between coexisting phases, such as minerals and melts. Interpreting radioactive disequilibrium... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

U-Th-Ra Fractionation During Weathering and River Transport

Research Article| January 01, 2003 U-Th-Ra Fractionation During Weathering and River Transport F. Chabaux; F. Chabaux Centre de Géochimie de la Surface (CNRS/ULP), Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig 67084 Strasbourg Cedex, France Search for other works by this author on: GSW Google Scholar J. Riotte; J. Riotte Centre de Géochimie de la Surface (CNRS/ULP), Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig 67084 Strasbourg Cedex, France Search for other works by this author on: GSW Google Scholar O. Dequincey O. Dequincey Centre de Géochimie de la Surface (CNRS/ULP), Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig 67084 Strasbourg Cedex, France Search for other works by this author on: GSW Google Scholar Author and Article Information F. Chabaux Centre de Géochimie de la Surface (CNRS/ULP), Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig 67084 Strasbourg Cedex, France J. Riotte Centre de Géochimie de la Surface (CNRS/ULP), Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig 67084 Strasbourg Cedex, France O. Dequincey Centre de Géochimie de la Surface (CNRS/ULP), Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig 67084 Strasbourg Cedex, France Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2003) 52 (1): 533–576. https://doi.org/10.2113/0520533 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F. Chabaux, J. Riotte, O. Dequincey; U-Th-Ra Fractionation During Weathering and River Transport. Reviews in Mineralogy and Geochemistry 2003;; 52 (1): 533–576. doi: https://doi.org/10.2113/0520533 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search The potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960’s (e.g., Rosholt et al. 1966; Hansen and Stout 1968). This interest results from the dual property of the nuclides of the U and Th radioactive series (1) to be fractionated during water-rock interactions and (2) to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters. Therefore, the study of radioactive disequilibria in surface environments should help to bring information about the nature, the intensity... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Study of the radioactive disequilibrium of the volcanic rocks from the Trindade Island (Brazil)

perature allochtonous (Socorro-Guaxupe nappe). The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain with extensional structures in normal shear zones and compressive structures exhibiting E-NE transport. Tourmaline leucogranites in stratiform bodies and diatexitic gneisses occur frequently. There is a metamorphic transition, north to south, from the kyanite zone (and metabasic rocks in eclogite facies) to kyanite and sillimanite coexistence, until kyanite broken and exclusive sillimanite presence. The S2 texture foliation on mineral assemblage, suggests a decompressive path, where metabasics boudins preserve paragenese eclogite facies (Omp-Pl-Grt-Rt-Qtz) with simplectite textures between Cpx-Pl-Qtz. Externally, corroded garnets wrapped by plagioclase coronas, in amphibole matrix, represent the amphibolite facies reequilibration (MgHbl-Pl-Grt-Zo-Ilm-Ttn). Metapelites (RtKy-Grt-Ms-Bt-Pl-Qtz) may show Sil-Pl-Qtz coronitic intergrowths around garnets. The maximum temperatures for the distinguished metamorphic domains (14 samples), vary from 710 ± 30◦C (Ky domain) to 730 ± 50◦C (Sil domain). Although the pression vary at 17 ± 0.5 to 6 ± 0.5kbar. P-T conditions at 720 ± 50◦C and 11.5 ± 2.5kbar marks the kyanite assemblage re-equilibration (amphibole facies). P-T conditions 625 ± 55◦C and 6.8 ± 2kbar represent the cooling path (Ky-Sil fields). The high pression decompressive path suggests an extrusional process, immediately after buried at about 60km deep. Fast exhumation, process controlled by convergent events, derived by the Sao Francisco plate subductions and tectonic erosion take these units, isothermally, to upper levels (20-33 km). Later, the metamorphic path shift toward a near-isobaric cooling though in a slower extrusional process. P-T path continuity through three metamorphic domains suggests a sin-metamorphic process (S2 foliation) as a continuous effect S1 × S2, without diacronism or ensuing event superposition. However, large amount of leucogranites denotes a regional thermal event, subsequent and superimposed, responsible for quartz static polygonization in schists/gneisses and for Ms breakdown with microcline crystallization (730oC/6kbar) in the vicinity of large granite plutons. — ( December 14, 2001 ) . ∗ Supported by FAPESP 00/11389-6 and 98/15624-8. STUDY OF THE RADIOACTIVE DISEQUILIBRIUM OF THE VOLCANIC ROCKS FROM THE TRINDADE ISLAND (BRAZIL) Rosana N. Santos and Leila S. Marques Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de Sao Paulo, 01065-970, Sao Paulo, SP. Departamento de Fisica, Pontificia Universidade Catolica de Sao Paulo, 01303-050, Sao Paulo, SP. Presented by Antonio C. Rocha-Campos

Precise geochronology of phoscorites and carbonatites:: The critical role of U-series disequilibrium in age interpretations

We present the results of a comparative study of several geochronometer minerals (baddeleyite, zircon, apatite, phlogopite and tetraferriphlogopite) and isotopic systems (U-Pb, Th-Pb and Rb-Sr) from phoscorites (magnetite-forsterite-apatite-calcite rocks) and carbonatites of the Kovdor ultramafic-alkaline-carbonatite massif, Kola Peninsula, Russia. Uranium, thorium and their decay products are extremely fractionated by minerals that crystallise from carbonatite and phoscorite magma. We obtain high-precision ages from different chronometers, compare their accuracy, and evaluate the role of geochronological pitfalls of initial radioactive disequilibrium, differential migration of radiogenic isotopes, and inaccurate decay constants. Apatite yielded concordant U-Th-Pb ages between 376 and 380 Ma. The accuracy of the apatite 238U- 206Pb ages is, however, compromised by uncertainty in the amount of radiogenic 206Pb produced from initial excess 230Th. The 235U- 207Pb ages are relatively imprecise due to large common Pb correction and the uncertainty in the initial Pb isotopic composition. The Th-Pb system yields a more precise age of 376.4 ± 0.6 Ma. Zircon from two carbonatite samples is characterised by moderate to low U contents, high Th contents, and very high Th/U ratios up to 9000. The 206Pb*/ 238U systems in the zircon are strongly affected by the presence of excess 206Pb*, produced by decay of initial 230Th. The 208Pb*/ 232Th ages of zircon from both carbonatite samples are uniform and yield a weighted average of 377.52 ± 0.94 Ma. Baddeleyite U-Pb analyses are 3 to 6% normally discordant and have variable 207Pb*/ 206Pb* apparent ages. Eleven alteration-free baddeleyite fractions from three samples with no evidence for Pb loss yield uniform 206Pb*/ 238U ages with a weighted average of 378.54±0.23 Ma (378.64 Ma after correction for initial 230Th deficiency), which we consider the best estimate for age of the phoscorite-carbonatite body of the Kovdor massif. The 206Pb*/ 238U ages of baddeleyite fractions from five other samples spread between 378.5 and 373 Ma, indicating a variable lead loss up to 1.5%. The anomalously old 207Pb/ 235U and 207Pb/ 206Pb ages are consistent with the presence of excess radiogenic 207Pb* in the baddeleyite. We interpret this as a result of preferential partitioning of 231Pa to baddeleyite. Fifteen phlogopite and tetraferriphlogopite fractions from five carbonatite and phoscorite samples yielded precise Rb-Sr isochron age of 372.2 ± 1.5 Ma, which is 5 to 7 m.y. younger than our best estimate based on U-Th-Pb age values. This difference is unlikely to be a result of the disturbance or late closure of Rb-Sr system in phlogopite, but rather suggests that the accepted decay constant of 87Rb is too high. Comparative study of multiple geochronometer minerals from the Kovdor massif has revealed an exceptional complexity of isotopic systems. Reliable ages can be understood through systematic analysis of possible sources of distortion. No single geochronometer is sufficiently reliable in these rocks. Th-Pb and Rb-Sr can be a very useful supplement to U-Pb geochronometry, but the routine use of these geochronometers together will require more precise and accurate determination of decay constants for 232Th and 87Rb.

Heat from radioactive elements in young volcanics by γ-ray spectrometry

Radioactive heat production data for volcanic rocks ranging in age from 430 ka to the historical time and cropping out on the three main islands of the Aeolian arc (Southern Italy) are presented. They were derived from uranium, thorium and potassium concentration measured from γ-radiations originating from the decay of 214Bi ( 238U series), 208Tl ( 232Th series) and the primary decay of 40K. Concentration results from a NaI(Tl) detector, compared with independent determinations by means of mass spectrometry, reveal significant secular radioactive disequilibrium between 214Bi and 238U, in the rhyolitic rocks, which is ascribable to their young age. The use of the low-energy portion of the spectrum, where there are a number of γ-rays produced by 234Th, minimises the disequilibrium problem in the uranium determination. The Th/U ratio remains almost constant at 3.3 in all the rock types. Due to their low isotopic content, basalts show a low radioactive heat production rate (0.6 μW m −3), whereas the highest values (6.6–7.1 μW m −3) are found in trachytic and rhyolitic lavas.

Origin of 226Ra– 230Th disequilibria in arc lavas from southern Chile and implications for magma transfer time

Improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface. The rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 238U series. Young lavas from southern Chile, in which 238U– 230Th disequilibria have been measured [Sigmarsson et al., Nature 346 (1990) 163–165; Sigmarsson et al., Nature 394 (1998) 566–569], were analyzed for 226Ra abundances. The disequilibrium between 226Ra and 230Th in these lavas is found to correlate with 238U– 230Th disequilibria and 10Be/Be [Morris et al., Nature 344 (1990) 31–36]. These correlations strongly suggest that the excess of 226Ra over 230Th is due to the addition of a slab-derived fluid to the magma source, since Ra and U are fluid-mobile elements and the cosmogenic 10Be is most likely derived from the subducting Nazca plate beneath the Andes. The largest slab signature is observed in the lavas of Villarrica volcano, which is the most active volcano in South America. A model for subduction fluxing is discussed, in which the U series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle, but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge. Repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 10Be/Be–B/Be correlation for different arcs [Morris et al., Nature 344 (1990) 31–36]. The fact that excesses of 226Ra and 238U over 230Th are correlated indicates that linear arrays on the ( 230Th/ 232Th)–( 238U/ 232Th) diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts. The 226Ra– 230Th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer, or less than 8000 years. This disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m/year. Finally, the correlations of ( 226Ra /230Th) with ( 238U/ 232Th) and 10Be/Be in Andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied.