Trace element transport rates in subduction zones: evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas (original) (raw)
Related papers
Journal of Petrology, 1998
Basin, whereas the subduction signature and volatile (CO 2 and F) of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean contents increase eastwards towards the modern arc. These obsediments from DSDP Sites 204 and 275, and ODP Site 596, servations suggest that depletion is due to melt extraction during are integrated with existing geochemical data for lavas from the Lau back-arc extension and volcanism, together with a long 'residence Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) time' of mantle material within the mantle wedge. The upper mantle and the extinct Lau Ridge arc, giving new insights into the beneath the northernmost end of the Tonga arc and Lau Basin petrogenesis of lavas in an active arc-back-arc system. Geochemical contains an ocean-island basalt (OIB) component derived from the variations in Tonga-Kermadec arc lavas are the result of (1) Samoa plume to the north. This is reflected in high concentrations differences in the amount and composition of the material being of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi subducted along the arc, and (2) pre-existing heterogeneities in the and Niuatoputapu islands at the northern end of the Tonga arc. upper mantle. Differences in the material being subducted beneath Pb isotopes also suggest an LR-SMC contribution into Tafahi and the arc have an important influence on the chemistry of the arc Niuataputapu. Trace element and isotope modelling is used to lavas. At the Kermadec Trench,~1 km thick layer of sediment is investigate the combined effects of varying mantle source depletion being subducted beneath the arc, compared with~200 m at the and subduction on the geochemistry of the arc lavas. The results Tonga Trench. This results in the high Th/U and more radiogenic suggest that the arc lava geochemistry can be explained largely by Pb isotope compositions of Kermadec lavas compared with Tonga the balance between a relatively constant subduction input of Pb, lavas. The latter have Pb isotope compositions intermediate between Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0•001-0•005 those of Pacific sediments and Pacific mid-ocean ridge basalt weight fraction of the Stolper & Newman (1994, Earth (MORB), suggesting that much of the Pb in these lavas is derived and Planetary Science Letters, 121, 293-325] 'H 2 O-rich from subducting Pacific Ocean crust. This is supported by the Pb component' composition), into the overlying, but variably depleted isotope signatures of the subducting LR-SMC, which are also mantle wedge. observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are KEY WORDS: Tonga-Kermadec arc lavas; trace elements; isotopes; petrogenesis; mantle plumes generally less depleted than the modern arc lavas. In the back-arc
Chemical Geology, 2010
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 N 120 km, 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 143 m of the subducting basaltic crust of the Louisville Seamount Chain. If this minimum estimate is representative of subduction zones worldwide, where 7 km thick oceanic crust containing 0.5 ppm 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.
U-Th Isotopes in Arc Magmas: Implications for Element Transfer from the Subducted Crust
Science, 1997
Uranium-thorium isotope results from island arc volcanic rocks were used to investigate the rates of transfer of fluids and sediments from the downgoing slab. Uranium, but not thorium, is readily mobilized in the fluid. A negative array between thorium/cerium and neodymium-143/neodymium-144 indicates that significant amounts of the thorium in arc rocks are derived from subducted sediments, although perhaps only about 30 percent of the thorium in subducted sediments is returned to the crust in this way. The transfer times for fluid through the mantle wedge are about 30,000 to 120,000 years, whereas those for sediment melts may be several million years. The low average uranium/ thorium ratios of bulk crust primarily reflect different crustal generation processes in the Archaean.
Element transport from slab to volcanic front at the Mariana arc
Journal of Geophysical Research, 1997
We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97-1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near-constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid-oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U-230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.
New insights into the origin of O–Hf–Os isotope signatures in arc lavas from Tonga–Kermadec
Chemical Geology, 2009
Tonga-Kermadec Island arc lavas Samoa 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 18 O does not increase northwards as would be expected from the~4 fold increase in subduction rate if slab-derived fluids had high 18 O/ 16 O 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. 187 Os/ 188 Os 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 mantlederived magmas with minimal interaction with the arc crust.
Geochimica et Cosmochimica Acta, 2002
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 ( 238 U-230 Th-226 Ra and 235 U-231 Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238 U-230 Th-226 Ra and 235 U-231 Pa 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, 230 Th excesses and 226 Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226 Ra, this inverse correlation between 230 Th and 226 Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226 Ra and 230 Th excesses also vary with lava composition: 226 Ra excesses are negatively correlated with Na 8 and La/Yb and positively correlated with Mg#. Conversely, 230 Th excesses are positively correlated with Na 8 and La/Yb and negatively correlated with Mg#. Th/U, 230 Th/ 232 Th, and 230 Th excesses are also variable and correlated to one another. 231 Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na 8 . The isotope ratios 143 Nd/ 144 Nd, 176 Hf/ 177 Hf, 87 Sr/ 86 Sr, and 208 Pb/ 206 Pb 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, 230 Th, and 226 Ra 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.
Th, Pb, and Sr isotope variations in young island arc volcanics and oceanic sediments
Earth and Planetary Science Letters, 1991
Destructive plate margin rocks have (23°Th/232Th) values in the range 0.5-2.5, which is similar to those in MORB and OIB. Approximately 60% of the available data on subduction-related rocks plot within 10% of the (23°Th/E32Th)-(238U/232Th) equiline, and most of the remaining analyses are displaced to high (238U/23°Th). Moreover, despite the large range of Th isotope ratios in sediments and altered MORB in the subducted slab, the majority ( -70%) of destructive margin rocks plot within the Th-Sr and Th-Pb mantle arrays defined by MORB-OIB. Th/U ranges from 1.0 to 6.6 in the rocks analysed, and both the measured and the source Th/U inferred from initial (23°Th/23:Th) correlate with 2°8pb*/2°6Pb*. Thus, the major (six-fold) difference in Th/U between different arc suites was established several 100 Ma prior to subduction, and it does not appear to reflect element fractionation processes associated with Recent subduction.