226Ra Excesses Research Articles

UThRa systematics in Kilauea and Mauna Loa basalts, Hawaii

We describe 226Ra 230Th 238U (dis)equilibria and RbBaThULa concentration relationships in historical and prehistoric lavas from Kilauea and Mauna Loa. ( 226 Ra 230 Th ), ( 230 Th 232 Th ), Th/U, Ba/Rb and Ba/La ratios [but not (Rb,Ba,La)/(Th,U) ratios] are essentially identical in both volcanoes, whereas the absolute concentrations (after correction for olivine crystallization) differ by up to a factor of 2, in response to varying melt fractions. This shows that bulk partition coefficients of these elements are significantly smaller than melt fractions. Very small or absent 230Th 238U disequilibrium implies very small or negligible magmatic fractionation between Th and U. 226Ra 230Th disequilibria are significantly larger (∼ 20% excess 226Ra on average) but are also independent of melt fraction. The combination of significant RaTh fractionation together with small or absent ThU fractionation provides constraints on recently proposed models to explain U-series disequilibria during partial melting and melt extraction. Instantaneous melt extraction models are rejected: (a) because they are inconsistent with experimentally determined partition coefficients; and (b) more generally because they would require significant covariation of ( 226 Ra 230 Th ) with melt fraction. On the other hand, dynamic melting models involving slow fractional melting or melt infiltration within the garnet stability region, followed by rapid movement through the lithosphere, are consistent with the results and yield melt porosities between 10 −3 and 10 −2 for plume upwelling velocities of 1 m yr −1. In addition, we tentatively proposed alternative models for creating the Ra excesses in the magma. One such process involves the mobilization of Ra within the volcanic edifice, subsequent advection toward and redeposition within the roof region of the magma chamber, and finally incorporation into the magma itself. Another mechanism for incorporating excess Ra in the magma might be transport of very small amounts of carbonate fluids or carbonatite melts (containing very large excesses of 226Ra) into partially molten regions in the mantle. Given the currently available data and state of knowledge about magma extraction processes, there is no obvious preference for either the purely magmatic models or those involving “extraneous” fluids in the mantle or within the volcanic edifice.

Consequences of melt transport for uranium series disequilibrium in young lavas

Radioactive disequilibrium of 238U nuclides is commonly observed in young lavas and has often been used to infer the rates of melting and melt migration. However, previous calculations do not actually include melt transport. Here we explore the behaviour of short-lived radionuclides in a new calculation that includes the fluid dynamics of melt segregation. We emphasize that series disequilibrium results from the differences in residence time of parent and daughter nuclides. Unlike previous models, contrasts in residence times are controlled by differences in transport velocities caused by melt separation and continued melt-solid interaction throughout the melting column. This “chromatographic” effect can produce larger excesses of both 230Th and 226Ra within the same physical regime compared to previous calculations which do not include melt transport. Using this effect to account for U-series excesses leads to radically different inferences about the rates of melt migration. Where previous models require rapid melt extraction, our calculation can produce larger excesses with slow melt extraction. Nevertheless, reproducing the large ( 226Ra/ 230Th) activity ratios observed in fresh mid-ocean ridge glasses is still problematic if the residence times are controlled solely by bulk equilibrium partitioning. While it still remains to be shown conclusively that the large 226Ra excesses are produced during melting, our calculation only requires differences in transport velocities to produce secular disequilibrium. Thus we speculate that other processes, such as crystal surface interaction, may also contribute to the production of the observed excesses.

238U-230Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites

The paper describes 238U-series nuclides and 230Th/232Th ratios measured by mass spectrometry in mineral separates of young Mt. Shasta andesites and dacites. The results constrain the timing of recent calc-alkaline magma fractionation at this volcano. Hotlum, Misery Hill and Black Butte rocks show small, < 13% 230Th-238U and < 6% 226Ra-230Th, disequilibria. Plagioclase have 7–26% 226Ra excesses, magnetite and groundmass have 4–5% 226Ra deficits, and pyroxenes have equilibrium (226Ra/230Th) activity ratios. Internal (230Th)-(238U)and Ba-normalized (226Ra)-(230Th) isotope diagrams for Hotlum and Black Butte dacites suggest that closed-system Th-U and Ra-Th fractionation occurred less than 10,000 years ago. Significant 226Ra-230Th disequilibria in the Black Butte dacite strongly suggests that this rock erupted more recently than 9400 years ago. Results for Hotlum andesites suggest a longer pre-eruption crystal residence time compared to the dacites. There may also have been recent open Ra-Th system changes in the melt composition. Initial Th/U ratios for the rocks are low (2.43–2.57), similar to those in mid-ocean ridge basalts (MORB), and preclude significant assimilation of crust with markedly different Th-U composition.

238U- and 232Th-series chronology of phonolite fractionation at Mount Erebus, Antarctica

Uranium, thorium, radium, and barium abundances and 234U 238U and 230Th 232Th isotopic ratios determined by thermal ionization mass spectrometry and ( 228Th) ( 232Th ) activity ratios determined by alpha spectrometry are used to date anorthoclase growth and infer magma chamber residence times of phonolites erupted in 1984 and 1988 from Mount Erebus, Antarctica. The 1984 and 1988 glasses have slightly different ( 230Th) ( 232Th ) ratios but both have a 10% excess of ( 230Th) over ( 238U) and equilibrium ( 228Th) values. By comparing these data and Pb-isotopic data reported in Sun and Hanson (1975) to similar data for oceanic basalts, the duration of differentiation from basanite to phonolite is limited to less than 150,000 years. The anorthoclase separates have ( 230Th) ( 238U ) ratios exceeding those of the associated glasses but have ( 230Th) ( 232Th ) ratios like those of the glasses. Both glasses are depleted in 226Ra with respect to 230Th by about 25%, whereas associated anorthoclase separates have extreme excesses of 226Ra over 230Th and ( 228Th) ( 232Th) = 2.2. On a plot of ( 226Ra)/Ba vs. ( 230Th)/Ba, the glass-anorthoclase pairs produce isochrons averaging 2380 y, which represents the average age of anorthoclase growth in the shallow magma system at Erebus. The implied residence time of phonolite magmas in the shallow magma chamber system of Erebus is about 3000 y. Final crystal growth occurred after intrusion into the converting lava lake less than decades before eruption.

The behavior of the uranium decay chain nuclides and thorium during the flank eruptions of Kilauea (Hawaii) between 1983 and 1985

The concentrations of members of the 238U decay chain and 232Th have been determined for the lavas that erupted on the East Rift Zone of Kilauea Volcano, Hawaii (Puu Oo) between January 1983 and January 1985. There was a decrease during the first 180 days in the abundances of all nuclides, following the behavior of the incompatible elements. (230Th/238U) varies with (232Th/238U) yielding a batch process age for the source magma of 127,800 ±28,500 (2σ) y, similar to East Pacific Rise basalts. No (226Ra/230Th) disequilibrium was evident at Puu Oo although Haleakala and Loihi show significant excesses of (226Ra) over (230Th). The initial (210Pb) excess relative to (226Ra) implies strong incompatibility of 210Pb probably with the help of chloride complexing, and the deficiency in later episodes indicates volatilization from the melt mediated by the formation of volatile chloride compounds.

238U [sbnd]230Th [sbnd]226Ra disequilibria in young Mount St. Helens rocks: time constraint for magma formation and crystallization

We use 238U-series nuclides and 230Th/ 232Th ratios measured by mass spectrometry to constrain processes and time scales of calc-alkaline magma genesis at Mount St. Helens, Washington. Olivine basalt, pyroxene andesites and dacites that erupted 10–2 ka ago show 3–14% ( 230Th) ( 238U) and 6–54% 226Ra 230Th disequilibria. Mineral phases exhibit robust ( 226Ra) ( 230Th) fractionation. Plagioclase has large 65–280% ( 226Ra) excesses, and magnetite has large 65% ( 226Ra) deficits relative to ( 230Th). Calculated partition coefficients for Ba, Th, and U in mineral-groundmass pairs, except Ba in plagioclase, are low (⩽ 0.04). Correlation between ( 226Ra/ 230Th ) activity ratios and rm/BaTh element ratios in the minerals suggests that 226Ra partitions similar to Ba during crystallization. Internal ( 230Th) ( 238U) isochrons for 1982 summit and East Dome dacites and Goat Rocks and Kalama andesites show that closed Th U system fractionation occurred 2–6 ka ago. Apparent internal isochrons for Castle Creek basalt (34 ka) and andesite (27 ka) suggest longer magma chamber residence times and mixing of old crystals and young melt. Mineral ( 226Ra) ( 230Th) disequilibrium on Ba-normalized internal isochron diagrams suggests average magma chamber residence times of 500–3000 years. In addition, radioactive ( 226Ra/ 230Th ) heterogeneity between minerals and groundmass or whole rock is evidence for open-system Ra Th behavior. This heterogeneity suggests there has been recent, post-crystallization, changes in melt chemical composition that affected 226Ra more than 230Th. Clearly, magma fractionation, residence and transport of crystal-melt before eruption of chemically diverse lavas at Mount St. Helens occurs over geologically short periods.

226Ra excesses in mid-ocean-ridge basalts and mantle melting

The discovery of radioactive disequilibrium among isotopes of the 238U decay chain in recent volcanic rocks1–7 has enhanced our understanding of both the timescale and processes of melting in the mantle. Excesses of 230Th (half life 75,200 yr) relative to its parent 234U in some of these rocks indicate that thorium is preferentially partitioned into the melt phase, and constrain the combined duration of melting and magma transport to less than a few hundred thousand years. Recent observations of 226Ra and 228Ra excesses over their respective parents 230Th and 228Th in individual subaerial volcanoes have led to even more precise determinations of the timescale of magmatic processes3,6. Here we report the observation and implications of 226Ra (half life 1,620 yr) excesses in mid-ocean-ridge basalts. These excesses are probably produced during melting in the mantle, and the timescale they impose suggests that they are controlled by kinetic processes, rather than chemical equilibrium. Regardless of the process, the 226Ra excesses constrain the length of the magma transfer times from melting through eruption (and sample collection) to less than about a thousand years.

Uranium series dating of Allan Hills ice

Uranium 238 decay series nuclides dissolved in Antarctic ice samples were measured. Ice from the Allan Hills, Cul de Sac site, that contains a large concentration of fine volcanic glass shards, has high 226Ra, 230Th, and 234U activities but similarly low 238U activities compared to Antarctic ice samples without volcanic shards. The 226Ra, 230Th, and 234U excesses are proportional to the shard content. The 238U decay series results are consistent with the assumption that alpha decay products recoiled into the ice from the fine shards. The age of the Cul de Sac ice is (325±75) × 103 yr from this method of uranium series dating.

Radionuclide migration over recent geologic time in a granitic pluton

Measurements of isotope activity ratios in the 238U decay series have been made for core samples obtained from boreholes up to 1-km depth in the Eye-Dashwa Lakes granite pluton, Atikokan, northwest Ontario, Canada. This work was done to determine the significance of recent radionuclide migration in plutonic rocks for the Canadian Nuclear Fuel Waste Management Programme. Most samples displayed secular equilibrium between 238U, 234U, 230Th and 226Ra, indicating that there has been no significant radionuclide migration in these samples over periods as long as the last 1 Ma. Disequilibrium was observed in some altered near-surface and deeper fracture-filling mineral samples with 234U deficiencies of up to 20%. A similar deficiency of 226Ra was found at ∼ 1-km depth in a gypsum-infilled core sample. These results indicate that U and Ra leaching by groundwater has occurred within periods 1 Ma and 8 ka ago, respectively. One instance of 226Ra excess was found at 94-m depth, indicating deposition of 226Ra in the last 8 ka. All cases of disequilibrium involved fracture-infilling minerals or altered granite cores from fracture zones, indicating that radionuclide migration occurred via fractures. Examples were found, however, of altered samples or mineral infillings that showed no detectable disequilibrium. These locations have been closed to migration in the last 1 Ma. Relative age estimates are made for some of the infilling minerals, based on their U-series isotopic characteristics.

Uranium, thorium, and radium in ocean water and deep-sea sediments

From the data of 25 deep-sea cores it is possible to show that the U/Th ratio of sediments is nearly constant at a low CaCO 3-content. This result and reflections about some other points have led to the assumption that U and Th are being transported into the sediment through the intermediary of minerals. During the transport U (preferably 234U), Ra, and some Th are leached by ocean water. This model provides for a possibility to interpret the activity ratio 234U/ 238U< 1 in deep-sea sediments with low carbonate content as well as the activity ratio 228Th/ 232Th∼ 15 in ocean water. Likewise a part of the 226Ra excess (compared with the Io in solution) may be caused by that leaching.