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.     

(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.     

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.     

Determining melt productivity of mantle sources from U-Th and U-Pa disequilibria; an example from Pico Island, Azores

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

Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones

Mineral/melt trace element partition coefficients were determined for rutile (TiO₂) for a large number of trace elements (Zr, Hf, Nb, Ta, V, Co, Cu, Zn, Sr, REE, Cr, Sb, W, U, Th). Whilst the high field strength elements (Zr, Hf, Nb, Ta) are compatible in rutile, other studied trace elements are incompatible (Sr, Th, REE). In all experiments we found D_Ta > D_Nb, D_Hf > D_Zr and D_U > D_Th. Partition coefficients for some polyvalent elements (Sb, W, and Co) were sensitive to oxygen fugacity. Melt composition exerts a strong influence on HFSE partition coefficients. With increasing polymerization of the melt, rutile/melt partition coefficients for the high field strength elements Zr, Hf, Nb and Ta increase about an order of magnitude. However, D_Nb/D_Ta and D_Hf/D_Zr are not significantly affected by melt composition. Because D_U ? D_Th, partial melting of rutile-bearing eclogite in subducted lithosphere may cause excesses of ²³⁰Th over ²³⁸U in some island arc lavas, whereas dehydration of subducted lithosphere may cause excesses of ²³⁸U over ²³⁰Th. From our partitioning results we infer partition coefficients for protactinium (Pa) which we predict to be much lower than previously anticipated. Contrary to previous studies, our data imply that rutile should not significantly influence observed ²³¹Pa-²³⁵U disequilibria in certain volcanic rocks.     

Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

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.     

The effect of assimilation, fractional crystallization, and ageing on U-series disequilibria in subduction zone lavas

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.     

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using U, Th, and Pa excesses

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, ε_Nd = 8.4-10.5, ε_Hf = 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.     

Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: inferences from 238U-230Th-226Ra and 235U-231Pa disequilibria

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.     

U-series disequilibria in volcanic rocks from the Canary Islands: Plume versus lithospheric melting

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

A U-series study of lavas from Kamchatka and the Aleutians: constraints on source composition and melting processes

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

Mantle dynamics and mantle melting beneath Niuafo’ou Island and the northern Lau back-arc basin

A suite of young volcanic basaltic lavas erupted on the intra-plate island of Niuafo’ou and at active rifts and spreading centres (the King’s Triple Junction and the Northeastern Lau Spreading Centre) in the northern Lau Basin is used to examine the pattern of mantle flow and the dynamics of melting beneath this complex back-arc system. All lavas contain variable amounts of a subduction related component inherited from the Tonga subduction zone to the east. All lavas have higher ⁸⁷Sr/⁸⁶Sr, lower ¹⁴³Nd/¹⁴⁴Nd and more radiogenic Pb isotope compositions than basalts erupted at the Central Lau Spreading Centre in the central Lau Basin, and are interpreted as variable mixtures of subduction-modified, depleted upper mantle, and mantle residues derived from melting beneath the Samoan Islands which has leaked through a tear in the subducting Pacific Plate beneath the Vitiaz Lineament at the northern edge of the Lau Basin. Our data can be used to map out the present-day distribution of Samoan mantle in this region, and show that it influences the compositions of lavas erupted as far as 400 km from the Samoan Islands. The distribution of Samoan-influenced lavas implies south- and southwest-wards mantle flow rates of >4 cm/year. U-series disequilibria in historic Niuafo’ou lavas have average (²³⁰Th/²³⁸U) = 1.13, (²³¹Pa/²³⁵U) = 2.17, (²²⁶Ra/²³⁰Th) = 2.11, and together with major and trace element data require ∼5% partial melting of mantle at between 2 and 3 GPa, with a residual porosity of 0.002 and an upwelling rate of 1 cm year⁻¹. We suggest that intraplate magmatism in the northern Lau Basin results from decompression melting during southward flow of mantle from beneath old (110–120 Ma), relatively thick Pacific oceanic lithosphere to beneath young (<5 Ma), thinner oceanic lithosphere beneath the northern Lau Basin.     

Subduction and melting processes inferred from U-Series, Sr-Nd-Pb isotope, and trace element data, Bicol and Bataan arcs, Philippines

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.     

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-series evidence for crustal involvement and magma residence times in the petrogenesis of Parinacota volcano, Chile

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     

The extent of U-series disequilibria produced during partial melting of the lower crust with implications for the formation of the Mount St. Helens dacites

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.     

Uranium-series fractionation in mafic extrusives

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

Distinguishing lower and upper crustal processes in magmas erupted during the buildup to the 7.7 ka climactic eruption of Mount Mazama, Crater Lake, Oregon, using 238U–230Th disequilibria

Uranium-series isotope ratios determined for 35 volcanic rocks and 4 glass separates erupted from ~36 to 4.8 ka at Mt. Mazama, Crater Lake, Oregon, identify both ²³⁰Th-excess and ²³⁸U-excess components. U–Th isotope compositions cover a wide range, exceeding those previously measured for the Cascade arc. Age-corrected (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) activity ratios range from 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % ²³⁰Th-excess to 8.8 % ²³⁸U-excess), respectively. The most distinctive aspect of the data set is the contrast in U–Th isotope ratios between low and high Sr (LSr, HSr) components that have been previously identified in products of the 7.7 ka caldera-forming climactic eruption and preclimactic rhyodacite lavas. The LSr component exclusively contains ²³⁸U-excess, but the HSr component, as well as more primitive lavas, are marked by ²³⁰Th-excess. ²³⁰Th-excesses such as those recorded at Mt. Mazama are commonly observed in the Cascades. Melting models suggest that high ²³⁰Th-excesses observed in the more primitive lavas evolved through mixing of a mantle melt with a partial melt of a mafic lower crustal composition that contained garnet in the residuum that was produced through dehydration melting of amphibolite that was initially garnet free. Dehydration melting in the lower crust offers a solution to the “hot-slab paradox” of the Cascades, where low volatile contents are predicted due to high slab temperatures, yet higher water contents than expected have been documented in erupted lavas. The ²³⁸U-excess observed at Mt. Mazama is rare in Cascade lavas, but occurs in more than half of the samples analyzed in this study. Traditionally, ²³⁸U-excess in arc magmas is interpreted to reflect slab fluid fluxing. Indeed, ²³⁸U-excess in arcs is common and likely masks ²³⁰Th-excess resulting from lower crustal interaction. Isotopic and trace element data, however, suggest a relatively minor role for slab fluid fluxing in the Cascades. We propose that ²³⁸U-excess reflects melting and assimilation of young, hydrothermally altered upper crust. The processes related to generating ²³⁸U-excess are likely important features at Mt. Mazama that accompanied development of a large-scale silicic magma chamber that led to the caldera-forming eruption.     

Petrogenesis and 230Th-238U disequilibrium at Mt. Shasta,California, and in the Cascades

Petrogenesis at Mt. Shasta is dominated by mixing of magmas and/or assimilation of wall rock, as is shown by petrographic, major and trace element chemistry, and ²³⁸U-²³⁰Th disequilibrium data. At least three end- members are involved in these mixing processes. Lavas of very young Cascades lavas, from Mt. Garibaldi in the north to Lassen Peak in the south, are characterized by a large range of thorium isotopic ratios, although series of samples from single volcanoes are characterized by approximately constant (²³⁰Th/²³²Th). There is a monotonic decrease in this ratio from Crater Lake south through Lassen Peak, perhaps reflecting increasing thickness of the underlying crust. Th/U fractionation in Cascades lavas, as evidenced by (²³⁰Th/²³⁸U) 1, is in the opposite sense to that in most island arc lavas. This trend suggests that fluid transport, which is thought to produce uranium enrichment in island arc, is lacking or somehow modified in the petrogenesis of the Cascades lavas.     

Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from U-Th-Ra systematics

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.