5:15 PM - 7:15 PM
[SCG55-P13] Sr–Nd–Pb isotopic compositions of basalts along the Marie Celeste Transform in the Central Indian Ridge -Preliminary results-
Keywords:Mid-ocean ridge, Oceanic crust, Mantle heterogeneity
The morphology, lithology, crustal structure, seismicity and hydrothermal activity of global mid-ocean ridges are highly variable. To build a comprehensive understanding, most previous studies have focused on the investigations along the axis of a mid-ocean ridge. In contrast, the MOWALL (Moho Observation along transform fault WALLs) project aims to reveal the temporal variations in oceanic crust formation mechanisms through systematic rock sampling along the Marie Celeste Transform (MCTF) in the Central Indian Ridge (CIR). Our primary working hypothesis is that the lithological and chemical heterogeneity of the source mantle critically controls melt production to form oceanic crust. To characterize source mantle geochemistry, the Sr–Nd–Pb isotopic compositions of basalts are commonly used tools. Therefore, we conducted reconnaissance isotopic measurements for ten basalt samples collected along the MCTF during the research cruise of R/V Hakuho-maru (KH-24-4 cruise) from Oct 12 to Nov.11 in 2024 (Okino et al., JpGU2025).
The samples used in this study were collected from ten different sites (one sample from each site) spaced approximately 20–40 km apart, covering a range of approximately 190 km along the MCTF (Nimura et al., JpGU2025). First, 25 g chips were cut from massive, fresh parts of the dredged rock samples on the basis of megascopic observations. Initial sample preparation, including the removal of seawater contamination from the chips and the powdering method, followed the procedure of Machida and Ishii (2003). Before isotope analyses, samples with high loss-on-ignition (>2.0%) were acid-leached following the procedure of Weis et al. (2005) to remove hydrous phases resulting from interactions with seawater or seafloor weathering. The sequential extraction of Pb, Sr, and Nd from the same sub-sample was performed via a combination of procedures described in Tanimizu and Ishikawa (2006) for Pb and Machida et al. (2009) for Sr and Nd. Sr and Nd were extracted from the residual materials taken during Pb separation. Isotopic analyses of bulk-rock Pb were performed using the multi-collector ICP-MS (MC–ICP–MS, NEPTUNE plus, Thermo Fisher Scientific) with nine Faraday collectors at the Chiba Institute of Technology (CIT). The bulk-rock Sr and Nd isotopes were subsequently analysed via a thermal-ionization mass spectrometer (TIMS; TRITON plus, Thermo Fisher Scientific) with nine Faraday collectors at the CIT.
The Nd and Pb isotopes of basalts from the MCTF are relatively enriched compositions. They are similar to basalts from Rodrigues Ridge (a topographic high connecting the CIR with a volcanic trail of the Réunion hotspot) and the “Intermediate series” of Mauritius Island. Moreover, this signature is identical to that of basalts along the CIR axis at 18–20°S, particularly in the segments adjacent to the MCTF (segments 16 and 17). In contrast, the Sr isotopic compositions of basalts from the MCTF show a wider range of variation compared to those from the CIR axis at 18–20°S, although the samples with the lowest 87Sr/86Sr values (less radiogenic) are similar to those from the CIR axis. Therefore, we conclude that compositional variation is regulated by mantle heterogeneity commonly observed around the CIR and MCTF. The radiogenic Sr signatures are probably explained by the remaining effects of interactions with seawater. Further comprehensive chemical analysis of additional samples, not only from the dredge sites in this study but also from other sites along the MCTF, will be conducted to investigate the nature of mantle heterogeneity and its relationship with variations in melt production at the mid-ocean ridge.
The samples used in this study were collected from ten different sites (one sample from each site) spaced approximately 20–40 km apart, covering a range of approximately 190 km along the MCTF (Nimura et al., JpGU2025). First, 25 g chips were cut from massive, fresh parts of the dredged rock samples on the basis of megascopic observations. Initial sample preparation, including the removal of seawater contamination from the chips and the powdering method, followed the procedure of Machida and Ishii (2003). Before isotope analyses, samples with high loss-on-ignition (>2.0%) were acid-leached following the procedure of Weis et al. (2005) to remove hydrous phases resulting from interactions with seawater or seafloor weathering. The sequential extraction of Pb, Sr, and Nd from the same sub-sample was performed via a combination of procedures described in Tanimizu and Ishikawa (2006) for Pb and Machida et al. (2009) for Sr and Nd. Sr and Nd were extracted from the residual materials taken during Pb separation. Isotopic analyses of bulk-rock Pb were performed using the multi-collector ICP-MS (MC–ICP–MS, NEPTUNE plus, Thermo Fisher Scientific) with nine Faraday collectors at the Chiba Institute of Technology (CIT). The bulk-rock Sr and Nd isotopes were subsequently analysed via a thermal-ionization mass spectrometer (TIMS; TRITON plus, Thermo Fisher Scientific) with nine Faraday collectors at the CIT.
The Nd and Pb isotopes of basalts from the MCTF are relatively enriched compositions. They are similar to basalts from Rodrigues Ridge (a topographic high connecting the CIR with a volcanic trail of the Réunion hotspot) and the “Intermediate series” of Mauritius Island. Moreover, this signature is identical to that of basalts along the CIR axis at 18–20°S, particularly in the segments adjacent to the MCTF (segments 16 and 17). In contrast, the Sr isotopic compositions of basalts from the MCTF show a wider range of variation compared to those from the CIR axis at 18–20°S, although the samples with the lowest 87Sr/86Sr values (less radiogenic) are similar to those from the CIR axis. Therefore, we conclude that compositional variation is regulated by mantle heterogeneity commonly observed around the CIR and MCTF. The radiogenic Sr signatures are probably explained by the remaining effects of interactions with seawater. Further comprehensive chemical analysis of additional samples, not only from the dredge sites in this study but also from other sites along the MCTF, will be conducted to investigate the nature of mantle heterogeneity and its relationship with variations in melt production at the mid-ocean ridge.