Banded Iron Formations – an insight into early life on Earth

While the earth formed some 4.5 billion years ago in the Hadean Eon, most of the rocks we see nowadays are much younger than that. Looking at changing rocks through time we can see a number of distinct environments and time periods represented, such as the impressive exposures of white chalk from the Cretaceous Period, but relatively few opportunities to study the very oldest rocks on the planet remain. On Earth, through the combined actions of metamorphism, erosion and remelting of rocks, Hadean rocks are in very short supply. More samples exist from Earth’s next Eon, the Archaean, including fascinating examples like these that imply a very different world. Banded iron formations (BIFs for short) are distinct, layered, and often heavily deformed rocks. Typical BIFs show repeating, consecutive, iron-rich and iron-poor layers; bands of a few millimetres to centimetres of black, silver or grey iron oxides such as haematite (Fe2O3) or magnetite (Fe3O4) alternate with layers of sediment lacking in iron, like shale or chert that are often red in colour, producing a beautiful layered effect. These formations are therefore an excellent indicator of the Earth’s early environmental history.

About 2.4 billion years ago, oxygen first appeared in Earth's atmosphere, the product of organisms called cyanobacteria which first developed the process of photosynthesis around that time. Prior to this, oxygen generally reacted with dissolved iron or organic matter in the oceans, but once these sources became oversaturated O2 started to fill the atmosphere; this is often referred to as the Great Oxygenation Event, the first accumulation of biologically induced oxygen.

Without oxygen in the atmosphere, iron does something we're not familiar with today - it actually dissolves in water and can be held in the oceans like salt. Once oxygen began building up in the atmosphere, suddenly all this iron became insoluble, started precipitating out and sinking to the sea floor. This appears to have been a periodic process; periods of abundant dissolved iron alternated with periods of limited dissolved iron and formation of cherts and shales. Once enough oxygen was present to use up the iron dissolved in the oceans, BIFs could no longer form, so the planet no longer makes them today. Their age usually means that they have been subject to a number of deformation processes, producing beautiful folded effects in specimens. Banded iron formations truly are a unique insight into early planetary history. ZM

Further information:

Genesis of Banded Iron Formations: http://econgeol.geoscienceworld.org/content/89/6/1384.abstract Banded Iron Formations: http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Banded_iron_formation.html

Image credit: http://www.flickr.com/photos/jsjgeology/14872616219/

Figure: Folded jaspilite BIF (Hamersely Group, Neoarchean to Paleoproterozoic, ~2.47-2.55 Ga; Hamersley Range, Western Australia)

Original caption:

Original caption: “Road trip through Banff and Jasper National Parks, Canada. September 2018.GoPro HERO5 Session.”

Mary Ellen Jasper

A beautiful rock, made of life, consisting of a banded iron formation 1.85 billion years old made of microcrystalline silica containing red haematite inclusions sandwiched between the grey haematite of the precipitated iron. What makes this rock so interesting is that the red jasper is the fossilised remnant of stromatolites, some of the earliest complex organisms in geological history. They were formed by cyanobacterial microbial mats, growing sinuously over and through layers of precipitated sediments in an ever higher stack as the sediments fell rhythmically, creating the lovely shapes of the red jasper.

They come from the Mary Ellen mine in Minessota, in a layer of rocks called the Biwabik iron formation. Originally precipitated in a shallow sea, the chemical reason for the richness in iron started with the very photosynthetic bacteria that formed the stromatolites. As their lifestyle released oxygen into the sea, the reduced iron oxidised, and being insoluble, precipitated on the sea bottom, accompanied by plenty of silica that fossilised the mats into jasper as they grew.

The wiggling of the columns is thought to represent the tracking of the sun's position through the seasons, showing that the strategy adopted by sunflowers has along evolutionary history. Similar formations elsewhere have been used to prove the inference that the number of days in the year has shrunk through time as the Earth's rotation has slowed. These rocks also testify to one of the major events in global history, when free oxygen gradually appeared and poisoned off the existing ecosystem (who remain as some extremophile bacteria), thus paving the way for the evolution of the complex oxygen breathing life that is the most familiar feature of today's biosphere.

Our past post on banded iron formation http://tinyurl.com/p5ob9a6 and stromatolites http://tinyurl.com/nh48h3z

Loz

Image credit: Captain Tenneal

http://www.agatelady.com/photo-gallery-mineral-of-the-month-january-2012.html

Jurassic snails

The Morrison Formation is one of the iconic fossil bearing stacks of sedimentary rock of the western USA, famed for its late Jurassic dinosaurs amongst others. Deposited between 155 and 148 million years ago. The rocks are a typical sedimentary mixture, including lake mudstones, river sandstones and limestones, with many of the fossils focussed in the lithified sands and green silts that were once swamps, rivers and floodplains of a life filled landscape. Its name comes from a town in Colorado where fossils were first found and which became a major site in the vicious inter palaeontologist bone wars of the 19th century (see http://bit.ly/2ToEvJy for more on this). The basin that filled with sediments formed during a mountain building event called the Nevadan Orogeny, a precursor to the later events that uplifted the Rockies.

Here we have a more humdrum fossil from the formation, namely a clump of freshwater snails that were replaced with red agate/jasper, a microcrystalline quartz containing particles of mud whose iron content has oxidised to rust (seehttp://bit.ly/2MCaPGr for an explanaiton). The specimen measures 3.8 x 3.2 x 2.2 cm and came from Colorado.

Loz

Image credit: Joe Budd/Rob Lavinsky/iRocks.com

Carnelian

The orange to bright red variety of microcrystalline silica, both as agate (banded) and chalcedony has been used for over 7,000 years in jewellery and seals. Coloured by iron, the hue varies from a dull brown (aka sard) to a bright red, depending on the presence of other impurities and the quantity of iron present. The name comes from the Latin for a kind of cherry of similar colour. Unlike jasper, carnelian is usually translucent. Antique gems were often carved, as personal seals, amulets and protective stones to be worn in jewellery. They have turned up in archaeological digs as far apart as Knossos (the Minoan capital in Crete), the Harrappan cities of the Indus Valley, in Egyptian jewellery from the first dynasties onwards, ancient Assyria and all over the Hellenistic/Roman world. tones were often recycled, and many carved antique gems now grace crowns and monstrances of the medieval era.

Sources include Brazil, India, Russia, the USA and Uruguay, where it lies here and there all over the country, or rather, I have found bits everywhere I've been so far.

Loz

Image credit: Lech Darski

http://www.minerals.net/gemstone/carnelian_gemstone.aspx

Spotted jasper sphere Jasper is a form of microcrystalline silica coloured by the presence of impurities such as iron oxide (seehttp://tinyurl.com/ov9b8js for a detailed description of its formation). Up to 20% of the stone can be made of foreign substances, forming a wide variety of patterns. While it is uncertain how this particular pattern formed, it certainly makes for a lovely specimen. Loz Image credit: Heritage Auctions http://www.minerals.net/gemstone/jasper_gemstone.aspx http://www.gemselect.com/gem-info/jasper/jasper-info.php

The meat shaped stone Carved during China's last dynasty (1644-1911) to resemble a piece of fatty pork with its crackling fried in soy sauce, this piece of jasper has been cleverly used by the carver. The surface of the skin has been stained to resemble skin, veins and hair follicles carved in. The piece is one of the treasures of the National Palace Museum in Taiwan. Loz Image credit: National Palace Museum