Ra Disequilibria Research Articles

What Causes Excess Deepening of the Sediment Mixed Layer in the Deep Ocean?

AbstractThe sediment mixed layer (SML) in the deep ocean is an important interface with a rich diversity of benthic organisms. With increasing ocean mineral exploration, and eventual mining, the effect of sediment mixing on deep ocean ecosystems has raised considerable concern. We evaluate the distribution patterns and driving factors of SML depth in deep ocean nodule fields using naturally occurring 210Pb–226Ra isotopes. Results show that average SML depth has increased in Mn‐nodule fields since the end of the last century. SML processes are associated with significant desorption of 226Ra from sediments, resulting in a departure from radioactive equilibrium. By estimating possible driving factors, we conclude that anthropogenic exploration activities, rather than natural physical and/or biological drivers, are the most likely mechanism for intensified sediment mixing. 210Pb–226Ra disequilibria may be a potential tracer for quantifying the impact of human exploration on deep‐ocean sediment mixing and associated biological and geochemical effects.

U–Th–Ra variations in Himalayan river sediments (Gandak river, India): Weathering fractionation and/or grain-size sorting?

Understanding the origin of U–Th–Ra variations in the Ganga river sediments is a prerequisite for correctly using U-series nuclides to constrain the sediment transport times in Himalayan rivers. For this purpose, U, Th, and Ra concentrations, along with 238U–234U–230Th–226Ra radioactive disequilibria, were analyzed in bank, bedload and suspended sediments from the Gandak river, one of the main tributaries of the Ganga river. The data confirm that U and Th budgets of the Himalayan sediments are significantly influenced by minor resistant minerals, such as zircon, garnet and Ti-bearing minerals, the dissolution of which required the use of a high-pressure acid digestion process. Most importantly, the results indicate that the variations in (238U/232Th) and (230Th/232Th) activity ratios and 238U–234U–230Th–226Ra disequilibria in sediments along the river alluvial plain mainly reflect modifications in the mineralogical and grain-size compositions rather than the degree of weathering during transport. The (238U/232Th) and (230Th/232Th) activity ratios in the bank and bed sediments are related to variations in the minor primary minerals strongly enriched in U and Th (i.e., zircon, REE-bearing minerals and Ti-bearing minerals), whereas the activity ratios in the suspended load are related to variations in the proportions of clay, Fe-oxyhydroxides and the silt-sand fraction, which contains U- and Th-bearing minor minerals. The data also indicate that 238U–234U–230Th–226Ra disequilibria are strongly influenced by secondary mineral phases: the 230Th budget is likely mainly controlled by Fe-oxyhydroxides, and the 226Ra budget is likely mainly controlled by clay minerals. Therefore, the variations in the 238U–234U–230Th–232Th system in the sediments of the Gandak river cannot simply be interpreted as the result of fractionation due to chemical transformation of the bulk sediment during its transport within the alluvial plain and/or the result of radioactive decay. Consequently, they cannot be used to infer long sediment transport times within the Gandak plain (10–100ka), as previously proposed. Such analytical and interpretative artifacts are certainly not specific to the present study on the Gandak basin. These issues will certainly be encountered anytime this technique is applied to alluvial systems in which the U and Th budgets of the sediments are influenced by “heavy” minerals that can be sorted during the transport of sediments within the plain.

Regolith evolution on the millennial timescale from combined U–Th–Ra isotopes and in situ cosmogenic 10Be analysis in a weathering profile (Strengbach catchment, France)

U–Th–Ra disequilibria, cosmogenic in situ 10Be concentrations and major and trace element concentrations have been analyzed in a 2 m-deep weathering profile sampled at the summit of the granitic Strengbach catchment (France). The data have been used to independently estimate both the long-term regolith production and denudation rates and the weathering and erosion rates. Modeling of the 238U–234U–230Th–226Ra disequilibrium variations in the lower part of the profile yields a regolith production rate of 12±4 mm/kyr (30 ± 10 T/km2/yr), while modeling of the high-resolution 10Be concentration profile leads to an exposure age of 19.7±2.2 kyr, an inherited concentration of 15,000 ± 1,000 at/g in quartz and a mean denudation rate of 22 ± 10 mm/kyr (37 ± 15 T/km2/yr). The consistency between production and denudation rates suggests that, on a millennial timescale, the regolith mass balance at the summit of the catchment is close to a steady state, even if the watershed may have been impacted by Quaternary climatic changes and by recent anthropogenic perturbations (e.g., 20th century acid rain and recent afforestation efforts). The results also indicate that physical erosion is likely the dominant long-term process of regolith denudation in the catchment. Furthermore, the comparison of the long-term production and denudation rates and of weathering and erosion rates determined from the depth profile analyses with the current weathering and erosion rates estimated at the outlet of the watershed based on monitoring of the water chemistry and sediment fluxes suggests that physical erosion may have varied more than the chemical weathering flux during the last 150 kyr. Although very few other sites with U-series, in situ 10Be and stream monitoring data are available for comparison, the current data suggest that (1) the mass balance steady state of regolith might be commonly achieved in soil mantled landscapes, and (2) physical erosion has varied much more than chemical weathering in mid-mountain catchments over the last 10–150 kyr. These results highlight the importance of the combined analysis of U-series nuclides and in situ 10Be in the same weathering profile for the determination of key geomorphic parameters, which are important to constraining landscape stability and the responses of landscapes to natural or anthropogenic forcing.

Mid-ocean ridge basalt generation along the slow-spreading, South Mid-Atlantic Ridge (5–11°S): Inferences from 238U–230Th–226Ra disequilibria

U-series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid-ocean ridges. Here we present the first 238U–230Th–226Ra isotope data for a suite of 83 basalts sampled between 5°S and 11°S along the South Mid-Atlantic Ridge. This section of the ridge can be divided into 5 segments (A0–A4) and the depths to the ridge axis span much of the global range, varying from 1429 to 4514m. Previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts. Accordingly, this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials. 226Ra–230Th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their 230Th values do not require any age correction. The U–Th isotope data span a significant range from secular equilibrium up to 32% 230Th excess, also similar to the global range, and vary from segment to segment. However, the (230Th/238U) ratios are not negatively correlated with axial depth and the samples with the largest 230Th excesses come from the deepest ridge segment (A1). Two sub-parallel and positively sloped arrays (for segments A0–2 and A3 and A4) between (230Th/238U) and Th/U ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies. Despite abundant evidence for source heterogeneity, there is no simple correlation between (230Th/238U) and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions >350kyr prior to partial melting to produce the basalts. In our preferred model, the two (230Th/238U) versus Th/U arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous peridotite source regions.

Timescales of degassing and conduit dynamics inferred from 210 Pb– 226 Ra disequilibria in Volcán de Colima 1998–2010 andesitic magmas

Abstract Determining the timescales of magma degassing is essential for understanding the mechanisms controlling the eruption style and the dynamics of magmatic systems. Towards this end, we measured 210 Pb– 226 Ra disequilibria in andesite lavas erupted from Volcán de Colima between 1998 and 2010. ( 210 Pb/ 226 Ra) 0 activity ratios range from 0.86 to 1.09, and are best explained in terms of 222 Rn degassing and accumulation. The range in 210 Pb deficits indicates that the timescales of 222 Rn degassing did not exceed 11 years. 210 Pb excesses are rare and small (<10%), which signifies that 222 Rn degassing is more effective than 210 Pb accumulation in this intermediate system despite the relatively low gas output at the surface. The absence of significant 210 Pb excesses strongly suggests that the volcanic activity results from episodic ascent of small magma batches through the vapour-saturated section of the magmatic system. Overall, the degassing models based on 210 Pb– 226 Ra disequilibrium suggest an open and complex subvolcanic magmatic system comprising several conduits in which multiple magma batches reside for up to 10 years. Shifts from effusive to explosive Vulcanian eruptive phases are not related to changes in degassing mode on timescales resolvable using 210 Pb– 226 Ra disequilibria.

Regolith formation rate from U-series nuclides: Implications from the study of a spheroidal weathering profile in the Rio Icacos watershed (Puerto Rico)

A 2m-thick spheroidal weathering profile, developed on a quartz diorite in the Rio Icacos watershed (Luquillo Mountains, eastern Puerto Rico), was analyzed for major and trace element concentrations, Sr and Nd isotopic ratios and U-series nuclides (238U–234U–230Th–226Ra). In this profile a 40cm thick soil horizon is overlying a 150cm thick saprolite which is separated from the basal corestone by a ∼40cm thick rindlet zone. The Sr and Nd isotopic variations along the whole profile imply that, in addition to geochemical fractionations associated to water–rock interactions, the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts. The mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile. The upper part of the profile is probably associated with a different weathering system (lateral weathering of upper corestones) than the lower part, which consists of the basal corestone, the associated rindlet system and the saprolite in contact with these rindlets. Consequently, the determination of weathering rates from 238U–234U–230Th–226Ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile, i.e. the rindlet zone and the lower saprolite. Similar propagation rates were derived for the rindlet system and the saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of U-series nuclides in the rindlet-saprolite subsystem. The consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that, in the Rio Icacos watershed, the weathering system has reached a geomorphologic steady-state. Our study also indicates that even in environments with differential weathering, such as observed for the Puerto Rico site, the radioactive disequilibrium between the nuclides of a single radioactive series (here 238U–234U–230Th–226Ra) can still be interpreted in terms of a simplified scenario of congruent weathering. Incidentally, the U–Th–Ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered.

Shelf‐basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.

Constraints on crystal storage timescales in mixed magmas: Uranium-series disequilibria in plagioclase from Holocene magmas at Mount Hood, Oregon

Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales, especially in cases where crystals may derive from multiple sources. We report here 230Th–226Ra model ages of two distinct populations of plagioclase from low silica dacites from Mount Hood, Oregon, a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge, mixing, and incorporation of plagioclase derived from mafic and silicic end-member magmas. We have measured trace element concentrations and 238U–230Th–226Ra disequilibria in four plagioclase size fractions from the Timberline (1500a) and Old Maid (215a) eruptive sequences. After correction for groundmass and apatite contamination, average 230Th–226Ra model ages of large (>500μm) plagioclase are >4.5ka (Timberline) and >5.5ka (Old Maid), with ages of cores that are >10ka in each case, indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions. These model ages are longer than timescales of repose between eruptions, indicating that these crystals resided in the sub-surface over multiple eruptions, likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas, remobilizing a fraction of the large plagioclase crystals during each eruptive event. After correction for large plagioclase contamination, small (<500μm) plagioclase, derived from mafic magmas, have high (226Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusion), leading to 230Th–226Ra model ages that are <~3ka for Old Maid and undefined for Timberline separates. However, the preservation of significant 230Th–226Ra disequilibria require that the majority of crystals in the separate are young (<<10ka). The high (226Ra)/[Ba] could potentially be explained by rapid crystallization immediately prior to and/or during mixing events, consistent with evidence of rapid crystallization of rims. Rapid crystallization of mafic intrusions may trigger eruption at Mount Hood by producing a partially-crystalline mafic magma capable of mixing with a reheated silicic crystal mush.

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

Measurements of 238U– 230Th– 226Ra disequilibria, Sr–Nd–Pb–Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr–Nd–Pb–Hf isotopes ( 87Sr/ 86Sr = 0.70319–0.70322; 143Nd/ 144Nd = 0.51302–0.51305; 206Pb/ 204Pb = 19.04–19.06; 207Pb/ 204Pb = 15.53–15.54; 208Pb/ 204Pb = 38.61–38.65; 176Hf/ 177Hf = 0.28311–0.28312). All the samples possess ( 230Th/ 238U) disequilibrium with 230Th excesses, and they show systematic variations in ( 230Th/ 232Th) and ( 238U/ 232Th) ratios. The highest 226Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have ( 226Ra/ 230Th) ratio that are close to equilibrium. The 238U– 230Th– 226Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of 230Th and 226Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent–daughter nuclides in 230Th– 238U and 226Ra– 230Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.

Crustal and mantle influences and U–Th–Ra disequilibrium in andesitic lavas of Ngauruhoe volcano, New Zealand

The andesitic volcano Ngauruhoe, which is located within the Tongariro Volcanic Complex at the southern end of the Taupo Volcanic Zone in North Island, New Zealand, has been constructed over the past 5 ka and last erupted in 1975. Nearby Ruapehu volcano has a much longer eruptive history extending back beyond 230 ka B.P. The magmas erupted at both volcanoes have been predominantly medium-K basaltic andesites and andesites, which evolved through polybaric crystal fractionation and assimilation processes that took place within complex, dispersed magmatic storage systems. Despite their close spatial proximity, the two volcanoes show geochemical contrasts suggesting that in each case both the mantle-derived parental magmas and the crustal assimilants were different. Variations in major and trace element data for Ngauruhoe lavas indicate control by crystal fractionation and assimilation (AFC) but the data are difficult to reconcile with derivation from a single batch of compositionally unique, mantle-derived parental basalt. Geochemical variation can be approximated by generalised AFC models using average basement or crustal xenolith compositions but precise mathematical modelling is limited because of the sensitivity of models to the selection of a particular parental composition and the difficulty of determining the exact nature of the crustal assimilant compositions that might have been involved in each specific case. U–Th isotopic data for Ngauruhoe volcanic rocks show disequilibrium that defines a positively inclined array lying to the right of the equiline on a 230Th/ 232Th versus 238U/ 232Th diagram. U-series data for Ngauruhoe and post 1945 AD Ruapehu eruptives show similar patterns of disequilibrium but the Ngauruhoe data define an array with a different slope and a different intercept on the equiline. 230Th/ 238U ratios in Ngauruhoe lavas range from close to secular equilibrium (0.979) to 0.864 with the youngest (post 1870 AD) eruptives showing the least disequilibrium. 230Th/ 238U and 87Sr/ 86Sr ratios are correlated; the samples with the highest 87Sr/ 86Sr values show the least disequilibrium. There is also a correlation between eruption age and Nd and Pb isotopic composition; the oldest samples tend to show the least radiogenic isotopic ratios. These data, in combination with major and trace element abundance data indicate that Ngauruhoe U–Th disequilibrium is determined by variations in parental magmatic compositions and AFC. Unlike the mature, long-lived Ruapehu system, Ngauruhoe's eruptive cycles can be directly connected to periodic magma recharge from the lower crust and mantle. For Ngauruhoe eruptives, 226Ra/ 230Th varies from 1.218 to 1.492. This compares with a range of 0.972 to 1.186 for Ruapehu lavas erupted between 1945 and 1996. Reconciling the 226Ra/ 230Th data with decay during fractional crystallisation requires relatively low rates of fractionation (1–5 × 10 − 5 /year) and short (1000–2000 years) fractionation times.

A new method for U–Th–Pa–Ra separation and accurate measurement of 234U– 230Th– 231Pa– 226Ra disequilibria in volcanic rocks by MC-ICPMS

A new method for the chemical separation and MC-ICPMS measurements of U–Th–Pa–Ra disequilibria in volcanic samples from a single sample aliquot is presented. The accuracy and precision of our techniques is assessed by replicate analyses of the synthetic U and Th isotope standards IRMM-184 and IRMM-35, and the secular equilibrium rock standards BCR-2, W-2a and TML. A measure of true sample reproducibility including the errors related to sample processing and the chemical separation is obtained from analysis of multiple digestions of the rock standards. 234U/ 238U, 230Th/ 238U, 231Pa/ 235U and 226Ra/ 230Th activity ratios on ten separate dissolutions of USGS rock standard BCR-2, for example, reproduce to 0.4%, 1.4%, 1.1% and 2.4% (2 SD), respectively. All elements (U–Th–Pa–Ra) are separated from a single spiked sample aliquot and the elemental concentrations are measured on the same solutions as the isotopic compositions. Analytical artefacts from determination of elemental and isotopic concentrations on different sample aliquots due to sample heterogeneity are therefore avoided. In addition, our new chemical separation protocol is time-efficient and allows for rapid processing of volcanic samples with less than 25 fg of Pa and Ra.

Reappraisal of fluid and sediment contributions to Lesser Antilles magmas

We present new U-series disequilibrium and radiogenic isotope data for 7 mafic lavas from the Lesser Antilles arc. These are combined with published data in an internally consistent model that quantitatively estimates the amount of sediment and fluid added to the source of the Lesser Antilles arc system. Some lavas form an array consistent with bulk sediment addition (0.2–2%) whereas others appear to require addition of 0.4–2% sediment melt, particularly in the south of the arc. Evidence for both bulk sediment and sediment melt addition can be found within both the northern and central sections of the arc suggesting a thermal structure whereby the upper portions of the subducted sediment pile lie close to their solidus beneath much of the arc. Addition of up to 5% fluid derived from altered oceanic crust to these sediment enriched mantle wedge source regions can simulate the majority of the lavas on a plot of 207Pb/ 204Pb versus Ce/Pb. By taking into account the range in calculated wedge compositions and allowing for some mobility of Th in the fluid, the same model can also account for much of the observed range of U–Th–Ra disequilibria, especially if the eclogitic residue contains trace amounts of rutile. The implication of this more complex model is that the time scales for fluid addition and differentiation could be significantly shorter than those estimated in some previous studies.

Gas transport model for the magmatic system at Mount Pinatubo, Philippines: Insights from ( 210Pb)/( 226Ra)

Measurements of 226Ra– 210Pb disequilibria in eruptive products have the potential to track the accumulation or loss of volatiles in magmatic systems on timescales of decades because the intermediate nuclide, 222Rn, follows the gas phase. We present measurements of 210Pb– 226Ra disequilibria for whole-rock samples representing a time sequence of tephra through the June 15, 1991 cataclysmic eruption of Mount Pinatubo volcano, Philippines. Mount Pinatubo volcano is a dacitic system that did not significantly vent gases at the surface prior to eruption, and we can, therefore, isolate the 210Pb– 226Ra disequilibria due to gas accumulation without complications due to 222Rn loss during degassing. Pyroclastic samples have ( 210Pb)/( 226Ra) 0 ranging from 1.01 to 1.10; averaging 1.06. A sample of the post-climactic dome has ( 210Pb)/( 226Ra) 0 = 1.12. Previous uranium-series degassing studies have suggested that 210Pb excesses are created by rapid volatile transport (carrying the intermediate daughter 222Rn) and subsequent volatile accumulation and decay of 222Rn to 210Pb. However, bubbles in viscous dacite magma cannot rise at speeds needed to provide a flux of 222Rn large enough to cause measurable disequilibria in the 210Pb– 226Ra system. In addition, there is little evidence for magmatic sources large enough to supply a rapid flux of 222Rn. Therefore, we present a model in which 210Pb– 226Ra disequilibria is established during basaltic recharge of the Pinatubo reservoir. The relatively low viscosity of basaltic magma allows for differential gas motion and the production of 210Pb excess in localized basaltic melt. Transport of volatiles and 210Pb-rich basalt through a crystal matrix and the formation of bubble plumes in the dacitic reservoir produces a mixed magma with 210Pb excess. Through this mechanism, the timescale of gas transport and accumulation is constrained, not by the half-life of 222Rn (3.8 days), but rather by the half-life of 210Pb (22.6 years). Bubble plume motion preserves disequilibria and creates a zone of eruptable dacite with 210Pb excess alleviating the need for gas transport on very short time-scales. Using the rate of decay of 210Pb coupled with published trace element data, we present a quantitative investigation of this new conceptual model and propose that changes in 210Pb values with time may suggest changing conditions of magma supply at volcanoes.

Mantle melting and magma supply to the Southeast Indian Ridge: The roles of lithology and melting conditions from U-series disequilibria

Intermediate-spreading Southeast Indian Ridge (SEIR) basalts display geographic gradients from 90°E to 118°E in Th/U, ( 230Th/ 232Th) and 238U– 230Th– 226Ra disequilibria ( 230Th-excesses of 1 to 21% and 226Ra-excesses of 0 to 160%). Highly correlated ( 238U/ 232Th) and ( 230Th/ 232Th) ( r 2 = 0.94) span nearly the entire global MORB range; basalts from three of four ridge morphologies form subparallel, vertically stacked arrays on an equiline diagram. ( 226Ra/ 230Th) is inversely correlated with ( 230Th/ 238U) within sub-regions of the study area. There is a convex ( 230Th/ 238U) along-axis pattern (lower 230Th-excesses in the west and east, at shallowest and deepest depths), such that ( 230Th/ 238U) does not correlate with axial depth as described elsewhere, despite the ~ 2300 m west-to-east axial depth increase. κ and κ Pb also vary along axis from κ < κ Pb in the west to κ ≅ κ Pb in the east. Variations in axial depth, crustal thickness, U-series disequilibria, κ and κ Pb are best explained by forward model scenarios in which the physical characteristics of melting (such as melting rate, degree of melting, porosity and melt initiation depth) vary along axis in response to an inferred long wavelength temperature gradient and small variations in the underlying source lithology (pyroxenite veins). This leads to decreased melt supply from west to east, a concomitant change in axial morphology and axial magma chamber depth, and systematic long wavelength geochemical gradients. Melting rate covaries with melt supply in the western and central regions, decreasing eastward along the SEIR, but supply and rate vary inversely from the central to the eastern regions, because cooler eastern mantle contains a few percent of fusible pyroxenite veins that melt productively to generate melts with high Th–U–Ra concentration but low 230Th-excess and 226Ra-excess. The veins contribute only a small proportion of the total melt volume from mantle that is > 95% peridotite. Th, Pb and He isotopic gradients are consistent with a changing proportion of enriched pyroxenite veins along axis having a roughly 0.5 to 1 Gyr reservoir age.

238U‐230Th‐226Ra‐210Pb‐210Po, 232Th‐228Ra, and 235U‐231Pa constraints on the ages and petrogenesis of Vailulu'u and Malumalu Lavas, Samoa

We report 238U‐230Th‐226Ra‐210Pb‐210Po, 232Th‐228Ra and 235U‐231Pa measurements for a suite of 14 geologically and geochemically well‐characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent‐daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas (&lt;8 Ka) resurfacing both its summit crater and lower flanks. 210Pb and 210Po measurements indicate that several flows have erupted within its summit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half‐lives of 230Th, 231Pa, and 226Ra, we interpret their 238U ‐230Th, 235U‐231Pa and 230Th ‐ 226Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. (230Th/238U) &gt; 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (238U/232Th) and (230Th/232Th) is attributed to long‐term source variation. The Samoan basalts are all alkaline basalts and show significant 230Th and 231Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (230Th/238U), (231Pa/235U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that (230Th/238U) and (231Pa/235U) of many OIBS can be explained by simple time‐independent models. Interpretation of the 226Ra data requires time‐dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U‐Th‐Ra and U‐Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant 210Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these 210Pb deficits. The young coeval volcanism on Malumalu and Vailulu'u suggests the Samoa hot spot is currently migrating to the northeast due to dynamic interaction with the Tonga slab.

Experimental study of radium partitioning between anorthite and melt at 1 atm

We present the first experimental radium mineral/melt partitioning data, specifically between anorthite and a CMAS melt at atmospheric pressure. ^(226)Ra disequilibria are an important chronometer of recent magmatic activity. Ion microprobe measurement of coexisting anorthite and glass phases produces a molar D_Ra = 0.040 ± 0.006 and D_(Ra)/D_(Ba) = 0.24 ± 0.05 at 1400 °C. Our results indicate that lattice strain partitioning models fit the divalent (Ca, Sr, Ba, Ra) partition coefficient data of this study well, supporting previous work on crustal melting and magma chamber dynamics that has relied on such models to approximate radium partitioning behavior in the absence of experimentally determined values.

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.

Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages

Research Article| May 01, 2007 Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages Kenneth W.W. Sims; Kenneth W.W. Sims 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Robert P. Ackert, Jr.; Robert P. Ackert, Jr. 2Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Frank C. Ramos; Frank C. Ramos 3Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA Search for other works by this author on: GSW Google Scholar Robert A. Sohn; Robert A. Sohn 4Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Michael T. Murrell; Michael T. Murrell 5Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Search for other works by this author on: GSW Google Scholar Donald J. DePaolo Donald J. DePaolo 6Department of Earth and Planetary Science, MC4767, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (5): 471–474. https://doi.org/10.1130/G23381A.1 Article history received: 20 Sep 2006 rev-recd: 26 Dec 2006 accepted: 04 Jan 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Kenneth W.W. Sims, Robert P. Ackert, Frank C. Ramos, Robert A. Sohn, Michael T. Murrell, Donald J. DePaolo; Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages. Geology 2007;; 35 (5): 471–474. doi: https://doi.org/10.1130/G23381A.1 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 SocietyGeology Search Advanced Search Abstract We present 238U-230Th -226Ra disequilibria and cosmogenic 3He and 36Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The 238U-230Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/–20 ka; 2σ). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the 3He concentration (47.5 ± 5 ka; 2σ) and the 36Cl concentration (41.2 ± 8.8 ka; 2σ) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the 3He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model 36Cl age and, as a result, the 36Cl age is significantly younger than the U-Th isochron age and erosion-corrected 3He ages; this discordance is attributed to a lack of closed-system behavior in the 36Cl system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tracing pre-eruptive magma degassing using ( 210Pb/ 226Ra) disequilibria in the volcanic deposits of the 1980–1986 eruption of Mount St. Helens

Disequilibria between 210Pb and 226Ra can be used to trace magma degassing, because the intermediate nuclides, particularly 222Rn, are volatile. Products of the 1980–1986 eruptions of Mount St. Helens have been analysed for ( 210Pb/ 226Ra). Both excesses and deficits of 210Pb are encountered suggesting rapid gas transfer. The time scale of diffuse, non-eruptive gas escape prior to 1980 as documented by 210Pb deficits is on the order of a decade using the model developed by Gauthier and Condomines (Earth Planet. Sci. Lett. 172 (1999) 111–126) for a non-renewed magma chamber and efficient Rn removal. The time required to build-up 210Pb excess is much shorter (months) as can be observed from steady increases of ( 210Pb/ 226Ra) with time during 1980–1982. The formation of 210Pb excess requires both rapid gas transport through the magma and periodic blocking of gas escape routes. Superposed on this time trend is the natural variability of ( 210Pb/ 226Ra) in a single eruption caused by tapping magma from various depths. The two time scales of gas transport, to create both 210Pb deficits and 210Pb excesses, cannot be reconciled in a single event. Rather 210Pb deficits are associated with pre-eruptive diffuse degassing, while 210Pb excesses document the more vigorous degassing associated with eruption and recharge of the system.

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.