238U–230Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift

doi:10.1016/j.chemgeo.2006.05.002

Chemical Geology

Volume 234, Issues 1–2, 30 October 2006, Pages 148-168

https://doi.org/10.1016/j.chemgeo.2006.05.002 Get rights and content

Abstract

Trace element and U-series isotope analyses are presented for a suite of recent (< 10 ka) basalts from the axial portion of the Kenya rift. Samples from throughout the rift have LREE-enriched patterns with HREE > 10 × chondrite and the LREE between 60 and 200 × chondrite. REE fractionation is consistent with melting a garnet lherzolite source region with between 2% and 6% modal garnet. Other trace element ratios are distinct from OIB, notably Zr/Hf which ranges from 43 to 48, whilst at a given Zr content the Zr/Hf ratio is significantly greater than that found in OIB. (²³⁸U/²³²Th) range from 0.362 to 1.036, (²³⁰Th/²³²Th) from 0.503 to 1.109, with (²³⁰Th/²³⁸U) ranging from 0.783 to 2.966. All but two samples are in ²³⁰Th excess or in secular equilibrium. Samples with elevated (²³⁸U/²³²Th), also have Rb/Cs > 120, but unexceptional ²⁰⁸Pb^⁎/²⁰⁶Pb^⁎ and hence κ_Pb values. These samples have experienced U and Cs loss and are excluded from further consideration. Of the unaltered samples, all have (²³⁸U/²³²Th) generally lower than OIB, with maximum values of < 0.8, and some < 0.6. The maximum (²³⁰Th/²³⁸U) is 1.39, similar to OIB. Although none of the basalts has a primary composition, (²³⁰Th/²³⁸U) does not vary systematically with indices of fractionation, and comparison with evolved rocks from Kenya indicates that ²³⁸U–²³⁰Th disequilibrium in the basalts is not the product of fractionation and crustal residence, but a product of melt generation. The maximum (²³⁰Th/²³⁸U) that can be generated by batch melting, assuming a source mineralogy consistent with the REE variation is 1.05 and so the variation in (²³⁰Th/²³⁸U) is attributed to more complex models of melt generation and/or transport. Both dynamic melting and equilibrium porous flow suggest mantle upwelling rates of ≤ 2 cm year^− 1. It is suggested that the Kenya basalts represent melts derived from lithospheric mantle that has been thermally reactivated by and incorporated into the underlying (East African) mantle plume.

Introduction

Continental basaltic magmatism reveals a compositional diversity that is somewhat greater than that observed in the oceanic realm. While in part this relates to the interaction of mafic magmas with the more easily fusible parts of the continental crust, much compositional diversity has been attributed to magma sources located in the continental mantle lithosphere—that part of the mantle attached to the base of the continents, forming part of the plate structure that does not take part in mantle convection. Petrological investigations indicate that lithospheric melting occurs at slightly lower temperatures than melting in the asthenosphere because it is controlled by volatile-bearing phases, notably amphibole, phlogopite and, at greater depths, carbonate (e.g. Gallagher and Hawkesworth, 1992). Moreover, basalts from numerous CFB provinces and extensional continental regimes reveal compositional trends that are consistent with a pattern of early lithospheric melts followed by plume-related or asthenospheric magmas once the lithosphere has thinned (Perry et al., 1987, Fitton et al., 1991, Turner et al., 1996).

Physical models of continental magmatism that include lithospheric melting imply a period of conductive heating from an underlying mantle plume, leading to the generation and accumulation of melt within the mantle followed by extraction (Gallagher and Hawkesworth, 1992, Leeman and Harry, 1993). Alternatively, heating of the lithosphere may result in so-called thermal erosion during which lithospheric mantle is incorporated into the underlying asthenosphere or mantle plume where it is either removed from the system by convection or contributes to plume-related magmatism (Davies, 1994, Thoraval et al., 2006). These alternatives probably represent two extremes of a spectrum of processes but each implies a contrasting melting process. The first model is static and more akin to simple batch melting, whereas the second involves a dynamic system in which melt is extracted by compaction of a convecting matrix, similar to the predominant process operating beneath mid-ocean ridges and within mantle plumes. U-series isotopes have the potential to distinguish between these processes (e.g. McKenzie, 1985, Iwamori, 1994, Elliott, 1997, Zou and Zindler, 2000, Asmerom et al., 2000), and so inform physical models of plume–lithosphere interaction during continental extension and break-up.

Within the Kenya rift, mafic melts of variable composition have been generated over the past 35 Ma and reflect the interaction between the underlying mantle plume with the overlying lithosphere (e.g. Macdonald et al., 2001). Geophysical studies have revealed the detailed structure of the crust and mantle both along the rift and across the strike of the rift, and in particular, the presence of low velocity asthenospheric mantle with up to 6% partial melt beneath the axis of the Kenya rift (Green et al., 1991). Earlier investigations into the petrology of the Kenya basalts interpreted them as resulting from melting in a mantle plume (e.g. Latin et al., 1993). By contrast, more recent studies have demonstrated the strong control exerted by the lithosphere on the location, volume and composition of erupted magmas and have shown that the lithosphere acts as a major source of magma in many parts of the Kenya rift (e.g. Macdonald et al., 2001, Le Roex et al., 2001, Spath et al., 2001). The Kenya rift therefore offers an opportunity to investigate the processes that ultimately lead to the production of mafic melts from the mantle lithosphere. Are they produced by simple conductive heating of the lithosphere, or is the mantle lithosphere incorporated into a convecting, dynamic regime that has the physical properties of asthenosphere but the compositional characteristics of the mantle lithosphere?

Existing U-series data from the Kenya rift based on α-spectrometry reveal considerable ²³⁰Th excess over ²³⁸U (Black et al., 1998), implying a melting regime that extends into the garnet stability field, consistent with trace element, especially REE fractionation (Macdonald et al., 2001). However, the data do not readily distinguish between the different melting regimes and so the process whereby the lithosphere is remobilised during plume–lithosphere interaction and extension remains unclear. Here we present new mass spectrometric analyses of trace element abundances and (²³⁸U/²³⁰Th) disequilibrium from recent (< 10 ka) basalts from the Kenya rift that address these problems. They allow us to distinguish between batch and dynamic melting processes in the source regions of basalts with lithospheric origins, and emphasise the differences in source composition between the Kenya rift and plume-related magmatism generally.

Section snippets

Geological background and sample description

The African rift is one of the clearest currently active examples of continental magmatism related to extension above a mantle plume on Earth. The elevated topography and negative Bouguer gravity anomalies across the whole of East Africa (Ebinger et al., 1989), shallow and deep seismic tomography (Nyblade et al., 2000, Weeraratne et al., 2003) and the volume of magmas (Latin et al., 1993), testify to the presence of a thermal anomaly in the mantle that has been variously attributed to the

Analytical methods

Trace elements were determined by ICPMS analysis at the Open University, using an Agilent 7500s instrument fitted with a standard quartz spray chamber and a Babington nebuliser. 100 mg of sample were dissolved in 1 ml HF with 2 ml HNO₃ in screw-top Teflon beakers overnight at a temperature of ∼ 120 °C. After further reaction with HNO₃ and HCl to remove fluorides, samples were taken up in 3 ml concentrated HNO₃ and the clear solutions diluted to 100 ml in 18 MΩ water to give a final solution in

Trace elements

Trace element analyses are tabulated in Table 2, together with published major element and Sr, Nd and Pb isotope analyses from earlier studies (Rogers et al., 2000, Macdonald et al., 2001) for completeness. All samples have LREE-enriched patterns with HREE > 10 × chondrite and the LREE between 60 and 200 × chondrite (Fig. 3). Small positive Eu anomalies reflect the possible effects of plagioclase accumulation, as do Sr/Nd ratios > 16. REE fractionation is clearly illustrated on a plot of La/Yb

Comparison with evolved magmas from the Kenya rift and crustal storage times

Two recent studies of variations in the behaviour of the U-series isotopes in felsic magmas within the Kenya rift have shown that, in contrast to their mafic counterparts, they have limited ranges in U–Th disequilibrium, and these are also illustrated in Fig. 7. The peralkaline rhyolites from the Greater Olkaria Volcanic Complex (GOVC) (Heumann and Davies, 2002) fall close to the equiline, whereas trachytes from Longonot (Rogers et al., 2004) define a near horizontal trend with (²³⁰Th/²³²Th)

Comparison with OIB and other continental mafic volcanic rocks

The Kenya basalt analyses are compared with similar data from selected OIB and continental mafic volcanics in Fig. 8, from which a number of features are apparent. Firstly, the Kenyan samples generally lie between OIB and continental volcanics, with intermediate (²³⁸U/²³²Th) ratios. Secondly, with two exceptions, all samples show an excess of ²³⁰Th over ²³⁸U although the maximum (²³⁰Th/²³⁸U) ratio is 1.39, no more extreme than values from OIB and MORB. Thirdly, the range of (²³⁸U/²³²Th) in the

Lithosphere in the melting regime

We have shown elsewhere that the radiogenic isotope characteristics of the basalts within the Kenya rift vary with the age of the underlying lithosphere (Rogers et al., 2000), and that critical aspects of the trace element composition of all Kenya basalts, e.g. the Zr/Hf, Nb/Ta ratios, are also different from those typical of OIB and not simply a function of melt fraction. These differences are not caused by crustal interaction as crustal samples tend to have low Zr/Hf and Nb/Ta ratios (David

Conclusions

Basalts from the Kenya rift result from melting of a lherzolite mantle with 2–6% garnet in the source region. Trace elements are distinct from OIB (notably low U/Th, high Nb/Ta and Zr/Hf ratios and low Zr contents), suggesting either an origin in the mantle lithosphere or that the mantle source has incorporated a lithospheric component that dominates the trace element and hence isotope characteristics of the erupted lavas. After screening for the effects of alteration and U mobility, U-series

Acknowledgements

The authors would like to thank Craig Lundstrom and Tanya Furman for their constructive reviews, Roberta Rudnick for similarly constructive editorial comments and David Peate for comments on an earlier draft and access to his compilation of MORB and OIB U-series data. Many thanks also to Andy Lloyd for the figures. U-series and trace element research at the Open University is funded by NERC as part of the Research Facilities Infra-structure and grant GR3/JREI/149 respectively. [RR]

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Magmatic activity in the Tanzania Craton-influenced regions of the East African Rift System (EARS) exhibits clear evidence of derivation from a variety of metasomatic sources. These metasomatic sources, which may span a range of ages, have likely been preserved within the lithospheric mantle. The influence of these metasomes on the composition of the rift magmas represents the single greatest heterogeneity in the composition of erupted lavas within the EARS. The earliest widespread magmatic event in the craton-influenced regions of the EARS south of Lake Turkana is the Samburu Phase (ca. 20–17 Ma), which manifests as alkaline volcanics derived from the lithospheric mantle. This phase represents the initial destabilization of the continental lithosphere as a precursor of rift development. The Flood Phonolite Phase (ca. 16–12 Ma) was most pronounced in regions south of those impacted by the prior Samburu Phase, and likely represent the continued southward expansion of lithospheric destabilization. The Mid-Miocene Resurgence Phase (ca. 12–9 Ma) is poorly represented in the Southern Kenya Rift, though magmatic activity is recorded in the Western Branch of the EARS at Kivu during this time. In contrast, the Early Rift Development Phase (ca. 9–4 Ma) is particularly well-represented in the Kenya Rift. Large-scale magmatic events include: commencement of activity in the Northern Tanzania Divergence (NTD) and Mt. Kenya, Rungwe Volcanic Province, and the Southern Kenya Rift. The Stratoid Phase (ca. 4 Ma – 0.5 Ma) commences with significant magmatic events at ca. 3 to 2.5 Ma within the Southern Kenya Rift and NTD. This phase manifests as basalts and trachytes that flood the newly formed graben in the Southern Kenya Rift. This phase also correlates with the main phase of magmatism in the NTD, the commencement of volcanism at Virunga, and the intermediate shield phase in the Rungwe Volcanic Province. The Axial Phase (0.5 Ma to present) is defined by narrow axial grabens in the Kenya Rift with central volcanoes and interposing basaltic cinder cones. In the less mature regions of the EARS, alkaline volcanoes dominate. Three distinct pulses of magmatism and extension initiated in Turkana commencing at ca. 23 Ma, 12 Ma, and 4 Ma; the consistent temporal lag in magmatic activity in the Southern EARS following each pulse reveals the southward expansion of these extensional events. Lavas at any given location within the rift exhibit a consistent temporal sequence characterized by a shift from Type II lavas (lithospheric mantle derived) to Type IV lavas (hybrid lithospheric mantle – sub-lithosphere derived) to Type III lavas (sub-lithosphere derived), requiring a progressive change in the reservoirs contributing to rift magmatism from lithospheric to sub-lithospheric. When examined in aggregate, these data paint a picture of progressive rift development and attendant lithospheric thinning, mediated by pulses of extension originating in Turkana.
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The Cenozoic magmatism of East Africa: Part V – Magma sources and processes in the East African Rift
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Thus, a SCLM source with asthenosphere-like melting behaviors may be needed. Rogers et al. (2006) suggested that these observations were consistent with thermo-mechanical erosion of the SCLM during rifting. This model envisages material from the SCLM being incorporated into the upper mantle and melting.
The generation of magmas in the East African Rift System (EARS) is largely the result of either: (A) melting of easily fusible compositions located within the lithospheric mantle due to thermobaric perturbations of the lithosphere, or (B) melting of the convecting upper mantle due to decompression caused by thinning of the plate during extension. Melt generated from amphibole- or phlogopite-bearing metasomes within the lithospheric mantle yields alkaline, silica-undersaturated lavas, while more silica-saturated lavas are primarily a function of melting material within the convecting upper mantle. Sourcing of silica-undersaturated melts within the lithospheric mantle is consistent with the observed tendency for initial melts within any given region to exhibit trace element characteristics consistent with melting of lithospheric metasomes, likely reflecting the initial destabilization and thinning of the lithospheric mantle. With continued lithospheric thinning, the trend towards more silica-saturated compositions coincides with a shift towards compositions interpreted as melting of the convecting upper mantle. Contributions from these two sources may oscillate where extension is pulsed – melts of the convecting upper mantle are favored during periods of plate thinning; melting of either existing or recently formed metasomes may be favored during periods of relative extensional quiescence. The isotopic systematics of East African magmatism reveals significant complexity as to the specific reservoirs that may participate in the melting processes noted above. The lithospheric mantle beneath East Africa has undergone enrichment through the percolation of sub-lithospheric derived melts and fluids over an extended interval, which close to the Tanzania craton has resulted in a layered lithospheric mantle exhibiting extreme isotopic ratios. Elsewhere, the lithospheric mantle has also undergone enrichment but given the more juvenile age of this lithosphere, less extreme isotopic values have developed. Material rising from the African Large Low Shear Velocity Province (LLSVP) has also metasomatized the lithospheric mantle, and thus lavas exhibiting a trace element signature linked to melting within the lithospheric mantle may exist as any number of reservoirs or mixtures of the same. Material derived from the convecting upper mantle incorporates the Afar Plume endmember, a depleted mantle endmember, and some form of lithospheric endmember. The isotopic characteristics of magma suites from throughout the region form arrays that broadly converge on the composition of the Afar Plume, despite some complexity where the plume material has formed a hybrid plume-lithosphere component. The convergence of these arrays strongly supports a model whereby the prevalent composition of material rising from the African LLSVP beneath the EARS is broadly equivalent to the composition of the Afar Plume.
14 C and U-series disequilibria age constraints from recent eruptions at Sete Cidades volcano, Azores
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Accurate dating of young (<10 ka) volcanic rocks poses numerous challenges yet determining eruption ages is critical to understanding magmatic evolution and hazards associated with active volcanic centers. ¹⁴C dating is the preferred technique for dating young geologic materials, but a few studies have suggested that volcanic degassing can result in erroneously old ¹⁴C dates (e.g. Pasquier-Cardin et al., 1999). Finding an isotopic system with which to accurately constrain eruptive recurrence intervals at recently active volcanic centers is critical to assessing future volcanic hazards. In this study, we have obtained ¹⁴C data from modern terrestrial gastropods on São Miguel island, Azores to assess the current impact of volcanic degassing on ¹⁴C ages near active and extinct volcanic centers, and for one active volcanic center, we have compared ¹⁴C of paleosols with ²²⁶Ra-²³⁰Th maximum eruptive ages for pumices immediately overlying the respective paleosols.
¹⁴C ages of the modern terrestrial gastropod Cornu aspersum demonstrate the effects of dilution from a range of current outgassing levels at three active trachytic stratovolcanoes on São Miguel island: Sete Cidades, Fogo, Furnas, as well as the extinct Povoação volcano. A modern gastropod sampled at Furnas, where there is evidence of extensive present-day degassing, has a ¹⁴C age of 394 ± 97 cal BP. At Fogo volcano, also a location with active degassing, the ¹⁴C age of a modern gastropod is 1324 ± 60 cal BP. Both of these ages clearly show effects of dilution of atmospheric ¹⁴C due to volcanic degassing. In contrast, there is currently no apparent outgassing occurring at Sete Cidades or Povoação, and modern gastropod ages from these volcanoes are post-bomb.
We also obtained ¹⁴C data from paleosols and ²²⁶Ra-²³⁰Th ages associated pumices from the youngest eruptive sequence of Sete Cidades volcano, the Pepom P1–P17 deposits, the ages of which are poorly constrained but thought to be <5000 years (Queiroz, 1997). ²²⁶Ra-²³⁰Th disequilibrium in a P1 glass separate indicates a maximum eruptive age of 4440 ± 195 years. ¹⁴C ages of paleosols below P1 and P3 are older than those derived from ²²⁶Ra-²³⁰Th dating, at 7293 ± 141 cal BP and 7698 ± 172 cal BP, respectively. We interpret the ¹⁴C ages as being erroneously old due to abundant volcanic degassing at the time of eruption. Similarly, but to a lesser extent, the ²²⁶Ra-²³⁰Th maximum eruptive age of glass separated from the P8 eruption is 2659 ± 55 y, whereas the age of the paleosol sampled directly beneath P8 is older (3054 ± 194 y BP). In contrast, paleosols sampled at multiple field localities from below the most recent eruptive deposit (P17) are within error of each other (476 ± 56 cal y BP and 585 ± 68 cal y BP), while ²²⁶Ra-²³⁰Th maximum eruptive ages in glass separates from two different P17 sampling sites are 762 ± 14 y and 730 y ± 15 y. This is consistent with the observation that Sete Cidades volcano is experiencing limited hydrothermal activity at the present time and likely had similarly low levels of degassing just prior to the last eruption. The average eruptive recurrence interval for the P1–P17 deposits, based on ²²⁶Ra-²³⁰Th maximum eruptive ages from P1 and P8 and ¹⁴C ages from P17, is ~230 years. This recurrence interval is similar to the ~200 y eruptive recurrence interval at Furnas volcano and much shorter than the ~1200 y eruptive interval at Fogo volcano. This has important implications with respect to volcanic hazards and should be considered in any future volcanic hazard planning.
Petrogenesis and timing of mafic magmatism, South Taimyr, Arctic Siberia: A northerly continuation of the Siberian Traps?
2016, Lithos
Citation Excerpt :
The analyses of internal basalt standard BOB-1 indicate that precision over the period of analytical work was 8% RSD or better (Table S3.3). Trace elements for a subset of three extrusive and four intrusive samples (Table 1) were analysed using an Agilent 7500a ICP-MS instrument at the Open University, UK according to methods described by Rogers et al. (2006). Detection limits for most trace elements are generally < 10 ppt, which is equivalent to < 10 ppb in the rock sample.
The Siberian large igneous province (LIP) forms the world's most extensive continental exposure of basalt and has several sub-provinces surrounding it, which may be genetically related. The Taimyr peninsula of north Siberia is one of these sub-provinces and is frequently assumed to be the northerly continuation of the basalts exposed at Noril'sk, the best-studied area of the Siberian LIP. However, the correlation is uncertain.
We present new major and trace element data from 35 samples of extrusive and intrusive rocks from Taimyr, with Sr and Nd isotope data from a subset of ten. The Taimyr rocks fall into two groups with low (~ 7 wt.%) and elevated (~ 9 wt.%) MgO concentrations. The high-MgO rocks display a restricted range of initial ⁸⁷Sr/⁸⁶Sr (0.705 to 0.706) and ¹⁴³Nd/¹⁴⁴Nd (0.5122 to 0.5124) ratios, and share bulk silicate earth normalised rare earth element patterns strikingly similar to data observed in the ore-related Noril'sk intrusions. The remaining low-MgO group samples have a broader range with higher Sr and lower Nd isotope values and higher incompatible trace element ratios (e.g., Th/Ta > 5.3 and La/Sm_n > 1.7) similar to the crustally-contaminated Nadezhdinsky and Morongovsky suite basalts of the Noril'sk region. The major and trace element data for both groups are consistent with a process of fractional crystallisation coupled with small degrees of assimilation of incompatible-element-enriched lower crust involving different contaminants. Trace element model calculations indicate a process of magma formation at large degrees of partial melting and at pressures of less than 3 GPa, probably within the garnet–spinel transition zone or the spinel stability field of the asthenospheric mantle. We obtained an argon plateau age of ~ 252 (252.7 ± 1.5) Ma and a ~ 239 Ma total fusion age from a Taimyr lava and intrusive sample, respectively, confirming that volcanism is only partly contemporaneous with the activity of the Siberian LIP. Although this is in agreement with previous interpretations, we argue that the age difference between both events is only ~ 13 Ma and probably less (~ 5 Ma) although further investigation of the relationship is required. Our data allow correlation with distinct Noril'sk members and most importantly to the ore-bearing (Ni–Cu) intrusions implying that whole rock chemistry could have value as a prospecting tool in Taimyr.

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238U–230Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift

Abstract

Introduction

Section snippets

Geological background and sample description

Analytical methods

Trace elements

Comparison with evolved magmas from the Kenya rift and crustal storage times

Comparison with OIB and other continental mafic volcanic rocks

Lithosphere in the melting regime

Conclusions

Acknowledgements

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²³⁸U–²³⁰Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift