Rift–plume interaction reveals multiple generations of recycled oceanic crust in Azores lavas
Introduction
The isotopically enriched character of Azores islands compared to MORB has long been recognized (White and Schilling, 1976, White et al., 1979), as well as the influence of the Azores melting anomaly on the composition of lavas erupted along the mid-Atlantic ridge (MAR) (Schilling et al., 1983, Dosso et al., 1999, Gale et al., 2011, Hamelin et al., 2013). MORB and OIB lavas from the Azores plateau (37 °N–41 °N; Fig. 1) all show incompatible element enrichments and often high 3He/4He values compared to the depleted MORB Mantle (DMM), which suggest the presence of undegassed mantle material in their source (Moreira et al., 1999, Madureira et al., 2014). A puzzling aspect however, is the large variability in radiogenic isotopes observed in Azores lavas, and the lack of overlap with typically recognized mantle endmembers such as EM-I, EM-II and HIMU (Hawkesworth et al., 1979, Dupré et al., 1982, Zindler and Hart, 1986, Haase and Beier, 2003, Millet et al., 2009, Madureira et al., 2011, Beier et al., 2012, Genske et al., 2016).
The radiogenic isotope systematics of the Azores lavas are complex, with multiple distinct linear trends in the various isotopic spaces, often on the scale of a single island (Fig. 2; e.g. Beier et al., 2007, Elliott et al., 2007, Millet et al., 2009). Beier et al. (2008) interpreted these trends as binary mixing lines rather than mantle isochrons based on the lack of correlation between 232Th/238U values inferred from 208Pb/206Pb isotope ratios and from measured Th and U concentrations for each trend. This implies that binary mixing between distinct mantle components dominates the mantle source(s) of Azores lavas, yet the various components are not always available for melting and are spatially distributed heterogeneously (Beier et al., 2008, Beier et al., 2012, Beier et al., 2013). An additional complexity arises from the lack of clear age progression in the magmatism (Beier et al., 2015), and from the apparent control of eruptive centers by the tectonic features of the MAR – Terceira Rift triple ridge system (Fig. 1a; Georgen and Sankar, 2010, Hildenbrand et al., 2014, Sibrant et al., 2016). The Azores plume is thought to be associated with a deep-rooted thermal anomaly in the mantle (Montelli et al., 2006, Boschi et al., 2007, French and Romanowicz, 2015), although some have argued that this thermal anomaly is presently rather weak (Silveira et al., 2006, Beier et al., 2012, Métrich et al., 2014).
The trends defined by Azores lavas in various radiogenic isotope spaces converge at a common composition (Madureira et al., 2005, Beier et al., 2007, Beier et al., 2008, Millet et al., 2009; Fig. 2). This component is best identified in the Nd-Pb isotopic space, at 206Pb/204Pb ∼ 19.3, 207Pb/204Pb ∼ 15.55, and 143Nd/144Nd ∼ 0.51290. This “common” component appears to mix with at least three different endmembers: (1) The “Terceira/São Jorge” endmember, present mostly in Terceira, Graciosa and São Jorge, with relatively high Pb isotopic values (206Pb/204Pb ∼ 20.5, and 207Pb/204Pb ∼ 15.65). (2) The “São Miguel” endmember, with relatively high Sr, Pb and low Nd and Hf isotopic values and no equivalent worldwide in terms of its radiogenic isotope composition, with 87Sr/86Sr and 144Nd/143Nd values between EMII and HIMU (Zindler and Hart, 1986, Beier et al., 2007). This endmember has been interpreted as recycled Achaean oceanic crust and/or metasomatised lithospheric mantle (Schaefer et al., 2002, Beier et al., 2007, Elliott et al., 2007, Turner et al., 2007, Moreira et al., 2012). (3) The “Faial” endmember, present in in most Faial lavas and some from Graciosa, São Jorge. This endmember is characterized by 206Pb/204Pb ∼ 19.1 but at relatively high 207Pb/204Pb ∼ 15.63 (Fig. 2a) compared to the rest of the Azores lavas.
In contrast to their complex Sr-Nd-Pb systematics, the up till now more limited Hf-isotope data from Azores lavas define a broad single array in Nd-Hf isotopic space with the steepest slope of all plume provinces (Salters et al., 2011), entirely below the mantle array reference line (Chauvel et al., 2008, Vervoort et al., 2011), with São Miguel lavas making up most of this trend. The only other oceanic basalt provinces that plot distinctly below the mantle array are lavas from St. Helena, and Rurutu, Tubuai and Mangaia (Cook-Austral; Salters et al., 2011, Hanyu et al., 2013, Hanyu et al., 2014, Nebel et al., 2013) the type-localities of the HIMU mantle component. However, São Miguel lavas have distinctly higher 207Pb/204Pb for a given 206Pb/204Pb compared to HIMU, as well as much higher 87Sr/86Sr and lower 143Nd/144Nd (Fig. 2; see also Beier et al., 2007). This implies deviations below the mantle array in Nd-Hf space are not an exclusive property of HIMU type sources.
In order to better constrain the Nd-Hf isotope systematics of the Azores plume and the origin of its steep array in Hf-Nd space, we report 176Hf/177Hf isotope ratios (along with ancillary Nd, Pb, and Sr isotope and trace elements) on 41 previously well-characterized subaerial and submarine samples from the islands of Graciosa, Faial, Terceira, Pico, São Jorge, São Miguel and from the João de Castro seamount. We show that lavas from João de Castro define a second, steeper array than São Miguel in Nd-Hf space. We model the distinct isotopic endmembers observed in the Azores lavas and discuss the significance of a negative deviation from the mantle array in Nd-Hf isotopic space in terms of mantle source. Lastly, we explore the spatial expression of Azores endmembers along the ultraslow Terceira Rift revealed from the isotope systematics, and discuss the composition of erupted lavas in the context of a plume–ridge interaction.
Section snippets
Analytical methods
We carried out whole-rock 176Hf/177Hf measurements on 41 samples from the islands of Graciosa, Faial, Terceira, Pico and São Jorge, and from the João de Castro seamount (sample locations in Fig. 1). Most samples have already been characterized for major, trace elements and several for Nd-Pb-Sr isotopic ratios (Turner et al., 1997, Beier et al., 2008, Beier et al., 2012; Table 1). We also carried out Nd, Pb, Sr isotope measurements and whole-rock trace element abundance measurements where
Results
For the purpose of our discussion and data presentation, each island is considered as an individual eruptive center (Beier et al., 2008), and submarine samples are grouped with samples from the closest island (Fig. 1). All samples are alkaline, forming a continuum in composition from basalts to trachybasalts. In an attempt to assess the origin of the different endmembers among Azores islands, we discuss the entire published dataset for each island in this section (data aggregated in the
Discussion
Our new data constrain the εNd–εHf slope of Central and Western Azores at 1.79 (R2 = 0.76) and at 2.05 for all Azores islands (R2 = 0.92), with the entire Azores dataset plotting below the mantle array (Fig. 3a). The twin steep arrays formed by João de Castro and São Miguel lavas in Nd-Hf isotope space and their clear extension below the mantle array are unique in oceanic magmas. Moreover, the slope of João de Castro lavas (2.99) is also the steepest recognized in any group of oceanic magmas to
Conclusions
- (1)
New Hf isotopic data reveal that Azores lavas from São Miguel and João de Castro seamount form two distinct arrays, which both extend below the mantle array in εNd–εHf space. Such double array in Nd-Hf space has not been previously recognized in oceanic magmatic provinces. Isotopic modeling shows the São Miguel and João de Castro endmembers are consistent with Archean (2.5–3.0 Ga) recycled oceanic crust, and the typical heterogeneity observed along ridge segments (D-MORB with E-MORB) can
Acknowledgements
Many thanks to Wayne Buckley, Elizabeth Bair, Carl Frisby and Stevie Henrick at USC for their help with isotope analyses. This work was partially funded by NSF grants OCE-1624315 to Michael Bizimis and OCE 1537135 to Gene Yogodzinski and Michael Bizimis. Our sincere thanks to Executive Editor Marc Norman and Associate Editor Andreas Stracke for their handling of this manuscript, as well as two anonymous reviewers whose comments greatly improved an earlier version of this manuscript.
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