Geochemical variability of MORBs along slow to intermediate spreading Carlsberg-Central Indian Ridge, Indian Ocean

Abstract

ICP-MS trace element data from the Carlsberg Ridge MORBs from two different locations is presented. Using this data the intra-ridge geochemical variations of the Carlsberg Ridge - Central Indian Ridge MORBs is reassessed. Trace element contents of the Carlsberg Ridge MORBs are similar to the Rodriguez Triple Junction MORBs [e.g., LIL and REE spidergrams and (La/Sm)N ratio etc.]; both closely resemble the average N-MORB. However, the MORBs from the northern- and southern Central Indian Ridge, significantly vary in composition from the average N- and E-MORBs. Most Carlsberg Ridge- Central Indian Ridge MORBs show significantly less fractionation in FeO sub(t), in the MgO-CaO-FeO sub(t) diagram, compared to those of the Mid Atlantic- and East Pacific Rise MORBs. Furthermore, the depleted LREE and nearly flat HREE patterns of the Carlsberg Ridge - Central Indian Ridge MORBs (along with their low compatible trace element (Ni, Cr, Sr) variability compared with the increasing incompatible trace element (Y, Zr) contents in the log-log plots, as well as their increasing incompatible trace element ratios) favor partial melting dominated processes for their petrogenetic evolution. Our (Sr, Nd, Pb) isotope data review shows that the Carlsberg Ridge-Central Indian Ridge MORBs are derived from a depleted mantle source that was variously contaminated by continental crust, perhaps during the third stage of Gondwana break up between 155 and 135 Ma. Such contamination could also have occurred later during the strike slip movement along a mega fracture (a member of the Davie Transform Faults in the Somali Basin that broke Gondwanaland into the East and West Gondwanas). The sup(208)Pb/sup(204)Pb versus sup(206)Pb/ sup(204)Pb plot of above mentioned MORBs suggests that the depleted mantle source of the Rodriguez Triple Junction MORBs was contaminated by approx. 21 percent lower continental crust, whereas the mantle source of the Central Indian Ridge MORBs was contaminated by upper continental crust, which are approx. 19 percent for the Carlsberg Ridge and Northern Central Indian Ridge MORBs and approx. 32 percent for the Southern Central Indian Ridge MORBs. The contaminated mantle sources were compositionally similar to the Al-depleted Komatiite basalt and significantly enriched in Rb, Ba, La and Ce over the depleted mantle.