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doi:10.2204/iodp.proc.344.205.2016

Results and discussion

Major and trace element composition and Sr-Nd-Pb isotopic ratios are reported in Tables T1, T2, and T3 and are shown in Figures F2, F3, F4, F5, and F6.

Site U1381

LOI for basement samples from Site U1381 ranges from 1.55 to 4.72 wt% with an average value of 2.97 wt% (Table T1). These LOI values show that these rocks have suffered from seafloor weathering. This is consistent with petrographic characteristics (i.e., such as the presence of secondary hydrous/alteration minerals—clay minerals such as smectite—in the samples; see the “Expedition 344 summary” and “Methods” chapters [Harris et al., 2013a, 2013b]); Expedition 334 Scientists, 2012a). SiO₂ abundances range from 49.0 to 53.5 wt% (Table T1). In the total alkalis [Na₂O + K₂O] versus SiO₂ (TAS) diagram (Fig. F2), all samples from this site are basaltic and plot within the field of subalkalic (tholeiitic) rock series. All samples show high light REE to heavy REE ratios (LREE/HREE), with (La/Yb)_N(normalized to chondrite) = 2.2–3.1 (Table T2), similar to typical enriched mid-ocean-ridge basalt (E-MORB) (Sun and McDonough, 1989). There is no Eu anomaly in these rocks (Fig. F3A). Almost all samples show relative depletions of Ba, K, Pb, Sr, and Y. Some other large ion lithophile (LILE) and HFSE elements are slightly enriched (two times for some samples) (Fig. F3B), as is typical for E-MORB (Sun and McDonough, 1989). Isotopically, the Site U1381 samples show more enriched characteristics (with ⁸⁷Sr/⁸⁶Sr(i) ratios of 0.70357–0.70406 and ¹⁴³Nd/¹⁴⁴Nd(i) ratios of 0.51290–0.51298 (Table T3) than those from the Galapagos Islands and hotspot track (Hoernle et al., 2004) and plot between the field of depleted MORB mantle (DMM) or East Pacific Rise (EPR)/Galapagos spreading center (GSC) crust (Hoernle et al., 2000; Janney and Castillo, 1996) and enriched mantle-2 (EM2) (Zindler and Hart, 1986) (Fig. F5). All Pb isotope data show slight variations (²⁰⁶Pb/²⁰⁴Pb(t) = 18.770–18.993, ²⁰⁷Pb/²⁰⁴Pb(t) = 15.586–15.602, and ²⁰⁸Pb/²⁰⁴Pb(t) = 38.621–38.761) (Table T3) and basically plot between EPR/GSC crust and Cocos Ridge (Hoernle et al., 2004, 2008; Gazel et al., 2009) (Fig. F6). Because the samples have been carefully leached, the isotopic compositions provided above may basically represent the actual compositions.

Hole U1414A

LOI values of basaltic basement samples from Hole U1414A show large variations, ranging from 0.54 to 6.73 wt% with an average value of 2.26 wt% (Table T1). This variation shows that most samples suffered seafloor weathering. This is consistent with petrographic characteristics, such as the presence of secondary hydrous/alteration minerals (e.g., smectite) in the samples (see the “Expedition 344 summary” and “Input Site U1414” chapters [Harris et al., 2013a, 2013c]). In addition to one sample (344-U1414A-56R-2W, 22–28 cm) with a higher SiO₂ of 57.4 wt% (Fig. F2), the contents of SiO₂ for all other Hole U1414A samples range from 48.5 to 54.4 wt% (Table T1). In the TAS diagram (Fig. F2), similar to the case for Site U1381 samples, all Hole U1414A samples are basaltic and plot within the field of subalkalic (tholeiitic) rock series. All samples from this hole show high LREE/HREE ratios, with (La/Yb)_N(normalized to chondrite) = 1.9–4.1 (Table T2), similar to E-MORB (Sun and McDonough, 1989). There is no Eu anomaly (Fig. F4A). A few samples are slightly enriched in K, Pb, and Sr, and almost all samples generally show relative depletions of Ba, K, Pb, Sr, and Y and slight enrichment of some incompatible elements (e.g., Nb, Ta, La, and Ce) (Fig. F4B). Again, this is typical for E-MORB (Sun and McDonough, 1989). Isotopically, the Hole U1414A samples are more inhomogeneous (with ⁸⁷Sr/⁸⁶Sr(i) ratios of 0.70275–0.70406 and ¹⁴³Nd/¹⁴⁴Nd(i) ratios of 0.51278–0.51302) (Table T3) than those from Site U1381. In addition to one sample (344-U1414A-51R-4W, 126–134 cm) plotting close to EPR/GSC crust, all other samples plot between the field of DMM or EPR/GSC crust and EM2 (Fig. F5). All Pb isotope data (²⁰⁶Pb/²⁰⁴Pb(t) = 18.424–19.283, ²⁰⁷Pb/²⁰⁴Pb(t) = 15.570–15.609, and ²⁰⁸Pb/²⁰⁴Pb(t) = 38.360–38.911) (Table T3) show more variability than those from Site U1381 (Fig. F6). Isotopic compositions indicate mantle heterogeneity below the Cocos Ridge (at least for the drilled part of the ridge). A new petrogenetic model will be necessary for interpreting the observable differences in isotopic compositions between the present work and previous studies for other parts of the Cocos Ridge.

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