3112矿床深部矿体偏镭的放射性同位素研究

应用铀、钍、锕铀系中~(228)Ra、~(230)Th、~(231)Pa、~(232)Th等同位素与~(38)U、~(226)Ra之间存在量关系探索了3112矿床深部矿体偏镭的原因。结果表明该矿床深部偏镭是外来镭后期叠加所致。分析数据显示,严重的偏镭现象难以用局部偏铀地段的镭流失予以补偿。这就提示有隐伏镭源体铀存在的可能性。本文所述方法也适用于地表放射性异常性质的判别研究。     

Partial melting processes above subducting plates: Constraints from Pa-U disequilibria

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.     

(Pa/U)-(Th/U) of young mafic volcanic rocks from Nicaragua and Costa Rica and the influence of flux melting on U-series systematics of arc lavas

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.     

Fractionation of Th, Pa, and Be induced by particle size and composition within an opal-rich sediment of the Atlantic Southern Ocean

This study centers on the question: How sensitive are ²³¹Pa/²³⁰Th and ¹⁰Be/²³⁰Th to sediment composition and redistribution? The natural radionuclides ²³¹Pa, ²³⁰Th and ¹⁰Be recorded in deep sea sediments are tracers for water mass advection and particle fluxes. We investigate the influence of oceanic particle composition on the element adsorption in order to improve our understanding of sedimentary isotope records. We present new data on particle size specific ²³¹Pa and ¹⁰Be concentrations. An additional separation step, based on settling velocities, led to the isolation of a very opal-rich phase. We find that opal-rich particles contain the highest ²³¹Pa and ¹⁰Be concentrations, and higher ²³¹Pa/²³⁰Th and ¹⁰Be/²³⁰Th isotope ratios than opal-poor particles. The fractionation relative to ²³⁰Th induced by the adsorption to opal-rich particles is more pronounced for ²³¹Pa than for ¹⁰Be. We conclude that bulk ²³¹Pa/²³⁰Th in Southern Ocean sediments is most suitable as a proxy for past opal fluxes. The comparison between two neighboring cores with rapid and slow accumulation rates reveals that these isotope ratios are not influenced significantly by the intensity of sediment focusing at these two study sites. However, a simulation shows that particle sorting by selective removal of sediment (winnowing) could change the isotope ratios. Consequently, ²³¹Pa/²³⁰Th should not be used as paleocirculation proxy in cases where a strong loss of opal-rich material due to bottom currents occurred.     

U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower ²³²Th/²³⁰Th (∼10,000) than those immediately adjacent to the banks (24,000-31,000) and a (²³¹Pa/²³⁰Th) near the production ratio (∼0.1). The change in ²³²Th/²³⁰Th and (²³¹Pa/²³⁰Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of ²³⁰Th and ²³¹Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both ²³²Th/²³⁰Th (∼13,000) and (²³¹Pa/²³⁰Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ²³⁴U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ²³⁴U. Modern marine carbonates are shown to have ²³²Th/²³⁰Th ratios that reflect the local seawater in which they formed.     

Th/U/U and Pa/U ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry

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/²³⁸U_initial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have ²³⁴U/²³⁸U_initial 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.     

U-TH-PA-RA study of the Kamchatka arc: new constraints on the genesis of arc lavas

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.     

U-series disequilibria in Kick’em Jenny submarine volcano lavas: A new view of time-scales of magmatism in convergent margins

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.     

Assessment of USGS BCR-2 as a Reference Material for Silicate Rock U-Pa Disequilibrium Measurements

The accuracy of ²³¹Pa-²³⁵U measurements can be readily assessed using a secular equilibrium reference material (RM), but a secular equilibrium RM is also required to calibrate the ²³³Pa spike used in ²³¹Pa determinations. The only silicate RM commonly accepted to be in secular equilibrium is Table Mountain Latite (TML) and so an additional reference is required. Our measurements on the widely available USGS BCR-2 (Basalt Columbia River) rock powder yielded (²³¹Pa/²³⁵U) = 0.997 ± 0.013 2s (n = 10), indicating its value as a secondary reference to test the fidelity of U-Pa determinations. Such a reference material additionally provides a useful check on data reduction, which our literature survey highlights can lead to discrepancies of up to 53% between reported (²³¹Pa/²³⁵U) activity ratios and corresponding U and Pa concentration data.