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Influence of lithospheric structure on Os isotopes of Mid-Ocean Ridge basalts TEXT SIZE: A A A

 

Sr-Nd-Pb isotopic compositions of Mid-Ocean Ridge basalts (MORB) are used to infer their source compositions because crustal contamination of MORB magmas is thought to have an insignificant effect on these isotopic systems. However, Re-Os systematics of MORB could be affected by assimilation and fractional crystallization (AFC) processes. Oceanic crusts at different ridges may have varied Os isotopic compositions. Oceanic crust samples at fast-spreading ridges have low 187Os/188Os and 187Re/187Os ratios, but those from ultra-slow spreading ridges have high 187Os/188Os and 187Re/188Os ratios. Under this circumstance, MORB from different ridges with different spreading rates may have largely variable Os isotopic compositions due to AFC processes. However, there are only three analyses available for MORB from ultra-slow spreading ridges. Thus, whether MORB from different spreading ridges have varied Os isotopic compositions, and to what extent the Os variations in MORB can reflect their source nature, remain unclear.

Ph.D. student Yang Yang and her supervisors Tai-Ping Zhao and Mei-Fu Zhou at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences carried out Re-Os isotopic analyses on MORB from the ultra-slow spreading Southwest Indian Ridge (SWIR). This study shows that the samples have much larger ranges of 187Os/188Os ratios than previously reported N-MORB. A group of MORB have 187Os/188Os ratios negatively correlated with MgO (Fig. a), clearly indicating the effect of AFC processes. Quantitative modeling suggests assimilation of ~ 3% -11% altered oceanic crust. N-MORB from the ultra-slow and slow- spreading ridges show larger ranges of 187Os/188Os ratios than those from fast-spreadings, reflecting different structures and chemical compositions of oceanic crusts due to different spreading rates of the ridges.

For the first time, this study discovered N-MORB with enriched Os isotopic compositions from the SWIR and proposes the correlation between spreading rates and Os isotopic systematics in MORB. This study contributes to a better understanding on the control of lithospheric structure on the chemical variations of MORB.

 

Fig. a. Plots of 187Os/188Os versus MgO for MORB from the SWIR. AFC: assimilation and fractional crystallization; FC: fractional crystallization.

Fig. b. Variation of 187Os/188Os with full spreading rate for global N-MORB. Open circles are MORB data in this study, and diamonds are literature data.

 

This study has been published in LITHOS. (Yang, A.Y., Zhao, T.-P., Zhou, M.-F., Deng, X.-G., Wang, G.-Q., Li, J., 2013. Os isotopic compositions of MORB from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes. Lithos 179, 28-35.)

 

(Supplied by Science and Technology Department, CASGIG and Key Laboratory of Mineralogy and Metallogeny, CAS)

 

 
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