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Source of island arc lavas

Explore the formation of island arc lavas and why Beryllium, Boron, and Beryllium-10 are crucial in identifying and determining the subducted component and recycling processes in subducting plates. Discover how chemical variations, involving continental crust, impact the Be-B classifications and the utility of Be isotopes and B-Be systematics in this context.

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Source of island arc lavas

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  1. Source of island arc lavas By using the limitations of Be isotopes and B-Be systematics.

  2. Formation of island arcs lavas

  3. Why is B, Be and Be10 used? • 10Be and B data is unique in subducting plates. • 10Be and B is mostly from the subduction component. • Chemical variation in mantle and involvement of continental crust does not affect Be-B classifications. • This makes Be-B good for identifying and determining the subducted component and recycling processes of subducting plate.

  4. Beryllium isotope (10Be) • Radioactive (half-life = 1.5 Myr) • Short half-life causes measurements to be taken in young sediments. • Concentrated in clay rich ocean sediments. • This causes a high concentration of 10Be in the upper ocean sediments. • Little 10Be in mantle derived magmas. (below detection limits)

  5. Boron • Boron is almost the same as 10Be. • High concentration in ocean sediments. • Low concentrations of B in MORBs and IOBs. • Boron is enriched in the altered part of the of oceanic crust. • 10Be and B with Be is used together. • Be is the normalizing element.

  6. Two-component mixing lines (from studies of 10Be, Be and B) in each arc confirms that subducted slab is homogeneous • High ratios of 10Be/Be and B/Be differentiates the subducted component from the rest. • B is not stored in sub-arc mantle. • Chemical fractionating takes place as subducting slab moves past the region of magma generation.

  7. Results • Figure 1 (10 Be-B mixing diagram) • Linear trends indicates mixing between two materials. Figure 1

  8. Mixing of mantle and fluids derived from surface sediments and fluids from deeper sediments correlates to that of the subducted component in figure 1. • The above shows that the fluids are homogeneous by using the isotope data alone. • Even though 10Be ratio decreases down a sediment column

  9. This graph is a mixture between the trends in figure 1 and with different fields for mantle and crustal resevoirs. Conclusion Figure 2

  10. Bulk mixture of oceanic crust and pelagic sediments are end-members of the subducted component of island arc lavas. • The other end member can also be fluids which was derived from pelagic sediments alone. • The end result is that mixing of the subducted component and homogeneous mantle would produce linear mixing trends. • This confirms that island arc lavas are a mixture of mantle material and a subducted component.

  11. References • Morris, J.D., Leeman, W.P. and Tera, F. 1990. “The subducted component in island arc lavas: constraints from Be isotopes and B-Be systematics.” • http://www.nasa.gov/multimedia/imagegallery/image_feature_589.html • http://www.britannica.com/eb/art-2737

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