#komatiite

Genesis of Mineral Deposits field trip

After the Precambrian Geologic History of Brazil field trip, we went to Genesis of Mineral Deposits field trip, that also occurred in Minas Gerais.

Rounded boulder of metamorphosed BIF

Metamorphosed BIF with Chevron folds, in situ

View of Serra da Piedade

Refolded fold in a hematite enriched BIF

Sunset over Serra da Piedade

Sequence of different igneous rocks

Cataclasite

Metakomatiite

Ultramafic breccia

Between 3.5 and 3.2 Ga years ago, during the Paleoarchean era of the Archean Eon, the Earth didn’t look much like it does today. The atmosphere lacked oxygen meaning there was no protective ozone layer, the oceans, the crust had just formed, oceans were forming, and the first living things (cyanobacteria) were slowly creating free oxygen.

In the midst of all this the Barberton Greenstone Belt was also forming. This belt is found in South Africa east of Pretoria. It is a sequence of mafic to ultramafic lava and metasedimentary rocks (metamorphic rocks that still contain sedimentary structures) that were deposited during the Archean Eon. They are the oldest igneous rocks on earth.

The area underwent two tectonic episodes of terrane accretion, that is two periods of material being added to the tectonic plate at a subduction zone. Imagine it similar to pushing sand in sandbox to form a wall around your sandcastle. This process helped to build new continents throughout geologic time. It is hypothesized that the greenstone belts formed from a passive margin oceanic crust becoming part of a subduction undercut margin.

About 3.1 Ga, a granite batholith was intruded into the belt and partially metamorphosed into gneiss.

Why is it called the greenstone belt? Well, if you recall from earlier lessons (why is magma formed and what is it made of?) mafic and ultramafic rocks are primarily made of olivine, a green mineral. One of the main types of ultramafic rock found here are komatiites, named for the Komati River that flows through the area. It is a mantle-derived volcanic rock with an extremely high magnesium content and very low silicon, potassium and aluminum content.

Komatiites are rare because almost all of them formed during the Archean when it is though the mantle was starting to cool. During this time period, the Earth had much higher heat production simply due to all the residual primordial heat of planetary accretion. In fact, eruption temperatures were thought to be up to 1600 degrees C (2912 degrees F). The lava would have flowed with a viscosity close to water.

They are composed of forsterite (magnesium-rich olivine),

calcic and chromian pyroxenes,

anorthite (calcium feldspar)

and chromite.

They often take on spinifex texture meaning the crystals look like spinifex grass, long and blade-like.  

Thanks for coming to today’s lesson! Tune in tomorrow to learn more about Patagotitan! Fossilize you later!

komatiite: can you please just be a normal person for once

fire opal:

Komatiite

This rock is the metamorphosed remnant of a lava flow that erupted about 3.5 billion years ago. The rock is a komatiite, a type of igneous rock the earth doesn’t produce today.

When the Earth’s mantle melts, it doesn’t melt all the way, it does something called “partially melting”. You can do an experiment like this: take water and freeze it, then add a tiny bit of salt. The ice will start melting, but not melt all the way. If you add more salt, it will melt more; the first bit of salt might melt 1% of the ice, more salt might make 10% of it melt, but it’ll take a lot of salt to make it melt 100%.

You can do the same experiment with heat, but it’s harder to pour an exact amount of heat into ice. As a solid made of more than one compound begins to melt, it “partially melts”, with some elements moving in to the molten portion and other elements staying in the solid.

Near the Earth’s surface, olivine is a very stable mineral. Olivine is the most abundant mineral in the upper mantle and it doesn’t melt easily; every other mineral in the upper mantle melts before olivine. The basalts that erupt at the mid-ocean ridge have a little bit of olivine in them, but not much. Basalts that erupt at Hawaii or Iceland carry more olivine in them; that seems to suggest that the mantle beneath Hawaii or Iceland is melting to a higher extent than the mantle beneath mid-ocean ridges. Hawaii’s mantle is hotter, so more of the olivine melts, and that olivine shows up in the lava erupted at the surface.

3.5 billion years ago, Earth’s mantle was hotter. The planet has spent 4.5 billion years removing heat through convection and volcanism, so igneous rocks forming billions of years ago formed from hotter mantle. Those partial melts would have even more olivine in them; the defining characteristic of komatiites.

These 3.5 billion year old igneous rocks formed by melting the mantle by maybe 40 or 50%; lavas like those erupted in Hawaii might represent 20 or 25% of the mantle melting. These have much more olivine in them, and they erupted much hotter.

When these lavas got to the surface, they began cooling rapidly and the olivine component in the melt began rapidly crystallizing. The olivine started forming at single spots and grew out rapidly from that spot, forming needles. This pattern of olivine needles is known as a spinifex texture and it is distinctive of komatiites.

The magma starts off extremely hot, with lots of olivine component in it. That melt gets to the surface and starts cooling rapidly, causing olivine crystals to grow rapidly. That pattern has survived all the metamorphism that happened at this site. 3.5 billion years ago, an extremely hot lava flow erupted in what is today South Africa and today that rock tells us how hot the mantle was when the rock formed.

-JBB

Image credit: James St. John https://flic.kr/p/oTGLxC