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

Formation of island arcs lavas

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.

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)

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.

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.

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

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

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

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.

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