Lecture 9: Oceanic Basalts

What we think we know:
The depleted upper mantle is broadly complementary to the continents.
The uppermost mantle (MORB source) is fairly homogeneous and
devoid of primordial He.
The OIB source is more heterogeneous and includes both enriched and
primordial material.
Possible enriched components include subducted ocean crust,
terrigeneous and pelagic sediments, delaminated continental mantle
and intra-mantle differentiates.
The heterogeneities need to persist for Ga to evolve their isotopic
signatures. Until recently this was thought to require them to be large
but this is now less clear.
A source of high 3 He/ 4 He is required and this is most likely primordial
mantle that has never melted. Whether this is a discrete layer or
admixed into the mantle remains controversial