230Th Excesses Research Articles

A resilient ice cover over the southernmost Mendeleev Ridge during the late Quaternary

The presence of a late Quaternary ice sheet/ice shelf over the East Siberian Sea has been proposed in several papers. Here, we further document its duration/resilience based on the sedimentary, bulk mineralogical, and geochemical (organic matter content and its stable isotopic composition, U‐Th series) properties of a core raised from the southernmost Mendeleev Ridge. The chronostratigraphy of the studied core was mainly built from the 230Th excess (230Thxs) distribution and decay downcore. At the core‐top, peaking 230Thxs values during the early MIS 3 and mid‐MIS 1 encompassing an MIS 2 hiatus were observed. As documented in several papers, these peaks suggest seasonally open ice conditions over proximal continental shelves. Below, the interval spanning MIS 4 and possibly MIS 5d records major ice‐rafting events illustrated by overall high coarse‐fraction contents. Underlying MIS 5e, down to MIS 11, the sediment depicts relatively low sand (1.7±2.5 dw%), high clay (33.5±4.7 dw%), and very low organic carbon (0.10±0.06 dw%) contents, and low δ13Corg values (−24.3±0.9‰). This section is interpreted as recording fine sediment transport by deep currents and/or meltwater plumes below a resilient ice cover, only interrupted by a few short‐duration events. These events include (i) detrital carbonate pulses assigned to deglacial events along the NW Laurentide Ice Sheet margin (Termination (T) III), and (ii) intervals with some planktonic foraminifer occurrences, likely relating to their advection from open areas of the Arctic Ocean (MIS 5e, 9 and 11). All Terminations, but TII and the early MIS 3, show peaking Mn/Al values linked to the submergence of Arctic shelves under a rising sea level. We conclude that the resilient ice cover, likely an ice shelf, has been present over the southern Mendeleev Ridge during most of the interval after the Mid‐Pleistocene Transition and was favoured by the low summer insolation of the MIS 14 to 10 interval.

Challenging the hypothesis of an Arctic Ocean lake during recent glacial episodes

ABSTRACTThe Arctic Ocean is one of the last frontiers on Earth with many unknowns about its geological and climate history and considerable speculation on its role in the Earth's climate and ocean system. It has been proposed recently that it was occupied by a freshwater body of more than 9.5 × 106 km3 underneath a thick ice mass during part of glacial isotopic stages 6 and 4. We argue that such a dramatic scenario, implying replacement of marine waters by freshwater throughout the entire Arctic Ocean and Nordic Seas, is physically implausible. The very low 230Th excesses (230Thxs) observed in sediments from these intervals were used as evidence for the presence of a U‐depleted overlying freshwater column. We show here that they may simply result from short, sporadic sedimentary pulses, below multiyear sea ice or ice shelves, linked to deglacial ice streaming and surging events interrupting long‐duration sedimentary gaps. Due to this sporadic sedimentation regime, interpolating time from 230Thxs data or between benchmark ages in sedimentary sequences would simply be erroneous.

Importance of permeability and deep channel network on the distribution of melt, fractionation of REE in abyssal peridotites, and U-series disequilibria in basalts beneath mid-ocean ridges: A numerical study using a 2D double-porosity model

A plethora of observations have been made at mid-ocean ridges and mantle sections of ophiolites: presence of tabular replacive dunites, highly depleted LREE in residual abyssal peridotites, U-series disequilibria in fresh basalts, crustal thickness, and highly attenuated seismic and magnetotelluric structures beneath spreading centers. These independent observations can become more powerful if they are jointly analyzed in a self-consistent model. The difficulty for such a model is to resolve and evaluate the effect of fine scale petrologic feature such as dunite channels in a tectonic scale geodynamic model. Here we present a two-dimensional double-porosity ridge model that consists of low-porosity residual lherzolite and harzburgite matrix and high-porosity interconnected channel network. The geodynamic simulation features a spatially distributed channel network and anisotropic permeability that gradually develops as the upwelling mantle is deformed. We compute the porosity, melt and solid flow fields for several choices of channel distribution and permeability model. We use the calculated porosity distribution and velocity fields to model the variations of REE in residual mantle and U-series disequilibria in melts.The present study underscores the importance of deep channel networks and permeability model to the interpretation of first order geophysical and geochemical observations at mid-ocean ridge spreading centers. The anisotropic permeability of channels can enhance melt focusing by 60%, resulting in thicker crust. The attenuated seismic and magnetotelluric structures require a channel network starting from 60 km depth beneath the ridge axis. The depleted REE patterns in clinopyroxenes in residual abyssal peridotites are also consistent with melt extraction into channels starting from 60 km depth. Although deep channels are conducive to producing U-series disequilibria in eruptible melts, the present model still cannot explain the full ranges of the observed U series disequilibria data in MORB samples. Additional factors, processes, and models are discussed. And finally, we found that, within the uncertainty of the permeability, the porosity varies by three folds and the excess of 230Th varies by up to two folds.

U-series disequilibria of trachyandesites from minor volcanic centers in the Central Andes

Young trachyandesite lavas from minor volcanic centers in the Central Andes record the magma differentiation processes at the base of the lower continental crust. Here we report U-series disequilibrium data for the historical lavas from the Andagua Valley in Southern Peru to define the time-scale and processes of magmatism from melting in the mantle wedge to differentiation in the crust. The Andagua lavas show (230Th)/(238U), (231Pa)/(235U), and (226Ra)/(230Th) above unity except for one more evolved lava with 230Th depletion likely owing to fractional crystallization of accessory minerals. The 226Ra excess indicates that the time elapsed since magma emplacement and differentiation in the deep crust is within 8000years. Based on the correlations of U-series disequilibria with SiO2 content and ratios of incompatible elements, we argue that the Andagua lavas were produced by mixing of fresh mantle-derived magma with felsic melt of earlier emplaced basalts in the deep crust. Because of the lack of sediment in the Chile-Peru trench, there is no direct link of recycled slabs with 230Th and 231Pa excesses in the Andagua lavas. Instead, 230Th and 231Pa excesses are better explained by in-growth melting in the upper mantle followed by magma differentiation in the crust. Such processes also produced the 226Ra excess and the positive correlations among (226Ra)/(230Th), Sr/Th, and Ba/Th in the Andagua lavas. The time-scale of mantle wedge melting should be close to the half-life of 231Pa (ca. 33ka), while it takes less than a few thousand years for magma differentiation to form intermediate volcanic rocks at a convergent margin.

210Pb-226Ra disequilibria in young gas-laden magmas

We present new 238U-230Th-226Ra-210Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant 238U and 226Ra excesses, whereas those from the ocean islands have moderate 230Th and 226Ra excesses, reflecting mantle melting in the presence of a water-rich fluid in the former and mantle melting by decompression in the latter. Differentiation to erupted compositions in both settings appears to have taken no longer than a few millennia. Variations in the (210Pb/226Ra)0 values in all settings largely result from degassing processes rather than mineral-melt partitioning. Like most other ocean island basalts, the Terceira basalt has a 210Pb deficit, which we attribute to ~8.5 years of steady 222Rn loss to a CO2-rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium (210Pb/226Ra)0 values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of 222Rn in a gas phase. We infer that degassing during decompression of water-saturated magmas either causes these magmas to crystallize and stall in reservoirs where they reside under conditions of near stasis, or to quickly rise towards the surface and erupt.

Contrasting magmatic cannibalism forms evolved phonolitic magmas in the Canary Islands

Volcanic sequences on ocean islands record the temporal evolution of underlying magmatic systems and provide insights into how silicic crust is produced away from convergent margins. Assimilation has often been suspected to contribute, but the detection of such a process and its evolving maturity during migration across a mantle plume is less well documented. Here we present new major and trace element and Sr-Nd-Pb-U-Th-Ra-Pa isotope data that facilitate comparison of basanite to phonolite evolution on Tenerife (Canary Islands) with that shown by published data from La Palma. On both islands, (230Th/238U) ratios decrease with differentiation from parental magmas with 230Th excess toward different, silicic contaminants in secular equilibrium. On La Palma, this is inferred to reflect assimilation of small amounts of mafic wall rock. On Tenerife, both (230Th/238U) and (231Pa/235U) ratios decrease toward 1 with increasing differentiation, and this is accompanied by a subtle increase in Pb isotope ratios. At the same time, (226Ra/230Th) ratios change from 1 (a hitherto unreported magnitude). The Tenerife assimilant is thus constrained to be a partial melt of syenite formed in equilibrium with residual feldspar. The differences reflect a primarily deeper, more mafic magma system beneath La Palma during its late shield-building stage, whereas recent magmatic evolution at Tenerife occurs primarily at lower temperatures in small, shallower magma systems formed during its post–basaltic shield stage. Differentiation takes millennia or less.

A new approach for constraining the magnitude of initial disequilibrium in Quaternary zircons by coupled uranium and thorium decay series dating

Abstract We have measured 238U–206Pb, 235U–207Pb, and 232Th–208Pb ages on Quaternary zircons by laser ablation, single-collector, magnetic sector inductively coupled plasma mass spectrometry (LA-ICP-MS). To obtain reliable ages for Quaternary zircons, corrections for initial disequilibrium associated with deficits and excesses of both 230Th and 231Pa relative to secular equilibrium resulting from differential partitioning during zircon crystallization or source melting must be made. In contrast, the 232Th–208Pb decay system is clearly advantageous for samples affected by disequilibrium because the 232Th decay system lacks long-lived intermediate daughter isotopes. Conventionally, the initial disequilibrium for the 238U and 235U decay series has been determined by the distribution ratio between the melt and zircon (i.e., ƒTh/U = (Th/U)Zircon/(Th/U)Melt and ƒPa/U = (Pa/U)Zircon/(Pa/U)Melt). In our study, these correction factors were determined from comparison of the measured 238U–206Pb and 235U–207Pb ages with 232Th–208Pb ages obtained for three zircons of known eruption and, in some cases, zircon crystallization ages (Kirigamine Rhyolite, Bishop Tuff, and Toga Pumice). The resulting correction factors are ƒTh/U = 0.19 ± 0.14 and ƒPa/U = 3.66 ± 0.89 (Kirigamine Rhyolite), ƒTh/U = 0.24 ± 0.20 and ƒPa/U = 3.1 ± 1.2 (Bishop Tuff), and ƒTh/U = 0.28 ± 0.17 and ƒPa/U = 3.04 ± 0.99 (Toga Pumice). Although the uncertainties of these f values are relatively large, our results support the adequacy of the conventional approach for correction of initial disequilibrium. A recent study published results that apparently show zircon crystallization ages are younger than the eruption age of Bishop Tuff. It seems to be difficult to eliminate these discrepancies, even if the Th/U partitioning and disequilibrium generated during partial melting are taken into account for recalculation of its zircon age. However, magma chamber process and history of Bishop Tuff are too complex to obtain accurate zircon ages by U–Pb method. To overcome this, therefore, the Th–Pb zircon dating method is a key technique for understanding complex, pre-eruptive magma processes, and further efforts to improve its precision and accuracy are desirable.

Origin of low δ26Mg basalts with EM-I component: Evidence for interaction between enriched lithosphere and carbonated asthenosphere

This study presents stable Mg isotopic data for Cenozoic potassic basalts from Wudalianchi and Erkeshan in northeastern China to determine the interactions between upwelling carbonated asthenosphere and enriched lithospheric mantle. Although the Wudalianchi and Erkeshan basalts have variable MgO contents of 4.45 to 9.47wt.%, they exhibit a homogeneous Mg isotopic composition with δ26Mg values ranging from −0.57‰ to −0.46‰ and averaging −0.51±0.06‰ (2SD, n=18). This Mg isotopic composition is lighter than that of the average mantle (δ26Mg=−0.25±0.07‰) but similar to late Cretaceous (<110Ma) and Cenozoic basalts from the North China Craton and the South China Block (δ26Mg=−0.60 to −0.35‰). The high CaO/Al2O3 and Ba/Rb, and low Hf/Hf∗ ratios of the Wudalianchi and Erkeshan basalts are typical characteristics of carbonatitic metasomatism, suggesting that the light Mg isotopic composition could derive from involvement of recycled sedimentary carbonates in the mantle source. The high Dy/Er ratios (2.55 to 2.75) and excess of 230Th (230Th/238U=1.24 to 1.33) suggest presence of garnet in a relatively deep mantle source. Additionally, the seismic tomographic observations show the existence of the stagnant Pacific slab in the mantle transition zone (410–660km) under eastern China. This carbonated mantle source should be located in the asthenosphere. However, compared to other low δ26Mg basalts from eastern China with MORB-like Sr–Nd–Pb isotopic compositions and OIB-like trace element features, the Wudalianchi and Erkeshan basalts exhibit EM-I Sr–Nd–Pb isotopic compositions combined with high SiO2, Ba, K, Pb and LREE contents, high Ba/Th and low Ce/Pb ratios. These geochemical features require a contribution from another mantle source, most likely an EM-I lithospheric mantle. Therefore, an asthenosphere-lithosphere interaction model is proposed for determine the origin of the Wudalianchi and Erkeshan basalts. The original melt was derived from partial melting of the carbonated asthenospheric mantle that was metasomatized by carbonate melt from the stagnant Pacific slab in the mantle transition zone. This ascending melt with a low δ26Mg signature subsequently interacted with the EM-I lithospheric mantle. The interaction dissolved pyroxene and crystallized olivine, releasing LILE, REE, Sr and Pb from dissolved minerals into the melt. Simultaneously, Mg was inherited by newly formed olivine. Therefore, this interaction modified the trace elements and the Sr–Nd–Pb isotopic compositions of the melt toward EM-I geochemical features but preserved the light Mg isotope signatures.

Os and U–Th isotope signatures of arc magmatism near Mount Mazama, Crater Lake, Oregon

Interaction of mantle melts with the continental crust can have significant effects on the composition of the resulting melts as well as on the crust itself, and tracing this interaction is key to our understanding of arc magmatism. Lava flows and pyroclastic deposits erupted from ∼50 to 7.7 ka at Mt. Mazama (Crater Lake, Oregon) were analyzed for their Re/Os and U–Th isotopic compositions. Mafic lavas from monogenetic vents around Mt. Mazama that erupted during the buildup to its climactic eruption have lower 187Os/188Os ratios (0.1394 to 0.1956) and high 230Th excess ((230Th/238U)0 of 1.180 to 1.302), whereas dacites and rhyodacites tend to have higher 187Os/188Os ratios (0.2292 to 0.2788) and significant 238U excess ((230Th/238U)0 of 0.975 to 0.989). The less radiogenic Os isotope compositions of the mafic lavas can be modeled by assimilation of young (∼2.5 to 7 Ma), mafic lower crust that was modified during regional extension, whereas the more radiogenic Os isotope compositions of the dacites and rhyodacites can be attributed to assimilation of older (∼10 to 16 Ma), mid to upper crust that acquired its composition during an earlier period of Cascade magmatism. Production of Th excesses in the lower crust requires very young garnet formation accompanying dehydration melting in the lower crust at less than a few 100 ka by heat from recent basaltic magma injection. The results from this study suggest that the combination of Os and Th isotopes may be used to provide insights into the timescales of evolution of the continental crust in arc settings, as well as the influence of the crust on erupted magmas, and suggest a link between the age and composition of the lower and upper crust to regional tectonic extension and/or earlier Cascade magmatism.

238U–230Th–226Ra Disequilibria Constraints on the Magmatic Evolution of the Cumbre Vieja Volcanics on La Palma, Canary Islands

A suite of 48 samples, including both historical and prehistoric lavas and some plutonic rocks, have been analysed from the Cumbre Vieja rift, La Palma, Canary Islands. Additionally, mineral–melt partition coefficients have been measured for clinopyroxene, plagioclase, amphibole, titanite and apatite in selected rocks. The lavas range from basanite to phonolite (SiO2 = 41·2–57·5 wt % and MgO = 10–0·8 wt %) in composition and form coherent, curvilinear major and trace element arrays in variation diagrams, irrespective of eruption age. The mafic lavas have typical ocean island incompatible trace element patterns and Sr, Nd and Pb isotope compositions show little variation but have a HIMU-type character. Generation of the parental magmas is inferred to have involved ∼4% dynamic melting of a garnet lherzolite source that may have previously been metasomatized by melts derived from a recycled mafic component containing residual phlogopite. The major process of differentiation to phonotephrite involved fractional crystallization of basanitic magmas that evolved along the same liquid line of descent under similar pressure–temperature conditions. Numerical simulations using the MELTS algorithm suggest that this occurred across a temperature interval from c. 1320 to 950°C at 400 MPa and an oxygen fugacity equivalent to quartz–fayalite–magnetite (QFM), with an initial H2O content of 0·3 wt %. The later stages of differentiation (<5 wt % MgO) were dominated by mixing with partial melts of young syenites formed from earlier magma batches. All of the lavas are characterized by 230Th and 226Ra excesses and (230Th/238U) decreases with decreasing Nb/U and increasing SiO2, with no accompanying change in (226Ra/230Th). To explain the observations, we propose a model in which there was a significant role for amphibole, and more importantly accessory titanite, in decre'asing Nb/U, Ce/Pb and Th/U ratios and increasing or buffering (226Ra/230Th) ratios during the later stages of differentiation and magma mixing. These processes all occurred over a few millennia in small magma batches that were repeatedly emplaced within the mid-crust of the Cumbre Vieja rift system prior to rapid transport to the surface.

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.

Melting versus contamination effects on 238U–230Th–226Ra and 235U–231Pa disequilibria in lavas from São Miguel, Azores

The island of São Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The ‘enriched’ signatures (e.g., radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238U–230Th and 235U–231Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238U–230Th–226Ra and 235U–231Pa disequilibria are presented for sixteen mafic lavas (<50wt.% SiO2) that encompass the full range of São Miguel's long-lived radiogenic isotope variability. However, primary U-series signatures of mafic lavas can be influenced by assimilation of more differentiated products. Notably, syenite nodules and cumulate alkali feldspar xenocrysts have been identified in São Miguel basalts. We argue that secondary contamination of basalts is most evident in the hugely variable 230Th–226Ra disequilibria (~18% deficits to >300% excesses of 226Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of São Miguel display slightly higher 230Th and 231Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D¯UU and D¯Th, coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of São Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure.

238U–230Th equilibrium in arc magmas and implications for the time scales of mantle metasomatism

Large excesses of 238U and 226Ra relative to 230Th characterize many arc magmas and are commonly interpreted to represent recent addition of slab-derived fluid to the mantle wedge beneath the arc. A significant proportion of arc magmas are, however, in 238U–230Th radioactive equilibrium. This is generally thought to result from “buffering” of the young slab fluid U-series signal by a sediment component in secular equilibrium. Here we present new 238U–230Th–226Ra and 235U–231Pa measurements for historic andesites from Volcán de Colima, Mexico. In all lavas (230Th/238U) are in equilibrium, whereas (231Pa/235U) and (226Ra/230Th) are significantly greater than one. These data demonstrate that arc magmas with (230Th/238U) equilibrium can have significant 231Pa and 226Ra excesses, precluding ageing of the magmas in the crust as the cause of 230Th/238U equilibrium. Quantitative modeling of metasomatic and melting processes further indicates that addition of sediment melts to a depleted mantle wedge produces significant 230Th excesses and that 238U excesses induced by recent addition of fluids derived from the altered oceanic crust are not sufficient to compensate these 230Th excesses. U-series activity ratios in Colima magmas are best explained by models in which the metasomatised mantle returns to secular equilibrium before melting, implying a time lag ⩾350 kyr, with subsequent production of 231Pa and 226Ra excesses by in-growth during melting rather than by addition of slab fluids. Investigation of a global compilation of U-series data in arc magma indicates that our model proposed for Colima applies to most arc magmas in or near (230Th–238U) equilibrium. The time lag between mantle metasomatism and melting appears to vary between hundreds years to more than 350 kyr in subduction zones. We posit that the absence of U/Th elemental fractionation during melting of arc sources in (230Th/238U) equilibrium reflects a higher fO2 compared to MORB sources that yield magmas with 230Th excesses.

U-series disequilibrium in young Tengchong volcanics: Recycling of mature clay sediments or mudstones into the SE Tibetan mantle

We report U-series disequilibrium data in the youngest volcanic rocks from Maanshan, Dayingshan and Heikongshan volcanoes in the Tengchong volcanic field, representing the only 3 volcanoes from the Indo-Asian suture zone (southwestern Tibet to western Yunnan) that are young enough to preserve 238U–230Th disequilibrium. The most striking feature of these young Tengchong lavas is their extremely low (230Th/232Th) (0.303 to 0.376) and (238U/232Th) (0.289 to 0.360) activity ratios (or ultra-high Th/U concentration ratio). These young lavas also show small to moderate (4% to 10%) 230Th excesses. Such 230Th excesses, together with tomographic results, suggest that partial melting initiated at depths greater than 75km in the garnet stability field. U-series isotope data, together with major and trace element and Nd–Sr–Pb isotope data, indicate that Tengchong lavas are derived from partial melting of an enriched subcontinental lithospheric mantle. The ultra-high melt Th/U concentration ratios of 9.5±0.7 further indicate recycling of continentally derived clay-rich mature sediments or mudstones into the SE Tibetan mantle. The materials with ultra-high Th/U ratios may come from the clay-rich mature sediments from Indian Ocean or Neo-Tethyan Ocean or the mudstones/shales from the subducted Indian continental plate.

Melt generation beneath Arctic Ridges: Implications from U decay series disequilibria in the Mohns, Knipovich, and Gakkel Ridges

We present new 238U–230Th–226Ra–210Pb, 235U–231Pa, and Nd, Sr, Hf, and Pb isotope data for the slow- to ultraslow-spreading Mohns, Knipovich, and Gakkel Ridges. Combined with previous work, our data from the Arctic Ridges cover the full range of axial depths from the deep northernmost Gakkel Ridge shallowing upwards to the Knipovich, Mohns, and Kolbeinsey Ridges north of Iceland. Age-constrained samples from the Mohns and Knipovich Ridges have (230Th/238U) activity ratios ranging from 1.165 to 1.30 and 1.101 to 1.225, respectively. The high 230Th excesses of Kolbeinsey, Mohns, and Knipovich mid-ocean ridge basalts (MORB) are erupted from ridges producing relatively thin (Mohns, Knipovich) to thick (Kolbeinsey) oceanic crust with evidence for sources ranging from mostly peridotite (Kolbeinsey) to eclogite-rich mantle (Mohns, Knipovich). Age-constrained lavas from 85°E on the Gakkel Ridge, on the other hand, overlie little to no crust and range from small (∼5%) 230Th excesses to small 238U excesses (∼5%). The strong negative correlation between (230Th/238U) values vs. axial ridge depth among Arctic ridge basalts is controlled not only by solidus depth influence on 238U–230Th disequilibria, but also by variations in mantle source lithology and depth to the base of the lithosphere, which is expected to vary at ultra-slow spreading ridges. Small 231Pa excesses (65% excess) in age-constrained basalts support the presence of eclogite in the mantle source for this region. Conversely, the ultraslow-spreading Gakkel Ridge basalts are homogeneous, with Sr, Nd, and Hf radiogenic isotopic signatures indicative of a long time-averaged depleted mantle source. The Gakkel samples have minimum (226Ra/230Th) ratios ranging from 3.07 to 3.65±3%, which lie along and extend the global negative correlation between 226Ra and 230Th excesses observed in MORB. The new 230Th–226Ra data support a model for global MORB production in which deep melts record interaction with shallower materials. This scenario requires either mixing with shallow-derived melts, or melt-rock reaction with shallower rocks in the lithosphere or crust.

Dacite formation at Ilopango Caldera, El Salvador: U‐series disequilibrium and implications for petrogenetic processes and magma storage time

Ilopango Caldera was formed ∼1810 years ago by eruption of the Tierra Blanca Joven (TBJ) dacite (70 km3) in central El Salvador. A subsequent eruption in 1880 produced a cluster of dacite domes in the center of Lago Ilopango that contain olive‐bearing enclaves of basaltic andesite. The purpose of this study is to use trace element, isotope, and U‐series data from the TBJ and 1880 eruptions to assess petrogenesis and the timescale of magma storage. We find that although the range of trace element data in the TBJ dacite can be reproduced by simple crystal fractionation of a plagioclase‐ and amphibole‐rich mineral assemblage, the87Sr/86Sr and 207Pb/204Pb data suggest that the 1880 basaltic andesite enclave has a different source than the dacites. This is consistent with U‐series data that show the TBJ dacites have lower (230Th/232Th) than the 1880 basaltic andesite enclave (1.5 versus 1.6, respectively). All Ilopango rocks have 230Th excesses, and the range in (238U/232Th) of the TBJ dacites can be modeled by crystal fractionation of a mineral assemblage including accessory zircon and allanite from a magma that is similar in composition to the enclave. Mineral isochrons yield crystallization ages of &lt;10,000 years, and disequilibrium (226Ra/230Th) values suggest a similarly short residence time. Our data suggest that the large volumes of erupted dacite at Ilopango Caldera are generated very rapidly.

Rapid directional changes associated with a 6.5 kyr-long Blake geomagnetic excursion at the Blake–Bahama Outer Ridge

Geomagnetic excursions are recognized as intrinsic features of the Earth's magnetic field. High-resolution records of field behaviour, captured in marine sedimentary cores, present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability. We present here the highest resolution record yet published of the Blake geomagnetic excursion (∼125ka) measured in three cores from Ocean Drilling Program (ODP) Site 1062 on the Blake–Bahama Outer Ridge. The Blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate (∼10cmka−1) to allow detailed reconstruction of the field behaviour at this site during the excursion. Palaeomagnetic measurements of the cores reveal rapid transitions (<500yr) between the contemporary stable normal polarity and a completely reversed state of long duration which spans a stratigraphic interval of 0.7m. We determine the duration of the reversed state during the Blake excursion using oxygen isotope stratigraphy, combined with 230Th excess measurements to assess variations in the sedimentation rates through the sections of interest. This provides an age and duration for the Blake excursion with greater accuracy and with constrained uncertainty. We date the directional excursion as falling between 129 and 122ka with a duration for the deviation of 6.5±1.3kyr. The long duration of this interval and the fully reversed field suggest the existence of a pseudo-stable, reversed dipole field component during the excursion and challenge the idea that excursions are always of short duration.

Extreme 230Th excesses in magnesian andesites from Baja California

Four Late Pleistocene magnesian andesites (locally called “bajaites”) from Baja California have been dated by the unspiked K–Ar method and analyzed for major, trace elements and Sr, Nd, Pb, Th and U isotopes. They display very large 230Th excesses (50% to 120%) relative to 238U. They also have very high Ba and Sr contents and Sr/Y and La/Yb ratios, together with suprachondritic Nb/Ta ratios (22 to 26). From correlations between 230Th excess and major elements, trace element and isotopic ratios, we show that the unusual geochemical signature of these lavas can be ascribed to very low partial melting degrees of a mantle source containing residual rutile. This mantle source derived from interactions between the Baja California lithospheric mantle and older magnesian andesite melts carrying a sedimentary component, which were formed during the Late Miocene opening of the Baja California asthenospheric window. During the Quaternary, this metasomatized source experienced dehydration melting triggered by the hot thermal regime due to the uprise of the subslab Pacific asthenosphere beneath Baja California and the Gulf of California.

The influence of source heterogeneity on the U–Th–Pa–Ra disequilibria in post-glacial tholeiites from Iceland

We investigate the relative influence of mantle upwelling velocity and source heterogeneity on the melting rates recorded by 230Th–238U, 231Pa–235U and 226Ra–230Th disequilibria in post-glacial tholeiites from Iceland’s main rift areas. The measured (230Th/238U) ratios range from 1.085 to 1.247, the (231Pa/235U) ratios from 1.333 to 1.925, and the (226Ra/230Th) ratios from 0.801 to 1.218. A general positive correlation between 230Th excesses and distance from the inferred plume centre is consistent with a model of decreasing mantle upwelling velocity with increasing distance from the plume axis. However, the model is not substantiated by the (231Pa/235U) data as the correlation with distance from the plume centre is weak. On the scale of individual eruption centres, the observed U-series are influenced by variations in melt transport time, source porosity, and local variations in mantle upwelling velocity. Broad correlations between (230Th/238U) and (231Pa/235U) and highly incompatible trace element ratios for samples from the Western Volcanic Zone provide, however, evidence for a significant underlying effect of source heterogeneity on the U-series data. Low 230Th and 231Pa excesses in enriched samples from the Western Volcanic Zone with high U/Th, Nb/U and Nb/La indicate that partial melts from an enriched source component, characterised by high melt productivity but low bulk DU/DTh, influence the U-series systematics of the erupted melts. These results re-affirm the presence of comparatively larger abundances of enriched material in the mantle source beneath the South Western Rift of Iceland, which has been suggested based on relationships between highly incompatible element and Pb isotope ratios in Icelandic basalts. Overall, our results highlight the importance of lithological heterogeneity on the melting behaviour of the upper mantle and the composition of oceanic basalts.

Insights into the Galápagos plume from uranium-series isotopes of recently erupted basalts

Uranium-series isotopes (238U-230Th-226Ra-210Pb), major element, trace element, and Sr-Nd isotopic data are presented for recent (<60 years old) Galapagos archipelago basalts. Volcanic rocks from all centers studied (Fernandina, Cerro Azul, Sierra Negra, and Wolf Volcano) display 230Th excesses (4%–15%) and steep rare earth element (REE) patterns indicative of residual garnet during partial melting of their mantle source. Rare earth element modeling suggests that only a few percent of garnet is involved. Correlations between (238U/232Th), radiogenic isotopes and Nb/Zr ratio suggest that the U/Th ratio of these Galapagos volcanic rocks is primarily controlled by geochemical source variations and not fractionation during partial melting. The lowest (230Th/238U) ratio is not observed at Fernandina (the supposed center of the plume) but at the more geochemically “depleted” Wolf Volcano, further to the north. Small radium excesses are observed for all samples with (226Ra/230Th) ranging from 1.107 to 1.614. The 226Ra-230Th disequilibria do not correlate with other uranium-series parent-daughter nuclide pairs or geochemical data, suggesting modification at shallow levels on timescales relevant to the half-life of 226Ra (1600 years). The combination of 226Ra and 210Pb excesses is inconsistent with interaction of magma with cumulate material unless decoupling of 210Pb (or an intermediate daughter, such as 222Rn) occurs prior to modification of Ra-Th disequilibria. An intriguing correlation of (210Pb/226Ra)0 with Nb/Zr and radiogenic isotopes requires further investigation but suggests possible control via magmatic degassing and accumulation that may somehow be related to source heterogeneities.