November 18, 2011

Diamond mines of Ithaca?

Geology of New York: A Simplified Account startled me with mention that there was kimberlite in the Ithaca area.

Kimberlite? In Ithaca? The stuff named after Kimberley, South Africa, and its 1870s diamond rush? An igneous rock coming through our sedimentary rocks?

But there it is, in the words of Terry Engelder:

During the Mesozoic Era [65-250 million years ago], some north-south joints in central and western New York became the passageways for magma that moved upward from the earth's mantle. The magma solidified to form kimberlite dikes, which are most concentrated in the vicinity of Ithaca. (Kimberlite is a dark-colored igneous rock.) Most of the dikes are a few centimeters thick, but some reach several tens of centimeters in thickness. (137)

It turns out that there are no diamonds in Ithaca's kimberlite:

However, one mineral that is found in the South African kimberlites but has never been found in those from New York is diamond. Furthermore, the mineral assemblage in our "kimberlites" indicates that they formed in the uppermost portion of the mantle, and were too cool for diamonds to be stable. Therefore, diamonds are not expected to be present in any of the currently known New York dikes.

Their description suggests bigger dikes:

These unusual rocks occur as narrow dikes that formed as molten rock from the upper portion of the earth's mantle pushed upwards into joints (vertical cracks) in the surrounding rock, then cooled, and solidified. The dikes range in size from an inch (2.5 cm.) or less, to 195 feet (60 m.) in width.

That site has great pictures if you scroll down, and more information. It's not just Ithaca. Visible kimberlite goes up to Syracuse, and:

There are undoubtedly many more dikes buried below glacial cover, under water, or that just haven't been recognized yet.

I'm not sure whether to think that kimberlite filled many of the gaps underneath us, or to worry that there were many gaps, and large ones, and that the kimberlite probably didn't find all of them outside of a very narrow belt. From what I've seen on the surface, I'm guessing most joints remain open.

But no diamonds, sorry!

Posted by simon at November 18, 2011 8:35 AM in
Note on photos