Primitive neon isotopes in Terceira Island (Azores archipelago)

doi:10.1016/j.epsl.2005.02.030

Earth and Planetary Science Letters

Volume 233, Issues 3–4, 15 May 2005, Pages 429-440

https://doi.org/10.1016/j.epsl.2005.02.030 Get rights and content

Abstract

We present the first neon data, as well as new helium data, on Terceira Island (Azores archipelago, Portugal). Clear ²⁰Ne and ²¹Ne excesses compared to air are observed (²⁰Ne/²²Ne > 11.2) and moreover, the samples show a more primitive ²¹Ne/²²Ne ratio than MORB, confirming that the Azores hotspot can be considered as sampling a “primitive”, relatively undegassed, reservoir. Most ⁴He/³He isotopic ratios range between 80,000 and 63,500 (∼ 9 to 11.5 R/Ra), being similar to those previously reported by [1] [M. Moreira, R. Doucelance, B. Dupré, M. Kurz, C.J. Allègre, Helium and lead isotope geochemistry in the Azores archipelago, Earth Planet. Sci. Lett. 169 (1999) 189–205] . A more primitive ⁴He/³He ratio of ∼ 50,000 (R/Ra ∼ 15) was obtained in one sample, but we cannot completely exclude the possibility of a cosmogenic ³He contribution for this sample. Our study illustrates that the neon systematics can be more capable than helium to constrain the ultimate origin of hotspots in geodynamic settings dominated by plume–ridge interaction.

Introduction

From the collection of helium isotopic data in the last 20 yrs, significantly different ⁴He/³He signatures emerged for the MORB and OIB sources (e.g. [2], [3] and references therein). ⁴He production results from radioactive decay of U and Th and different time-integrated isotopic ratios can be produced by heterogeneous mantle degassing, through distinct depletion of He as compared with U and Th. MORB samples, considered to come from the upper mantle, present a narrow range of ⁴He/³He ratios with a mean value around 85,000 to 90,000 (R/Ra = 8 to 8.5; where Ra is the atmospheric ³He/⁴He ratio of 1.384 × 10⁻⁶). A significantly larger variation is found for OIB, but ⁴He/³He ratios as low as 20,500 (R/Ra ∼ 35) and 15,000 (R/Ra ∼ 50) measured in samples from Loihi seamount and Iceland hotspot (Baffin island) respectively, are interpreted as reflecting the contribution of a more primitive and less degassed source believed to be the lower mantle [4], [5], [6], [7], [8]. This interpretation is in agreement with the geophysical evidence linking the hotspot activity in these areas to deeply rooted mantle plumes [9], [10], [11]. The existence of these ⁴He/³He contrasts led to the proposition of a two-layer mantle model where the upper mantle is degassed, while a relatively undegassed reservoir is sampled by deep mantle plumes generating OIB [8], [12].

More recent studies on noble gases have shown that the two-layer mantle model is confirmed by neon isotopic data (e.g. [13], [14], [15], [16]). ²⁰Ne and ²²Ne are almost primitive isotopes (the nucleogenic productions of ²⁰Ne and ²²Ne can be neglected in mantle rocks) and similar ²⁰Ne/²²Ne ratios were found for MORB and OIB samples [13], [14], [15], [17], [18], [19], [20], [21]. Different ²¹Ne/²²Ne ratios are, however, assigned to MORB and OIB sources. Since ²¹Ne derives from α particle-induced nuclear reactions such as ¹⁸O(α, n)²¹Ne [22], its production is connected with radiogenic He formation. The higher ²¹Ne/²²Ne ratio observed in MORB as compared with OIB samples (for a given ²⁰Ne/²²Ne ratio), is compatible with a higher time-integrated (Th + U)/He ratio as previously predicted from the ⁴He/³He signatures for the more degassed upper mantle (e.g [14], [16]). Moreover, in oceanic islands where the mixing between primitive mantle and MORB sources or magmas is believed to occur, it has been shown that Ne can be more sensitive than He in detecting contributions from relatively undegassed sources [16], [20], [23], [24], [25], [26].

Notwithstanding that Azores archipelago emerges in the vicinity of the Mid-Atlantic Ridge, it has been considered as a hotspot resulting from a mantle plume [27], [28], [29] being characterized by a buoyancy flux similar to that reported, for example, for the Canary Islands (e.g. [30]). Nevertheless, mantle plumes have been the locus of an intense debate and controversy still exists concerning their ultimate origin: D” layer, bottom of mantle transition zone and even the asthenosphere have been considered as possible solutions (e.g. [31]). Noble gas isotope geochemistry is a key tool to clear up this question given its ability to identify the incorporation of relatively undegassed lower mantle portions in plumes. The Azores was firstly classified as one of the “low ³He hotspots” characterized by ⁴He/³He ratios higher than the mean MORB ratio [32]. However, Moreira et al. [1] showed that Azores Islands have ⁴He/³He ratios both higher and lower than MORB values. Higher values (⁴He/³He > 140,000; R/Ra < 5.2) are represented by rocks from the Eastern part of S. Miguel, while more primitive ratios were recorded in Terceira samples (around 64,000; R/Ra = 11.3). Using Pb isotopic data, these authors considered the S. Miguel end-member (²⁰⁶Pb/²⁰⁴Pb = 20.00; ²⁰⁷Pb/²⁰⁴Pb = 15.75; ²⁰⁸Pb/²⁰⁴Pb = 40.33) resulting from the presence in the mantle source of subcontinental lithosphere delaminated during the opening of the Atlantic. For the Terceira end-member, the radiogenic lead isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 20.02; ²⁰⁷Pb/²⁰⁴Pb = 15.64) coupled with the unradiogenic ⁴He/³He signatures, were assigned to mixing between ancient recycled oceanic crust and high ³He material from the lower mantle. The influence of such low ⁴He/³He plume signature on the Mid-Atlantic ridge basalts was demonstrated by [33], who reported a significant decrease of the MORB ⁴He/³He ratio in the vicinity of the Azores archipelago. However, given that the helium anomaly (⁴He/³He = 64,000; R/Ra = 11.3) measured in [1] was not sufficient to clearly constrain the origin of the Azores hotspot, we decided to analyze both helium and neon isotopes in new olivine phenocrysts sampled in Terceira lavas. Concerning Ne, these are the first data published for Azores archipelago.

Section snippets

Geology of Terceira and sample locations

The Azores archipelago is located in the North Atlantic near the triple junction where the Eurasian, African and American plates meet (Fig. 1). The boundary between the Eurasian and African plates is currently considered as represented by the NW–SE lineament of S. Miguel–Terceira–Graciosa or, alternatively, by the direction defined by S. Miguel–S. Jorge Islands. Several models have been proposed to describe the plate kinematics along this boundary: pure extension [34], [35], [36], [37], pure

Analytical procedure

Noble gases were mainly measured from green to yellow olivine phenocrysts, representing 18 samples of basaltic to mugearitic composition (see Table 1 and Appendix A). For one sample (PMT-24), a fraction of clinopyroxene was also analysed. Olivine was preferentially chosen given its significantly lower U and Th contents as compared with clinopyroxene (e.g. [49]) preventing the in-situ production of radiogenic ⁴He and nucleogenic ²¹Ne. Moreover, clinopyroxenes are more likely to retain crustal He

Origin of the Azores hotspot

Anomalous topography, gravity, crustal thickness, S-wave velocities and geochemical signatures have been used as evidence for the existence of a plume in the Azores region ([11], [27], [61], [62], [63] and references therein). However, the relatively shallow character of the S-wave negative anomalous zone (< 300 km), the lack of a well defined hotspot track, the recent 3D re-interpretation of admittance in the area [64] and some geochemical arguments (e.g. [65]) have been used against a deep

Conclusions

We presented the first neon isotopic ratios on the Azores archipelago together with new helium data for Terceira Island. Ne and He isotopic data argue for a primitive mantle contribution to Terceira magmas, thus endorsing the models invoking the role of a deep mantle plume in the generation of the Azores Islands. We propose that the Terceira He and Ne signatures result of mixing between two types of magmas: MORB and solar-like plume. As indicated by the positioning of Terceira samples close to

Acknowledgements

This work was partially financed by FCT, through the project STAMINA (Study of the Tectono-magmatic Interplay in the Azores) (PDCTM/PP/MAR/15255/99), and by a CRUP/CPU joint research program. The authors are indebted to M. Kurz for the He analyses performed at WHOI. PM and JM are indebted to G. Queiroz, J. Gaspar and T. Ferreira by their guidance and support during the field work, while N. Lourenço, M. Miranda, P. Silva and P. Moita are thanked for fruitful discussions on the Azores geology and

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Citation Excerpt :
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Primitive neon isotopes in Terceira Island (Azores archipelago)

Abstract

Introduction

Section snippets

Geology of Terceira and sample locations

Analytical procedure

Origin of the Azores hotspot

Conclusions

Acknowledgements

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