Source of island arc lavas

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
Conclusion

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

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