Origin of seamount volcanism in northeast Indian Ocean with emphasis on Christmas Island

The Northeast Indian Ocean has been a central point of research in the recent past due to its intraplate geophysical and geochemical characteristics. It is dominated by sub-aerial volcanic islands and submerged guyots and two islands, namely, Cocos (Keeling) Island and Christmas Island. Christmas Island, the focus of this study, consists of limestone and mafic intraplate volcanics. The origin of most of the features in northeast Indian Ocean is not fully understood. Christmas Island has experienced multiple stages of intraplate volcanic activity as previously established by 40Ar/39Ar radioisotopic analyses of basalts from the island (Hoernl et al., 2011). Here, we present new 40Ar/39Ar ages where the rock samples from Waterfall Spring (WS), Ethel Beach (EB) & Dolly Beach (DB) on the east coast of the island yielded plateau and mini-plateau ages of 37.75±0.77 Ma, 37.10±0.66 Ma and 43.37±0.45 Ma respectively, whereas a sample from Flying Fish Cove (FFC) in the north of the island yielded a minimum age of 38.6±0.5 Ma. All these units are part of the Lower Volcanics Series. The samples from the west coast (Winifred Beach, WB) are younger with an age of 4.32 ± 0.17 Ma, and are part of the Upper Volcanic Series. This confirms two stages of volcanism at the island with a gap of around 38 Ma. The 40Ar/39Ar radioisotopic ages were overlayed on Gplates and seismic tomography models to determine its paleo motion. The present position of the island is 10.5°S, 105.5°E. During Eocene its reconstructed position was 30°S latitude. Seismic tomography models have highlighted a low velocity zone beneath the island during Eocene. Geochemically, the two volcanic suites (Upper & Lower) are mostly similar in their major and trace element composition. The majority of localities (WS, EB, and WB) are basanites; where as that from Dolly Beach is basaltic. The Dale's (west coast), are trachyte and appear evolved with high SiO2. They also have low Ba and Sr ~25ppm, whereas those from east coast have 550 - 900 ppm Sr. Despite an age difference of more than 38 Ma, there is no significant difference between Hf, Nd and Pb isotopic signatures of the Upper Volcanic and Lower Volcanic series which show an enriched component, interpreted by Hoernle et.al (2011) to be due to contamination by continental material. This signifies a common homogenous source for a period of more than 40 Ma in contrast to many OIBs. Dale's do have a distinct isotopic character from the Lower Volcanic Series and this signature is still under study. These geochemical analyses from the island broadly lie within those sampled by Hoernle et al. (2011) from their regional study. Here, we are looking at the island in detail. We present geochronological, geophysical and geochemical data from the island which addresses this model using Christmas Island as an example. Ref: Hoernle et al., 2011, Origin of Indian Ocean Seamount Province by shallow recycling of continental lithosphere, Nature Geoscience 4: 883-887