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

2.
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   

3.
We present U, Th, and Pa isotope data for young lavas from Costa Rica and Nicaragua in the Central American arc. Thorium isotopic ratios for Costa Rica and Nicaragua differ dramatically: Costa Rican lavas are characterized by low (230Th/232Th) (1 to 1.2) and, for four out of five lavas, (230Th/238U) greater than unity. Nicaraguan lavas have high (230Th/232Th) (2.2 to 2.7) and, for five of six samples, (230Th/238U) less than unity. All lavas have (231Pa/235U) greater than unity, with initial values ranging from 1.27 to 1.77, but those from Costa Rica have larger 231Pa excesses. There is a broad positive correlation between (231Pa/235U) and (230Th/238U) similar to the worldwide trend for arcs outlined by Pickett and Murrell (1997), although many of the Nicaraguan lavas skirt the high end of that trend. In greater detail, the Central American data appear to divide into separate high-(231Pa/235U) and low-(231Pa/235U) tiers. These tiers may be different because of either different residence times in the crust or different proportions of sedimentary components from the slab.Substantial (231Pa/235U) excesses (>1.5) in both Costa Rica and Nicaragua require a melting process that allows for enhanced daughter (231Pa) ingrowth. With increasing U addition, (231Pa/230Th) increases in a manner that cannot be explained adequately by aging of fluid components before partial melting and eruption. Thus, either some 231Pa is added from the slab, or melting-enhanced 231Pa ingrowth is greater in sources that have experienced a larger amount of slab-derived flux and a higher extent of melting. These observations can be explained if regions that have undergone greater extents of fluxing and melting have experienced these processes over a longer time interval than those that have had little flux added and little melt extracted. We propose a flux-ingrowth melting model in which corner flow in the mantle wedge supplies fresh hot mantle into a zone of slab fluid addition. Partial melting occurs in response to this fluxing. We assume critical melting at low porosity (∼10−3), rapid fluid flux to the melting region, and rapid melt transport. Solid mantle traverses the melting region over 105 to 106 yr, thereby allowing 231Pa and 230Th ingrowth from U retained in the residues of melt extraction. Magmas are aggregated from all parts of the melting regime, mixing melts from incipiently fluxed regions with those from sources that have experienced more extensive fluid addition, partial melting, and daughter nuclide ingrowth. With suitable assumptions about component addition from the slab, this flux-ingrowth model matches a wide range of U-series and trace element data from Costa Rican and Nicaraguan lavas, with required average extents of melting of ∼1 to 3% and 7 to 15%, respectively. Upwelling and/or extensive melt-rock reaction are not required to explain large (231Pa/235U) excesses in Central America or other arcs. On Th isotope equiline plots, the model produces linear arrays that resemble isochrons but that have no age significance. Instead, these arrays are generated by mixing of melts from sources that have experienced fluid addition and partial melting over a range of time intervals, as seems likely in arc source regions. Finally, the flux-ingrowth model predicts considerable 226Ra excesses for integrated magmas. If we assume that 226Ra is added continuously with the slab-derived fluid, the model predicts large and increasing (226Ra/230Th) with increasing melting and slab-component addition, without requiring the addition of a distinct late fluid.  相似文献   

4.
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.  相似文献   

5.
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   

6.
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   

7.
U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and (238U)/(232Th), (230Th)/(232Th), (230Th)/(238U) and (234U)/(238U) for 43 samples, most of which are minimally differentiated, along with (226Ra)/(230Th) and (231Pa)/(235U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial (230Th)/(238U) ranges from 1.1 to 1.6. (226Ra)/(230Th)o ranges between 0.9 and 1.8 while (231Pa)/(235U)o ranges between 1.0 and 2.0.Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume.Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia.  相似文献   

8.
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   

9.
《Geochimica et cosmochimica acta》1999,63(23-24):4119-4138
Measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr−1 for tholeiites and from 1 to 3 cm yr−1 for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr−1 for tholeiites and from 0.1 to 1 cm yr−1 for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.  相似文献   

10.
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   

11.
《Applied Geochemistry》2003,18(1):127-134
Based on U-series disequilibrium arguments there is good evidence for the presence of U-rich accessory minerals in the outer mantle. The very large excesses of 226Ra and 231Pa activity relative to 230Th, 238U, and 235U in most mid-ocean ridge basalts and some non-divergent plate-margin basalts are inconsistent with prevailing incompatibility models of U-series fractionation. Application of a fundamental principle of equilibrium balance reveals that in these instances more than half of the original U and Th remains behind in the residual outer mantle when basaltic magmas separate. One is forced to conclude that, in the outer mantle, U and Th do not occur primarily in major silicate minerals, where they would indeed be incompatible. Rather, they must be occurring as high concentration components in refractory accessory minerals. The precipitous concentration gradients bounding such minerals would allow for the operation of physical processes, such as alpha recoil and daughter diffusion, to produce paired disequilibrated phases. Daughter deficiencies would develop in the high concentration minerals, and daughter excesses in the surrounding low concentration major silicates. This would constitute a steady-state condition existing in the mantle before the onset of melting. Subsequent preferential melting of the matrix silicates would readily result in the disequilibria observed in basalts.  相似文献   

12.
U-Pb systems were examined in samples (ranging from 4 to 10 cm3 in volume) of ore material taken from along a 3.5-m profile across a zone of U mineralization exposed in an underground mine at the Strel’tsovskoe U deposit in eastern Transbaikalia. The behaviors of two isotopic U-Pb systems (238U-206Pb and 235U-207Pb) are principally different in all samples from our profile. While the individual samples are characterized by a vast scatter of their T(206Pb/238U) age values (from 112 to 717 Ma), the corresponding T(207Pb/235U) values vary much less significantly (from 127 to 142 Ma) and are generally close to the true age of the U mineralization. The main reason for the distortion of the U-Pb system is the long-lasting (for tens of million years) migration of intermediate decay products in the 238U-206Pb(RD238U) in the samples. This process resulted in the loss of RD238U from domains with high U concentrations and the subsequent accommodation of RD238U at sites with low U concentrations. The long-term effect of these opposite processes resulted in a deficit or excess of 206Pb as the final product of 238U decay. The loss or migration of RD238U are explained by the occurrence of pitchblende in association with U oxides that have higher Si and OH concentrations than those in the pitchblende and a higher +6U/+4U ratio. The finely dispersed character of the mineralization and the loose or metamict texture of the material are the principal prerequisites for RD238U loss and an excess of 206Pb in adjacent domains with low U concentrations. Domains with low U contents in the zone with U mineralization serve as geochemical barriers (because of sulfides contained in them) at which long-lived RD238U(226Ra, 210Po, 210Bi, and 210Pb) were accommodated and subsequently caused an excess of 206Pb. The 235U-207Pb system remained closed because of the much briefer lifetime of the 235U decay products. This may account for the significant discrepancies between the T(206Pb/238U) and T(207Pb/235U) age values. RD238U was most probably lost via the migration of radioisotopes at the middle part and end of the 238U family (starting with 226Ra). The heavy Th, Pa, and U radioisotopes (234Th, 234Pa, 234U, and 230Th) that occur closer to the beginning of 238U decay, before 226Ra, only relatively insignificantly participated in the process. Our results show that the loss and migration of RD238U are, under certain conditions, the main (or even the only) process responsible for the distortion of the U-Pb system.  相似文献   

13.
The 230Th/234U/238U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system 230Th/234U/238U ages is elevated 234U/238Uinitial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have 234U/238Uinitial values close to seawater, there is a need for age validation. Redundant 230Th/234U/238U and 231Pa/235U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both 230Th/234U/238U and 231Pa/235U ages in fragments as small as 500 mg. We have obtained excellent agreement between 230Th/234U/238U and 231Pa/235U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the 230Th/234U/238U age accuracy.Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby 238U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of 230Th/234U/238U and 231Pa/235U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for 231Pa and from undetermined sources currently limit the 231Pa/235U age uncertainty to about ±2.5%. U isotope data and 230Th/234U/238U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.  相似文献   

14.
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   

15.
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   

16.
Magmas erupted at mid-ocean ridges (MORB) result from decompression melting of upwelling mantle. However, the mechanism of melt transport from the source region to the surface is poorly understood. It is debated whether melt is transported through melt-filled conduits or cracks on short time scales (<∼ 103 yrs), or whether there is a significant component of slow, equilibrium porous flow on much longer time scales (>∼ 103-104 yrs). Radiogenic excess 226Ra in MORB indicates that melt is transported from the melting region on time scales less than the half life of 226Ra (∼1600 yrs), and has been used to argue for fast melt transport from the base of the melting column. However, excess 226Ra can be generated at the bottom of the melt column, during the onset of melting, and at the top of the melt column by reactive porous flow. Determining the depth at which 226Ra is generated is critical to interpreting the rate and mechanism of magma migration. A recent compilation of high quality U-series isotope data show that in many young basalts, 226Ra excess in MORB is negatively correlated with 230Th excess. The data suggest that 226Ra excess is generated independently of 230Th excess, and cannot be explained by “dynamic” or fractional melting, where observed radiogenic excesses are all generated at the base of the melt column. One explanation is that the negative correlation of activity ratios is a result of mixing of slow moving melt that has travelled through reactive, low-porosity pathways and relatively fast moving melt that has been transported in unreactive high-porosity channels. We investigate this possibility by calculating U-series disequilibria in a melting column in which high-porosity, unreactive channels form within a low-porosity matrix that is undergoing melting. The results show that the negative correlation of 226Ra and 230Th excesses observed in MORB can be produced if ∼60% of the total melt flux travels through the low-porosity matrix. This melt maintains 226Ra excesses via chromatographic fractionation of Ra and Th during equilibrium transport. Melt that travels through the unreactive, high-porosity channels is not able to maintain significant 226Ra excesses because Ra and Th are not fractionated from each other during transport and the transport time for melt in the channels to reach the top of the melt column is longer than the time scale for 226Ra excesses to decay. Mixing of melt from the high porosity channels with melt from the low-porosity matrix at the top of the melting column can produce a negative correlation of 226Ra and 230Th excesses with the slope and magnitude observed in MORB. This transport process can also account for other aspects of the geochemistry of MORB, such as correlations between La/Yb, αSm/Nd, and Th/U and 226Ra and 230Th excess.  相似文献   

17.
Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29-30, 2004 was analyzed for 238U- and 232Th-series nuclides to determine initial 230Th, 226Ra, 210Pb, 210Po, 228Ra, and 228Th activities. The initial (210Po/210Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its 210Po upon eruption. The (230Th/232Th) and (238U/232Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of 210Pb, 226Ra, and 230Th in the phonolite contrast with those of the trachyandesites, in that 210Pb and 230Th are both strongly enriched with respect to 226Ra. In addition, the phonolite had a small deficit in 228Ra with respect to 232Th. The Ra deficits likely resulted from partitioning into feldspars and hornblende in a time frame that extended over several decades to a century. These disequilibria can be explained by crystal fractionation at a decreasing rate through time at an average of 3-5 × 10−3 year−1. The calculated crystallization rate is about an order of magnitude faster than has been calculated for most other phonolites and trachytes, and about half that calculated for crystallization of the Makaopuhi lava lake. These data imply that the 2004 magma was not the differentiated cap of a much larger body that remained at depth. Instead, it was likely the residue of a relatively small body of more mafic magma that was injected into the crust southeast Tristan and underwent extensive and rapid crystal fractionation before it erupted.  相似文献   

18.
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   

19.
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   

20.
A suite of peralkaline trachytes from Longonot volcano, Kenya,which erupted during the last 6000 years, has been analysedfor major and trace elements, Pb and Nd isotopes, and U–Th–Radisequilibria. The lavas are divided into three stratigraphicgroups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas,designated LMx1 and LMx2, which, respectively, pre-date andpost-date the Lt2 lavas. Major and trace elements are consistent,with up to 37% within-group fractional crystallization of predominantlyalkali feldspar. The parental magma for the different trachytegroups had a more mafic composition—probably hawaiitic.Nd and Pb isotopes show minimal variation, both within and betweenmagma groups, and indicate that up to 10% comendite magma fromthe neighbouring Olkaria volcanic field may have intermixedwith the Longonot magma. (230Th/238U) disequilibria indicatethat limited U/Th fractionation occurred during the past 10kyr, whereas (226Ra/230Th) disequilibria reflect the effectof alkali feldspar fractionation >8 kyr ago in the Lt2a lavas,between 3 and 7 kyr ago in the Lt2b lavas and in the past 3kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2blavas are interpreted using a model that combines the equationsof radioactive decay and in-growth with Rayleigh crystallizationto give fractionation rates of about 0·2 x 10–4/yearfor the evolution of hawaiite to trachyte, but more rapid ratesof up to 3 x 10–4/year for fractionation within the trachytesequence. (226Ra/230Th) from two whole-rock–alkali feldsparpairs are interpreted to show the crystals formed at 5800 yearsBP (Lt2b) and 2800 years BP (Lt3), implying that phenocrystformation continued almost up to the time of eruption. The resultsstrongly indicate that fractionated magmas can be stored forperiods on the order of 1000–2500 years prior to eruption,whereas other magmas were erupted as fractionation was proceeding. KEY WORDS: trachyte; magma chambers; u-series; Kenya  相似文献   

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