• Marisa Acosta

Trinity Trip 2018

UO Graduate Students Ellen Olsen (left), Marisa Acosta (me; middle), and Michelle Muth (right) on a ledge above Kangaroo Lake, Oregon

We were privileged to be able to join Jessica Warren ( of the University of Delaware) and her lab group in the field for several days to learn about the Trinity Ophiolite.

The mantle is a new and wondrous place for me, so I was really excited to get out of my comfort zone. I was very surprised when I found that it's really analogous to the system I've been working on - fluid flow and alteration! There are even veins (though I have some reservations about whether or not it's a good thing to call the dunite flow channels veins - a philosophical blog for another day).

Below find images of my favorite outcrops & features.

One of the arguments for instances of porous flow during melt extraction from the mantle is the presence of clinopyroxenite 'veins' that were absorbed by the melt. The melt became saturated with Cr-spinel and precipitated it locally in planar arrays which mimick the original orientation of the clinopyroxenite vein.

Similarly, in this outcrop one can see a body of dunite (parental magma at the time of formation) which preferentially utilized the clinopyroxenite veins as flow pathways once it had intersected them.

A dunite melt extraction channel with massive clinopyroxenes in the center. This outcrop is interpreted as being a porous flow channel along the edges which channelized in the center. Plugging of the channel or cooling of the melt then led to pyroxene saturation and precipitation. There is also a thin selvage of harzburgite around the dunite channel, a common feature of this "veins".

There are many types and generations of serpentine in the Trinity, and the chrysotile veinlets were by far the most striking.

Pegmatitic gabbro with cores of pyroxene mantled by amphibole reaction rims, most likely hornblende.

Another example of a dunite channel with a massive clinopyroxene center (also has harzburgite grading into lherzolite selvage)

GIANT hornblende pegmatite. This rock was definitely my favorite.

Progressive development of mesh serpentinization texture.

I might make a couple of thin sections out of interest, time permitting this quarter.


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