Samples taken from the three main basaltic intervals were chemically analyzed. The samples were taken from Cores 210-1277A-1W, 3R, 4R, and 5R at the positions listed in Tables T1 and T2. The sample from Core 210-1277A-1W is from an isolated piece of basalt (Flow 4). The samples from Core 210-1277A-3R are from the two massive lava Flows 2 and 3. The sample from Core 210-1277A-4R is a piece of basalt interpreted as part of the underlying lava Flow 1, from which another sample was taken in Core 210-1277A-5R. The samples were carefully selected to cover the vertical range of basalt and to exclude detrital sediment, as well as hydrothermally veined and oxidized intervals. All the basalt from Core 210-1277A-1W, however, is strongly altered and rich in secondary carbonate.
Basalts were studied in six thin sections, representing a range of visible textures. Samples from Flow 4 are considerably more altered than those in the flows below. The uppermost sample (210-1277A-1W-1, 36–39 cm) exhibits a well-crystallized granular intersertal texture with microphenocrysts of plagioclase, together with small granules of opaque ore. Plagioclase crystals are relatively unaltered, whereas the mesostasis is mainly replaced by carbonate. Another sample from this flow (Sample 210-1277A-1W-1, 101–104 cm) is similar but contains occasional large plagioclase phenocrysts. Scattered small amygdules are dominantly carbonate and chlorite. A further sample from this core (Sample 210-1277A-1W-1, 38–41 cm) includes scattered microphenocrysts of augite partly replaced by calcite.
Samples from Flows 2 and 3 are less altered. The upper levels of Flow 3 (Samples 210-1277A-3R-1, 132–134 cm, and 3R-2, 36–39 cm) show devitrified glass (variolitic) texture with feathery plagioclase and rare ferromagnesian phenocrysts replaced by chlorite, carbonate, and opaque ore minerals. Beneath this, the basalt of Flow 2 (e.g., Sample 210-1277A-3R-2, 106–109 cm) is more crystalline, with a granular intersertal texture defined by plagioclase and clinopyroxene (augite) microphenocrysts. Many individual plagioclase crystals are altered and the mesostasis is partly chloritized, but secondary carbonate is much less abundant than in the overlying flow. No basalts from Flow 1 were studied in thin section.
Nine basalt samples were analyzed for major and trace elements by X-ray fluorescence (XRF) at the Grant Institute of Earth Science, University of Edinburgh (United Kingdom) using the method of Fitton et al. (1998) (Table T1). Loss on ignition varies from 3.43 to 26.37 wt%. High CaO values (e.g., 130 wt%) are attributed to the abundance of secondary calcite, as observed in thin sections (see below).
The precision and accuracy of chemical analysis by a standard XRF technique can be variable; however, high-precision data can be obtained, especially when using a specifically developed and tested XRF technique, as documented by Fitton et al. (1998). This method has been applied to basalts from a wide range of areas and ages including Iceland and the East Greenland rifted margin. Where samples are suspected to contain very low values of incompatible trace elements, the analytical conditions and calibrations for these elements are optimized for low concentrations, as appropriate. Background positions are placed as close as possible to peaks, and long count times are used at both peak and background positions. Where background count rates are measured on either side of the peak, as in most trace element determinations, the count time is divided equally between the two positions. Matrix corrections and spectrometer calibrations were carried out as specified by Fitton et al. (1998). Trace element precision has been previously estimated by repeatedly analyzing several samples from ODP Leg 152, yielding a precision of sons of analytical values with published international standards indicate high levels of accuracy (Fitton et al., 1998). Comparisons show that precision and accuracy of basalt analyses using the above modified XRF technique compare favorably with inductively coupled plasma–mass spectrometry (ICP-MS) data (see table 2 of Fitton and Godard, 2004). In the present case, almost identical results were obtained for a duplicate analysis of one sample (see Table T1). As a result, the interpretations below are justified by the analytical precision and accuracy of the available XRF data.
To confirm and extend the results from the XRF analysis, seven of the samples were analyzed for major elements, trace elements, and rare earth elements (REEs) by ICP-MS using a whole-rock technique (Table T2). The analysis was done at ACME Analytical Laboratories Ltd., Vancouver, Canada. Crossplots of the data for individual element oxides or elements (not shown here) confirm that very similar values were obtained for both the XRF and ICP-MS methods. Of the elements that are most important for discrimination of tectonic settings of eruption, Nb values are slightly higher as analyzed by ICP-MS than by XRF, whereas Zr values are slightly lower. Such small differences, however, do not affect the interpretations given below.
Initial interpretation of the major element and trace element data was carried out using the XRF data. To help assess bulk chemical composition the samples were plotted on a Zr/Ti vs. Nb/Y diagram (Fig. F7). This allows rock classification using only immobile elements. All of the samples plot in the andesite/basalt field. One of the samples (AR130 in Table T1), however, was excluded from further consideration, as it contains numerous small carbonate veins based on visual inspection and was therefore considered to be more altered (although its inclusion would not change the interpretation).
Because all of the samples are altered to variable degrees, any interpretation of the tectonic setting of eruption must rely on the relative abundances of major and trace elements that are considered to be immobile under conditions of low-grade hydrothermal alteration typical at Site 1277 (Pearce and Cann, 1973). The large ion lithophile (LIL) elements Sr, K, Rb, and Ba are typically mobile compared to the high field–strength elements (HFS) Nb, Nd, Ti, Y, and Cr (Pearce, 1982). Recent work on the East Greenland margin (Leg 152) also has shown that K, Rb, and Ba are relatively mobile, whereas Si, Sc, Al, Fe, Zn, V, and Nb are less mobile (Larson et al., 1998). Yttrium, normally considered to be immobile in such seafloor settings, was found to be immobile at Site 917 but mobile at Site 918. The mobility was explained by the breakdown of clinopyroxene in these basalts (Larson et al., 1998). The Site 1277 basalts commonly contain clinopyroxene phenocrysts, most of which appear in thin section to be unaltered except in Flow 4.
Samples were also plotted on several geochemical diagrams that are known to be useful for discrimination of the tectonic settings of basaltic rocks. On a Ti/Zr vs. Y plot (Fig. F8A) the basalts mainly plot in the field of mid-ocean-ridge basalt (MORB), with one sample just into the field of within-plate basalt and one just within the island arc basalt field. On a V vs. Ti diagram (Fig. F8B) all of the samples plot in the field of ocean floor basalt. On a Ti/100 vs. Zr vs. 3Y diagram (Fig. F8C) all of the samples plot in the combined field of MORB, calc-alkaline basalt, and island arc tholeiites. On a 2Nb vs. Zr/4 vs. Y diagram (Fig. F8D) all of the basalts plot in the combined normal (N-)MORB-volcanic arc field (i.e., Field D).
Plotted on MORB-normalized spider plots (Fig. F9A), the immobile elements exhibit a MORB trend, with a slight relative depletion of Nb, which is characteristic of depleted MORB. The basalt from the uppermost Flow 4 (Core 210-1277A-1R) is slightly richer in the HFS element Cr compared to basalt from the underlying Flows 2 and 3 (Cores 210-1277A-3R through 5R), but the other HFS elements are slightly depleted compared to these flows. The relative enrichment of the LIL elements Sr, K, Rb, and Ba in all of the flows can be attributed to alteration because these elements are typically mobile as noted above, although a crustal influence on the melt cannot be excluded (see below).
When the three least-altered samples (<10 wt% CaO; see Table T1) from the two lower volcanic levels (Flows 2 and 3; Cores 210-1277A-3R through 5R) are plotted together they define a tight grouping (Fig. F9B). Nb shows a slight increase in enrichment relative to MORB but a decrease compared with the enrichment of higher molecular weight immobile trace elements (e.g., Ce). This "negative Nb anomaly" is unlikely to result from differential alteration because Nb is widely believed to be immobile under prevailing seafloor conditions (e.g., Pearce, 1982, 1983). Also, as noted above, the precision and accuracy of the analysis using the increased count time technique (Fitton et al., 1998) allows the negative Nb anomaly to be considered a real feature rather than an artifact of the analysis.
As noted above, seven samples were also analyzed by a whole-rock ICP-MS method (Table T2) to extend the results from the XRF analysis. The samples plot within or near the MORB field on most of the standard tectonic discrimination diagrams (e.g., Zr/Y vs. Zr [Fig. F10A], Ti vs. V [Fig. F10B], and Ti/100 vs. Zr vs. 3Y [Fig. F10C]). In a Hf/3 vs. Th vs. Ta plot (Wood, 1980) the samples plot in the enriched (E-)MORB/within-plate tholeiite, N-MORB, and island arc tholeiite fields (Fig. F10D). The spider plot of sample/MORB vs. element indicates near-MORB to slightly enriched patterns (Fig. F11A). The low normalized values of Nb relative to Th and Ce in particular, which are also relatively immobile elements, confirm that a negative Nb anomaly is present, as indicated by the XRF data. Finally, chondrite-normalized plots show N-MORB-like or slightly enriched patterns (Fig. F11B).
The analytical results from Site 1277 can be compared with the compositions of basalts from several representative areas, including the Iberia margin, Southwest Indian Ridge, East Pacific Rise, Goban Spur, Hatton Bank, Reykjanes Ridge, and the compositional array of Iceland basalts (Fig. F12). The Site 1277 basalts are chemically similar to basalts from the Goban Spur and lie within the field of East Pacific Rise basalts and some Southwest Indian Ridge basalts. However, they differ from the enriched basalts that characterize the Iceland array, as these exhibit higher degrees of mantle melting that is widely believed to relate to a North Atlantic mantle plume (Kempton et al., 2000).
As noted above, the Newfoundland margin basalts plot within or close to the field of basalts from Goban Spur (Deep Sea Drilling Project [DSDP] Leg 80), which is early formed oceanic crust located near the base of the continental shelf, southwest of the British Isles (de Graciansky, Poag, et al., 1985). In this area a narrow ocean–continent transition zone is characterized by faulted Hercynian basement cored at Site 549, passing laterally into "transitional" crust at Site 551, and then into early formed ocean crust at Site 550. Below Upper Cretaceous chalk at Site 551A, 58.9 m of basaltic flows and pillows with pink and white calcareous infillings was recovered. In addition, 33 m of basalts, pillow lavas, hyaloclastite, and minor limestones of inferred late Albian age was recovered at Site 550. Because the Newfoundland basalts plot within parts of the much wider compositional ranges of basalts from the East Pacific Rise and the Southwest Indian Ridge, they are compatible on chemical grounds alone with an origin related to seafloor spreading; however, their occurrence above exhumed mantle and being interbedded with serpentinite-rich mass flows at Site 1277 differs strongly from "normal" oceanic crust.
The Site 1277 basalts are generally more depleted than those from the Iberia margin, which range from MORB to transitional (T-)MORB and E-MORB. Published REE values of Iberia basalts analyzed by ICP-MS (Seifert et al. 1997; Cornen et al., 1999) are variable, ranging from 3.20 to 29.5 ppm. As a result, the compositional field on the Nb/Y vs. Zr/Y diagram (Fig. F12) is large and overlaps with some enriched plume-influenced basalts (e.g., Iceland) and is also similar to some continental flood basalts. Care should be taken when interpreting the Iberia analyses, as these were made on clasts within serpentinite breccias and they could have been derived from several different tectonic settings related to rifting and early seafloor spreading, as summarized in the following section.
Volcanic rocks related to contemporaneous subduction (e.g., volcanic arc basalts) typically exhibit a well-defined negative Nb anomaly, as seen in modern lavas erupted above subduction zones (e.g., Pearce, 1982). However, the subduction component inferred from chemical analysis may not always relate to contemporaneous magmatism. Where subduction fluids interact with mantle, a subduction chemical signature may be incorporated into lithospheric mantle. This subduction component may be retained until it is released from the lithospheric mantle during some later tectonic event (e.g., an extensional event unrelated to the original subduction setting). A good example is the late Cenozoic basaltic magmatism of the western United States, which shows evidence of a lithosphere-hosted subduction component (e.g., Fitton et al., 1988). Previously, small negative Nb anomalies in basalts from Gorringe Bank on the south Iberia margin were interpreted as the effect of "contamination" of the melt by subcontinental mantle or by delaminated mantle lithosphere (Cornen et al., 1996), suggesting that the principle of an inherited subduction component may be widely applicable. As a cautionary note, however, small negative Nb anomalies may also occur and be unrelated to known subduction (e.g., in certain mid-ocean-ridge settings because of normal variations in partial melting). In principle, Nb may also be preferentially incorporated within certain Ti-rich minerals independent of subduction (e.g., rutile-bearing pyroxenites, as locally sampled from beneath the Iberia abyssal plain) (see Cornen et al., 1999).
It is likely that the negative Nb anomaly seen in the MORB-normalized plots of Site 1277 basalts represents a chemical signature that was inherited from subcontinental mantle that was affected by an earlier subduction event in the region. The probable origin of the subduction influence would be related to closure of the Iapetus or Rheic oceans. Similar conclusions are reached by Müntener and Manatschal (2006) based on the composition of the underlying peridotites.
The results from Site 1277 can also be compared with analytical data for two sills of alkaline diabase of Albian–Cenomanian age that were recovered from the lowest levels of the adjacent Site 1276 (Fig. F2). The resulting MORB-normalized patterns were interpreted to reflect alkaline "hotspot"-type magmatism >25 m.y. younger than the likely age of magmatism at Site 1277 (Hart and Blusztajn, 2006). Sr, Nd, and Pb isotopes and the relative depletion of Nb and Ta in the sills, as seen on normalized multielement plots, are suggestive of an additive component of crustal material. It is therefore possible that the composition of melts along the Newfoundland margin, including both Sites 1276 and 1277, was affected by the presence of continental crust and/or exhumed lithospheric mantle. Seismic refraction studies suggest that the ocean–continent transition zone of the Newfoundland margin is likely to include thinned continental crust that was emplaced during the later stages of continental breakup (Van Avendonk et al., 2006).
Assuming that the MORB at Site 1277 has incorporated a lithosphere-hosted subduction component, this has important implications for basalts that were erupted during continental breakup and are now preserved in orogenic belts exposed on land. The Site 1277 basalts represent one of the few known examples of basement emplaced during the very earliest stage of seafloor spreading (Fitton, in press). These settings are by their nature rarely available for study in modern submarine rifted margins, which are covered by thick deep-sea sediments. Instead, they are more commonly encountered in tectonically emplaced ancient rifted margins (e.g., Robertson, 2007).
In several orogenic belts (e.g., Tethys and Iapetus oceans) the tectonic setting of rifting is controversial. For example, the Triassic volcanic rocks of Greece, related to rifting of the Neotethys Ocean, have been explained in terms of either normal rifting unrelated to subduction (Dornsiepen and Manutsoglu, 1996) or as subduction-related (back-arc) rifting (Pe-Piper and Piper, 2002). The presence of a subduction signature, notably a negative Nb anomaly, has been taken as evidence that this rifting occurred in a contemporaneous subduction-related setting (Pe-Piper and Piper, 2002, and references therein). However, the field geological setting in the region is compatible with a continental rift setting unrelated to active subduction (Robertson et al., 1991; Robertson, 2006). Another example is rifting that opened the Iapetus Ocean, as preserved in Quebec, an area where a subduction-like chemistry has been explained by crustal assimilation during rifting and continental breakup (Camiré et al., 1995). The chemical analyses of the Site 1277 basalts show that distinctive negative Nb anomalies can occur in rifted margin settings, unrelated to contemporaneous subduction.