226Ra Excesses Research Articles

On the recent bimodal magmatic processes and their rates in the Torfajökull–Veidivötn area, Iceland

Historical bimodal composition eruptions spanning Torfajökull central volcano and neighbouring Veidivötn fissure swarm in southeastern Iceland provide clues about the nature and timing of basalt and rhyolite petrogenesis and eruption at an active divergent plate boundary. This study focuses on lavas and tephras of the last two regional eruptions in 871 and 1477 AD, using samples that approximate mafic and felsic endmember compositions relative to regional mixing trends in literature data from the area. Whole rock and mineral U–Th–Ra isotopic compositions demonstrate both rapid petrogenetic timescales and limited compositional variation in the basalt and rhyolite magma sources. Rhyolites display the greatest 230Th excesses (≤ 17%) at only slightly lower ( 230Th/ 232Th) activity ratios than co-eruptive Veidivötn basalts. Both magma types display small but significant 226Ra excesses (≤ 10% and ≤ 60% in rhyolites and basalts, respectively). U-series isotopic and trace element data are consistent with crustal melting dominated by young (≤ 60 kyr), K-metasomatized mafic protoliths similar in composition to the Veidivötn tholeiites as source of the 871 and 1477 rhyolites. Young protolith ages are inconsistent with published models that call on old materials as source rocks for Icelandic rhyolites (e.g., old silicic segregation lenses or isostatically subsided lavas). Zero-age U–Th and few-ka Ra–Th mineral-whole rock isochrons indicate that crystals formed shortly prior to eruption, consistent with petrographic and compositional indicators that they are phenocrysts, and suggest that the rhyolite melts are of Late Holocene age.

A 210Pb– 226Ra– 230Th– 238U study of Klyuchevskoy and Bezymianny volcanoes, Kamchatka

Lavas from Klyuchevskoy and Bezymianny volcanoes, Kamchatka, appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust. This fractionation may have occurred on timescales significantly less than 1000 years if observed 226Ra excesses largely reflect variable residual porosity in the source melting region. Unlike most arc lavas, those with the highest MgO contents and Ba/Th ratios have the lowest 226Ra excess. Forward models suggest that those portions of the source which had undergone the greatest addition of U by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity. At Kluchevskoy, a change from eruption of high-MgO to high-Al 2O 3 basaltic andesites around 1945 is reflected in an increase in size of 226Ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime. The eruption of andesites at Bezyminanny, simultaneous with the eruption of basaltic andesites at Klyuchevskoy, further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time. Thus, the melting processes beneath Klyuchevskoy and Bezyminanny are demonstrably complex. They have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the Central Kamchatka Depression. Finally, the 210Pb data are, with one or two exceptions, in equilibrium with 226Ra, suggesting that there was restricted relative magma-gas movement in this highly productive magmatic system.

An excess 226Ra chronology for deep-sea sediments from Saanich Inlet, British Columbia

To further explore the efficacy of 226Ra (excess) dating for deep-sea sediments, previously dated varve sediments from Saanich Inlet were investigated. Ages obtained using 226Ra (excess) are comparable to the varve ages in the upper 20–25 m of the sedimentary record, but radiometric ages for those sediments older than c. 4000 yr BP are significant underestimates. This results from major changes in sedimentation within Saanich Inlet around 4000 yr BP linked to rising sea levels, with younger sediments characterised by a higher biogenic contribution resulting from the establishment of an anoxic fjord environment. The older sediments were deposited in a shallow water inlet characterised by variable Ra mass balance and non-radiogenic losses. Therefore, while 226Ra (excess) can produce reliable dates, its application may be limited where the relative significance of authigenic and allogenic input and bottom water anoxia has been variable and where closed-system behaviour is compromised.

Recent mobilization of U-series radionuclides in the Bernardan U deposit (French Massif Central)

Several nuclides of the 238U ( 234U, 230Th, 226Ra, 210Pb) and 235U ( 231Pa, 227Ac) decay series have been measured through gamma spectrometry in the U-bearing episyenites of the Bernardan mine (northwest Massif Central, France), in order to study recent mobilizations in the U-deposit and their time-scales. ( 230Th/ 238U) and ( 231Pa/ 235U) ratios show that U has been little affected during the last 350 ky, especially in the U-rich samples. In contrast, 226Ra was subjected to a general redistribution, with a clear tendency for Ra to be lost from the U-rich samples and added to U-poor samples. The highest Ra excesses are present in low-U samples, and are related to adsorption onto iron hydroxides. Ra appears to have been redistributed within the mineralised episyenitic bodies, which seem to remain in 226Ra– 238U radioactive equilibrium, a result consistent with the earlier study of Leroy [1984. Episyénitisation dans le gisement d'uranium du Bernardan (Marche): Comparaison avec des gisements similaires du Nord-Ouest du Massif Central français. Miner. Depos. 19, 26–35]. As shown by the 226Ra and 210Pb data, the Ra redistribution was very active between 8 and 0.1 ky, and this might be a consequence of the hydrological changes in groundwater circulation accompanying the Holocene climate warming.

238U– 230Th– 226Ra– 210Pb constraints on the genesis of high-Mg andesites at White Island, New Zealand

On White Island, New Zealand, the intensified period of strombolian–volcanian and phreatomagmatic explosive activity that commenced in March 1977 led to eruption of unusually primitive, high-Mg andesites. These are Fo 80–93 olivine-saturated rocks that have MgO contents up to 10% (Mg# = 65–71) and SiO 2 of 56–58%. They have incompatible trace element characteristics that are typical of arc rocks. 87Sr/ 86Sr, 143Nd/ 144Nd and 176Hf/ 177Hf ratios (0.7049–0.7053, 0.51282–0.51266 and 0.28301–0.28298, respectively) are consistent with subducted sediment addition and/or crustal input but there is no clear correlation of either isotope ratio with MgO. The rocks have modest (3–10%) 238U excesses at low ( 230Th/ 232Th) ratios (0.697 to 0.722). 226Ra– 230Th disequilibria are also restricted but, unusually, include both 226Ra excesses and deficits with ( 226Ra/ 230Th) = 0.94–1.07. ( 210Pb/ 226Ra) o ranges from 0.98 to 1.52 requiring gas accumulation that may increase over time and with decreasing MgO. Sr/Y and Tb/Yb ratios are both low and relatively invariant at 8 and 0.3, respectively, and along with the 238U excesses preclude an origin in which residual garnet was involved. The occurrence of some 226Ra deficits suggests the presence of residual amphibole during partial melting for some samples. Rapid magma ascent (to preserve the 226Ra disequilibria) limits the amount of possible melt–wall rock interaction that might reduce source-derived Tb/Yb ratios and in the mantle or raise 87Sr/ 86Sr in the crust. The White Island high-Mg andesites did not form by partial melting of eclogite in the subducting Pacific plate. Their primitive, olivine-saturated compositions suggest that their source was peridotitic and experimental data suggest that equilibration with peridotite at low temperatures at 0.5–1.5 GPa and in the presence of elevated alkalis can reconcile the high SiO 2 and MgO of the rocks. This may reflect stalling of the magmas beneath the continental lithosphere for a few 1000 years.

The scale and origin of the osmium isotope variations in mid-ocean ridge basalts

The decay of 187Re to 187Os provides a unique tracer of recycled lithosphere in the Earth's upper mantle, and the radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) have been taken to indicate the presence of recycled oceanic crust in the mantle source. However, others have attributed the data to contamination by seawater or an age effect (due to 187Re decay following igneous crystallisation). This study presents new Re–Os, U–Th–Ra and B isotope data for normal-MORB (N-MORB) glass (including 18 samples analysed previously). The scale of the variation in 187Os/ 188Os observed here in N-MORB glass ranges from 0.126 to 0.148, with a mean value of 0.133 ± 0.009 (2 σ). Previously published Os isotope data for MORB is shown to have been affected by an analytical artefact, and in particular the very radiogenic Os isotope compositions reported earlier are now shown not to exist. Nevertheless, some radiogenic Os isotope compositions are still seen in the data obtained here. Coupled Th and Ra excesses appear to rule out an age effect, at least, for those samples studied, however, δ 11B covariations with 187Os/ 188Os are consistent with the assimilation of older oceanic crust (affected by seawater contamination). Individual sulphides (less likely to be affected by such contamination) are always less radiogenic than their glass host, with 187Os/ 188Os values ranging from 0.123 to 0.131. This indicates that sulphide precipitation occurred prior to contamination of the surrounding melt, but the data also suggest that low-Os sulphides have been affected by contamination through partial equilibration with that melt. Taken together, these indicate that dominant process responsible for the radiogenic Os isotope compositions of N-MORB glass is contamination by older oceanic crust. Although the influence of age effects and the presence of recycled material cannot be completely ruled out, Os-rich sulphides yield sub-chondritic Os isotope compositions which point rather to a long-term depletion in Re in the MORB source likely to have resulted from formation of the continental crust over much of Earth's history.

Plume–lithosphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from 238U– 230Th– 226Ra and Sr–Nd–Pb isotope systematics

Precise measurements of 238U– 230Th– 226Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr–Nd–Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess 238U– 230Th– 226Ra disequilibria with 230Th (18–24%) and 226Ra (9–21%) excesses, and there exists a positive correlation in a ( 226Ra/ 230Th)–( 230Th/ 238U) diagram. The extent of 238U– 230Th– 226Ra disequilibria is markedly different in lavas of individual eruption ages, although the ( 230Th/ 232Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the ( 230Th/ 238U)–( 206Pb/ 204Pb) and ( 226Ra/ 230Th)–( 206Pb/ 204Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high D U and D U/ D Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios ( 206Pb/ 204Pb > 20.47) due to late Mesozoic metasomatism.

Diffusive fractionation of U-series radionuclides during mantle melting and shallow-level melt–cumulate interaction

U-series radioactive disequilibria in basaltic lavas have been used to infer many important aspects of melt generation and extraction processes in Earth’s mantle and crust, including the porosity of the melting zone, the solid mantle upwelling rate, and the melt transport rate. Most of these inferences have been based on simplified theoretical treatments of the fractionation process, which assume equilibrium partitioning of U-series nuclides among minerals and melt. We have developed a numerical model in which solid-state diffusion controls the exchange of U-series nuclides among multiple minerals and melt. First the initial steady-state distribution of nuclides among the phases, which represents a balance between diffusive fluxes and radioactive production and decay, is calculated. Next, partial melting begins, or a foreign melt is introduced into the system, and nuclides are again redistributed among the phases via diffusion. U-series nuclides can be separated during this stage due to differences in their diffusivity; radium in particular, and possibly protactinium as well, can be strongly fractionated from slower-diffusing thorium and uranium. We show that two distinct processes are not required for the generation of 226Ra and 230Th excesses in mid-ocean ridge basalts, as has been argued previously; instead the observed negative correlations of the ( 226Ra/ 230Th) activity ratio with ( 230Th/ 238U) and with the extent of trace element enrichment may result from diffusive fractionation of Ra from Th during partial melting of the mantle. Alternatively, the ( 226Ra/ 230Th) disequilibrium in mid-ocean ridge basalts may result from diffusive fractionation during shallow-level interaction of mantle melts with gabbroic cumulates, and we show that the results of the interaction have a weak dependence on the age of the cumulate if both plagioclase and clinopyroxene are present.

A 210Pb– 226Ra– 230Th– 238U study of Klyuchevskoy and Bezymianny volcanoes, Kamchatka

Lavas from Klyuchevskoy and Bezymianny volcanoes, Kamchatka, appear to show a link between the extent of partial melting in their mantle source region and the subsequent degree of fractionation suffered by the magmas during passage through the crust. This fractionation may have occurred on timescales significantly less than 1000 years if observed Ra excesses largely reflect variable residual porosity in the source melting region. Unlike most arc lavas, those with the highest MgO contents and Ba/Th ratios have the lowest Ra excess. Forward models suggest that those portions of the source which had undergone the greatest addition of U by fluids from the subducting plate also underwent the greatest extents of partial melting at the highest residual porosity. At Kluchevskoy, a change from eruption of high-MgO to high-Al2O3 basaltic andesites around 1945 is reflected in an increase in size of Ra excess which seems to require a simultaneous decrease in residual porosity and suggests a rapid changes in the melting regime. The eruption of andesites at Bezyminanny, simultaneous with the eruption of basaltic andesites at Klyuchevskoy, further suggests that different degree melts produced at differing residual porosity can be formed and extracted from the melt region at the same time. Thus, the melting processes beneath Klyuchevskoy and Bezyminanny are demonstrably complex. They have clearly been influenced by both fluid addition from the subducting plate and extension and decompression beneath the Central Kamchatka Depression. Finally, the Pb data are, with one or two exceptions, in equilibrium with Ra, suggesting that there was restricted relative magma-gas movement in this highly productive magmatic system. ! 2007 Elsevier Ltd. All rights reserved.

U–Th–Ra disequilibria and the time scale of fluid transfer and andesite differentiation at Arenal volcano, Costa Rica (1968–2003)

To evaluate the time scale of fluid transfer and andesite differentiation at Arenal volcano in Costa Rica, we have measured trace-element concentrations and U-series disequilibria in whole rocks and mineral separates (pyroxene, plagioclase, magnetite) from lavas of the current eruption (1968 to 2003) by HR-ICP-MS, TIMS and PIMMS techniques. Whole rock and mineral separate analyses show a small but measurable variation in ( 230Th)/( 232Th) (1.10 to 1.18). In contrast, ( 230Th)/( 238U) range from 0.91 to 1.04 reflecting the moderate spread in Th/U. Stage 1 (1968–1971) whole rocks and mineral separates have both higher ( 230Th)/( 232Th) and ( 238U)/( 232Th) than to younger stage 2 lavas (1971 to present), which have lower, nearly constant ( 230Th)/( 232Th) and lower, slightly variable ( 238U)/( 232Th). 226Ra excesses exist in both whole rocks and mineral separates with ( 226Ra)/( 230Th) ranging between 0.94 and 4.8. Whole rock ( 226Ra)/( 230Th) are largest early in the eruption and decrease in the later lavas, which are influenced by newer recharge material. 238U– 230Th whole rock and mineral data produce an inclined array on an equiline diagram, which we interpret to represent progressive melting of a variably fluxed mantle wedge and a Nicaraguan sediment component, and subsequent mixing. 238U– 230Th internal isochrons suggest that minerals grew instantaneously with respect to the half-life of 230Th. Whole rock and mineral separate ( 226Ra)/( 230Th) data indicate that melts were produced, transported, differentiated and erupted in < 8 kyr. Mineral ( 226Ra)/Ba–( 230Th)/Ba model ages are calculated and corrected for melt inclusions and glass adherents in the mineral fractions, and for the differential partitioning of Ra and Ba. Plagioclase model ages and 238U– 230Th isochron ages suggest that plagioclase could be as young as a few years or as old as several centuries upon eruption.

226Ra/230Th excess generated in the lower crust: Implications for magma transport and storage time scales: Comment and Reply: COMMENT

[Dufek and Cooper (2005][1]; hereafter DC) present an interesting and provocative paper on the origins of 226Ra excess in arc lavas. They develop an incongruent continuous melting model to argue that 226Ra excess can be created by partial melting in the lower crust. However, despite the novelty of

Plume–ridge interaction studied at the Galápagos spreading center: Evidence from 226Ra– 230Th– 238U and 231Pa– 235U isotopic disequilibria

New 238U– 230Th– 226Ra and 231Pa– 235U disequilibria data measured by TIMS are presented for ridge-centered MORB glasses dredged during the R/V Sonne 158 cruise at the Galápagos or Cocos-Nazca Spreading Center (GSC) between 86.0°W and 92.3°W. The application of U-series isotopes to the GSC region, situated a few hundred kilometres to the north of the Galápagos hotspot, allows assessment of fundamental questions related to the dynamics of plume–ridge interaction. These include (1) the relationship between long-lived source variations, U-series disequilibria and extent of differentiation, (2) partial melting during solid upwelling, and (3) the nature and rates of plume–ridge mass transfer. The along axis U-series disequilibria variation show gradational patterns that locally are correlated with geochemical and isotopic parameters such as La/Sm, Tb/Yb, 206Pb/ 204Pb and 143Nd/ 144Nd as well as major element compositions. The correlation of ( 230Th)/( 238U) with radiogenic isotopes and Tb/Yb provides constraints on the plume source influence on the melting process, reflecting an increase in the amount of melting at depth in the presence of garnet or aluminous clinopyroxene. Moreover, the correlation between U-series signatures, radiogenic isotopes, incompatible element abundance and MgO content indicates a causative relationship between the melting of plume source materials and how these lavas differentiate at shallow depths. We speculate that this involves loss of alkalis from ascending melts to shallow peridotite and crustal gabbro, resulting in increased olivine fractionation from the magmas. The U-series data place stringent constraints on the timing of plume–ridge mass transfer and thus distinguish whether mass transfer occurs by movement of melts or solid mantle. Within the likely conditions proposed by the model of (Braun and Sohn [EPSL 213 (2003): 417–430] and with knowledge of ( 231Pa)/( 235U) and ( 230Th)/( 238U) observed in Galápagos Islands lavas [A. Saal, personal communication], we show that all 226Ra excess will be lost and the initial 231Pa and 230Th excesses will be largely decayed. Therefore, we conclude that the plume influence on the GSC lavas results from a solid mantle flow process instead of through migration of plume-derived melts to the ridge.

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.

Basanite to phonolite differentiation within 1550–1750 yr: U-Th-Ra isotopic evidence from the A.D. 1585 eruption on La Palma, Canary Islands

U-Th-Ra disequilibria of basanites, tephrites, and phonolites from the A.D. 1585 eruption on La Palma, Canary Islands, constrain magma differentiation times in an ocean-island rift zone. The insignificant difference in (230Th)/(232Th) implies differentiation from basanite to phonolite in <15 k.y. 226Ra has a half-life of 1600 yr, however, and permits higher temporal resolution; (226Ra)/(230Th) disequilibria are highest in the phonolites (46%–54%) and basanites (44%–47%) and lowest in the tephrites (38%–41%). The higher 226Ra excesses in the end-member compositions model basanite-phonolite differentiation within 1550–1750 yr at a rate of 0.04% fractional crystallization per year. Such a short time interval is in sharp contrast to the ∼200 k.y. proposed for phonolite differentiation on the neighboring island of Tenerife and could reflect different volcanic systems, with a mantle-fed rift system on La Palma versus a crustal magma reservoir on Tenerife.

226Ra/230Th excess generated in the lower crust: Implications for magma transport and storage time scales

Excesses of 226 Ra in arc magmas have been interpreted as resulting from flux melting of the mantle above subducting slabs, followed by fast ascent rates of magma from slab to surface (up to 1000 m/yr). However, we demonstrate that incongruent melting of the lower crust could either maintain or augment mantle-derived 226 Ra excesses, and so reduce inferred vertical transport rates. We developed an incongruent, continuous melting model, and both the incongruent melting reaction and ingrowth effects contribute to the 226 Ra excess. In particular, we found that dehydration melting of amphibolite can produce modeled 226 Ra excesses greater than 300%. Mixtures of such amphibolite dehydration melts with mantle melts will likely retain a 238 U excess (subducted slab) signature. This amphibolite dehydration melting process will also produce elevated ratios of light rare earth elements to heavy rare earth elements, similar to those observed in several arc settings, which may distinguish these magmas from those with 226 Ra excesses produced by slab dewatering alone.

Magma evolution and ascent at volcanic arcs: constraining petrogenetic processes through rates and chronologies

Large 226Ra excesses in a number of mafic arc magmas, and geophysical observations of earthquake hypocenters locations, indicate that mafic melts can get transferred from source to surface within days to ∼1 kyr. Decreasing 226Ra– 230Th disequilibria with increasing SiO 2 in co-magmatic suites of individual arc volcanoes suggest that magma differentiation often occurs on timescales of a few thousand years in closed systems, or less in open systems. However, the rapid decrease of global 238U– 230Th disequilibria with increasing SiO 2 within basaltic andesite compositions suggests that rapid closed system differentiation does not typically produce more evolved compositions from mafic parental magmas with large 238U excesses. In the case of rapid magmatic evolution, open system processes are frequently involved in the production of andesites and dacites, and the data imply that the open system component is close to 238U– 230Th secular equilibrium. MELTS modeling of magma evolution at Santorini in the Aegean arc provides corroborating evidence from an individual well-studied arc volcano, that closed system fractional crystallization fails to produce the more evolved compositions, and increasing 87Sr/ 86Sr ratios with increasing SiO 2 from andesites to rhyolites suggests assimilation of old crust is involved. These observations can be conceptualized through thermal modeling of basaltic sill injections into the lower and upper crust. Melt production in deep crustal hot zones provides an explanation for low 238U– 230Th disequilibria of the evolved magmatic component, as significant accumulation of evolved melts through partial re-melting of previously intruded basalts requires >100 kyr incubation time at typical magmatic flux rates. Thermal modeling also provides a framework for understanding assimilation of old upper crustal rocks in more mature magmatic systems. Further insights into magmatic evolution can be gained from dating of minerals from mafic, intermediate and felsic arc magmas. (a) Many U–Th mineral isochrons in mafic arc magmas yield ages significantly older than those obtained from Ra–Th mineral isochrons from the same samples. A crystal size distribution case study from Soufrière, St. Vincent, indicates that this may result from the abundance of cumulate xenocrysts within more mafic arc magmas. (b) The majority of U–Th mineral isochrons in intermediate arc magmas yield ages within error of eruption age, indicating that other dating techniques that may be more suitable for deciphering the shorter timescales yielded by these compositions. In a case study of crystals of remobilized andesites from the Soufrière Hills volcano, Montserrat, intra-crystalline trace element diffusion modeling yields crystallization timescales of the order of 10–1000 years. Varying temperature-time histories of individual crystals can be interpreted to reflect multi-stage magma ascent and the development of ephemeral subvolcanic magma reservoirs. (c) U–Th mineral isochrons from felsic arc magmas yield very variable ages from >200 ka to shortly prior to eruption. A case study from the Taupo Volcanic Zone has shown that both remobilization of previous intrusives and melting of old country rock are occurring, giving rise to a this range in ages. Both intermediate and felsic magmas therefore can be shown to have multi-stage ascent histories. Future work will have to address the reasons for the apparent inter-volcano variability in time-averaged subvolcanic magma volumes within the arc crust. These include differences in magma supply rate; thickness, composition and thermal structure of the crust; and magma composition and volatile content, which may vary between different tectonic settings.

The extent of U-series disequilibria produced during partial melting of the lower crust with implications for the formation of the Mount St. Helens dacites

The extent to which U-series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for U, Th and Ra. We show that partial melting of mafic lower crust can only produce small disequilibria between 238U, 230Th and 226Ra. Crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect. Consequently, U-series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent, unless the timescales of silicic magma production within the crust are short compared to the half-life of 226Ra. Our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust. We show that the 226Ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and/or the involvement of mantle derived basalts less than a few thousand years old. Application to Mount St. Helens suggests that crystallisation of young mantle-derived magma is likely to be the dominant process in the formation of these dacites.

U-series disequilibria in MORB from the Garrett Transform and implications for mantle melting

Abstract Here, we report 238U–230Th–231Pa–226Ra disequilibria and 87Sr/86Sr measurements in 11 mid-ocean ridge basalt (MORB) glasses from the Garrett Transform (∼13°30′S latitude on the East Pacific Rise [EPR]) whose compositions range from primitive, depleted high-MgO basalts to evolved basalts. U and Th concentrations of samples range between 3 and 75 ppb and between 6 and 220 ppb, respectively, with a corresponding large variation in Th/U (1.5–2.9). (230Th)/(232Th) varies from 1.2 to 1.6 such that (230Th)/(238U) range from ∼15% excess 230Th in a high-Th/U evolved sample to ∼25% excess 238U in a high-MgO sample with low Th/U. Out of 11 samples, 7 have 238U excess, an unusual feature for MORB. All samples have 226Ra excesses, with (226Ra)/(230Th) varying between 1.3 and 3.8 constraining ages since eruption to Comparing the Garrett Transform to the Siqueiros Transform shows a remarkable correspondence between sample setting, composition and disequilibria systematics. Both settings produce linear trends of (230Th)/(238U) as a function of Th/U, consistent with mixing between two melts derived from different depths in the melting column. The mixing relationships are identical in both locations: The most incompatible rich samples with the highest Th/U and 230Th excess come from the ridge–transform intersection (RTI), whereas the most incompatible element poor basalts with the lowest (230Th)/(238U) and Th/U are erupted along leaky transform faults. Samples with intermediate Th/U and (230Th)/(238U) all come from within intra-transform spreading centers, consistent with the spreading centers acting to homogenize these diverse magmas. The cause of variation in Th/U could reflect either melting processes or different long-lived sources. No clear indication exists within these data.

The effect of weathering regime on uranium decay series and osmium in two soil profiles

Two soil profiles from the United States with radically different emplacement and climatic histories were analyzed for U, Th and members of the 238U decay series (234U, 230Th, 226Ra, 210Pb), 137Cs and osmium isotopes. The arid New Mexico profile is developed on an approximately 250, 000 years old colluvium while the temperate New Hampshire profile is formed on till after the last glaciation at about 10, 000 years ago. Both the profiles show significant 234U/238U 230Th/234U and 226Ra/230Th disequilibria, however, in the New Hampshire profile, the disequilibria are far more pronounced in mid-depths (20–50 cm). High Os concentration with highly radiogenic 187Os/188Os is another characteristic of the mid-depths of the New Hampshire profile. This layer, particularly at about 30–40 cm depth has the characteristics of a soil developed on black shale, as evidenced from both the high U and Os concentrations and the large excess of 230Th over 238U. This profile clearly shows that the regolith on which the contemporary soil is developing was not homogeneous. The presence of measurable excess 226Ra activity over 230Th activity in both profiles suggests the need for a source of 226Ra external to the regolith in both cases. Atmospheric deposition of 226Ra is a possible source for this 226Ra excess and brings to light the important role of atmospheric deposition of nuclides and their transport in the soil profile in pedogenic processes. It also shows that regolith developed by glacial processes need not be homogeneous, thereby confounding the understanding of vertically modified soil profiles.

Ba, Ra, Th, and U in marine mollusc shells and the potential of 226Ra/Ba dating of Holocene marine carbonate shells

The geochemistry of Ba, Ra, Th, and U and the potential of using 226Ra/Ba ratios as an alternative dating method are explored in modern and Holocene marine mollusc shells. Five modern shells of the Antarctic scallop Adamussium colbecki collected from the present day beach and six radiocarbon dated specimens from Holocene beach terraces of the Ross Sea region (Antarctic) between 700 and 6100 calibrated yr BP old have been analysed by mass spectrometry. In clean shells 226Ra concentrations and 226Ra/Ba ratios show a clear decrease with increasing age, suggesting the possibility of 226Ra dating. Limiting factors for such dating are Ba and 226Ra present in surface contaminants, and ingrowth of 226Ra from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using 232Th. Sub-samples from the same shell with different proportions of contamination form a mixing line in a 226Ra/Ba– 232Th/Ba graph, and the 226Ra/Ba of the pure shell can be derived from the intercept on the 226Ra/Ba axis. Contaminant corrected 226Ra/Ba ratios of late-Holocene 14C-dated samples fall close to that expected from simple 226Ra excess decay from seawater 226Ra/Ba values. 226Ra ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of post mortem U uptake into the shell makes a correction for 226Ra ingrowth from secondary U difficult to achieve. In the A. colbecki shells, 226Ra ingrowth from such secondary U becomes significant only when ages exceed ∼2500 yr. In younger shells, 226Ra/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between 226Ra and Ba in the ocean can be confirmed for that time scale, the 226Ra/Ba chronometer may enable the reconstruction of variability in sea surface 14C reservoir ages from mollusc shells and allow its use as a paleoceanographic tracer.