The Earth Story

The famous Carrara Marble quarry in Northern Italy

lucalocatelliphoto

Marble quarry on Isle of Iona

This quarry sits on the Isle of Iona off the Western Coast of Scotland. These rocks were quarried for several centuries, from the 1700s to the early 1900s, for building stones and decorative pieces.

The beauty of metamorphism

Most garnets (barring the chromian Pyropes and a few of the iron bearing Almandines which are mantle minerals) are born in metamorphic events, either the baking and stewing in the resulting expelled juices incurred when rocks are intruded by hot molten rock, or more often in the slower and more spectacular mountain building events, where bands of different minerals record bands of different pressure temperature conditions at different depths as the crust stacked up in layers atop itself. Calcium grossular garnets can form in skarns, which are limestones stewed by granites, though the emerald green Tsavorites (see https://bit.ly/1ygBzM5) formed in a mountain building event, as does the king of green garnets, Demantoid (see https://bit.ly/1oE5O31).

In the red garnets there is a solid solution series, with the mantle sourced pyrope garnets (along with chrome diopside a diamond indicator mineral that erupts in the same kimberlite that carries the diamonds up from the mantle), at one end, the mostly metamorphic iron Almandine garnets in the middle and these that form mostly in Earth distilled granitic rocks or metamorphosed sediments (which tend to be aluminium rich enough for garnet formation) . Here we have orange Spessartite/ine garnets(aka Mandarin or Malaya garnet in the gem trade), named after their type location, the town of Spessart in Germany. The colour is caused by manganese, and it can also be recognised by its inclusions in a hand lens, which consist of veil patterns of fluid and crystalline inclusions. Major sources include the Umba valley in Tanzania and Kenya (also famous for corundums), Nigeria and Namibia. Colours are natural as these gems are not treated.

Garnets in a rock are used by geologists as both geobarometers and geothermometers to gauge the ambient temperature and pressure conditions in which the rock formed, and hence the grade of the metamorphic event. In the 8x4x2cm specimen in the photo mined in Namibia, multiple crystals of gemmy garnet dot a mica schist, in which the many flakes of mica that characterise rocks at this metamorphic range of pressure temperature conditions are also clearly visible. The full mineralogy of the rock is a function of the conditions and the geochemistry of the original rock, known as the protolith.

Loz

Image credit: Marco Frigerio‎

http://www.minerals.net/mineral/spessartine.aspx http://www.minerals.net/gemstone/spessartite_gemstone.aspx

Mica fish in a mylonitic rock. Excercise: take a look at the rock and figure out how the rock was strained!

Magnificent Kyanite

Are these the world's oldest fossils?

In the sceptical world of science the adage is often repeated that exceptional claims require exceptional evidence, and these tiny needles of hematite (see http://bit.ly/2ctVrsX) found in some of Canada's older rocks (dated to between 3.77 and 4.28 billion years ago) have stirred up a healthy controversy in the early life community. They were found in ancient much metamorphosed cherts, near pure microcrystalline silica originally deposited as sinters in long gone hot springs, testimony to our planet's primeval volcanism.

The research team think that these features are the remnants of early bacteria or archaea that fed on chemical energy rather like the communities found around black smoker vents at oceanic spreading ridges today. If their absolute age is at the upper end of the date range it implies that life began very shortly after the Earth-Moon system formed and the first oceans condensed. The problem is simple, once one tries to reach back that far, there are very few rocks remaining that haven't been chewed up and recycled repeatedly in the rock cycle. Those that do remain have been repeatedly squished and baked in mountain building events, having repeated pulses of altering fluids pass through them along the journey through geological time, so any features within are difficult to interpret with confidence, rather like the bacteria in the still controversial Martian meteorite.

The Nuvvuagittuq supracrustal belt in Quebec contains some of the world's oldest rocks, and their features geochemistry implies a submarine vent formation since they seem to have both basaltic pillow laves and the remnants of the associated hydrothermal system. This environment has long been posited one of the potential places where chemistry and geology somehow turned into biology, so this formation is one of the few available suspects available for analysis.

Along with the shape (reminiscent of filamentous iron metabolising bacteria some half a millimetre long) the team suggest several other lines of evidence in favour of their hypothesis, including distinctive structures formed by several filaments linked together by a central blob of heamatite that are like bacterial communities living in similar places today and the presence of associated minerals including carbonate and apatite rosettes suggestive of past life. Spheroidal nodules are thought to be the remains of decayed bacteria and contain chemicals that are usually the result of putrefaction. Carbon isotopes are also suggestive of possible organic origin, since biological reactions are lazy by nature and favour the less energy intensive C12 isotope to the heavier C13, and select it preferentially out of the environment leading to an imbalance in the natural ratio.

Others have criticised the findings, pointing out that the uniform seeming orientation of the structures is very unlikely to be of organic origin, and suggesting that metamorphism was a more likely source. Others say that these structures are very large for a supposed life form so primitive, living in an energy restrictive anoxic environment. There have already been several false or still contested positives in this particular scientific quest.

Either way, these structures may remain inconclusive, but a lot of ongoing research is focussed on this question and those few remaining truly ancient rocks, both in order to understand the history and origin of this great mystery called life and to glean some impression of what might be possible on other planets in the solar system and beyond.

Loz

Image credit: 1: Matthew Dodd, 2: Dominic Papineau http://bit.ly/2lrU2qr http://bit.ly/2lYoRUR http://bit.ly/2mB5moO http://bit.ly/2mxkCCu http://bit.ly/2mxnAam Original article: http://go.nature.com/2lAvzUE

The Gemlands.

Some country names light an avid fire in any coloured gem lover's heart: Madagascar's multiple riches, Sri Lanka's sapphires or moonstones, South Indian aquamarines, Tanzanian rubies, tanzanite, garnets and Alexandrite, Kenya's tsavorite and corundums, Nigeria's and Mozambique's tourmalines.

All these gems are children of metamorphism, when intense heat and pressure transformed or melted the crust in mountain building events, distilling granites that spat out pegmatites as they cooled and focussing fluids filled with the rare elements necessary for gem formation through widespread fault systems.

All these countries were once part of Gondwana, and their gems have a common origin, formed in a Precambrian event known as the Pan-African orogeny. This enormous burst of mountain building happened between 800-600 million years ago as the Gondwanan supercontinent assembled from the debris of Pangaea's predecessor: Rodinia.

Africa was squeezed, heated and transformed from both sides, as they was impacted by continental collisions. Two ancient oceans closed, the western one bringing South America barging in, and the eastern carrying India, Antarctica, Sri Lanka and Madagascar behind it.

Huge mountain belts thousands of miles long were pushed up as the Earth buckled and heated, while the lower crust melted and produced granites that spread around these newly minted gemlands in an orgy of transformation. Rare elements were concentrated from many different rock types as they partly melted, and were eventually incorporated into the last remnants of the granitic magmas known as pegmatites.

These cooled slowly, allowing large gemmy crystals of many minerals to grow, such as from beryls, alexandrite and tourmaline. The chemistry of the original magmas decided each region's bounty, and as always in the minerals game, some parts got better endowed than others.

Other gems, including corundums (ruby and sapphire), tanzanite and many types of garnet formed through the direct metamorphism of rocks crushed under rising mountains or hot mineralised fluids given off by baked rocks precipitating crystals in veins.

The Mozambique fold belt formed deep within one of the longest mountain chains as Archaean rocks were transformed, extending down the length of Africa from Arabia to Antarctica, and including all the gem producing areas listed above. Deformation from these events extended all the way to Western Australia, at the time way over on the other side of Antarctica.

Composed of high grade gneisses (amphibolite to granulite facies), intruded by many kinds of granitoid rocks, they form the matrix of the region I call the gemlands. Pegmatite fields abound, as do placer deposits and strata bearing metamorphic gem crystals.

These chunks of crust have since been spread around the globe by continental drift, after the rifting of Gondwana into its constituent parts during the Cretaceous. They were slowly uncovered by aeons of further uplift and erosion cycles.

Different levels of the crust are exposed in the various countries, due to uneven erosion, in turn related to variations in climate as they drifted around the globe (or in Africa's case, maybe traumatised by the collision remaining mostly still while the rest moved away). As a result, in Kenya and Tanzania, gems are found in rivers and mined directly from the rock. In Sri Lanka, the original source rocks were eroded away, and the gems are found in old river gravels many metres below ground.

Loz

Sri Lanka sapphire crystal, Image credit: Rob Lavinsky/irocks.com

http://www.utdallas.edu/~rjstern/pdfs/PanAfricanOrogeny.pdf A map of the Pan African fold belts, http://web.earthsci.unimelb.edu.au/antarctica/images/pan-african.gif Firewall access: http://www.sciencedirect.com/science/article/pii/030192689090071W

Sheared magma chamber

These rocks are part of Mount Shimansky, one of several mountain peaks found in Palmer Land – a name for the southern portion of the Antarctic Peninsula.

The dark spots you see are known as mafic enclaves. The main rock type in this frame is a diorite – a coarse grained igneous rock consisting of mostly quartz and plagioclase, with minerals like amphibole and biotite likely making up the darker spots (papers I’ve found haven’t given precise petrography but those minerals are common in this type of rock). This coarse grained igneous rock represents magma that cooled slowly beneath the Earth’s surface, likely over a period of tens to hundreds of thousands of years. In other words, at one point this was a magma chamber.

The darker rocks floating in the diorite are amphibolites. This is a common metamorphic rock type produced when basaltic igneous rocks are metamorphosed. The big blob of igneous diorite at this site forced its way up through basaltic igneous rocks and in the process it heated them, metamorphosing them and likely picking up a few chunks as it went. Some of these chunks could also have been basaltic magma that intruded into the magma chamber – available data doesn’t constrain that. What’s important – these amphibolites have a higher melting temperature than the surrounding diorite. In other words, as the diorite forced its way up, those darker rocks stayed solid and floated as clasts in the molten diorite.

The rocks have been heavily sheared since they formed. The rocks of Palmer Land were a tiny sliver of a continent that accreted onto the edge of Antarctica just over 100 million years ago during the Cretaceous. This process of continental collision is violent and can stretch and strain rocks. The end result at this site is the layered pattern in these rocks that we call a foliation.

-JBB

Image sources: https://www.flickr.com/photos/euphro/albums/72157626952681513

Reference: http://onlinelibrary.wiley.com/doi/10.1029/2011TC003006/full

What is metamorphism?

Earlier this month over at our blog page, http://the-earth-story.com/, I was gawking at the view a photographer had captured of the metamorphic rocks at the base of the Grand Canyon and received a question from a reader: what is a metamorphic rock? I’m hoping these two samples can answer that, because these two rocks have very similar compositions and histories, in fact both come from Alaska, but they look very different because one is a metamorphic rock.

At the surface of the Earth, rocks interact physically and chemically with water. Chemical reactions take material that came up from the inner part of the earth and change it into fine-grained particles like those found in soils. These fine grains can be moved around by rivers, carried to lakes or oceans, and eventually deposited to make thin layers like those you see in the first rock. This type of sedimentary rock is called a shale and it is the most common type of sedimentary rock on Earth, made of the tiniest grains of solids the planet makes.

Since shales are common, they regularly get caught in the way when mountains start growing. They can be trapped in-between two continents when they slam together, or they can be forced out of the way when magma blobs rise upwards to form a volcanic arc. When rocks that were made of minerals that were stable at the surface start to be heated and squeezed, at some point they turn into metamorphic rocks. Metamorphism is the process of changing the identity of a rock without taking that rock all the way to molten – it is a change that happens to the rock while it is still solid.

During metamorphism, several things happen. First, minerals that were stable at the surface break down to form new minerals. The large red crystals in the second rock are garnet porphyroblasts – large crystals that grew during metamorphism. These crystals grew because the temperature got too hot for clay minerals stable at the surface to stay there, so they broke down chemically and new minerals grew in their place.

The texture of rocks also changes during metamorphism. As mineral grains are heated, they begin to grow larger, eventually becoming large enough for you to see. The second rock is called a schist – one requirement for this type of rock is that mineral grains are roughly millimeter sized or larger. Often rocks like this one have a sheen to them from flat, platy minerals that reflect light back at the person holding them.

Finally, rocks that are caught in mountain ranges also feel pressures from many directions – they’re not just compressed from the top, there are forces from the sides. These forces cause the growing minerals to shift and reorient themselves, leaving planes and lines in the rock that weren’t there previously. These patterns are called foliations if the minerals have formed planes and lineations if they have formed lines.

Metamorphic rocks are common at the Earth’s surface, particularly in regions that have built and eroded mountain ranges. They are a record of rocks being shoved down into the Earth’s crust where it is somewhat hot, but not hot enough to melt them. They then ride back up to the surface and eventually wind up as samples like the one seen here, containing records of their trip into the depths.

-JBB

Image credits: Travis https://flic.kr/p/35TThx James St. John https://flic.kr/p/oF4yNP

I like this as an example of what planetary scientists do. Taking big scale pictures and measurements of Mars, looking for chemical trends that can be fit with understood Geologic Processes.

Thermodynamics

This metamorphic rock is extremely useful to find in the field as it catches a chemical reaction in progress. The pale blue mineral is kyanite and the pink mineral is andalusite, both polymorphs of aluminosilicate minerals. These minerals have the exact same chemical formula and will turn from one to the other depending on the pressure and temperature conditions. Kyanite generally is found at higher pressures than andalusite, while andalusite is found at lower pressures and generally higher temperatures (the third polymorph, sillimanite, forms at the highest temperatures).

If you look closely there are even single grains with pink at one end and blue at the other. This rock preserves conditions just after it crossed the boundary in pressure-temperature where kyanite reacts to form andalusite. The picture owner describes the kyanite as turning into andalusite, suggesting that the rocks were being metamorphosed to conditions where kyanite was stable and then pressure dropped; the reaction therefore literally tracks how the pressure and temperature of the rock changed during metamorphism.

This rock was found on the Isle of Mull, Scotland.

-JBB

Image credit: Anne Burgess http://www.geograph.org.uk/photo/2982832

Heat and pressure

Metamorphism is the part of the rock cycle where rocks change into different rocks when subjected to heat and pressure, which can come from a variety of sources. Examples might include a granite intruding into other rocks, heating them and altering them as element charged fluids are spat out during the slow cooling process (called contact metamorphism, usually resulting in aureoles of different sets of characteristic minerals around the intruding pluton), or the huge pressures generated during continental collision, when large slabs of rocks called nappes detach from the lower crust and pile up in slow motion like a pushed tablecloth. The rocks at the bottom of the heap (now called a mountain chain) are baked and crushed until the minerals start to change.

Metamorphism is divided into grades, each representing a band of pressure temperature conditions, and characterised by known suites of minerals issuing from different kinds of rock as they are transformed (known as the protolith). As the grade increases, so does the crystallinity of the rock, and traces of its original form, bedding, fossils and texture are gradually obliterated, while a set of lines called foliations is imposed on the rock at 90 degreses to the direction of dominant pressure.

The lower grades include rocks like slate (photo 1, see http://on.fb.me/1NZH6i5 and http://on.fb.me/1Q50mSO) and phyllite, and the layers in these rocks are easily seen, often cutting across original bedding lines and depositional features. These rocks are called low pressure/low temperature, and characterised by the gradual appearance of micas as the latter fades into schist (photo 2, whose characteristic glinty surfaces are produced by mica crystals, see http://on.fb.me/1LaXY5p) as the hellish conditions increase.

In the deepest parts of growing mountain ranges and other extreme environments the crystals flow and separate into the light and dark layers of gneiss (photo 3 see http://on.fb.me/1PwAvO9). In the example in the fourth photo, the stimulus came from an earthquake or grinding of blocks of rock along a fault. The rock here is called mylonite (see http://on.fb.me/1LaXegR) and was snapped along a shear zone in the Alps. Sometimes large crystals like the plagioclase feldspar form in metamophic rocks, and are called porphyroblasts (see http://on.fb.me/23SGp5C). They include famous gems like the subway garnet (see http://on.fb.me/1T8psk4).

After that comes rock melting and granites separating off to rise in the crust (rocks known as migmatites, see http://on.fb.me/20EUggU), leaving behind a residual crystalline rock formed of high pressure high temperature minerals. So there you have it, a basic natural history of squished and baked rocks.

Loz

Image credit: Slate: Daderot, Schist NPS; gneiss: Mike Beauregard; Mylonite: Outcropedia

Garnet porphyroblasts

A porphyroblast is an undeformed crystal in a metamorphic rock that is much larger size than the surrounding groundmass. Here there are large porphyroblasts of garnet, in addition to smaller ones of staurolite (the black, bladed crystals to the side of the garnet).

Garnet crystals are famous as porphyroblasts as they’re incredibly common. They form so often that it’s pretty normal to see large, cm-sized garnets with well formed, euhedral edges as seen here.

For large crystals like these to grow in a metamorphic rock, several things must happen. First, the growing crystal must be strong enough to avoid being broken or deformed by the stresses applied during metamorphism. Second, the crystal must grow fast enough. Mineral growth is a complicated process involving atoms moving to, sticking to, and bonding to the edge of a growing crystal, so each mineral will grow at a different rate. Finally, the mineral must be made of elements that are abundant enough to allow it to grow; even if a mineral wants to grow fast, it won’t be able to if the elements it needs to grow are extremely rare.

Garnet is made of common elements such as aluminum, calcium, magnesium, iron, and silicon. It forms as a product of common reactions during prograde metamorphism; breakdown of clay minerals produces garnet as a natural result of the chemical reaction. Finally, garnet is a strong mineral; in fact in some porphyroblasts it actually overgrows other minerals and preserves the metamorphic fabric of the growing rock as inclusions.

-JBB

Image credit: Macroscopic Solutions https://flic.kr/p/xtd37U

Read more: http://bit.ly/1mNrQ1T http://bit.ly/1OU5w3v

Metaconglomerate

This texture is a particularly neat one to find in the field. These rocks are clearly metamorphic; there is a strong foliation pattern developed in the rock, but there is still a strong remnant of the protolith in the variation in chemistry from one spot to another.

This rock in northern France started off as a sedimentary conglomerate containing large chunks of rock >10 centimeters in length. That rock was then squished, squeezed and heated to the point that the rocks began to flow. The big clasts in the conglomerate were flattened and new minerals grew, but the metamorphism wasn’t intense enough to fully homogenize the rock. The clast chemistries started off distinct from the surrounding groundmass and they remained distinct throughout the metamorphism.

The shape of the clasts seen now can be used to reconstruct what happened during the metamorphism. On average, the cobbles that went into the conglomerate can be assumed to have started off life either round or randomly oriented. Measuring their length now can allow scientists to estimate how much strain has happened to this rock and how its shape changed during metamorphism.

-JBB

Image credit: Graeme Churchard https://flic.kr/p/tHwTaN

Read more: http://bit.ly/1LVk9BY http://bit.ly/1Rh3JBS