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1.
Although most arc lavas have experienced significant magma differentiation, the effect of the differentiation process on U-series disequilibria is still poorly understood. Here we present a numerical model for simulating the effect of time-dependent magma differentiation processes on U-series disequilibria in lavas from convergent margins. Our model shows that, in a closed system with fractional crystallization, the ageing effect can decrease U-series disequilibria via radioactive decay while in an open system, both ageing and bulk assimilation of old crustal material serve to reduce the primary U-series disequilibria. In contrast, with recharge of refresh magma, significant 226Ra excess in erupted lavas can be maintained even if the average residence time is longer than 8000 years.The positive correlations of (226Ra/230Th) between Sr/Th or Ba/Th in young lavas from convergent margins have been widely used as evidence of fluid addition generating the observed 226Ra excess in subduction zones. We assess to what extent the positive correlations of (226Ra/230Th) with Sr/Th and Ba/Th observed in the Tonga arc could reflect AFC process. Results of our model show that these positive correlations can be produced during time-dependent magma differentiation at shallow crustal levels. Specifically, fractional crystallization of plagioclase and amphibole coupled with contemporaneous decay of 226Ra can produce positive correlations between (226Ra/230Th) and Sr/Th or Ba/Th (to a lesser extent). Therefore, the correlations of (226Ra/230Th) with Sr/Th and Ba/Th cannot be used to unambiguously support the fluid addition model, and the strength of previous conclusions regarding recent fluid addition and ultra-fast ascent rates of arc magmas is significantly lessened.  相似文献   

2.
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 208Pb/204Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. 238U and 226Ra 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.  相似文献   

3.
We use coupled 238U-230Th and 235U-231Pa disequilibria measurements from Pico Island, Azores to examine the melting behavior of the underlying mantle. U-series disequilibria in young, mafic lavas are dependent on the melting rate of their source, which in most cases is primarily controlled by its melt productivity. Mafic lithologies such as eclogite and pyroxenite have much higher melt productivities than peridotite and so U-series measurements may provide constraints on the mineralogy of the melting mantle. Recent Pico Mountain lavas show limited geochemical variations and a restricted range of U-series disequilibria with (230Th/238U) = 1.22-1.25 and (231Pa/235U) = 1.46-1.50. Using a simple, dynamic melting model of a homogeneous source, these results can be reproduced with melting rates of <1 × 10−4 kg/m3/a and melt porosities of <0.7% near the onset of melting. For a plausible range of upwelling rates, this implies that the melt productivity is <6%/GPa. This value is consistent with a garnet peridotite source but not with more highly productive mafic lithologies. Given independent evidence for the involvement of mafic lithologies such as recycled oceanic crust in Pico magmagenesis, we suggest a scenario in which initial eclogitic melts are dispersed through melt-rock reaction into a larger volume of surrounding peridotite. Subsequent re-melting of the resultant incompatible element enriched peridotite carries a geochemical signature of the mafic lithologies but not necessarily a record of their high melt productivity.  相似文献   

4.
Pliocene to recent volcanic rocks from the Bulusan volcanic complex in the southern part of the Bicol arc (Philippines) exhibit a wide compositional range (medium- to high-K basaltic-andesites, andesites and a dacite/rhyolite suite), but are characterised by large ion lithophile element enrichments and HFS element depletions typical of subduction-related rocks. Field, petrographic and geochemical data indicate that the more silicic syn- and post-caldera magmas have been influenced by intracrustal processes such as magma mixing and fractional crystallisation. However, the available data indicate that the Bicol rocks as a group exhibit relatively lower and less variable 87Sr/86Sr ratios (0.7036–0.7039) compared with many of the other subduction-related volcanics from the Philippine archipelago. The Pb isotope ratios of the Bicol volcanics appear to be unlike those of other Philippine arc segments. They typically plot within and below the data field for the Philippine Sea Basin on 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, implying a pre-subduction mantle wedge similar to that sampled by the Palau Kyushu Ridge, east of the Philippine Trench. 143Nd/144Nd ratios are moderately variable (0.51285–0.51300). Low silica (<55 wt%) samples that have lower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb, and moderately low Ce/Ce* ratios. Unlike some other arc segments in the Philippines (e.g. the Babuyan-Taiwan segment), there is little evidence for the involvement of subducted terrigenous sediment. Instead, the moderately low 143Nd/144Nd ratios in some of the Bicol volcanics may result from subduction of pelagic sediment (low Ce/Ce*, high Th/Nd, and high Th/Nb) and its incorporation into the mantle wedge via a slab-derived partial melt.  相似文献   

5.
Precise measurements of 238U-230Th-226Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr-Nd-Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess 238U-230Th-226Ra disequilibria with 230Th (18-24%) and 226Ra (9-21%) excesses, and there exists a positive correlation in a (226Ra/230Th)-(230Th/238U) diagram. The extent of 238U-230Th-226Ra disequilibria is markedly different in lavas of individual eruption ages, although the (230Th/232Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (230Th/238U)-(206Pb/204Pb) and (226Ra/230Th)-(206Pb/204Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high DU and DU/DTh, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios (206Pb/204Pb > 20.47) due to late Mesozoic metasomatism.  相似文献   

6.
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 Na8 and La/Yb and positively correlated with Mg#. Conversely, 230Th excesses are positively correlated with Na8 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 Na8. 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.  相似文献   

7.
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.  相似文献   

8.
We present new U-series isotope, 87Sr/ 86Sr, 143Nd/ 144Nd andtrace element data for a set of mafic, K-rich rocks from volcanoesin Central–Southern Italy. These shoshonitic to ultrapotassiclavas display strongly depleted high field strength element(HSFE) abundances with respect to other incompatible trace elementstogether with high but variable 87Sr/ 86Sr and low but variable143Nd/ 144Nd values. Such characteristics are thought to bedue to addition of subducted crust of variable amount and compositionto their mantle sources prior to magma genesis. Rocks from thenorthernmost region (i.e. Tuscan Magmatic Province and NorthernRoman Magmatic Province) display (230Th/ 238U) activity ratiosclose to radioactive equilibrium, suggesting that metasomatismof their sources occurred before 400 ka and recent melting tookplace at shallow depths, in the absence of garnet. A 238U excessof up to 27% has been measured in rocks from the NeapolitanDistrict. The occurrence of significant U excesses is a featureof arc magmas, but is typically seen in depleted lavas ratherthan in highly enriched rocks such as these (20 ppm Th). Thissignature requires a recent addition of a U-rich component tothe already strongly enriched mantle wedge beneath this regionof Italy. We suggest that a supercritical liquid, from deeplysubducted carbonate-rich sediments of the still-active Ionianslab, is responsible for generating a high-U, low-Th component,which produces the observed disequilibria. A 30% 230Th excessmeasured in a single unaltered sample from the Lucanian MagmaticProvince, along with a less marked negative HFSE anomaly, suggeststhe contribution of a deeper, garnet-bearing component in thegenesis of these magmas, plausibly related to the upwellingof asthenospheric mantle around the corner of the Ionian slab. KEY WORDS: U/Th disequilibria; potassic and ultrapotassic rocks; subduction: metasomatism; mantle melting; Central and Southern Italy  相似文献   

9.
Measurements of 238U-230Th-226Ra disequilibria, Sr-Nd-Pb-Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr-Nd-Pb-Hf isotopes (87Sr/86Sr = 0.70319-0.70322; 143Nd/144Nd = 0.51302-0.51305; 206Pb/204Pb = 19.04-19.06; 207Pb/204Pb = 15.53-15.54; 208Pb/204Pb = 38.61-38.65; 176Hf/177Hf = 0.28311-0.28312). All the samples possess (230Th/238U) disequilibrium with 230Th excesses, and they show systematic variations in (230Th/232Th) and (238U/232Th) ratios. The highest 226Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have (226Ra/230Th) ratio that are close to equilibrium. The 238U-230Th-226Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of 230Th and 226Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent-daughter nuclides in 230Th-238U and 226Ra-230Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.  相似文献   

10.
To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate = 1.8 cm/yr; 67°05′-70°26′N) display (230Th/238U) < 1 and (230Th/238U) > 1 with (230Th/238U) from 0.95 to 1.30 and have low U (11.3-65.6 ppb) and Th (33.0 ppb-2.40 ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e.g. 87Sr/86Sr = 0.70272-0.70301, εNd = 8.4-10.5, εHf = 15.4-19.6 (La/Yb)N = 0.28-0.84) encompass a narrow range of (230Th/232Th) (1.20-1.32) over a large range in (238U/232Th) (0.94-1.32), producing a horizontal array on a (230Th/232Th) vs. (238U/232Th) diagram and a large variation in (230Th/238U). However, the (230Th/238U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with (234U/238U) (1.001-1.031). Samples with low (230Th/238U) and elevated (234U/238U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium 234U/238U have high (230Th/238U) values (?1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the (230Th/238U) and (231Pa/235U) ratios in uncontaminated Kolbeinsey lavas, but low (231Pa/235U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment.  相似文献   

11.
In this study, we present Th–U disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the Nevados de Payachata volcano which erupted through ∼70 km of continental crust in the Central Volcanic Zone of the Andes (18°S, 69°W). Both lavas and mineral separates were analyzed by mass spectrometry for 238U–230Th disequilibria. The lavas are characterized either by 230Th enrichment or depletion relative to its parent nuclide 238U. Mineral separates are used to derive U–Th isochron ages and these ages compare favorably with inferred stratigraphic ages or K–Ar ages, although in one case the U–Th age is significantly older than the stratigraphic age. Despite relatively constant Sr, Nd, and Pb isotope ratios, the lavas display inverse trends in 230Th/238U versus Ce/Yb or Ba/Hf diagrams. These trends cannot be interpreted by simple two-component mixing. Rather, there must be three (and perhaps four components) involved in the genesis of the Parinacota lavas. A mantle wedge, a slab fluid, and a lower crustal component can be identified. A sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence. The existence of binary arrays can be explained by variable amounts of crustal material. The process of crust–mantle interaction must have been short enough to preserve U–Th disequilibrium (<300 ka). Received: 21 April 1999 / Accepted: 11 March 2000  相似文献   

12.
New data are presented for lavas from the Kamchatka Peninsula and the Aleutian arc. Radiogenic isotopes are strikingly homogeneous in the Kamchatka lavas and although incompatible trace element ratios exhibit much greater variability, much of this appears to result from shallow level, crystal fractionation. The data reveal little systematic across-arc change in radiogenic isotopes or trace element ratios. The Nd and Pb isotope data overlap those for Pacific MORB and limit the amount of sediment that could be incorporated in the mantle source region to <1% which is insufficient to account for the observed La/Ta ratios (50–68) in the high-MgO lavas. The lack of a positive correlation between La/Ta and depth to the slab suggests that melt–wall rock interaction was not important in controlling this ratio. Instead it is inferred that La/Ta increased during partial melting and that DLa/DTa = 0.11–0.06, possibly due to residual amphibole. Ba, U, Sr and Pb were added to the source by an aqueous fluid from the subducting slab and its inferred isotopic composition indicates that this fluid was derived from the altered oceanic crust. The addition of U resulted in a large range of (238U/232Th) from 0.79–2.48 similar to that observed in the Mariana and Lesser Antilles island arcs. However, (230Th/232Th) = 0.79–2.34, and the majority of samples lie close to the equiline indicating that the time since U/Th fractionation is generally ≥150 thousand years. The large width of the volcanic zone is assumed to reflect protracted fluid release from the subducting slab over the depth interval 170–380 km possibly coupled with extension across the Central Kamchatka Depression. The data from the Aleutians contrast strongly with those from Kamchatka. Radiogenic isotope data indicate that the Aleutian lavas contain a significant recycled sedimentary component, consistent with elevated 10Be/9Be ratios. The Aleutian lavas have (230Th/232Th) = 0.79–2.34 and exhibit a significant range of U/Th disequilibria [(238U/230Th) = 0.75–1.01]. However, 10Be/9Be is positively correlated with (238U/230Th) suggesting that the 10Be signal was carried by the aqueous fluid from the slab. The U/Th disequilibria for the Aleutian lavas lie close to a 30 thousand year reference line suggesting that this fluid was released from the slab ∼30 thousand years ago similar to recent estimates from the Lesser Antilles, Marianas, and Tonga-Kermadec island arcs from which it is inferred that fluid addition was the trigger for partial melting. Given that the rate of convergence in Kamchatka is similar to that in the Aleutians, Marianas and Tonga-Kermadec the inferred greater time since␣fluid release in Kamchatka requires further investigation. Received: 24 September 1997 / Accepted: 7 July 1998  相似文献   

13.
The off-rift central volcano of Öræfajökull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb–Nd–Sr isotopic data from Öræfajökull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr–Nd–Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-207Pb/204Pb, low-143Nd/144Nd end-member that resembles Öræfajökull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Öræfajökull exhibit the most Öræfajökull-like isotopic compositions, implying that a homogenised Öræfajökull source with positive ?207Pb is focused under the Öræfajökull centre and its associated flank zone. This then mixes laterally with the dominant negative-?207Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative Δ207Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in 206Pb/204Pb in mantle sources with negative Δ207Pb was probably generated by heterogeneity in 206Pb/204Pb and μ in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas.  相似文献   

14.
New major and trace element abundances, and Pb, Sr, and Nd isotopic ratios of Quaternary lavas from two adjacent volcanoes (South Pagan and the Central Volcanic Region, or CVR) located on Pagan Island allow us to investigate the mantle source (i.e., slab components) and melting dynamics within the Mariana intra-oceanic arc. Geologic mapping reveals a pre-caldera (780–9.4 ka) and post-caldera (<9.4 ka) eruptive stage for South Pagan, whereas the eruptive history of the older CVR is poorly constrained. Crystal fractionation and magma mixing were important crustal processes for lavas from both volcanoes. Geochemical and isotopic variations indicate that South Pagan and CVR lavas, and lavas from the northern volcano on the island, Mt. Pagan, originated from compositionally distinct parental magmas due to variations in slab contributions (sediment and aqueous fluid) to the mantle wedge and the extent of mantle partial melting. A mixing model based on Pb and Nd isotopic ratios suggests that the average amount of sediment in the source of CVR (~2.1%) and South Pagan (~1.8%) lavas is slightly higher than Mt. Pagan (~1.4%) lavas. These estimates span the range of sediment-poor Guguan (~1.3%) and sediment-rich Agrigan (~2.0%) lavas for the Mariana arc. Melt modeling demonstrates that the saucer-shaped normalized rare earth element (REE) patterns observed in Pagan lavas can arise from partial melting of a mixed source of depleted mantle and enriched sediment, and do not require amphibole interaction or fractionation to depress the middle REE abundances of the lavas. The modeled degree of mantle partial melting for Agrigan (2–5%), Pagan (3–7%), and Guguan (9–15%) lavas correlates with indicators of fluid addition (e.g., Ba/Th). This relationship suggests that the fluid flux to the mantle wedge is the dominant control on the extent of partial melting beneath Mariana arc volcanoes. A decrease in the amount of fluid addition (lower Ba/Th) and extent of melting (higher Sm/Yb), and an increase in the sediment contribution (higher Th/Nb, La/Sm, and Pb isotopic ratios) from Mt. Pagan to South Pagan could reflect systematic cross-arc or irregular along-arc melting variations. These observations indicate that the length scale of compositional heterogeneity in the mantle wedge beneath Mariana arc volcanoes is small (~10 km).  相似文献   

15.
U-series radioactive disequilibria in basaltic lavas have been used to infer many important aspects of melt generation and extraction processes in Earth’s mantle and crust, including the porosity of the melting zone, the solid mantle upwelling rate, and the melt transport rate. Most of these inferences have been based on simplified theoretical treatments of the fractionation process, which assume equilibrium partitioning of U-series nuclides among minerals and melt. We have developed a numerical model in which solid-state diffusion controls the exchange of U-series nuclides among multiple minerals and melt. First the initial steady-state distribution of nuclides among the phases, which represents a balance between diffusive fluxes and radioactive production and decay, is calculated. Next, partial melting begins, or a foreign melt is introduced into the system, and nuclides are again redistributed among the phases via diffusion. U-series nuclides can be separated during this stage due to differences in their diffusivity; radium in particular, and possibly protactinium as well, can be strongly fractionated from slower-diffusing thorium and uranium. We show that two distinct processes are not required for the generation of 226Ra and 230Th excesses in mid-ocean ridge basalts, as has been argued previously; instead the observed negative correlations of the (226Ra/230Th) activity ratio with (230Th/238U) and with the extent of trace element enrichment may result from diffusive fractionation of Ra from Th during partial melting of the mantle. Alternatively, the (226Ra/230Th) disequilibrium in mid-ocean ridge basalts may result from diffusive fractionation during shallow-level interaction of mantle melts with gabbroic cumulates, and we show that the results of the interaction have a weak dependence on the age of the cumulate if both plagioclase and clinopyroxene are present.  相似文献   

16.
Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ 18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.  相似文献   

17.
In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469-2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show 238U-230Th-226Ra disequilibria with excess 238U and 226Ra, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest (230Th/232Th) ratio compared to older Miyakejima eruptives, yielding the youngest 238U-230Th model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a (230Th/232Th)-(238U/232Th) diagram and (2) a positive linear correlation in a (226Ra/230Th)0 − 1/230Th diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of 230Th and 226Ra, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in (230Th/232Th) and (226Ra/230Th)0 ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr-Nd-Pb isotopic systems. In such cases, Th isotopes and (226Ra/230Th) ratio are excellent geochemical tracers of magmatic evolution.  相似文献   

18.
The extent to which U-series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for U, Th and Ra. We show that partial melting of mafic lower crust can only produce small disequilibria between 238U, 230Th and 226Ra. Crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect. Consequently, U-series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent, unless the timescales of silicic magma production within the crust are short compared to the half-life of 226Ra. Our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust. We show that the 226Ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and/or the involvement of mantle derived basalts less than a few thousand years old. Application to Mount St. Helens suggests that crystallisation of young mantle-derived magma is likely to be the dominant process in the formation of these dacites.  相似文献   

19.
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

20.
《International Geology Review》2012,54(12):1456-1474
We present new major element, trace element, and Sr–Nd–Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e.g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr–Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher 87Sr/86Sr for a given 143Nd/144Nd ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.  相似文献   

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