226Ra Excesses Research Articles

URANIUM‐SERIES AND LUMINESCENCE DATING OF VOLCANIC LITHIC ARTEFACTS*

U‐series dating was used to determine the growth rate of a feldspar‐to‐clay weathering rind in a mid‐Holocene Cascade–Olcott tradition andesite core, and luminescence dating (last exposure to sunlight) was used to date fine‐grained feldspars scraped from the surfaces of similar buried artefacts from a 14C‐sediment‐dated archaeological site (45KI464) on the wet western slope of the Cascade Mountains of Washington. For U‐series dating, we measured 226Ra excess (226Ra excess =226Ra –230Th) in five stratigraphic depth controlled rind scrapings. 238U, 232Th and 230Th were counted by alpha spectrometry, and 226Ra and 210Pb were counted by gamma spectrometry on each sub‐sample.

U-TH-PA-RA study of the Kamchatka arc: new constraints on the genesis of arc lavas

The 238U- 230Th- 226Ra and 235U- 231Pa disequilibria have been measured by mass spectrometry in historic lavas from the Kamchatka arc. The samples come from three closely located volcanoes in the Central Kamchatka Depression (CKD), the most active region of subducted-related volcanism in the world. The large excesses of 226Ra over 230Th found in the CKD lavas are believed to be linked to slab dehydration. Moreover, the samples show the uncommon feature of ( 230Th/ 238U) activity ratios both lower and higher than 1. The U-series disequilibria are characterized by binary trends between activity ratios, with ( 231Pa/ 235U) ratios all >1. It is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting. We suggest that they are more likely to reflect mixing between two end-members: a high-magnesia basalt (HMB) end-member with a clear slab fluid signature and a high-alumina andesite (HAA) end-member reflecting the contribution of a slab-derived melt. The U-Th-Ra characteristics of the HMB end-member can be explained either by a two-step fluid addition with a time lag of 150 ka between each event or by continuous dehydration. The inferred composition for the dehydrating slab is a phengite-bearing eclogite. Equilibrium transport or dynamic melting can both account for 231Pa excess over 235U in HMB end-member. Nevertheless, dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid-derived 226Ra/ 230Th. A continuous flux melting model is also tested. In this model, 231Pa- 235U is quickly dominated by fluid addition and, for realistic extents of melting, this process cannot account for ( 231Pa/ 235U) ratios as high as 1.6, as observed in the HMB end-member. The involvement of a melt derived from the subducted oceanic crust is more likely for explaining the HAA end-member compositions than crustal assimilation. Melting of the oceanic crust is believed to occur in presence of residual phengite and rutile, resulting in no 226Ra- 230Th disequilibrium and low 231Pa excess over 235U in the high-alumina andesites. Consequently, it appears that high-alumina andesites and high-magnesia basalts have distinct origins: the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle. It seems that there is no genetic link between these two magma types, in contrast with what was previously believed.

Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U‐230Th‐226Ra disequilibria of Miyakejima volcano, Izu arc, Japan

238U‐230Th‐226Ra systematics in lavas from Miyakejima volcano, Japan, are presented to estimate the timescale of magmatic processes beneath an island arc. Miyakejima volcano has four recent eruptive stages (Stages 1–4) starting >7000 BP. 238U‐230Th‐226Ra disequilibria observed in lavas with large 238U and 226Ra excesses imply metasomatism of depleted mantle by fluid‐related processes. This metasomatism is also suggested by trace element and Sr‐Nd‐Pb isotopic systematics in the same lavas. In the equiline diagram, the trends for two magmatic stages (Stages 1 and 2) are regarded as two different isochrons with a common initial (230Th/232Th) ratio, although the trend for Stages 3 and 4 is a magma mixing line. Our model calculations show that slab‐derived fluids can deliver some Th and a very rapid ascent time of the slab components in the mantle wedge (< 7 kyr) is inferred. This rapid ascent can be explained by nearly instantaneous material transport in the mantle wedge by a hydrofracture model for fluid and a channel flow model for melt. Such a timescale estimate is not increased even if melting processes that enhance 226Ra are taken into account. The age difference in the equiline diagram corresponds to the interval of individual fluid‐release events (13 kyr between Stages 1 and 2, and 5 kyr between Stages 2 and 3). Thus fluid release from the slab and subsequent magma generation occur as episodic events on a several‐kiloyear timescale.

The groundwater geochemistry of the Bengal Basin: Weathering, chemsorption, and trace metal flux to the oceans

Sixty-eight groundwater samples from the Ganges-Brahmaputra floodplain in the Bengal Basin were analyzed to assess the groundwater geochemistry, the subsurface hydrology, the buffering effects of sediments on trace metal concentrations and their isotopic compositions, and the magnitude of the subsurface trace element flux to the Bay of Bengal and to the global ocean. Samples obtained from depths of 10 to 350 m were measured for major and trace elements, dissolved gas, and tritium. On the basis of the 3He/ 3H ages, the groundwater at depth (30–150 m) appears to be continually replenished, indicating that this recharge of groundwater to depth must ultimately be balanced by a significant quantity of submarine discharge into the Bay of Bengal. Using the 3He/ 3H groundwater age–depth relationship to calculate a recharge rate of 60 ± 20 cm/yr, we estimate a subsurface discharge into the Bay of Bengal of 1.5 ± 0.5 × 10 11 m 3/yr, or 15% of the surface Ganges-Brahmaputra river (GBR) flux. Several trace elements, especially Sr and Ba, display elevated concentrations averaging 7 to 9 times the surface GBR water values. The submarine groundwater fluxes of Sr and Ba to the oceans are 8.2 ± 2 × 10 8 and 1.5 ± 0.3 × 10 8 mol/yr, or 3.3 and 1.2%, respectively, of the world total, or equal to the surface GBR Sr and Ba estimated fluxes. Our groundwater flux for Ba agrees with the estimate of Moore (1997) (3 × 10 8–3 × 10 9 mol/yr), on the basis of measured Ba and Ra excesses in the Bay of Bengal. Other trace metals, such as U and Mo, are at low but measurable levels and are not major contributors to the global flux in this river system. A comparison of the Sr and Ba concentrations, plus 87Sr/ 86Sr ratios in groundwater to the oxalate extractable fractions of a coastal sediment core, suggests that weathering of carbonates and minor silicates, coupled with cation exchange plus adsorption and desorption reactions, controls the trace element concentrations and 87Sr/ 86Sr isotopic compositions in both the groundwater and river water. Our data also imply that other coastal floodplains (e.g., the Mekong and the Irrawaddy rivers) that have high precipitation rates and rapid accumulation of immature sediments are likely to make significant contributions to the global oceanic trace metal budgets and have an impact on the Sr isotopic evolution in seawater.

Melting processes and fluid and sediment transport rates along the Alaska‐Aleutian arc from an integrated U‐Th‐Ra‐Be isotope study

A comprehensive data set for young lavas erupted along the Alaska‐Aleutian arc is used to examine how fluid and sediment transport rates and melting processes vary in response to systematic changes in subduction rate and dip along the arc and across the ocean‐continent boundary. Positive correlations between convergence rate, volcano volume, and 238U excesses suggest that magmatic output is closely linked to the size of the fluid flux which occurred <10 kyr prior to eruption. Sediment‐sensitive tracers like Th/Nb, Ce/Ce*, and 10Be/9Be also increase with convergence rate. However, the inferred 10Be/9Be ratio of this component is low relative to that in the incoming sediments, and this could reflect either sediment removal by accretion or storage in the mantle wedge for ∼0.5–1 Myr. Th/Nb and Th/Nd ratios exceed those expected from bulk sediment addition in the center of the arc suggesting that the enhanced sediment signal reflects transfer in a partial melt phase, whereas partial melts from the subducted oceanic crust appear to be restricted to the western tear in the Pacific plate. Therefore the thermal structure at the slab‐wedge interface in the center of the arc must lie close to, and possibly between, the sediment and basalt solidii and is thus constrained to be in the region of 670–740°C at 3 GPa. Elevated (230Th/232Th) ratios, the occurrence of 230Th excesses and only modest 226Ra excesses in many of the lavas are modeled as the products of dynamic melting effects superimposed upon a fluid‐fluxed mantle wedge. The transition from oceanic to continental lithosphere seems to play an important role in determining the relative effects of fluid addition and partial melting upon U‐Th disequilibria. We infer that the change from fast and steep plate subduction in the Aleutians to shallow and slow subduction, coupled with increasing lithospheric lid thickness, in Alaska together control slab and wedge temperatures and the rate of matrix flow through the melting region.

( 231Pa/ 235U)-( 230Th/ 238U) of young mafic volcanic rocks from Nicaragua and Costa Rica and the influence of flux melting on U-series systematics of arc lavas

We present U, Th, and Pa isotope data for young lavas from Costa Rica and Nicaragua in the Central American arc. Thorium isotopic ratios for Costa Rica and Nicaragua differ dramatically: Costa Rican lavas are characterized by low ( 230Th/ 232Th) (1 to 1.2) and, for four out of five lavas, ( 230Th/ 238U) greater than unity. Nicaraguan lavas have high ( 230Th/ 232Th) (2.2 to 2.7) and, for five of six samples, ( 230Th/ 238U) less than unity. All lavas have ( 231Pa/ 235U) greater than unity, with initial values ranging from 1.27 to 1.77, but those from Costa Rica have larger 231Pa excesses. There is a broad positive correlation between ( 231Pa/ 235U) and ( 230Th/ 238U) similar to the worldwide trend for arcs outlined by Pickett and Murrell (1997), although many of the Nicaraguan lavas skirt the high end of that trend. In greater detail, the Central American data appear to divide into separate high-( 231Pa/ 235U) and low-( 231Pa/ 235U) tiers. These tiers may be different because of either different residence times in the crust or different proportions of sedimentary components from the slab. Substantial ( 231Pa/ 235U) excesses (>1.5) in both Costa Rica and Nicaragua require a melting process that allows for enhanced daughter ( 231Pa) ingrowth. With increasing U addition, ( 231Pa/ 230Th) increases in a manner that cannot be explained adequately by aging of fluid components before partial melting and eruption. Thus, either some 231Pa is added from the slab, or melting-enhanced 231Pa ingrowth is greater in sources that have experienced a larger amount of slab-derived flux and a higher extent of melting. These observations can be explained if regions that have undergone greater extents of fluxing and melting have experienced these processes over a longer time interval than those that have had little flux added and little melt extracted. We propose a flux-ingrowth melting model in which corner flow in the mantle wedge supplies fresh hot mantle into a zone of slab fluid addition. Partial melting occurs in response to this fluxing. We assume critical melting at low porosity (∼10 −3), rapid fluid flux to the melting region, and rapid melt transport. Solid mantle traverses the melting region over 10 5 to 10 6 yr, thereby allowing 231Pa and 230Th ingrowth from U retained in the residues of melt extraction. Magmas are aggregated from all parts of the melting regime, mixing melts from incipiently fluxed regions with those from sources that have experienced more extensive fluid addition, partial melting, and daughter nuclide ingrowth. With suitable assumptions about component addition from the slab, this flux-ingrowth model matches a wide range of U-series and trace element data from Costa Rican and Nicaraguan lavas, with required average extents of melting of ∼1 to 3% and 7 to 15%, respectively. Upwelling and/or extensive melt-rock reaction are not required to explain large ( 231Pa/ 235U) excesses in Central America or other arcs. On Th isotope equiline plots, the model produces linear arrays that resemble isochrons but that have no age significance. Instead, these arrays are generated by mixing of melts from sources that have experienced fluid addition and partial melting over a range of time intervals, as seems likely in arc source regions. Finally, the flux-ingrowth model predicts considerable 226Ra excesses for integrated magmas. If we assume that 226Ra is added continuously with the slab-derived fluid, the model predicts large and increasing ( 226Ra/ 230Th) with increasing melting and slab-component addition, without requiring the addition of a distinct late fluid.

Consequences of diffuse and channelled porous melt migration on uranium series disequilibria

Magmas erupted at mid-ocean ridges (MORB) result from decompression melting of upwelling mantle. However, the mechanism of melt transport from the source region to the surface is poorly understood. It is debated whether melt is transported through melt-filled conduits or cracks on short time scales (<∼ 10 3 yrs), or whether there is a significant component of slow, equilibrium porous flow on much longer time scales (>∼ 10 3–10 4 yrs). Radiogenic excess 226Ra in MORB indicates that melt is transported from the melting region on time scales less than the half life of 226Ra (∼1600 yrs), and has been used to argue for fast melt transport from the base of the melting column. However, excess 226Ra can be generated at the bottom of the melt column, during the onset of melting, and at the top of the melt column by reactive porous flow. Determining the depth at which 226Ra is generated is critical to interpreting the rate and mechanism of magma migration. A recent compilation of high quality U-series isotope data show that in many young basalts, 226Ra excess in MORB is negatively correlated with 230Th excess. The data suggest that 226Ra excess is generated independently of 230Th excess, and cannot be explained by “dynamic” or fractional melting, where observed radiogenic excesses are all generated at the base of the melt column. One explanation is that the negative correlation of activity ratios is a result of mixing of slow moving melt that has travelled through reactive, low-porosity pathways and relatively fast moving melt that has been transported in unreactive high-porosity channels. We investigate this possibility by calculating U-series disequilibria in a melting column in which high-porosity, unreactive channels form within a low-porosity matrix that is undergoing melting. The results show that the negative correlation of 226Ra and 230Th excesses observed in MORB can be produced if ∼60% of the total melt flux travels through the low-porosity matrix. This melt maintains 226Ra excesses via chromatographic fractionation of Ra and Th during equilibrium transport. Melt that travels through the unreactive, high-porosity channels is not able to maintain significant 226Ra excesses because Ra and Th are not fractionated from each other during transport and the transport time for melt in the channels to reach the top of the melt column is longer than the time scale for 226Ra excesses to decay. Mixing of melt from the high porosity channels with melt from the low-porosity matrix at the top of the melting column can produce a negative correlation of 226Ra and 230Th excesses with the slope and magnitude observed in MORB. This transport process can also account for other aspects of the geochemistry of MORB, such as correlations between La/Yb, α Sm/Nd, and Th/U and 226Ra and 230Th excess.

Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria ( 238U- 230Th- 226Ra and 235U- 231Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238U- 230Th- 226Ra and 235U- 231Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, 230Th excesses and 226Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226Ra, this inverse correlation between 230Th and 226Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226Ra and 230Th excesses also vary with lava composition: 226Ra excesses are negatively correlated with Na 8 and La/Yb and positively correlated with Mg#. Conversely, 230Th excesses are positively correlated with Na 8 and La/Yb and negatively correlated with Mg#. Th/U, 230Th/ 232Th, and 230Th excesses are also variable and correlated to one another. 231Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na 8. The isotope ratios 143Nd/ 144Nd, 176Hf/ 177Hf, 87Sr/ 86Sr, and 208Pb/ 206Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, 230Th, and 226Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.

Uranium-series fractionation in mafic extrusives

Based on U-series disequilibrium arguments there is good evidence for the presence of U-rich accessory minerals in the outer mantle. The very large excesses of 226Ra and 231Pa activity relative to 230Th, 238U, and 235U in most mid-ocean ridge basalts and some non-divergent plate-margin basalts are inconsistent with prevailing incompatibility models of U-series fractionation. Application of a fundamental principle of equilibrium balance reveals that in these instances more than half of the original U and Th remains behind in the residual outer mantle when basaltic magmas separate. One is forced to conclude that, in the outer mantle, U and Th do not occur primarily in major silicate minerals, where they would indeed be incompatible. Rather, they must be occurring as high concentration components in refractory accessory minerals. The precipitous concentration gradients bounding such minerals would allow for the operation of physical processes, such as alpha recoil and daughter diffusion, to produce paired disequilibrated phases. Daughter deficiencies would develop in the high concentration minerals, and daughter excesses in the surrounding low concentration major silicates. This would constitute a steady-state condition existing in the mantle before the onset of melting. Subsequent preferential melting of the matrix silicates would readily result in the disequilibria observed in basalts.

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

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

Timing and trigger of arc volcanism controlled by fluid flushing from subducting slab

We present 238U-230Th-226Ra systematics in lavas from Miyakejima volcano, Izu arc, Japan, together with major element, trace element and Sr-Nd-Pb isotopic compositions, to estimate the timescale of magmatic processes beneath an island arc. 238U-230Th-226Ra disequilibria observed in Miyakejima lavas with large excesses of 238U and 226Ra to 230Th imply metasomatism of depleted mantle by fluid related processes. Our data reveal that fluid components released from the slab ascend rapidly through the mantle wedge (>50m/yr), and that individual stages of subaerial volcanism are induced by flushing of different fluid batches to the mantle wedge with a periodicity of 1-13kyr. This indicates that arc magmatism is controlled by pulsed release of fluid from the slab occurring on such a small timescale, which might be related to the occurrence of earthquakes in the subducting slab.

Precise two chronometer dating of Pleistocene travertine: The 230Th/ 234U and 226Ra ex/ 226Ra(0) approach

In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/ 234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/ 234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/ 234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e. 226Ra ex/ 226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/ 234U chronometer. Analytical solutions of the coupled 234U/ 230Th/ 226Ra radionuclide system predicted that the 226Ra ex/ 226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Ra ex/ 226Ra(0) ages agreed well with those derived from 230Th/ 234U, (ii) all data plot within uncertainty on the 226Ra ex/ 226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Ra ex/ 226Ra(0) should be applicable to materials which are suitable for 230Th/ 234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/ 234U for samples that have been formed a few thousand years ago.

238U– 230Th– 226Ra fractionation in historical lavas from the Azores: long-lived source heterogeneity vs. metasomatism fingerprints

This study reports 238U– 230Th– 226Ra nuclides measured by mass spectrometry, trace elements and 87Sr/ 86Sr isotopes on historical alkali basalts from the Azores region. The ( 230Th/ 232Th) activity in the alkali basalts spans a range (from 0.897 in Sao Miguel to 1.382 in Terceira island) that is almost as large as the global variation of mid ocean ridge basalts (MORB) and oceanic island basalts (OIB). The ( 230Th/ 238U) activity (10–40%), Th/U (2.92–3.74) and the 87Sr/ 86Sr (0.70354–0.70467) also display large variations. The ( 226Ra/ 230Th) activity varies from 0.84 in Sao Miguel to 1.60 in Pico island. Samples from Pico, Faial and Terceira islands are found to plot on the negative ( 230Th/ 238U) vs. ( 226Ra/ 230Th) correlation defined by the 1730–1736 eruption of Lanzarote, Canary islands [Sigmarsson et al., Earth Planet. Sci. Lett. 162 (1998) 137], except for the sample from Sao Miguel island. We propose that the negative correlation reflects a mixing between two end-member melts, one displaying a low ( 230Th/ 238U) and a high ( 226Ra/ 230Th) and the other end-member displaying a high ( 230Th/ 238U) and a low ( 226Ra/ 230Th). Numerical model suggests that the high 226Ra, low 230Th excesses end-member represented by samples from Pico island can be explained by partial melting of a garnet lherzolite. The other end-member represented by the Terceira lava is shown to result from the interaction of low-degree melts with the metasomatized mantle containing phlogopite. Phlogopite crystals in contact with a melt enriched in 226Ra and Ba have a ( 226Ra/ 230Th)≫1 and high Ba/Th because of crystal affinities for Ra and Ba relative to Th and U. After 8000 years, however, the unsupported 226Ra excess will have decreased down to 0 as phlogopite displays little amount of U and Th. So, during the process of interaction with phlogopite crystals, low-degree melts formed in the garnet stability field that are characterized by high ( 230Th/ 238U), ( 226Ra/ 230Th) and Ba/Th will loose the 226Ra excess, scavenged by phlogopite while the ( 230Th/ 238U) and Ba/Th will remain unaffected. One finds that this process may be valid over 250 years maximum so that only the first interacting melts may preserve a high ( 230Th/ 238U), low ( 226Ra/ 230Th) activity. On Sao Miguel island, it is shown that the low ( 230Th/ 238U), while the source is clearly enriched in Th, cannot be explained by a lack of garnet in the mantle neither by a higher degree of melting. The 226Ra deficit (15%) and low Ba/Th and Sr/Th are explained by partial melting of an enriched mantle in the presence of phlogopite.

Models of U-series disequilibria generation in MORB: the effects of two scales of melt porosity

Conflicting evidence exists about the extent of melt–mantle reaction as melt ascends beneath a mid-ocean ridge. On the one hand, uranium series (U-series) disequilibria in mid-ocean ridge basalts suggest that melts reactively flow towards the surface and continuously re-equilibrate with lower pressure mantle. On the other hand, trace elements in abyssal peridotites suggest fractional melting and isolation of ascending melt from surrounding lower pressure peridotite. Following a suggestion by Kelemen et al. (1997) [Kelemen, P.B., Hirth, G., Shimizu, N., Spiegelman, M., Dick, H.J.B., Philos. Trans. R. Soc. 355, 283–318], I construct a model for melting beneath mid-ocean ridges which consists of two materials having different melt porosity distributions in order to quantitatively assess whether the U-series and abyssal peridotite observations could actually be compatible. The melting column consists of a volumetrically dominant lherzolite which produces melt by decompression and a volumetrically minor dunite which serves as a conduit for fast melt ascent. A suction parameter S (after Iwamori (1994) [Earth Planet. Sci. Lett. 125, 1–16]) controls the transfer of melt between the lherzolite and dunite media. S can vary from 0 where none of the ascending melt flows through the dunite melt conduit (reactive porous flow melting) to 1 where the melt produced in the lherzolite is immediately transferred to the dunite (near-fractional melting). Transport times of U-series nuclides are accounted for in both porosity regimes based on calculated flux balances. Results show that the trace element patterns observed in abyssal peridotites require S to range from 0.5 to 0.8 depending on assumptions about the composition of the original source material. U-series models are evaluated in terms of three quantities: S, χ (the volumetric fraction of dunite), and a 2/ ηC which lumps together several constants relating to permeability. 226 Ra and 231 Pa excesses are most sensitive to a 2/ ηC (which affects the melt porosity within the melting region). 226 Ra excesses match the observed levels of 226 Ra excess in mid-ocean ridge basalts (MORB) at all values of S. 231 Pa excesses favor lower values of S as models with high S produce significantly less ( 231 Pa)/( 235 U) than that observed. There remains some disparity between the values of S required by the abyssal peridotite data and the observed 231 Pa excesses although further refinement of partition coefficients could resolve the difference. Within the models presented, the two geochemical observations are close to being reconciled if S=0.5 and the mantle permeability is at least 1×10 −14 m 2. However, a factor of two higher bulk partition coefficient for U ( D U=0.003) would allow the observed ( ( 231 Pa)/( 235 U) ) to be matched at S=0.5 while dropping the required bulk permeability to less than 1×10 −15 m 2.

Formula omitted]– [formula omitted] disequilibria and mantle melting processes: a discussion

This paper discusses the implications of 238 U– 230 Th disequilibria in MORB and OIB on mantle melting. It is shown that melting rates can only be calculated if the melting process is not batch melting. At present, however, the uncertainties in U and Th partition coefficients are such that, in most cases, batch melting cannot be discarded. Moreover, a simple argument based on the Th–Sr isotope correlation, suggests that batch melting might best approximate the actual melting process in the mantle, at least in the case of OIB. The existence of mantle heterogeneities (e.g., the “marble-cake” model of Allegre and Turcotte [Allegre, C.J., Turcotte, D., 1986. Implications of a two-component marble-cake mantle. Nature 323, 123–127]), when taken into account, suggests that complex melting models might not be necessary to interpret 238 U– 230 Th data. In particular, preferential melting of garnet pyroxenites could explain the origin of OIB, whereas MORB could derive from larger degrees of melting of garnet lherzolites. However, the large 231 Pa and 226 Ra excesses found in MORB are better explained by dynamic melting models, if they are actually produced through crystal-melt partitioning during mantle melting. In any case, it is worth emphasizing that derivation of melting rates is strongly model dependent, and should be taken with caution.

Melt generation beneath ocean islands: a U-Th-Ra isotope study from Lanzarote in the Canary Islands

New major and trace element data, and Sr, Nd, Pb, U, Th and Ra isotopes are presented for prehistoric and historic lavas from Lanzarote in the Canary Islands. These rocks are amongst the most primitive found on intraplate ocean islands ranging in composition from basanite to alkali basalt, with MgO contents >9.3% and Mg numbers >67. The youngest are from two of the three 1824 vents, the largest group of samples is from the best known eruption episode, the 1730–36 Timanfaya eruptions, a smaller group of samples are from the northeast Corona region (∼50 ky) and the oldest samples are from the Famara complex and basement massif. The rocks have some of the characteristics of HIMU OIB, including high Ce/Pb, Nb/Ce and low Nb/U and restricted 87Sr/ 86Sr (0.70209–0.70332). There is significant 230Th/ 238U disequilibrium ( 230Th excesses range from 6–76%) with some of the intermediate silica composition Corona samples showing the greatest disequilibrium. The historic samples exhibit 226Ra excess. The major and trace element data have undergone fractionation corrections to Mg numbers of 70, requiring <5% olivine fractionation, and these inferred primary compositions are used to evaluate a number of melt generation and mixing models. The fractionation-corrected compositions for the 1824 and the 1730–36 have been modelled as 1–4% melts of a source similar to primitive mantle. However, Yb is incompatible, and so the amounts of residual garnet were < ∼5%, suggesting that there was no significant contribution from garnet pyroxenite source rocks. Rather the REE and the FeO contents are both consistent with melting in the garnet-spinel transition, at depths of 60–70 km. ( 230Th/ 238U) increases slightly with increasing La/Yb in the younger rocks, and they require some form of dynamic melting model. In the preferred model the upwelling rate is kept constant, and the differences in the degrees of melting are attributed to different lengths of the melt column, with the smaller degree melts being extracted from greater depths. Strikingly, ( 226Ra/ 230Th) increases with increasing degrees of melting, and so it reflects the time since extraction from the melt column rather than variations in the melting processes. Intra-suite minor and trace element variations are due to magma mixing, and not to progressive changes in the degrees of partial melting, and it is envisaged that such magma mixing occurred during the dynamic melting processes. Dynamic melting at depths of 60–70 km suggests that the regional uplift around the Canary Islands is at least in part due to thermal erosion of the underlying lithosphere. Variations of average Ce/Yb, Tb/Yb, ( 230Th/ 238U), SiO 2 and lithospheric age for different OIB highlight how the smaller degree melts tend to be generated at greater depths, and the mean pressure of melting increases with the thickness of the lithospheric lid. However, there is no general link between ( 230Th/ 238U) and the lithospheric age or thickness, and hence the integrated degrees of melting. High buoyancy fluxes result in higher degrees of melting and low ( 230Th/ 238U) (Chabaux and Allegre, 1994), but for OIB generated within low buoyancy plumes, ( 230Th/ 238U) and the degrees of melting primarily depend on the depths of melt extraction. Differences in the average composition of low buoyancy OIB depend on the thickness of the overlying lithosphere (Ellam, 1992; Haase, 1996), and the differences within an OIB suite, such as between the 1730-36 and 1824 lavas on Lanzarote, depend on the depth of extraction from the melt column.

Gondwana dispersion and Asian accretion: IGCP 321 final results volume : I. Metcalfe, ed I. Metcalfe, (A.A. Balkema), Gondwana dispersion and Asian accretion. IGCP 321 final results volume, ISBN 9054104465, 1999, (361p)

We present new 238U-230Th-226Ra-210Pb-210Po, 87Sr/86Sr and 143Nd/144Nd isotopic data of whole-rock samples and plagioclase separates from volcanic deposits of the 2006 and 2010 eruptions at Merapi volcano, Java, Indonesia. These data are combined with available eruption monitoring, petrographic, mineralogical and Pb isotopic data to assess current theories on the cause of a recent transition from effusive dome-building (2006) to explosive (2010) activity at the volcano, as well as to further investigate the petrogenetic components involved in magma genesis and evolution. Despite the significant difference in eruption style, the 2006 and 2010 volcanic rocks show no significant difference in (238U/232Th), (230Th/232Th) and (226Ra/230Th) activity ratios, with all samples displaying U and Ra excesses. The 226Ra and 210Pb excesses observed in plagioclase separates from the 2006 and 2010 eruptions indicate that a proportion of the plagioclase grew within the decades preceding eruption. The 2006 and 2010 samples were depleted in 210Po relative to 210Pb ((210Po/210Pb)i < 1) at the time of eruption but were variably degassed (69%–100%), with the degree of 210Pb degassing strongly related to sample texture and eruption phase. In good agreement with several activity monitoring parameters, 210Po ingrowth calculations suggest that initial intrusion into the shallow magma plumbing system occurred several weeks to a few months prior to the initial 2010 eruption. The 2006 and 2010 samples show a wide range in (210Pb/226Ra) activity ratio within a single eruption at Merapi and are largely characterised by 210Pb deficits ((210Pb/226Ra) < 1). Assuming a model of complete radon degassing, the 210Pb deficits in the 2006 volcanic rocks indicate relatively longer degassing timescales of ∼2–4 years than those given by the 2010 samples of ∼0–3 years. The uranium-series and radiogenic isotopic data do not support greater crustal assimilation of carbonate material as the explanation for the more explosive behaviour of Merapi in 2010 (as has been previously suggested) and instead indicate that relatively rapid ascent of a more undegassed magma was the primary difference responsible for the transition in explosive behaviour. This interpretation is in good agreement with gas monitoring data, previous petrological studies (mineral, microlite and melt inclusion work) and maximum calculated timescale estimates using Fe-Mg compositional gradients in clinopyroxene, that also suggest more rapid movement of relatively undegassed magma in 2010 relative to 2006.

Insights into mid‐ocean ridge basalt petrogenesis: U‐series disequilibria from the Siqueiros Transform, Lamont Seamounts, and East Pacific Rise

Parent‐daughter disequilibria between (230Th)/(238U), (231Pa)/(235U) and (226Ra)/(230Th) (parentheses refer to activities) have been measured by thermal ionization mass spectrometry and inductively coupled plasma‐mass spectrometry in basalts from three tectonomagmatic settings of the East Pacific Rise (EPR) at 8°20′‐10°N. Mid‐ocean ridge basalts (MORB) from the Siqueiros Transform, the Lamont Seamounts, and the EPR ridge crest span a large compositional range from primitive, high‐MgO basalts with strong incompatible element depletions (DMORB) to typical normal MORB (NMORB) to rare incompatible element enriched basalts (EMORB) derived from a more enriched source isotopically. Concentrations of U vary from &lt;13 ppb in DMORB to &gt;400 ppb in EMORB while Th/U ranges from 2 in DMORB up to 3 in EMORB. The young‐looking high‐MgO basalts have (226Ra)/(230Th) that ranges from 3.2 to 4.2, while EMORB appear old being near secular equilibrium. Initial (231Pa)(235U) are very high (&gt;2.5) in all of the Siqueiros basalts. Three basalts from the Lamont Seamounts have low incompatible element concentrations and low Th/U and are in secular equilibrium for (226Ra)/(230Th) while the sample located closest to the ridge axis has significant 226Ra and 231Pa excesses and minor 230Th excess. DMORB lack 230Th excess, have high excesses of226Ra and 231Pa, and resemble experimentally determined melts of peridotite at 1 GPa, implying derivation from relatively shallow level melting of spinel lherzolite at low residual porosity. Disequilibria for all three parent‐daughter pairs are consistent with typical axial NMORB resulting from mixing of melts derived from heterogeneous sources, specifically 90–95% DMORB with 5–10% EMORB. The observation that all samples, regardless of tectonomagmatic setting, lie on the same mixing trend suggests that melting beneath seamounts and transforms is similar to melting beneath the ridge axis. Variations in 230Th excess over short spatial scales imply that garnet‐bearing mafic veins create all of the 230Th excess observed in typical NMORB.

Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction

Here, we present new mass spectrometry measurements of U–Th–Ra disequilibria and Sr isotopes for historical volcanics from the Karthala and La Grille volcanoes in Grande Comore Island, Comores Archipelago. Alkali basalts from the Karthala are characterised by large 230Th (33–47%) and 226Ra excesses (21–53%), and radiogenic Sr compositions (0.7034–0.7041). In contrast, La Grille basanites have less radiogenic 87Sr/ 86Sr (∼0.7032) with less 226Ra excesses (21%) but similar 230Th– 238U disequilibria (45%). The Karthala samples display much more scatter in the Ra–Th isochron diagram than in the Th–U isochron diagram, suggesting a decoupling of the two parent–daughter systems. Correlations between the Sr isotopes and U–Th–Ra disequilibria suggest a mixing relationship between La Grille and Karthala sources. La Grille basanites are shown to result from partial melting of the old metasomatised oceanic lithosphere beneath the archipelago whereas Karthala lavas are derived from the Comore plume. The presence of amphibole in the lithosphere is responsible for Ra–Ba, Ba–Th and Ra–Th fractionation during melting such that D Ba > D Th > D Ra. The partitioning of these elements clearly differs from partial melting of garnet–lherzolite where D Th > D Ra > D Ba. Part of the variations in the 226Ra excesses is attributed to radioactive decay between the source and the surface. Short residence times (<1000 year) preclude the presence of a large magma chamber and may be an argument for rapid segregation of the melts through cracks or veins during melt transport.

Short magma chamber residence time at an Icelandic volcano inferred from U-series disequilibria

THE questions of how quickly magmas can differentiate and how long they reside in magma chambers are fundamental to our understanding of magma evolution and the behaviour of volcanoes. Timescales of a few years to a few hundred years have been inferred from physical modelling of crystallization1,2, and from variations in crystal sizes and magma compositions3–5. Shortlived disequilibria between daughter nuclides of the 238U decay series yield information about the processes and timescales of magma differentiation4,6–14. Here I report excesses of 226Ra over 230Th and 210Pb that decrease with differentiation, in lavas from the Vestmannaeyjar volcanic system (Iceland). The decreasing disequilibria can be explained by crystal fractionation of alkali basalt to form hawaiite and mugearite and a 10-year magma chamber residence time. Such rapid differentiation and so short a residence time in a deep reservoir probably result from the injection of a small volume of alkali basalt into a relatively cold crust.