Geochemical and Sr-Nd-Pb-Mg isotopic compositions of volcanic rocks from Jeju Island, South Korea: Implications for the mantle source
Abstract
Jeju Island is an intraplate volcano on continental crust, located 90 km south of the Korean Peninsula. The volcanic rocks are dominantly alkaline suites (basalt to trachyte) with small amounts of intercalated tholeiitic (TH) basalts. The alkaline suites can be subdivided into two sub-suites: high-Al alkaline (HAA) and low-Al alkaline (LAA). We determined major and trace element concentrations, and Sr-Nd-Pb-Mg isotopic compositions for sixty samples from 4 boreholes and 9 outcrops to better understand the petrogenesis and mantle source characteristics. The whole-rock 40Ar/39Ar ages range from ca. 968 to 24 Ka. No age differences were found in the three groups, although the HAA suite began to form slightly earlier than the other suites. On a primitive mantle-normalized trace element distribution diagram, they exhibit typical OIB-like LILE enriched patterns without significant depletion in Nb-Ta. No meaningful correlation was observed between SiO2 and Sr-Nd-Pb-Mg isotope compositions. However, some samples, especially the TH and HAA suites, exhibit positive anomalies in Eu and Sr, coupled with elevated 87Sr/86Sr, Δ7/4Pb and Pb/Ce ratios, indicating an important role of ancient anorthosite assimilation during its evolution. Furthermore, they show negative anomalies in K and Rb. Also note that the basaltic rocks have fractionated Nb/Ta and Zr/Hf ratios with negative Ti anomalies. They have elevated Dy/Yb ratios falling within the range of melts derived from garnet lherzolite, but with Yb contents much higher than those of garnet lherzolite melts, instead plotting close to those of eclogite-derived melts. They also have high Fe/Mn ratios (> 60) and FeO/CaO-3MgO/SiO2 values (>0.45), similar to melts from pyroxenite/eclogite sources. The Sr-Nd-Pb isotopic compositions display a good linear array between Indian MORB mantle and enriched mantle type 2 (EM2). There are no discernible 143Nd/144Nd, Δ8/4Pb, and δ26Mg isotopic differences in the three suites, implying their derivation from a compositionally homogeneous source by different degrees of melting. The δ26Mg values (-0.53 to -0.20‰) extend to much lower values than normal mantle (δ26Mg = -0.25 ± 0.07‰). These observations suggest rutile-bearing carbonated eclogite component in the mantle source of Jeju volcanic rocks together with peridotite component.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V13B0106K
- Keywords:
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- 1037 Magma genesis and partial melting;
- GEOCHEMISTRYDE: 1038 Mantle processes;
- GEOCHEMISTRYDE: 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRYDE: 1065 Major and trace element geochemistry;
- GEOCHEMISTRY