GMPV
Geochemistry, Mineralogy, Petrology & Volcanology

Geochemistry, Mineralogy, Petrology & Volcanology

Are mantle melts heterogeneous on a centimeter scale?

Are mantle melts heterogeneous on a centimeter scale?

The mantle makes up the majority of the volume of the Earth, but there is still a lot about it that we don’t understand. This is because we can’t observe it directly – forget ‘Journey to the center of the Earth’ – even our deepest drill holes (about 12 km deep) are merely tickling the surface of the planet (about 6400 km to the center).

The journey to the centre of the Earth, by Édouard Riou (it’s not that easy). Source: wikimedia.org

Most of what we know about the mantle comes from secondary sources of information. For example, we can use magmas (liquid rock, which later become lavas when they erupt), which form when the mantle melts slightly. When these erupt onto the Earth’s surface, they bring with them some information about the rock (the mantle) from which they formed. This is just like how our DNA can tell us something (but not everything) about our parents. So by analysing the magma, we can start to piece together what the mantle was like where it formed. The problem here is that magmas are liquid, and can easily mix with other magmas, so the original information gets lost.

One thing we do know about the mantle, though, is it is not the same everywhere, it is heterogeneous. Some of the differences might be left over from four and a half billion years ago when the planet formed, others are due to rocks from the surface being pushed down into the Earth. The question is the scale of this heterogeneity –  is it heterogenous on the scale of hundreds of thousands of kilometers, kilometers, tens of meters, or even smaller?

The drill used to sample the rocks. Open your mouth and say ‘aaahh’. Credit: Sarah Lambart

A new perspective comes from a team from Cardiff University and the Vrije Universiteit of Amsterdam, led by Sarah Lambart (now at the University of Utah), and published in Nature Geoscience. The team took samples drilled from below the bottom of the North Atlantic ocean, and from these they extracted some minerals – clinopyroxene and plagioclase. These were selected as the most primitive minerals – that means the ones that formed first from a magma, and so they should most closely mirror the melt’s composition. From these millimeter sized crystals, they drilled out tiny samples of powder (the drill is a lot like a dentist’s drill, but with less fear and pain), which were then dissolved and analysed, to measure the concentration of some different isotopes (the same element, with different mass) of strontium and neodymium.

What they found was startling – these tiny samples were extremely variable in their isotopic compositions. The range was so big that it represented almost the whole variability of the North Atlantic – a whole ocean-sized range of chemistry in samples totalling about the size of a grain of rice. This implies that the mantle is able to preserve heterogeneous melts over a scale that is far smaller than previously thought. So, why do we not see this in the erupted lavas? Because they get mixed up, and the tiny variations are lost – this mixing has to happen somewhere between the site from where the crystals were sampled, and the sea floor.

So what’s next? Sarah says the answer might lie in experiments. By recreating the conditions of mantle melting, and simulating the movement of melt through solid rocks, we might be able to understand better how such tiny scale variations can be preserved in the mantle and then be completely lost by the time the melts erupt onto the ocean floor.

#mineralmonday : lithiophilite

#mineralmonday : lithiophilite

What is it? Lithiophilite, LiMnPO4

What’s it made of? Lithium (Li), manganese (Mn), phosphorus (P) and oxygen (O). The PO4 at the end of the formula makes this a phosphate mineral (phosphorus + oxygen = phosphate).

Lithiophilite crystals (the crystals that look blackish). From Rob Lavinsky, via wikimedia.org

What’s it’s structure? The way the different atoms are arranged in lithiophilite is described as orthorhombic, which means the crystal is built of lots of tiny cuboid-shaped atomic meshes (we call these individual building blocks unit cells). The mineral olivine, which makes up most of the top 440 km of the Earth, is also orthorhombic (shout out to olivine-my favourite mineral-which unfortunately is far too common for #mineralmonday).

Is it useful? Not directly. It’s far too rare to mine for lithium, the useful part of the mineral. But, if we swap out the manganese (Mn) for iron (Fe), we get another mineral called triphylite (LiFePO4), which can be grown in the lab, and is a battery material with some potential applications in electric cars and bikes.

How can a crystal be a battery? A battery is basically just a material that can provide a flow of electrons. This means that something in the battery needs to oxidise (oxidation means loss of electrons, and a gain of positive charge). The iron in the LiFePO4 can oxidise, similar to how pieces of iron will rust (oxidise) over time. It’s basically the lithium, which can move around in, and out of, the mineral, that makes this possible.

MG electric concept car, powered by a LiFePO4 battery. From jwhite65 via wikimedia.org

What is it named after? It’s a mineral that really really likes lithium, so I guess we could describe it as a lithiophile (filos, filia etc. being Greek for friend). So it’s a friend of lithium, which is convenient because it’s made of it. Just to keep things confusing, we also have the word ‘lithophile‘ in geology, which more or less means an element that concentrates in the rocks we find on the Earth’s surface (lithos = stone, in Greek again). And to round it off, the element lithium was also named after lithos, because it was originally found in rocks.

It seems like they could have used another word. But at least lithium and lithiophilite were both discovered in Greece, right? That would make sense… But no, lithium was discovered in Sweden and lithiophilite in the USA. But, they were both found in the 1800s before thesaurus.com, so let’s not be judgemental…

Do you have a favourite obscure mineral? Want to write about it? Contact us and give it a go!

What I learned from chairing my first EGU session

What I learned from chairing my first EGU session

By Emily Bamber (PhD Student, University of Manchester)

At this year’s EGU meeting I was invited to co-convene the GMPV 5.7 session ‘Magma ascent, degassing and eruptive dynamics: linking experiments, models and observations’. At first, I felt nervous, as a PhD student who has so far only attended and presented at a few conferences. Afterwards I felt happy to be part of a session which presents cutting-edge research in the field I love, and able to share this experience with the volcanology network. Although it can be daunting, to stand on a stage and present in front of a large and experienced audience, ultimately the audience is welcoming, and supportive, and just as excited as I was to learn about new and interesting ideas in volcanology.

Practically, this experience gave me invaluable knowledge in how sessions are organised and managed, and I was able to meet lots of new people in my field as a result. Later, I was able to continue the discussion during the presentation of my own research at the poster session. Convening is a great experience for a PhD student, and a valuable way to represent the early-career community. Here I share some tips which I learned along the way:

  • Record the deadlines for important milestones when organising the session, as these can arrive quickly!
  • Read all of the abstracts prior to the session
  • Have questions ready for speakers during the oral presentation session
  • Keep an eye on the time for oral presentations and communicate with EGU staff (it may be their first day too!)
  • Enjoy the experience!

Emily Bamber is a 2nd year PhD student at the University of Manchester in the UK. She studies eruption dynamics at basaltic systems, through petrological studies of natural products collected in the field.

Twitter: @EmilyCBamber

#mineralmonday: lazurite

#mineralmonday: lazurite

#mineralmonday: your weekly* dose of obscure mineralogy, every Monday** [*not guaranteed; **or possibly Tuesday-Sunday]

What is it? Lazurite. Take a deep breath, the formula is Na3CaAl3Si3O12S.

 

Lazurite, from Didier Descouens via wikimedia.org

That’s a lot of elements to digest, what does it mean? Well, the aluminium (Al) and silicon (Si) form tetrahedra (4-faced 3D triangular shapes), with oxygen (O) on the points. These are arranged in rings with the sodium (Na) and calcium (Ca), with the sulphur (S) sitting in the middle. This kind of structure is called a cyclosilicate, because of the cyclical (ring shaped) arrangment.

 

OK now I’m bored. Is it pretty? Not just pretty – but pretty on a biblical scale. Lazurite is what makes lapis lazuli blue – this has been a semi precious stone for thousands of years. It’s basically lazurite with a couple of other minerals (we’ll get there another day). Here is a shout out from the Old Testament (Exodus): “Under his feet there seemed to be a surface of brilliant blue lapis lazuli, as clear as the sky itself“.

Alright I’m sold, where can I get some for under my own feet? The main worldwide source of lapis lazuli, and hence lazurite, is a mining district in northern Afghanistan. It’s been mined through the times of the Greek, Roman, Mesopotamian and Egyptian empires. As well as being used for carvings, jewelery and apparently flooring, ground up lapis lazuli / lazurite was used for making the colour ultramarine, widely used by painters in the Renaissance.

Girl with a Pearl Earring, aka Girl with a Lazurite Blue Headscarf, made from natural ultramarine.

If more minerals were blue then geology would be way more interesting, right? Probably. There are only so many ways we can describe the colours grey and brown when looking at rocks. The colour comes from the sulphur, but exactly how the sulphur relates to the colour is still apparently up for some debate.

And who or what is lazurite named after? Simply, named for the colour blue. In Persian, this is lāzhward, which was originally the name of the place where the blue lapis lazuli stone was mined, which then ended up being used to describe the colour itself.

So the mineral was named after the colour, which was named after the town, which was named after the stone. I’m confused. Me too. It’s like time travel movies. Don’t think about it too much.

Do you have a favourite obscure mineral? Want to write about it? Contact us and give it a go!