The Earth Story

Gorgeous metamorphosed carbonate rock. The big crystals are garnets, the green minerals are epidote and chlorite, and the pale white stuff is calcite.

instageology Clorite and muscovite in a thin section. First picture seen in polar light and the second one in nlrmal light.

Quartz gemscape sculpture. In most gems inclusions are perceived as flaws, but sometimes they can grace them with extra beauty or an interesting geological story. These are known as scenic inclusions and are now highly sought after, though they were routinely discarded not that long ago. This sculpture of quartz with natural inclusions of green chlorite and a red mineral, possibly hematite, was created by gem artist Lawrence Stoller. Loz Image credit: Lawrence Stoller http://www.crystalworks.com/

Click the arrow at the side to see how metamorphic rocks change grain sizes under a microscope as they are heated hotter and longer

A new one on me..... A reader commenting on a recent post on the rare Russian mineral Charoite (see http://on.fb.me/1uLKomu) put me onto the track of this beautiful variety of clinochlore, a mineral in the chlorite group that forms when lavas are altered by hydrothermal fluids. Named Seraphinite in the gem trade after the proverbial highest order of angels in the bible, the beautiful version we share here (like Charoite) comes from a single known locality, both in the wild depths of the Siberian East. The name comes from its resemblance to the feathers that supposedly grace angelic wings. Other gemstones with a single source include Tsavorite garnet (http://on.fb.me/1ygBzM5) and Tanzanite, a unique variant on Zoisite (http://on.fb.me/1B8IMQy). Most chlorite minerals form from altered minerals in basaltic type lavas that contain low silica and minerals such as pyroxenes and amphiboles or during low grade metamorphism. This variety was first discovered in the 19th century, it formed in a rock called a skarn, which represents chewed up semi transformed limestones that surrounded an intruding iron rich granite that were baked and metasomatised by the resulting fluids. It is dark green to grey and chatoyant, the silvery fibres (composed of mica inclusions) reflect the light in a cat's eye if cut in the correct manner, and has a resulting pearly sheen. Being soft (2-4 on Mohs scale) its use in jewellery is limited, though cabochons have been set into low wear jewels such as pendants and cut for collectors. They have sometimes been stabilised with polymer, a fair and common practice, as long as it is properly disclosed all the way down the distribution chain. The lovely sample in the image measures 10x8x0.5cm.  So please keep these useful comments coming, so that I can carry on learning as much writing here as you do reading our posts. I'm always on the lookout for new and unusual rocks and minerals to write about.  Loz Image credit: Carion Mineraux http://www.gemselect.com/gem-info/seraphinite/seraphinite-info.php http://www.gemdat.org/gem-27184.html http://www.mindat.org/min-27184.html

THE ARCHEAN EON Each geologic eon marks a step in the planet’s history to, well, where we are now. The Archean evolves from the hell of the Hadean at ~ 3.8 billion years in the past, its dawn recorded by the oldest rocks preserved in the geologic record. The end of the Archean, at its boundary with the Proterozoic ~2.5 billion years ago, is marked by one of the greatest catastrophes ever witnessed by the planet – a disaster rooted in early life processes within the Archean and without which there would be no “us” today.  In the beginning of the Archean –  –The sun was still weak, not yet the energy powerhouse of today: the sun is estimated to have been “shining” at about 75% of its present level and strengthened to about 85% by the end of the Archean. –Earth’s heat flow was three times higher than today and cooled to twice by the beginning of the Proterozoic. This would mean that given very little encouragement, anything that might cause a shallow rupture in the incipient crust (perhaps an earthquake, a meteor strike, or convection in underlying magma) could cause yet another volcano to erupt.  –The remnant pieces of Archean lithosphere preserved in the geologic rock record begin to look like primitive versions of today’s. The surface of the ancient earth seemed to have two main kinds of crustal material. One kind was essentially “lava,” hardened at the cold surface of the world. This sort of rock is referred to as “mafic” because it’s rich in magnesium and iron (ferric) and, as this composition implies, it’s a rather heavy rock material. The other sort of crustal material consisted of the lightweight constituents, the “froth” if you will, that collected like scum above the mafic material. Because this is largely composed of feldspars, this is called “felsic.” Felsic scum is the parent of our continental cratons: ~7% of the rocks exposed on the earth today date from the scum of the Archean.  –From a slow start of rock cycle processes in the late Hadean, we can almost recognize plate tectonic activity in earnest within the Archean. There are no properly preserved tectonic plate margins, but there are ancient cratons that seemed to have merged with other cratons. Along the boundaries where these proto-continents welded together are elongate zones of somewhat younger mafic rocks: these are the “greenstone belts.” Greenstone belts, as their name implies, are composed largely of “green” rocks, that is, rocks rich in chlorite and epidote produced during the metamorphic alteration of mafic minerals. The original rock types (before they were “green”) include lavas erupted within seawater, sediments (both rock debris and chemical sediments), and coarser-grained mafic rocks from deeper levels of the ancient “greenstone” crust or mantle. Rock suites that look much like these, given the name “ophiolites,” have been forming ever since the Archean whenever new oceanic lithosphere is created and is preserved through plate collisions onto continental terrains. So, by the end of the Archean, the earliest plate tectonic processes seem to have been functioning.  –The division of the earth’s surface between continental and oceanic terrains in the Archean is speculation. Estimates range from ~5% to 40% continental material (presumably dry land) and the rest oceanic. Today, only ~25% of the earth is continental. The ocean basins of the time were filling with sediments such as greywackes (a sedimentary rock type composed of rock detritus from a close-by source), chemical cherts (cryptocrystalline quartz deposited in the Archean largely by inorganic processes) and some rather strange deposits of “banded iron formations.” At the very least, these sedimentary accumulations attest to the presence of liquid water as a means of erosion on continents and transport of rock detritus: there were rivers as well as oceans.  –Life began to test the waters of the ancient Archean. At first, simple bacteria-like organisms (one-celled organisms lacking cell nuclei such as Archaea and Prokaryotes) used sulfurous and methane-rich sources to obtain metabolic energy. Archean environments in which these anaerobic organisms lived were probably much like those where these creatures are still found today: within and in rocks beneath undersea volcanic vents and in hot springs. These life forms today are labeled “extremophiles” because they enjoy extreme environments.  .  –There may not have been photosynthesis at the beginning of the Archean, but there certainly was by its end. Enter the “cyanobacteria” (blue-green algae), simple organisms capable of using solar energy to thrive; a “waste” product of their metabolism was free oxygen. Fossil cyanobacteria are found in rocks of 3.5 billion years in age, but not until ~2.7 billion years did they really take over the Archean.  –The Archean becomes the Age of the Stromatolites. These almost-creatures are composed of layers of blue-green algae, happily existing off sunlight and carbon dioxide in the Archean seas: as they metabolize carbon dioxide via photosynthesis, layers of calcium carbonate (what limestone is made of) precipitated from their CO2 usage, blocking the sunlight, so they moved a “story” upwards into the light, and started another layer, and another, and another… resulting in football-size spheres of layers of limestone and bacterial gunk. The stromatolites built the first reefs on earth. –Oxygen was poison. The vast majority of life-forms of the early Archean were the extremophiles, happily metabolizing sulfur and methane (to which the addition of oxygen totally blocks the chemical reactions essential to their physiology), and these co-existed quite nicely with the stromatolites throughout most of the Archean. Their savior from free oxygen was …rust. When oxygen entered the Archean seawater, it quite nicely hooked up with iron ions floating about, which then precipitated out of the solution and collected in layers and layers of iron oxide (yes, rust) on the seafloor. These layers formed another famous rock suite of the Archean, the Banded Iron Formations. Banded Iron Formations are still exploited today as iron ore, all due to the activity of the blue-green algae of the Archean.  Towards the end of the Archean, with more and more oxygen being produced by cyanobacteria, more and more iron was precipitated from the oceans until – –There were not enough iron ions in seawater to neutralize oxygen. For the first time an excess of oxygen began to accumulate in the atmosphere. Oh, not much – just up to several percent compared to the ~21% of our modern atmosphere. Never the less, this is termed the Great Oxygenation Event.  Poised on the boundary between the Archean and Proterozoic, the future for all life on earth hinged on the ability of primitive organisms to survive the Great Oxygenation Event. The presence of oxygen caused a massive extinction to the prevailing anaerobic life forms, seemingly a “backfire” in evolution that wiped out all the seeming biologic progress made to that date. But, like a tendril reaching into the future, those blue green algae and a few chemotrophs survived and … well what happens next is a story for the next Eon, the Proterozoic.  The biologic implosion at the end of the Archean is the first, and thought by some to be the greatest, extinction event inflicted on earth’s inhabitants. It was not brought about by meteors or excessive volcanism or global warming – but by oxygen poisoning.  Annie R.  *Expect a summery of the next Eon, the Proterozoic, in about a week’s time. Photo credit: Pilbara Archean Craton (satellite image): http://www.jspacesystems.or.jp/ersdac/ASTERimage/ASTERimage_library_E.html  For the intrepid reader: http://www.facebook.com/photo.php?fbid=426202710774112&set=pb.352857924775258.-2207520000.1353490038&type=3&theater https://www.facebook.com/photo.php?fbid=436673956393654&set=a.352867368107647.80532.352857924775258&type=1&theater http://www.unlockingtheworld.com/resources/public/volcanoes/384-the-early-earth-and-plate-tectonics http://essayweb.net/geology/timeline/archean.shtml http://forces.si.edu/atmosphere/02_02_02.html http://imnh.isu.edu/exhibits/online/geo_time/geo_time_eons.htm http://ammin.geoscienceworld.org/content/90/10/1473.abstract http://www.sciteclibrary.ru/eng/catalog/pages/8684.html http://www.sciencedirect.com/science/article/pii/0012825294900256 http://www.cprm.gov.br/33IGC/1353714.html http://www.noahsarkzoofarm.co.uk/pages/research/07-warm-water/warm-water.php http://www.bbc.co.uk/science/earth/earth_timeline/first_life http://serc.carleton.edu/microbelife/extreme/extremophiles.html