Search

Altered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc. Based on a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes, we confirm that AOC‐derived fluid (or hydrous melt), rather than sediment‐derived melt or fluid, is responsible for the Sr‐Nd isotope decoupling (i.e., 87Sr/86Sr is "excessively" enriched relative to 143Nd/144Nd when compared to the "normal" mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr‐Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC‐derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr‐Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr‐Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones. Plain Language Summary: Altered oceanic crust (AOC) is an important subduction component that contributes to the generation of arc magmas. However, identifying this contribution to arc magmas is challenging for the lack of effective tracers. Nonetheless, AOC has two characteristic features: (a) Sr‐Nd isotope decoupling (i.e., enriched in 87Sr/86Sr relative to 143Nd/144Nd in contrast to the "normal" mantle array); and (b) relatively heavy Ba isotope compositions. Here, we analyze the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc, which are combined with a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes. Our results confirm that fluid (or hydrous melt) derived from subducted AOC, rather than sediment, is responsible for the Sr‐Nd isotope decoupling observed in island arc lavas. Strikingly, the magnitude of the Sr‐Nd isotope decoupling correlates well with the Ba isotope compositions of the arc lavas. Thus, our results establish an intimate link between subducted AOC, heavy Ba isotope ratios, and Sr‐Nd isotope decoupling signature in island arcs. This combination of Ba‐Sr‐Nd isotopes can therefore now be used as a powerful tracer to identify the contributions of subducted AOC to arc magmas. Our results also suggest that AOC and serpentinite show synergistic behavior during subduction recycling. Key Points: Global arc data confirm that the Sr‐Nd isotope decoupling of arc lavas is caused by AOC‐derived fluidDecoupled Sr‐Nd isotopes plus heavy Ba isotope is a good tracer for recycled altered oceanic crust (AOC) in arc magmasAOC and serpentinite show synergistic behaviors during subduction recycling [ABSTRACT FROM AUTHOR]

In the past few years, the so-called mélange model has been offered as an alternative to the long-standing model of enrichment of the subarc mantle by separate additions of fluid and sediment components from the subducting plate. In the mélange model, components from the subducting plate become physically mixed at the slab-mantle interface. Partial melting of the peridotite subsequently occurs after being hybridized by the mélange material that diapirically rises into hotter portions of the wedge. Here, we present the first Ba isotope study of lavas from the Tonga-Kermadec arc (southwest Pacific Ocean) and show that Ba isotopes distinguish between fluid and melt derived from different subducted components. This provides fresh constraints on the debate. Remarkable along-strike Ba isotope variations were observed and are best explained by contributions from variable proportions of sediment and altered oceanic crust (AOC) fluid from the subducting plate. Combined Ba-Sr-Pb isotope relationships indicate that sediment melt and AOC fluid were added to the source of the arc lavas separately at different times. This is inconsistent with the mélange model, at least in this arc [ABSTRACT FROM AUTHOR]

Assessing the impact of crustal assimilation on the composition of oceanic arc lavas is important if source composition is to be correctly interpreted. This is particularly the case in the Lesser Antilles where lavas encompass a very large range in radiogenic isotope compositions. Here we present new 176Hf/177Hf and trace element data for a suite of samples from St Lucia in the southern Lesser Antilles arc where assimilation of sediments located within the arc crust has been shown to influence significantly Sr-Nd-Pb isotope compositions. We show that a high rate of assimilation (r=0·8) of sediment is responsible for the co-variation of Th/Th*, La/Sm, 87Sr/86Sr, 206/207/208Pb/204Pb, 143Nd/144Nd and 176Hf/177Hf towards extreme 'continental' compositions. Lavas that escaped sediment assimilation have a typical oceanic arc signature and provide the best indication of mantle source characteristics beneath St Lucia. They display similar Ba/Th, La/Sm and Nd isotopic compositions to lavas further north in the arc, but with slightly more radiogenic Sr and Pb. Addition of less than 2% of local bulk subducting sediment, or less than 3·5% of sediment partial melt or fluid, to the mantle wedge can explain these compositions; these estimates are similar to those previously proposed for the northern arc. After correction for the effects of sediment assimilation, the St Lucia lavas have only slightly more radiogenic Pb and Sr isotope signatures compared with the northern islands; this can be attributed to differences in the isotopic composition of the local subducting sediment rather than to greater sediment input, as has been previously proposed. Comparison of St Lucia with the other southern Lesser Antilles islands suggests that similar mantle source compositions exist beneath Martinique, St Vincent and perhaps Bequia, whereas a more 'continental' source might characterize Ile de Caille, Kick 'em Jenny and Grenada. [ABSTRACT FROM AUTHOR]

New U-Th-Ra, major and trace element, and Sr-Nd-Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc-continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc-continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated Pb/Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. U and Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia. [ABSTRACT FROM AUTHOR]

The islands of Flores and Corvo in the Azores archipelago are the only two of nine subaerial volcanic edifices lying west of the Mid-Atlantic Ridge (MAR). This makes them important for constraining the evolution of this young (<40 Ma) oceanic plateau. The alkalic basalt suites from Flores and Corvo lie on a single liquid line of descent. Ankaramitic cumulates, with MgO contents up to ∼18 wt %, result from clinopyroxene-dominated polybaric crystallization. The parental magmas (MgO ∼ 11 wt %) are inferred to be low-degree partial melts (F = 3–5%) of enriched peridotite generated at depths of ∼80–90 km. These primary magmas commenced crystallizing at the lithosphere–asthenosphere boundary and this continued in conduits over a pressure range of ∼0·6–1·2 GPa. Only lavas with MgO < 3 wt % fractionated at shallow crustal levels. Nd and Sr isotope data reveal variations in the source of both magmatic systems, suggesting variable contributions from both enriched (E-) and depleted (D-) mid-ocean ridge basalt (MORB)-source mantle components. This is supported by the greater variability of incompatible trace-element ratios within the Flores lavas (e.g. Ba/Nd, La/Sm, Th/Nd), whereas those from Corvo exhibit a good correlation between key trace-element ratios [e.g. (La/Sm)N, Th/Nd] and Sr isotope ratios. Lavas from Flores display a greater variability in Sr and Nd isotope compositions and define a mixing array between an E-MORB source and a common Azores mantle source. The latter signature is restricted to lava suites from the north and east of Flores. We concur with the generally accepted notion that Flores and Corvo are derived from the same mantle plume as is responsible for the eastern Azores islands. However, there is evidence (different Nb/Zr, Ta/Hf and La/Sm, but homogeneous Sr and Nd isotopic composition) that these two islands are dominated by a source component that is not as evident in the eastern archipelago. [ABSTRACT FROM AUTHOR]

Fonualei is unusual amongst subaerial volcanoes in the Tonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km3 and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 1100–1000°C, 2–4 wt % H2O and oxygen fugacities 1–2 log units above the quartz–fayalite–magnetite buffer. Major and trace element data, along with Sr–Nd–Pb and U–Th–Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2–6 km) depths beneath the volcano. U–Th–Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia. The 165 year eruption period for the dacites implies that mixing occurred on a similar timescale, possibly during ascent in conduits. The composition of the dacites renders them unsuitable candidates as contributors to average continental crust. [ABSTRACT FROM PUBLISHER]

Abstract: 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 5ka and last erupted in 1975. Nearby Ruapehu volcano has a much longer eruptive history extending back beyond 230kaB.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 1945AD 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 1870AD) 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 (1000–2000years) fractionation times. [Copyright &y& Elsevier]

Abstract: O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ 18O does not increase northwards as would be expected from the ~4 fold increase in subduction rate if slab-derived fluids had high 18O/16O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. 187Os/188Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The extent and geochemical impact of crustal contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr–Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with ‘primitive’ sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of >380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104–105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years. [Copyright &y& Elsevier]

A suite of young volcanic basaltic lavas erupted on the intra-plate island of Niuafo’ou and at active rifts and spreading centres (the King’s Triple Junction and the Northeastern Lau Spreading Centre) in the northern Lau Basin is used to examine the pattern of mantle flow and the dynamics of melting beneath this complex back-arc system. All lavas contain variable amounts of a subduction related component inherited from the Tonga subduction zone to the east. All lavas have higher 87Sr/86Sr, lower 143Nd/144Nd and more radiogenic Pb isotope compositions than basalts erupted at the Central Lau Spreading Centre in the central Lau Basin, and are interpreted as variable mixtures of subduction-modified, depleted upper mantle, and mantle residues derived from melting beneath the Samoan Islands which has leaked through a tear in the subducting Pacific Plate beneath the Vitiaz Lineament at the northern edge of the Lau Basin. Our data can be used to map out the present-day distribution of Samoan mantle in this region, and show that it influences the compositions of lavas erupted as far as 400 km from the Samoan Islands. The distribution of Samoan-influenced lavas implies south- and southwest-wards mantle flow rates of >4 cm/year. U-series disequilibria in historic Niuafo’ou lavas have average (230Th/238U) = 1.13, (231Pa/235U) = 2.17, (226Ra/230Th) = 2.11, and together with major and trace element data require ∼5% partial melting of mantle at between 2 and 3 GPa, with a residual porosity of 0.002 and an upwelling rate of 1 cm year−1. We suggest that intraplate magmatism in the northern Lau Basin results from decompression melting during southward flow of mantle from beneath old (110–120 Ma), relatively thick Pacific oceanic lithosphere to beneath young (<5 Ma), thinner oceanic lithosphere beneath the northern Lau Basin. [ABSTRACT FROM AUTHOR]

Abstract: The processes involved in the formation and transport of partial melts above subducting plates remain poorly constrained relative to those at mid-ocean ridges. In particular, 238U–230Th–226Ra disequilibria, that might normally be used to constrain melting dynamics, tend to be swamped by the effects of fluid addition from the down-going plate. The 231Pa–235U system provides an exciting exception to this because the highly incompatible nature of Pa means that fractionation and in-growth during partial melting overwrite the effects of fluid U addition. We present 231Pa–235U data on 50 well-characterised lavas from seven subduction zones in order to examine partial melting processes. Measured (231Pa/235U) ratios are all >1 and 15% are >2. Overall (231Pa/235U) shows broad positive correlations with (230Th/238U) and La/Yb and negative trends against Ba/Th and (226Ra/230Th). These systematics can differ from arc to arc but suggest that (231Pa/235U) tends to be higher in sediment-rich arc lavas where the effects of fluid addition are muted and there is less of a 231Pa deficit for melting to overprint. We have explored the effects of decompression melting, frictional drag dynamic melting with and without ageing subsequent to fluid U addition to the wedge as well as flux melting models. Globally, average (231Pa/235U) appears to correlate negatively with convergence rate and so in the numerical models we use the local subduction rate for the rate of matrix flow through the melting zone. Using this assumption and reasonable values for other parameters, the melting models can simulate the overall range of (231Pa/235U) and some of the data trends. However, it is clear that local variations in some parameters, especially source composition and extent of melting, exert a major influence on 231Pa–235U disequilibria. Some data, which lie at a high angle to the modelled trends, may be explained by mixing between small degree hydrous melts formed near the slab and larger degree, decompression melts produced at shallow depth. [Copyright &y& Elsevier]

Abstract: The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga–Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2–10, (b) the top 1–22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be–Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ∼800 °C during the breakdown of amphibole±plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10–15% partial melts of depleted mantle after it has been enriched by the addition of 0.2–2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ∼800 °C by the time it attains ∼80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partial melting of the altered oceanic crust. [Copyright &y& Elsevier]

New data show that island arc rocks have (210Pb/226Ra)o ratios which range from as low as 0.24 up to 2.88. In contrast, (228Ra/232Th) appears always within error of 1 suggesting that the large 226Ra-excesses observed in arc rocks were generated more than 30 years ago. This places a maximum estimate on melt ascent velocities of around 4000 m/year and provides further confidence that the 226Ra excesses reflect deep (source) processes rather than shallow level alteration or seawater contamination. Conversely, partial melting must have occurred more than 30 years prior to eruption. The 210Pb deficits are most readily explained by protracted magma degassing. Using published numerical models, the data suggest that degassing occurred continuously for periods up to several decades just prior to eruption but no link with eruption periodicity was found. Longer periods are required if degassing is discontinuous, less than 100% efficient or if magma is recharged or stored after degassing. The long durations suggest much of this degassing occurs at depth with implications for the formation of hydrothermal and copper-porphyry systems. A suite of lavas erupted in 1985–1986 from Sangeang Api volcano in the Sunda arc are characterised by deficits of 210Pb relative to 226Ra from which 6–8 years of continuous 222Rn degassing would be inferred from recent numerical models. These data also form a linear (210Pb)/Pb–(226Ra)/Pb array which might be interpreted as a 71-year isochron. However, the array passes through the origin suggesting displacement downwards from the equiline in response to degassing and so the slope of the array is inferred not to have any age significance. Simple modelling shows that the range of (226Ra)/Pb ratios requires thousands of years to develop consistent with differentiation occurring in response to cooling at the base of the crust. Thus, degassing post-dated, and was not responsible for magma differentiation. The formation, migration and extraction of gas bubbles must be extremely efficient in mafic magma whereas the higher viscosity of more siliceous magmas retards the process and can lead to 210Pb excesses. A possible negative correlation between (210Pb/226Ra)o and SO2 emission rate requires further testing but may have implications for future eruptions. [Copyright &y& Elsevier]

U-series isotopes can provide unique insights into the physical processes of magma evolution by constraining the time scales over which they operate. This, however, requires rock suites that provide a clear and complete record of the liquid line of descent. Sangeang Api volcano, in the east Sunda arc, provides such an opportunity because it erupts potassic lavas (SiO2 ∼ 47–55%), which contain a spectrum of xenoliths interpreted to represent the cumulates complementary to the lavas. Combined, the cumulates and lavas span a large compositional range, which major and trace element modelling suggests reflects ∼70% polybaric crystallization that began at sub-Moho depths and continued into the upper crust. The parental magmas can be successfully modelled by ∼3% dynamic partial melting, in the presence of 0–4% residual garnet, of a mid-ocean ridge basalt (MORB) source enriched by ∼3% subducted Sunda sediment in addition to a contribution of fluid-mobile elements from the subducting slab. The effects of fluid addition and partial melting on U–Th disequilibria appear to be competing processes. A moderate range in Sr, Nd and Pb isotopes is interpreted to reflect a combination of source heterogeneity and <15% assimilation of Indian MORB crust. Neither interaction with metasomatized arc lithosphere nor the presence of enriched, plume-type mantle in the mantle wedge is required by our data. The cumulates and lavas have largely indistinguishable (230Th/232Th) over a wide range of U/Th ratios and thus any age differences are minimal relative to the half-life of 230Th. All whole rocks and minerals are characterized by 226Ra excesses and modelling of the 226Ra–230Th–Ba data suggests that magmatic evolution beneath this arc volcano occurs on time scales of ∼2000 years. Combining this with simple numerical calculations we estimate that the Sangeang Api magma chamber is ∼6–10 km3 in size, cooling rates are ∼0·05°C/yr and minimum crystal growth rates are (2–7) × 10−13 cm/s. It is possible that a significant proportion of the crystal growth and differentiation occurred during isobaric decompression of magmas ascending through conduits. An implication is that the net magmatic flux across the Moho is of magmas that are already significantly evolved from primary magmas and this may be significant for why average continental crust has an andesitic bulk composition even though the flux out of the mantle wedge is basaltic. [ABSTRACT FROM PUBLISHER]

The Early Cretaceous volcanic rocks of southern Uruguay comprise mafic and felsic volcanics. The position of these outcrops at the southern edge of the Parana´–Etendeka continental flood basalt province provides an opportunity to investigate possible lateral variations in both mafic and more evolved rock types towards the margins of such an area of plume-related magmatism. The mafic lavas are divided into two compositionally distinct magma types. The more voluminous Treinte Y Tre´s magma type is similar to the low-Ti basalts of the Parana´ flood basalt province. The Santa Luci´a magma type is a distinct and rare basalt type with ocean-island basalt type asthenospheric affinities (high Nb/La, low 87Sr/86Sri). The felsic volcanics are divided into two series, the Lavalleja Series and the Aigu¨a Series. The Lavalleja Series are chemically and isotopically similar to the Parana´–Etendeka low-Ti rhyolites, and are considered to be related to the Treinte Y Tre´s lavas by extensive fractionation and crustal assimilation. The Aigu¨a Series have low 143Nd/144Ndi and low 87Sr/86Sri and unlike the rhyolites of the Parana´, are interpreted as melts of pre-existing mafic lower crust that subsequently underwent extreme fractionation. The differences observed in the felsic suites may be linked to differences in the volumes of the associated basalts and the amounts of extension. [ABSTRACT FROM PUBLISHER]

Studies the Re-Os isotopes on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain and the Colorado Plateau of the southwestern United States. Need for caution in the use of Os isotopes as diagnostic tracer of subcontinental mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere.

Details a U-series study of lavas from Kamchatka and the Aleutians, which focuses on the source composition and melting processes. Description of the Kamchatka arc; Analytical techniques used in the study; Results and discussion.

Presents research which studied lavas from the Tonga-Kermadec island arc. Subduction processes; Mantle wedge coupled to descending plate; Fluid and sediment fluxes from ocean plate; Magma extraction occurring rapidly after partial melting; Geological background; Analytical results and implications; Implications for physical models.

Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reservoirs such as subducting sediments and altered oceanic crust. The Tonga–Kermadec arc–Lau back‐arc provides an end‐member of subduction zones with the coldest thermal structure on Earth. Reported here are Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. The arc and back‐arc lavas range from basalts to dacites in composition with SiO2 = 48.3–65.3 wt% over which Li concentrations increase from 2 ppm to 16 ppm. Li/Y ratios range from 0.08 to 0.77 and from 0.24 to 0.65 in the arc and back‐arc lavas, respectively. The majority of the lavas have δ7Li that ranges from 2.5 ‰ to 5.0 ‰ with an average of (3.6 ±0.7) ‰, similar to that reported from other arcs and there is no distinction between the arc front and back‐arc lavas. The pelagic sediments have variable Li concentrations (33–133 ppm) and δ7Li that ranges from 1.2 ‰ to 10.2 ‰ while the volcaniclastic sediments have an even greater range of Li concentrations (3.6–165 ppm) and generally higher δ7Li values (8–14 ‰). However, δ7Li in the lavas does not correlate with commonly used trace element ratio or isotope signatures indicative of slab‐derived fluids or the sediments. This is probably because the range of δ7Li in the lavas and sediments overlap. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust. Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reserviors such as subducting sediments and altered oceanic crust. Here we report Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust. [ABSTRACT FROM AUTHOR]

Primitive ocean island basalts (OIB) display a large variability in stable iron (Fe) isotopes, beyond what can plausibly be explained by partial melting and crystal fractionation processes. This Fe isotopic heterogeneity is widely ascribed to inheritance from various exotic mantle components of enriched crustal origin, i.e., subducted and resurfacing oceanic or continental crust, its underlying lithosphere or overlying sediments. These enriched mantle components are characterised by variations in radiogenic Sr-Nd-Pb-Hf isotopes. The extent to which the inherited Fe isotopic signature of subducted and recycled material influences the Fe isotopic composition of OIB and which role secondary processes accompanied with partial melting play is, however, not well understood. The eastern Azores island of São Miguel displays a systematic change from a mantle source less radiogenic in Sr-Nd-Hf-Pb isotopes in the West to a highly enriched source in the East. These variations are among the largest reported in an OIB, indicating variable source component additions to the melt that offer a natural laboratory to elucidate sub-oceanic island processes. Among these, the eastern São Miguel component is unique amongst global OIB in that it has both, extremely radiogenic 206Pb/204Pb and 87Sr/86Sr isotopic ratios. The spatial distribution, complexity and uniqueness of these isotopic source characteristics are ideally suited to gain insights into the spatial distribution of mantle components and their potential controls on Fe isotope systematics. Comparing δ57Fe prim , which is the isotopic composition calculated to primitive lavas along a liquid line of descent, with 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb and 208Pb/204Pb reveals that the depleted component of western São Miguel shows a heavy Fe isotopic composition ranging from δ57Fe prim = +0.09 to +0.18‰ (excluding one outlier at −0.02‰ δ57Fe prim), whereas the enriched component of eastern São Miguel shows lighter δ57Fe prim = +0.05 to +0.12‰. Both suites show a continuum of isotopic compositions between two apparent endmembers. The light Fe isotopic signature of eastern São Miguel lavas, coupled with elevated Rb/Sr and K 2 O, indicate K-rich mantle metasomatism in their source. The Fe isotopic composition of western São Miguel's lavas are likely related to a similar source, but have been overprinted by the active Terceira Rift. The latter adds low-degree mid-ocean ridge basalt (MORB)-type melts to the original OIB, likely in the form of a re-fertilised depleted mantle component that underpins the ultraslow spreading region. The two-component mix that contributes to western São Miguel lavas highlights that both enriched mantle components and shallow crustal features can add to the Fe isotopic signatures of erupting OIB, with spatial separation within single volcanic islands. This suggests that a careful evaluation of local geologic features, such as rifting, is required when OIB are probed for their mantle sources, and that heavy Fe isotopic signatures in OIB may not reflect deep, plume-source lithologies. In summary, São Miguel lavas are sourced by a mantle plume, but within a spatially controlled region accompanied by a plate-controlled melt associated with a rifting zone. This dichotomy adds another complication to the genesis of OIB, which however maybe elucidated through combined radiogenic-stable isotope systematics. [ABSTRACT FROM AUTHOR]

We present whole rock 187 Os/ 188 Os data for the most mafic lavas along the Lesser Antilles arc (MgO = 5–17 wt.%) and for the subducting basalt and sediments. 187 Os/ 188 Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between 187 Os/ 188 Os and Os concentrations and between 187 Os/ 188 Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed. 87 Sr/ 86 Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with 187 Os/ 188 Os. The assimilant is inferred to have a MORB-like 87 Sr/ 86 Sr with high Sr (>700 ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ∼2.5) and 187 Os/ 188 Os > 0.2. Such compositional features are likely to correspond to a plagioclase-rich early-arc cumulate. Given that assimilation affects lavas that were last stored at more than 5 kbar, assimilation must occur in the middle-lower crust. Only a high MgO picrite from Grenada escaped obvious assimilation (MgO = 17% wt.%) and could reflect mantle source composition. It has a very radiogenic 87 Sr/ 86 Sr (0.705) but a 187 Os/ 188 Os ratio that overlaps the mantle range (0.127). 187 Os/ 188 Os and 87 Sr/ 88 Sr ratios of the sediments and an altered basalt from the subducting slab vary from 0.18 to 3.52 and 0.708 to 0.714. We therefore suggest that, unlike Sr, no Os from the slab was transferred to the parental magmas. Os may be either retained in the mantle wedge or even returned to the deep mantle in the subducting slab. [ABSTRACT FROM AUTHOR]

Abstract: New high-precision (double-spike) Pb isotope analyses of lavas from Tafahi and Niuatoputapu, the northernmost islands of the Tonga–Kermadec Island Arc, are used to examine the source of Pb in these samples, and the relative timing of Pb addition from the subducting oceanic crust and subducting sediment. Lavas from these islands have distinctive, radiogenic Pb isotope compositions, which are inherited from the basaltic crust of the subducting Louisville Seamount Chain on the Pacific Plate. The subducting oceanic plate and the overlying upper mantle beneath northern Tonga therefore have very different Pb isotope compositions, allowing the proportion of Pb derived from each of these sources, and from subducting sediment to be estimated. We show that between 42% and 90% of the Pb in northern Tonga lavas is derived from the basaltic crust of the subducting Louisville Seamount Chain. The dominant source of mantle Pb in arc lavas (at least in northern Tonga), is therefore subducted basaltic oceanic crust, rather than the overlying mantle wedge. The orientation of the Pb isotope arrays constrain the relative timing of Pb addition from these different sources, and show that sediment Pb must be mixed with the upper mantle before Pb from the subducted oceanic crust is added during a separate event. This observation is consistent with the results of experimental studies, which suggest that altered oceanic crust and sediment are likely to lose Pb by dehydration or melting at different depths. Melting of sediment at depths >120km, followed by migration of these melts to shallower levels within the overlying mantle, where Pb-bearing fluids derived from dehydration of oceanic crust trigger mantle melting, could explain the observed mixing relationships. Mass balance calculations show that the Pb flux into the arc magma source corresponds to the amount of Pb contained in the uppermost 67 to 143m of the subducting basaltic crust of the Louisville Seamount Chain. If this minimum estimate is representative of subduction zones worldwide, where 7km thick oceanic crust containing 0.5ppm Pb is subducted, then the average Nd/Pb ratio of the oceanic crust that is recycled into the deep mantle is decreased by at least 3.5% as a result of subduction. [Copyright &y& Elsevier]

We present 176 Hf/ 177 Hf isotope ratios on 41 previously well-characterized subaerial and submarine samples from the Azores islands of São Miguel, Terceira, Graciosa, Faial, Pico and the João de Castro seamount (on the Terceira Rift). In ε Nd – ε Hf isotope space all Azores lavas fall below the mantle array reference line and do not overlap the proximal Atlantic MORB. Lavas from São Miguel and João de Castro form two distinct and well defined arrays extending below the mantle array, which has not been previously documented in other oceanic magmatic provinces. The Nd-Hf isotope compositions of João de Castro overlap those of HIMU type lavas, yet they lack the characteristically radiogenic Pb isotope ratios of HIMU. The combined Nd-Hf-Pb-Sr isotope systematics of both São Miguel and João de Castro endmembers can be explained by recycling of a single package of heterogeneous oceanic crust ranging from D-MORB to E-MORB in composition, with an age between 2.5 and 3.0 Ga, with no requirement for parent-daughter ratio modification during subduction. In contrast the Nd-Hf-Pb isotope systematics of lavas from São Jorge, Terceira, Graciosa, Pico and Faial are consistent with the presence of younger (<700 Ma) recycled crust that underwent low-temperature alteration and dehydration during subduction. There is no evidence in the erupted lavas for direct mixing between these two generations of recycled material within the plume. These data suggest that old recycling age and absence of sediments along with recycled oceanic crust are both required to develop isotopic compositions below the mantle array in ε Nd – ε Hf space. Our modeling shows that the compositional variability of erupted MORB is large enough that, given enough time, they can generate a wide range of isotope compositions such as observed in OIB. Lastly, lava compositions along the Terceira rift can be explained by a westward asthenospheric flux along a tilted lithosphere/asthenosphere boundary, where fertile components are exhausted by partial melting after ∼70 km of transport along the Terceira Rift. While this observation is broadly consistent with the plume source–ridge sink model, it also suggests that the lithosphere/asthenosphere boundary geometry can smear the view of the plume heterogeneity. [ABSTRACT FROM AUTHOR]