Re: Boffinspeek galore
Just a couple of additional points.
Ultramafic eruptions were common in the Archaean (which ended 2.5Gya) although they continued into the Neoproterozoic. There are still some ultramafic eruptions in and around the North West Pacific.
Serpentinite is the product of hydrous metamorphism of ultramafics at low temperatures. You can see some lovely examples at the Lizard in Cornwall where a slice of the ocean crust and the upper Mantle (an ophiolite) has been pushed up over the Cornish slates and shales.