The Second Question: How does the process of forming the magnetic minerals in the Bucks Lake pluton effect the paleomagnetic signature?

Back to The First Question

I don’t have the foggiest notion of the answer, but I do know that it is a question.

Hietanen reported the presence only of magnetite in the rocks she studied.

Here are scanned photos of polished sections of the opaque minerals in the pyroxene diorite and the quartz diorite, as seen in reflected polarized light in a metallurgical microscope. I have enhanced the contrast in the original 1962 photos.

Opaque minerals in the pyroxene diorite

Opaque minerals in quartz diorite

I prepared these specimens myself, mainly because I knew how to do it from working as a lab assistant for the Economic Geology course. Identifying opaque minerals through their reflected light properties was part of the training, so I felt confident in identifying these specimens as ilmenite-hematite exsolution intergrowths. The hematite is the brighter of the two phases, and is more abundant in the pyroxene diorite.

The presence of larger and smaller lamellae suggests a two stage exsolution process.

As reported by Hietanen, there is magnetite in all these rocks, some of it likely primary, some perhaps secondary.

So, the primary minerals that carry the paleomagnetic signature in these rocks are the ilmenite hematite association which formed from the melt as a single phase and separated into two phases while cooling, doing that twice. When during this history did they pass through the Curie temperature and acquire magnetization? Was that magnetization parallel to the earth field at that time or self-reversed?

Magnetite, by my count, makes up less than 10% of the magnetic minerals. Some of it may have formed post-consolidation, from changes in the ferromagnesian minerals. What is its role in carrying the magnetic signature of the pluton?

Like I said, I don’t have the foggiest notion.