Eclogites: A look at the mantle

Folded blueschist with bands of eclogite

Remember the rock with alternating bands of green and blue from the last post. The blue in this rock is the blue schist that I talked about in that post, but what about the green? The green is actually a different type of rock known as eclogite.

The term eclogite, like blueschist, can be used in two ways 1) a metamorphosed basalt that contains garnet and clinopyroxene and reaches very high pressures or 2) any rock type metamorphosed to the same metamorphic conditions. Eclogites are sometimes referred to as “Christmas tree rocks” because they are green rocks (clinopyroxene makes it green) that usually contain large reddish garnets (pyrope or almandine). Eclogites are basically what you get when you take a blueschist and increase the pressure. Sometimes, the pressure is so high that you can form coesite- essentially, the mineral quartz with a different crystal structure.

Eclogites are interesting because they form either in the mantle or at the base of the earth’s crust, so we get a peek at mantle rocks when we find them. Eclogites are rare on the surface because they easily undergo what geologists call “retrograde metamorphism” which means that once you change the conditions, it’s not likely to stay an eclogite for long unless you move it to the surface quickly. The easiest way to do this is by rapid uplift or exhumation of the rocks, but pieces (xenoliths) can be found in some igneous rocks that rapidly brought them to the surface (e.g. kimberlites). Just to complicate things even more, the rocks on this particular beach may actually be pieces of eclogite that have undergone “retrograde metamorphism” to become blueschist, preserving just some of the eclogite. Geologists can determine if something like this happened by putting very thin slices of the rocks under a microscope and looking for evidence that the minerals have reacted.

Sorry about all the geology terms, geologists love jargon!

One Schist, two schist…

Just north of Jenner, CA

I recently took a trip to Santa Rosa, CA for a friend’s wedding. In my free time, I did what any geologist would do, I went to explore some nearby geology! One of the coolest rocks that I came across was a piece of what is known as blueschist- a rock that will excite most geologists.

The term blueschist can be used in 2 ways: 1) a metamorphosed basalt that contains a blue amphibole, or 2) a metamorphic rock (that started out as any rock type) that reached certain conditions during metamorphism and therefore contains certain minerals. I can already hear you saying, “That doesn’t make any sense!” I know. Geology can be confusing even for geologists, so to keep things simple let’s start by looking at a sample that I found.

Sample of blueschist from near Jenner, CA

See how it is blue? It is a blue mineral that gives it that blue color. That mineral is likely either glaucophane or lawsonite. So now that we’ve covered why it’s blue, what about the schist part of the name? It just means that it doesn’t look like the rock it was before metamorphism, and it now has layers because the minerals have aligned. Likely, before metamorphism, this rock was a basalt. Now, if we took a rock that started out as something other than a basalt, and subjected it to the same conditions, it would form different minerals during metamorphism, and probably wouldn’t be blue at all, but it would have undergone “blueschist metamorphism”.

Blueschist “knocker”. Notice the alternating bands of green and blue. More on that later!

So what are these metamorphic conditions I’m talking about? Blueschists are formed at high pressure, but at a relatively low temperature. Since geologists think on different scales than pretty much anyone else, let’s suffice it to say the rocks are quickly buried deep in the earth’s crust, but they don’t get as hot as you might expect. This sounds pretty boring, but the only way we can still see that a rock was under the right conditions to make a blueschist (“blueschist metamorphism”) is if it is transported to the surface quickly without changing. If blueschist stays buried it will continue to heat up, and that heat will change it into something else. That means that something, like faulting, needs to occur before the rock has time to change. We’ll talk more about how these form in a future post!

Have you seen a blueschist? Share in the comments!

Fluid inclusions: Bubbles caught inside minerals

Did you know that  there are tiny “bubbles” trapped inside minerals? You usually can’t see them without a microscope, and they can be as small as a few microns across (A micron is 1/1000 of a millimeter). These bubbles are called fluid inclusions. You can find them in most transparent minerals, though the number of fluid inclusions in a sample varies widely.

Fluid inclusions in quartz

How do these fluid inclusions get there? They are voids formed because of tiny imperfections in the crystal lattice of a mineral. They can either form during crystallization (formation) of the mineral, trapping whatever fluid happens to be around at the time, or form in tiny “healed” fractures within the mineral crystal. They are like tiny time capsules that can tell geologists what the conditions were when they formed. If they form while the mineral is crystallizing, then they are considered primary fluid inclusions. Primary inclusions are interesting because they can give you an idea of the temperatures and pressures the mineral was experiencing while it was forming. They can also tell us about the composition of the fluid that was around. Secondary fluid inclusions form any time after the mineral has formed and are the result of a fluid moving through the rocks like during metamorphism or during groundwater movement.

Fluid inclusions inside quartz containing multiple phases

What’s inside them? Fluid inclusions can contain any combination of solid, liquid, and gas. While water and carbon dioxide are the most common components, methane, nitrogen and salts (such as halite or calcium chloride) can also be present. Because the mineral is (usually) no longer at the temperatures where it formed, we typically can see different phases of the same compound. For example, it is common to see both liquid water and water vapor or liquid CO2 and gas CO2. The solids present are typically salts, but there can be “accidental” inclusions of other tiny minerals. When the fluid was originally trapped, each of these components were part of one fluid, not the individual phases you see at room temperature. Together, these components can tell you the composition of the trapped fluid.

Fluid inclusion in quartz containing liquid water, liquid CO2, gas CO2, and a salt crystal

How do geologists study them? Geologists use a special stage attached to a petrographic microscope to complete what is referred to as microthermometry. These special stages are attached to an electronic heating source and a cooling source- typically liquid nitrogen. Tiny chips of the mineral are placed in the special stage and cooled until the inclusion is frozen (all solid) and then heated up slowly until the fluid is all one phase (either liquid or gas). The temperatures at which these phase changes occur is recorded. Because certain phase changes happen at known temperatures, any differences can tell us what compounds are present. These data can also be used to determine the temperatures and pressures where the fluid was trapped.

You can also determine the composition of the fluid by using laser ablation inductively coupled mass spectrometery (LA-ICP-MS). This can provide concentrations of the elements present, for example how much iron is in the fluid. To measure the chemistry, you “pop” the inclusion using a laser and then measure the chemistry of what is left. The mass spectrometer cannot measure everything though, some of the gasses can’t be measured accurately, so it’s important to do the microthermometry first.

Hiking in the Adirondacks- Mount Jo

Recently, one of my favorite places to visit is Adirondack Park. For those that don’t know, Adirondack Park is a large State Park in upstate New York. So what’s cool about it? The geology of course!

View from the top of Mount Jo with Mount Colden (Left) and Algonquin Peak (right). A smidge of Heart Lake is also visible.

Being so close to the Appalachian Mountains, it’s easy to think that the Adirondacks are related, but they are actually part of what geologists call the Canadian Shield (a hunk of old continent made of Precambrian igneous and metamorphic rocks). These rocks formed the core of an ancient mountain range formed during the Grenville Orogen about 1.0 to 1.2 billion years ago. These mountains were part of the formation of the supercontinent Rodinia. Those mountains are no longer there, but we see the rocks now because the dome-shaped set of mountains we know as the Adirondacks started uplifting about 5 million years ago. That means that although the rocks are very old, the mountains are geologically young. The reason for uplift is debated, but some people think it may have been started by of a hot spot.

In the park, there are lots of interesting rock types like gneisses, marbles, quartzites, migmatites, charnockites, and one of my favorites- anorthosites. Anorthosites are igneous rocks that are composed of 90-100% plagioclase feldspar. These particular anorthosites have experienced granulite facies metamorphism, but for ease, let’s just call them anorthosite. (Ok, I really like anorthosites. They are not that common on earth but they make up the light parts of the moon. Yep, the MOON.)  You will also find tons (pun intended) of glacial erratics left over from the last Ice Age. Glacial erratics are pieces of rock, some as big as a house, that were transported long distances by glaciers. Plymouth Rock is probably the best known glacial erratic to Americans, but only because the Pilgrims happened to land there.

In September 2013, I went for my first hike in the Adirondacks- a climb up Mount Jo (2,876 ft). The trail is a little over 2 miles in length and does require some scrambling in places, but I highly suggest it if you are looking for a short and fun hike in the High Peaks region of the Park.

This trail offers some beautiful examples of anorothosite and gabbroic anorthosite. If you take the steep Short Trail, you scramble over anorothosite boulders on your way to the summit. Look at that plag! This variety of plagioclase is likely andesine. (Forgive me for neglecting to add a scale- bad geologist!)

Large plagioclase crystals in an anorthosite boulder.

At the top of Mount Jo, the rocks are a little more gabbroic- that just means that there is a little less feldspar and more minerals like pyroxene. There were also some thin syenite dikes cross cutting the main rock (syenites are like granite, but without quartz). These dikes formed when another magma of a different composition than the main rock filled in fractures. They also seem to be more resistant than the surrounding rock, resulting in the relief you can see in my favorite rock photo which looks like a person waving their arms (again, forgot a scale).

Resistant syenite dikes at the top of Mount Jo

There is a lot of exciting geology  in Adirondack Park, and this is only a small part of it. There is still a lot for me to explore, and I hope to get some exploring in this year (Last summer was so busy there wasn’t any time to get any real hiking in). One of my goals for 2015 is to climb Mount Marcy- this highest peak in New York at 5,344 feet.

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Read More:

An excellent blog post on the geologic history of the Adirondacks  http://written-in-stone-seen-through-my-lens.blogspot.com/2012/12/the-adirondack-mountains-of-new-york.html

Information from the New York State Geological Survey http://www.nysm.nysed.gov/nysgs/nygeology/tectonic/02.html

Why should I care about geology?

Good question. Geology affects our everyday lives in ways that most people don’t realize. It’s easier for most people to see how biology, chemistry, and engineering are important to society, but what do rocks have to do with anything?

Let’s start first with what geology is. Simply put, geology is the study of the earth. If we look deeper into the subject, we see it is actually pretty complex. Geologists study earthquakes, volcanoes, landslides, and floods. They make the discoveries that lead to the extraction of resources such as oil, coal, and precious metals. They test water quality and track pollutants in groundwater. They also study the history of earth by examining rocks. There are as many types of geologists as there are flavors of ice cream.

So how does geology affect your life? The answer is completely. We use products that contain materials that were mined from the earth. We use stone in our homes and build levies to control rivers. Volcanoes eject ash into the air. Earthquakes level buildings. Homes built on floodplains are destroyed. Sometimes, the local geology can even affect the weather.

And thus I begin this blog in an attempt to promote the science that I love, to educate others so they won’t be ignorant of the world around them, and to (hopefully) entertain at least a few people for a short while.