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1.
We present data for U and its decay series nuclides 230Th, 226Ra, 231Pa, and 210Po for 14 lavas from Kick’em Jenny (KEJ) submarine volcano to constrain the time-scales and processes of magmatism in the Southern Lesser Antilles, the arc having the globally lowest plate convergence rate. Although these samples are thought to have been erupted in the last century, most have (226Ra)/(210Po) within ±15% of unity. Ten out of 14 samples have significant 226Ra excesses over 230Th, with (226Ra)/(230Th) up to 2.97, while four samples are in 226Ra-230Th equilibrium within error. All KEJ samples have high (231Pa)/(235U), ranging from 1.56 to 2.64 and high 238U excesses (up to 43%), providing a global end-member of high 238U and high 231Pa excesses. Negative correlations between Sr, sensitive to plagioclase fractionation, and Ho/Sm, sensitive to amphibole fractionation, or K/Rb, sensitive to open system behavior, indicate that differentiation at KEJ lavas was dominated by amphibole fractionation and open-system assimilation. While (231Pa)/(235U) does not correlate with differentiation indices such as Ho/Sm, (230Th)/(238U) shows a slight negative correlation, likely due to assimilation of materials with slightly higher (230Th)/(238U). Samples with 226Ra excess have higher Sr/Th and Ba/Th than those in 226Ra-230Th equilibrium, forming rough positive correlations of (226Ra)/(230Th) with Sr/Th and Ba/Th similar to those observed in many arc settings. We interpret these correlations to reflect a time-dependent magma differentiation process at shallow crustal levels and not the process of recent fluid addition at the slab-wedge interface.The high 231Pa excesses require an in-growth melting process operating at low melting rates and small residual porosity; such a model will also produce significant 238U-230Th and 226Ra-230Th disequilibrium in erupted lavas, meaning that signatures of recent fluid addition from the slab are unlikely to be preserved in KEJ lavas. We instead propose that most of the 238U-230Th, 226Ra-230Th, and 235U-231Pa disequilibria in erupted KEJ lavas reflect the in-growth melting process in the mantle wedge (reflecting variations in U/Th, daughter-parent ratios, fO2, and thermal structure), followed by modification by magma differentiation at crustal depths. Such a conclusion reconciles the different temporal implications from different U-series parent-daughter pairs and relaxes the time constraint on mass transfer from slab to eruption occurring in less than a few thousand years imposed by models whereby 226Ra excess is derived from the slab.  相似文献   

2.
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   

3.
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   

4.
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   

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.
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   

7.
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   

8.
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   

9.
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   

10.
We present U-series, Sr-Nd-Pb isotope, and trace element data from the two principal volcanic chains on Luzon Island, developed over oppositely dipping subduction zones, to explore melting and mass transfer processes beneath arcs. The Bataan (western) and Bicol (eastern) arcs are currently subducting terrigenous and pelagic sediments, respectively, which have different trace element and isotopic compositions. The range of (230Th/238U) disequilibria for both arcs is 0.85-1.15; only lavas from Mt. Mayon (Bicol arc) have 230Th activity excesses. Bataan lavas have higher 87Sr/86Sr and lower 143Nd/144Nd than Bicol lavas (87Sr/86Sr = 0.7042-0.7046, 143Nd/144Nd = 0.51281-0.51290 vs. 87Sr/86Sr = 0.70371-0.70391, 143Nd/144Nd = 0.51295-0.51301) and both arcs show steep linear arrays towards sediment values on 207Pb/204Pb vs. 206Pb/204Pb diagrams. Analysis of incompatible element and isotopic data allows identification of a sediment component that, at least in part, was transferred as a partial melt to the mantle wedge peridotite. Between 1% and 5% sediment melt addition can explain the isotopic and trace element variability in the rocks from both arcs despite the differences in sediment supply. We therefore propose that sediment transfer to the mantle wedge is likely mechanically or thermally limited. It follows that most sediments are either accreted, reside in the sub-arc lithosphere, or are recycled into the convecting mantle. However, whole-sale sediment recycling into the upper mantle is unlikely in light of the global mid-ocean ridge basalt data. Fluid involvement is more difficult to characterize, but overall the Bicol arc appears to have more fluid influence than the Bataan arc. Rock suites from each arc can be related by a dynamic melting process that allows for 230Th ingrowth, either by dynamic or continuous flux melting, provided the initial (230Th/232Th) of the source is ∼0.6-0.7. The implication of either model is that inclined arrays on the U-Th equiline diagram may not have chronologic significance. Modeling also suggests that U-series disequilibria are influenced by the tectonic convergence rate, which dictates mantle matrix flow. Thus with slower matrix flow there is a greater degree of 230Th ingrowth. While other factors such as prior mantle depletion and addition of a subducted component may explain some aspects of U-series data, an overall global correlation between tectonic convergence rate and the extent of U-Th disequilibria may originate from melting processes.  相似文献   

11.
The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjöll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as 238U-230Th-226Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., 87Sr/86Sr increases systematically with increasing SiO2 content; this suite is suggested to have evolved in a magma chamber located at ∼600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, 87Sr/86Sr tends to decrease slightly with increasing SiO2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at ∼100 MPa beneath the Öskjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (226Ra/230Th), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <∼3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used.  相似文献   

12.
The Alaska–Aleutian island arc is well known for eruptingboth tholeiitic and calc-alkaline magmas. To investigate therelative roles of chemical and temporal controls in generatingthese contrasting liquid lines of descent we have undertakena detailed study of tholeiitic lavas from Akutan volcano inthe oceanic Aleutian arc and calc-alkaline products from Aniakchakvolcano on the continental Alaskan Peninsula. The differencesdo not appear to be linked to parental magma composition. TheAkutan lavas can be explained by closed-system magmatic evolution,whereas curvilinear trace element trends and a large range in87Sr/86Sr isotope ratios in the Aniakchak data appear to requirethe combined effects of fractional crystallization, assimilationand magma mixing. Both magmatic suites preserve a similar rangein 226Ra–230Th disequilibria, which suggests that thetime scale of crustal residence of magmas beneath both thesevolcanoes was similar, and of the order of several thousandyears. This is consistent with numerical estimates of the timescales for crystallization caused by cooling in convecting crustalmagma chambers. During that time interval the tholeiitic Akutanmagmas underwent restricted, closed-system, compositional evolution.In contrast, the calc-alkaline magmas beneath Aniakchak volcanounderwent significant open-system compositional evolution. Combiningthese results with data from other studies we suggest that differentiationis faster in calc-alkaline and potassic magma series than intholeiitic series, owing to a combination of greater extentsof assimilation, magma mixing and cooling. KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales  相似文献   

13.
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   

14.
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   

15.
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   

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.
238U–230Th disequilibria and Sr and O isotope ratios have been measured in a suite of samples from most of the known prehistoric and historic eruptions of Hekla volcano, Iceland. They cover the compositional range from basaltic andesite to rhyolite. Recent basalts erupted in the vicinity of the volcano and a few Pleistocene basalts have also been studied. Geochemical data indicate that the best tracers of magmatic processes in Hekla are the (230Th/232Th) and Th/U ratios. Whereas most geochemical parameters, including Sr, Nd and O isotopes, could be compatible with crystal fractionation, (230Th/232Th) and Th/U ratios differ in the basalts and basaltic andesites (1.05 and 3.2, respectively) and in the silicic rocks, dacites and rhyolites (0.98 and 3.4–3.7, respectively). This observation precludes fractional crystallization as the main differentiation process in Hekla. On the basis of these results, the following model is proposed: basaltic magmas rise in the Icelandic crust and cause partial melting of metabasic rocks, leading to the formation of a dacitic melt. The basaltic magma itself evolves by crystal fractionation and produces a basaltic andesite magma. The latter can mix with the dacitic liquid to form andesites. At higher levels in the magma chamber, the dacitic melt sometimes undergoes further differentiation by crystal fractionation and produces subordinate volumes of rhyolites. Together all these processes lead to a zoned magma chamber. However, complete zoning is achieved only when the repose time between eruptions is long enough to allow the production of significant volumes of dacitic magma by crustal melting. This situation corresponds to the large plinian eruptions. Between these eruptions, the so-called intra-cyclic activity is characterized by the eruption of andesites and basaltic andesites, with little crustal melting. The magmatic system beneath Hekla most probably was established during the Holocene. The shape and the size of the magma chamber may be inferred from the relationships between the composition of the lavas and the location of the eruption sites. In a cross-section perpendicular to Hekla's ridge, a bell-shaped reservoir 5 km wide and 7 km deep appears the most likely; its top could be at depth of 8 km according to geophysical data.  相似文献   

18.
Deep-sea coral has proved useful for paleoceanographic reconstructions and for documenting 14C-ages of water masses using 230Th-ages. However, for precise and accurate U-series dating, further information on coral-age structure, growth rate and diagenetic evolution is still needed. To document such processes, we used U-Th-226Ra systematics in a 40 cm diameter, Lophelia pertusa specimen collected in 1912 from the Eastern Atlantic (Sea of the Hebrides). External parts of the specimen are thought to have been alive when collected whereas more internal parts were likely dead. The “live-collected” and “dead” parts of the skeleton were sampled and analyzed for their 230Th, 232Th, 234U, 238U, 226Ra and Ba contents by thermal ionization and multicollector inductively coupled plasma mass spectrometry. 230Th/234U ratios in the most recent parts yielded ages of 86 ± 6 a and 92 ± 9 a, in agreement with the date of recovery. The older parts yielded 230Th ages ranging from 169 ± 15 to 211 ± 10 a (n = 5), but had a 31% higher U content than more recent parts of the skeleton. This raises concerns about the possibility of secondary diagenetic U uptake, although an environmentally controlled U/Ca shift between coral growth stages cannot be ruled out. 226Ra/Ba measurements were made, and model- 226Ra/Ba ages averaging 250 ± 12 yr were calculated for the older part, assuming a constant initial 226Ra/Ba ratio in bottom waters. These ages are slightly older than 230Th-ages, suggesting either that 226Ra/Ba ratios of ambient-seawater changed over time or that a diagenetic phenomenon have affected the U-series system, or both. Scanning electron microscope observations revealed bioerosion and secondary biomineralization in the older part of the coral skeleton, supporting the hypothesis that diagenetic processes may have influenced the ages obtained by the U-series toolbox. Modeled U-series ages for such an open system are discussed. However, a comparison between 14C- and 230Th-ages performed on both pristine and bioeroded parts of the coral gives coherent values (ca 450 a) for the preindustrial 14C-reservoir age of North Atlantic waters. It remains to be determined, however, whether diagenesis occurs rapidly over a short period of time, or whether it continues for longer periods. In the latter case, diagenetic processes would hamper paleoceanographic interpretations as well as the precise calculation of 14C ages of deep-water masses, based on comparative U/Th- and 14C-chronologies.  相似文献   

19.
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   

20.
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   

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