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1.
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   

2.
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   

3.
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   

4.
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   

5.
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   

6.
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   

7.
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   

8.
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   

9.
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   

10.
《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   

11.
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   

12.
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   

13.
Datations of ancient lavas from the Chaîne des Puys through the 230Th-238U radioactive disequilibrium method confirm the eruption of several basaltic or slightly differentiated lavas around 40,000 years ago. The study of (230Th/232Th)0 initial ratios of these lava flows clearly demonstrates the influence of a crustal contamination of magmas superimposed to crystal fractionation. This contamination probably affects many trace elements, in particular, U, Th and Sr. A model based on the (230Th/232Th)0 initial ratio variations of non-contaminated lavas permits to consider that the first eruptions in the Chaîne des Puys could have occurred about 100,000 years ago.  相似文献   

14.
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   

15.
活火山是指1万年来有过喷发历史的全新世火山。火山的高分辨年代学对火山灾害评估和火山分类具有重要意义。对于缺乏历史记载的全新世火山,直接对火山岩进行同位素定年很困难。本文利用具有高时间分辨率的镭-钍-铀非平衡确定中国东部年轻火山的年龄。根据镭-钍-铀同位素,海南岛的马鞍岭和雷虎岭是全新世火山(马鞍岭:4.3ka;雷虎岭:4.7ka);镜泊湖火山(4.9ka)也是全新世火山;龙岗火山存在晚更新世和全新世活动(7.0ka,15.0ka);大兴安岭阿尔山和诺敏河Ra/Th非平衡消失但~(230)Th/~(238)U非平衡显著,属于晚更新世喷发(阿尔山:63ka;诺敏河:71ka)。海南岛的马鞍岭火山、雷虎岭火山和东北地区的龙岗火山、镜泊湖火山,是4座活火山。至于东北地区的阿尔山和诺敏河火山是否是活火山,有待测试更多样品的Ra/Th同位素。五大连池老黑山和火烧山有历史喷发记录,这与它们都存在显著Ra/Th非平衡一致。五大连池老黑山和火烧山的岩浆滞留年龄分别小于4.2ka和3.2ka,岩浆上升速率 18～23m/y。  相似文献   

16.
Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ18Oolivine = 5.17 ± 0.08‰; 87Sr/86Sr = 0.7029 to 0.7031; εNd = +5.7 to +7.1; 187Os/188Os = 0.1481 to 0.1750; 206Pb/204Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ18Oolivine = 4.87 ± 0.18‰; 87Sr/86Sr = 0.7031 to 0.7032; εNd = +5.0 to +6.4; 187Os/188Os = 0.1421 to 0.1460; 206Pb/204Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset.Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (?110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites.The HIMU affinities of the lavas, in conjunction with the low δ18Oolivine and high 206Pb/204Pb for La Palma, and elevated 187Os/188Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic 206Pb/204Pb, 187Os/188Os, elevated Re/Os and Pt/Os, and low-δ18O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).  相似文献   

17.
The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing.  相似文献   

18.
We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr-Nd-Hf isotope data. However, we focus in particular on Pb isotope data and 238U/204Pb and 232Th/204Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated 238U/204Pb, 232Th/238U and 232Th/204Pb in continental mantle roots and (c) the 238U/204Pb and 232Th/204Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM).Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU.U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between 206Pb/204Pb = 19.93-20.25, 207Pb/204Pb = 15.63-15.66 and 208Pb/204Pb = 39.72-40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. 238U/204Pb and 232Th/204Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle.The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower 238U/204Pb, 232Th/204Pb and 232Th/238U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high 232Th/238U also suggest that the metasomatism was due to carbonatitic liquids.When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high 238U/204Pb and 232Th/204Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high 238U/204Pb and 232Th/204Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have 232Th/204Pb ratios exceeding those known from terrestrial basalts.  相似文献   

19.
《Geochimica et cosmochimica acta》1999,63(23-24):4081-4099
New major and trace element data, and Sr, Nd, Pb, U, Th and Ra isotopes are presented for prehistoric and historic lavas from Lanzarote in the Canary Islands. These rocks are amongst the most primitive found on intraplate ocean islands ranging in composition from basanite to alkali basalt, with MgO contents >9.3% and Mg numbers >67. The youngest are from two of the three 1824 vents, the largest group of samples is from the best known eruption episode, the 1730–36 Timanfaya eruptions, a smaller group of samples are from the northeast Corona region (∼50 ky) and the oldest samples are from the Famara complex and basement massif. The rocks have some of the characteristics of HIMU OIB, including high Ce/Pb, Nb/Ce and low Nb/U and restricted 87Sr/86Sr (0.70209–0.70332). There is significant 230Th/238U disequilibrium (230Th excesses range from 6–76%) with some of the intermediate silica composition Corona samples showing the greatest disequilibrium. The historic samples exhibit 226Ra excess. The major and trace element data have undergone fractionation corrections to Mg numbers of 70, requiring <5% olivine fractionation, and these inferred primary compositions are used to evaluate a number of melt generation and mixing models.The fractionation-corrected compositions for the 1824 and the 1730–36 have been modelled as 1–4% melts of a source similar to primitive mantle. However, Yb is incompatible, and so the amounts of residual garnet were < ∼5%, suggesting that there was no significant contribution from garnet pyroxenite source rocks. Rather the REE and the FeO contents are both consistent with melting in the garnet-spinel transition, at depths of 60–70 km. (230Th/238U) increases slightly with increasing La/Yb in the younger rocks, and they require some form of dynamic melting model. In the preferred model the upwelling rate is kept constant, and the differences in the degrees of melting are attributed to different lengths of the melt column, with the smaller degree melts being extracted from greater depths. Strikingly, (226Ra/230Th) increases with increasing degrees of melting, and so it reflects the time since extraction from the melt column rather than variations in the melting processes. Intra-suite minor and trace element variations are due to magma mixing, and not to progressive changes in the degrees of partial melting, and it is envisaged that such magma mixing occurred during the dynamic melting processes. Dynamic melting at depths of 60–70 km suggests that the regional uplift around the Canary Islands is at least in part due to thermal erosion of the underlying lithosphere.Variations of average Ce/Yb, Tb/Yb, (230Th/238U), SiO2 and lithospheric age for different OIB highlight how the smaller degree melts tend to be generated at greater depths, and the mean pressure of melting increases with the thickness of the lithospheric lid. However, there is no general link between (230Th/238U) and the lithospheric age or thickness, and hence the integrated degrees of melting. High buoyancy fluxes result in higher degrees of melting and low (230Th/238U) (Chabaux and Allegre, 1994), but for OIB generated within low buoyancy plumes, (230Th/238U) and the degrees of melting primarily depend on the depths of melt extraction. Differences in the average composition of low buoyancy OIB depend on the thickness of the overlying lithosphere Ellam 1992, Haase 1996, and the differences within an OIB suite, such as between the 1730-36 and 1824 lavas on Lanzarote, depend on the depth of extraction from the melt column.  相似文献   

20.
The results of our combined U-Pb, Rb-Sr, and Sm-Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U-Pb isotope systematics of the Moon. The Rb-Sr, Sm-Nd, and 238U-206Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The 235U-207Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the 206Pb-207Pb system nor U-Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm-Nd, Rb-Sr, and 238U-206Pb systems. The initial 87Sr/86Sr of 10017 is 0.69941 ± 7 and the initial εNd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U-Pb isochrons, are 206Pb/204Pbi = 31 ± 11 and 207Pb/204Pbi = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100-600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U-Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.  相似文献   

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