A whiff of old life…..Rocks might live for a long time compared to our biblical three score y

A whiff of old life…..Rocks might live for a long time compared to our biblical three score years and ten, but in the same way that organisms live, die and decay, thereby replenishing the soils with organic materials and nutrients such as phosphorous (called life’s bottleneck by Isaac Asimov) for the next cycle of life, so do rocks go through cycles of existence. A lava erupted in an oceanic spreading ridge, or sediments deposited by river, wind or glacier can enter new incarnations as very different rocks after the heat and pressure of being buried in the Earth or involved in a mountain building event has had its effect.These transformations are called metamorphism, and are graded by the estimated temperature / pressure conditions under which they occur, with a variety of index minerals of similar composition providing the means to assess them. Garnets are one such family of minerals that range from chrome rich versions born in the mantle (called pyrope), through iron rich versions that usually come from baking the crust, to manganese rich ones that are distilled out of the crust by granites. The cokmposition of the granite allows us to deduce something about its mode of formation, whether erupted in a kimberlite from the mantle, metamorphosed, or born in a granitic pegmatite,In the sample in the photo we have a piece of old organic rich lake or sea bottom mudstone that has been roasted and squeezed in the world’s depths during a mountain building event. Its precise ratios of trace elements can help us deduce its formation conditions, and hence its probable burial depth when transformed. The original rock (known as the protolith) was oxygen poor and organic matter rich, as attested to by its dark colour. The anoxic conditions allow the organic matter to be preserved in the mud, often including chemical cues as to which plant or animal types provided the richness in the first place.Generally black mudstones or schists indicate a grim old time for oxygen breathing life in the immediate vicinity of their deposition, sometimes recording worldwide events, such as the cretaceous blackouts that produced much of the oil in the middle east. It also provides the ideal conditions for good preservation of organic matter. When less baked, the result is hydrocarbons of the types that fuel our economy, when the process goes on a bit too long, the rock gets baked into a schist like this one. The organic rich mud has been transformed into a mixture of dark and shiny graphite (pencil lead), probably mixed in with some baked clay and studded with iron rich almandine garnets that absorbed all the surplus elements that couldn’t fit into the pure carbon structure of the graphite. Bearing in mind that the rock is 90% graphite, it must have been very organic rich when first deposited.I see lenses of this type of rock every year when driving up to my skiing flat in the Alps, the Vanoise and Maiurienne represent the sea bottoms of the vanished ocean of Tethys that once separated Africa from Europe, and contain many deeply baked black and green anoxic mudstones, turned into gleaming schists, some green some black. Every time I ski over them, I remain very aware that I am sliding on a thin crust of frozen water, overlying old sea bottom muck with its roasted and squished in remnants of life. Understanding the rock cycle gives us the tools to understand the place where we live, and where it fits in the planet’s complex history. As I write this, I sit atop another part of Tethys, this one unbaked, the great limestone platform of western Europe that formed form the skeletons of plankton back in the Jurassic.Loz#MineralMondayImage credit: Treasure Mountain Mining -- source link

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