Strontium and S isotopes show enriched signatures relative to most other Lower Zone rocks. This would match the PGE content of the rocks. Alternatively, the chills could represent a komatiitic magma derived from the asthenosphere that underwent assimilation of the quartzitic floor accompanied by crystallization of olivine and chromite.
This model is consistent with the lithophile elements and the elevated Sr and S isotopic signatures of the rocks. However, in order to account for the high Pt and Pd contents of the magma, the mantle must have been twice as rich in PGE as the current estimate for PUM, possibly due to a component of incompletely equilibrated late veneer.
Cited 17 times in Scopus. View in Scopus. Download Statistics. The first continental slivers were forming. The oldest of these that survives is at Amitsoq, south-east Greenland, a combination of volcanic and sedimentary rock. Plate tectonics was beginning although still in its infancy: full blown plates with their subductions would not exist until another billion years. But mantle plumes existed, probably more plentiful and faster than nowadays. These threw up volcanic plateaus and islands, and these became the cores of the new continents.
The best recorded of the origin stories of the continents comes from the Southern African craton. This was once the heart of Gondwana. But the region was much older than this. The oldest rock here is a staggering 3. It takes us back into a different era, called the Archean, a period lasting until 2.
The Earth was a different planet. The atmosphere was not conducive to life, with high CO2, probably mixed with methane to give a photochemical haze in the air. But the ocean already existed. The oceanic crust was divided into numerous microplates, where everything was more active than now. The interior was hotter. There were no continents yet but there were plenty of oceanic islands. The location of the oldest rocks of the South African craton are in Barberton. They are called the Barberton greenstone and they tell a story about how the continents formed.
The rocks form a belt stretching from northeast to southwest, roughly by 60 km with rocks covering an age range of million years. The region is at the border of Swaziland, just south of the famous Kruger park. The southern parts of the Kruger park are dense with wildlife and tourists. The river is full of hippos and crocodiles and the wooded area hides the elephants well. They have right of way, even if suddenly appearing from the bush, and they know it. They happily chase the drivers to let them know who is boss.
The real Kruger is not here: that is much further north where the land gets wilder and the animals more dispersed. But if you want to quickly get your African safari in, the southern Kruger is hard to beat. The birdlife is also superb. Sometimes the animals get out of the park, and you may suddenly find a hippo in your pond. Nothing to do but call the park service and ask whether they do collect. The landscape looks ancient. But it is nothing on the Barberton area where the rock are many times older.
Not much has survived, of courses. The originally horizontal layers have turned on edge in timeless upheavals. This is why the belt is narrow. The whole area is embedded in ancient graniodorite which has survived better — partly because it was underground. It is all part of the story. The Barberton mountains are a green land: in the wet season it is covered in bush and some forest.
It is an ancient greenstone, but that does not refer to the vegetation. The chlorite is a later metamorphic addition to the rocks, but it shows its origin as oceanic crust. The original rock consists of three groups, called Onverwacht, Fig Tree, and Moodies. Onverwacht is the oldest of the three. It is mostly volcanic. The rocks of the Onverwacht group show how volcanic rocks erupted onto an ocean floor. Just like in modern oceans, the water cools the surface of the lava and the pressure prohibits any explosive activity.
The result is pillow lavas. Indeed, the Onverwacht group shows plenty of these pillows in the region. There is little doubt how this land began, deep under water. Pillow lavas in Barberton. The most common origin of pillow lavas is a mid-oceanic spreading centre. That has also been suggested for the Barberton rocks. But it has been questioned. The eruptions began 3. That seems too long for a spreading centre. The layers also are extensive and not faulted. The suggestion is that this was a volcanic plateau or submarine shield volcano. It is not known whether plate tectonics in its modern form, with spreading centres and subduction zones, had already developed on Earth.
It may have been an environment more like Venus, with areas of volcanic activity and deeper basins without, but without larger scale motion from one such area to another. The origin of plate tectonics is still debated. In the modern world, pillow lavas produce basalt. But although the Onverwacht group has a minor basaltic component, the majority were a different, much hotter type of lava. This type was first identified in the Barberton belt, along the southern part where the river Komati flows. It thus was given the name komatiite. Nowadays, oceanic basalt is melted in shallow magma reservoirs at temperatures of at most C.
The komatiite was formed several hundred degrees hotter, up to C. They came from a mantle that was notably hotter than nowadays. The eruptions were not continuous: they were interrupted by epochs when thin layers of sediments collected, composed of iron-rich and silica-rich mud and volcanic tuffs. After the pillow lavas stopped forming, the sedimentation kept forming, now including carbonate layers which buried the lavas.
The carbonate shows that the ocean was much like that of today, with abundant calcium carbonates. The most notable is a dacitic layer, 2 km thick, dated at 3. This has been interpreted as the subsiding volcanic peak, in fairly shallow water. Fine-grained mafic ash, Onverwacht Group showing cross-bedding stratification. Now a second pile of volcanic rocks formed, but with a composition of basalt and dacite.
This is the upper Onverwacht group which formed around 3. The magma did not come from as hot a chamber as the earlier komatiites. The dacite suggests that the magma chambers had time to cool and differentiate. It has been suggested that the newly formed oceanic crust was subjected to some subduction, partly melted, and percolated back into the crust.
Now things quieted down for a while, until the Fig Tree group formed between 3. It is a volcaniclastic sedimentary sequence that is capped by felsic volcanic rocks that formed in deep- to shallow-water to alluvial environments. Some of the deposits show evidence for turbidites: underwater landslides. In these slides, the coarse material sand reaches farthest and mud the least: mud therefore forms the upper slope. The mud stones are quite black from the amount of graphite.
This suggests that the volcanic islands were bounded by a trench into which the slides went down. The Moodies Group was deposited next, between 3. It consists of shallow-marine to fluvial sandstone and conglomerate with minor shale and banded iron-formation. It shows some banding that is typical of tidal bays, where even the neap-spring tide cycle can be seen. Counting the layers shows that there were only 18 days in a tidal month, a combination of the Earth spinning faster and the Moon being closer than nowadays. Silicified cross-bedded and wave-rippled sandstone overlain by chert-slab conglomerate; Buck Reef Chert , central Barberton Mountain Land.
What happened? The island arc and trench suggest that plate tectonics was beginning to behave more like the modern Earth. The partial melt had formed a graniodorite felsic which had been emplaced in the oceanic crust. This reduced the density of the crust, thickened it, and caused the area to rise.
When the oceanic floor began to subduct, this region was too buoyant for that. This lower density material formed the core of the island and morphed it into young continental crust. The lithosphere thickened and formed the km deep keels of the modern continents. The Barberton area was not a single terrane: it was several distinct regions which formed in different places and only came together during the last phase of the Moodie group.
This process went on not just in the Barberton area. Similar events took place elsewhere in the world. The Pilbara area is an example, with the same age as the area here. The next phase came when series of these new island arcs began to amalgamate.
Canadian Journal of Earth Sciences - 9(10) - PDF
This formed the core of the new cratons. The earlier granite was remelted, and now formed large emplacements of over 60 kilometers in size. The process would continue for another million years. By the end, the world was full of microcontinents. The first continental collisions occurred. Mountain building began. Microbial laminations interspersed with sandstone and overlying conglomerate of a fluvial-supratidal sandplain, exposed in the Moodies Group of the Saddleback Syncline, central Barberton Greenstone Belt.
Angular green clast is composed of altered ultramafic rock. This and previous photos: 35th International geological congress, , Cape Town. There was another event which left its scars. The spherules are the size of sand grains, and formed from condensation of rock vapour in the atmosphere. Effectively, these were liquid rock drops which formed in the atmosphere. It rained rock. There are four such layers in the Barberton greenstone, but this layer number 2 stands out, at the border between the Onverwacht and Fig Tree groups.
Knives Made of Frozen Feces Are Kinda Crappy
It is present in the southern area of Barberton. The same layer is seen in the Pilbara area. It contains chromium indicative of an extraterrestrial origin. This was a meteorite impact, from a carbonaceous chondrite. And not just any impact. Spherules from layer S2. Source: Lowe et al. Based on the thickness of the layer, the impactor has been estimated as between 37 and 55 km in diameter.
This is considerably larger than then the KT event. The impact caused dikes to form in the Onverwacht group. Some of the spherules found their way into these dikes, suggesting they were still open when the rock rain arrived. Gradation of the spherules suggests passing tsunami waves. The area was still deep under water, but this did not protect against currents caused by the tsunamis, or the earthquakes.
The moment of the earthquakes has been estimated at a minimum of M A crater of some km across may have formed. However, this would have been on Archean oceanic crust of which little survives, thousands of kilometers away from the Barberton rocks. There was a second impact of similar size 30 million years later. These are likely the two largest impacts the Earth has suffered over the past 3. Were they related? That seems plausible. Perhaps a large asteroid had broken in two following a collision, and both fragments ended up with orbits with intersected the Earth.
A game of russian roulette followed which only had losers. One more thing is worth pointing out. The S2 layer is exactly at the change-over between the Onverwacht and Fig Tree group. In fact, so is the S3 layer, as the change happened a little later in the north of Barberton where the S3 layer is seen.
The Fig Tree group is one of felsic volcanisms, and the onset of internal melt which formed continental crust. Perhaps these impact had a role in this. They formed large cracks where magma chambers could collect, may even have induced melt themselves, and ended the epoch of komatiite. Was this a worldwide change? It probably would have happened over time any way, but these massive impacts may have accelerated the change. Komatiite is an ultramafic magma.
Basalt does this: it is a property of mantle material, and mafic magma thus indicates that there is a conduit to the mantle. The mantle is not normally melted, so heat needs adding. That can be done either in a spreading centre where mantle material can upwell from deep because of a lack of pressure above , or it can through heat from a hot spot.
The hot spot can be shallow most are or it can be a proper mantle plume. The lack of silica makes the magma to be of low viscosity: it flows easily, and over long distances. It is also dense, and that makes it harder to erupt over the less dense continental crust without some significant heat input. Ultramafic is even closer to the composition of the mantle, and has very low silicate content. In the modern world, this happens when magma chambers collect olivine from the mantle and slowly become more and more mafic. But in the early Earth, the olivine from the mantle could erupt directly and this is komatiite.
It is silicate and aluminium poor and has a very high melting temperature. Its viscosity approaches that of water, so it flowed every more easily than basalt. The aluminium is not always the same: some komatiites are depleted in aluminium, but others are not. The undepleted ones are older. The melting temperature of komatiite depends on its precise composition, and it appears those changed over the archaean. The melt happens at km depth, at a mantle temperature of C. This is believed to be appropriate for the Al-rich komatiites of Barberton, and those elsewhere in the world at that time.
The mantle was hotter than it is now, due to higher level of radioactives, but not this hot. So where did the komatiites come from? The temperatures of a few hundred degrees above the normal mantle suggests the presence of mantle plumes.
The origin of spinifex texture in komatiites
The komatiites are believed to have formed in the tails of superplumes. This puts the evidence for the extensive pillow lava under new light. These regions may have been the ancient, subsea equivalents of modern flood basalts. The hotter mantle would have had lower viscosity than the modern mantle, making plumes easier to form and faster to rise. But as the mantle cooled, things changed.
The komatiites show lower MgO, less Al and Cr. They would have melted at lower temperatures, C, and lower depth of km. By the end of the archaean, 2. The youngest komatiites erupted million years ago on Gorgona island. But these are lower MgO, and the melting temperature was much lower than hat of the pure komatiites of the Archaean.
They are part of our new, mature world. The hotheaded days of the Earth are well and truly over. All that is left is a memory of greenstone belt. Barberton is the graffiti of our youth. Just an Fyi The weather in the Hawaiian Islands are forecast to get a little bad. This may cause some signals we watch go a little wierd? Hawaii is hot enough to potentially erupt komatiite, if magma ascended quickly from depth into kilauea it likely would be erupting at a temperature high enough to keep olivine as a liquid in the magma which would most likely classify it as a komatiite or something similar as opposed to normal basalt.
The plume head under the big islands is over C and a few hundred degrees hotter than the average mantle temperature. Hawaii is most likely the biggest mantle plume on earth today, and probably the only one which actually has a solid connection to the earths core, making it more like the ancient archean plumes. Turtlebirdman Iceland plume is pretty migthy too And New research suggest it too goes down to core boundary. This is the size of the Iceland plume. Here are the rocks that dont transmitt P waves. But Hawaii is currently hotter But in my first link you can see the size of the Iceland Hotspot.
Currently only the ERZ is still active the main part of the SWRZ is probably going to be functionally extinct until the caldera fills again and hosts a large volume lava lake and that will probably take decades or longer. It is fortunate no one lives on the SWRZ because eruptions there can be like eruptions on nyiragongo, sudden lava lake draining events which would be scary to be anywhere downslope from. It takes much more than days. The magma erupted has arrived to a near-surface reservoir by rising from the rift zone km I imagine is a safe bet. In the rift zone the pit craters show that there are areas where magma can be stored, mostly in the UERZ.
Even if the conduit was a perfect narrow tube it would still take a lot of time to make all the journey from the summit area to the MERZ. The DI events show that pressure changes in the summit extend into the MERZ almost instantly but the transfer of magma takes much more, I imagine days to years depending on the eruption rate. Years for Puu oo. It is strange why during the eruption in may the summit response to the collapse at Pu u oo took so long. Last year if you ignore the initial small eruptions and take only the flows erupted after May 18 to be the real stuff, then it took about 2 weeks for summit lava to erupt at fissure 8, about the same time as in The lava erupting in fissure 8 was not stored rift lava it was almost entirely deep sourced very primitive lava from the lower half of the summit storage complex that had never been close to the surface before, so it likely takes around 2 weeks to flow the entire length of the east rift.
Interestingly this is about the length of time between the last fountain from kilauea iki and the start of the eruption at kapoho so it is probable that those two eruptions should be counted as one continuous event. I said it would depend on the rate. I think would be a bad example to go off because kilauea was still recovering from the quake and the eruption might have been triggered passively by south flank movement. The magma in that eruption is thought to have come from an intrusion in that preceded the second phase of that eruption. When the flank moved a bit in the summit drained into that space and the stored magma escaped due to gas pressure.
In both eruptions the high fountain stage was not faster than the slow effusion it was just not continuous so when eruptions did happen they were fast. By primitive you mean rich in olivine? That olivine would be more likely to come from deep rift storage, in historic times the summit has practically never erupted olivine phenocryst rich magma, being the only exception I know. Magma can be enriched in mafic crystals through differentiation, as you know, and it is how modern ultramafic magmas are tipically generated as Albert explained. The magma erupted was clearly of ERZ origin which means that at the rate the eruption was going and its duration magma from the summit abandoned the reservoir but never got to the eruption site and was left stranded at some point in the way, there is a permanent magma body dikes, conduits or magma chambers through a large portion of the rift zone and is where the lava erupted is directly drawn from, only once the eruption goes on for a long enough time and enough intensity might it start erupting melt from the summit.
Still, happened after a major disturbance to the east rift, much more than last year, so it likely took longer than normal and with more places for the magma to get stuck on the way. This did happen in but that was because of much lower supply rate while most of the hotspot was feeding to mauna loa. But you are correct too If Kilaueas rise magmas very quickly it will emerge at C and pretty much flow like komatites.
Kilauea is really really hot And 50 km down its maybe C As hot as Hawaiis hotspot is These rocks are likley near completely molten Most of Big Islands interior is molten. The hotspot have made one huge partial melting pool at over C that may contain as much as many many tens of km3 magma thats 80 km below the island This is the pure melt pool that feeds Kilauea and Mauna Loa Kilaueas halemaumau conduit is directly connected with it siphones magma from 70 km below thats may be as hot as C.
The hotspot rises in the mantle and decompress And plume head ends up with being molten Hawaii haves a real plume head as the hawaiian swell suggest not just a plume stem like some other hotspots are. How much melt the hawaiian hotspot contains is unknown but its likey many many s of km3 of materials below the seafloor in sourthen part of the island chain. And this deep stuff is so hot it can almost melt raw iron. There are some pretty hot magmas on earth. They can even produce a modern form of komatiite, but with lower temperatures than the archaean komatiite. The effect is that the melt may form at C.
That is still hundreds of degrees cooler than the Barberton komatiite: it is not quite the same stuff! What I find amazing is how low the viscosity was, similar to that of water. The old komatiite would have have come down at top speed, and very thin layers. On the other hand the density is very high which means that it could not easily erupt at any altitude. It would also need to rise very fast from the deep chambers, otherwise it adds crustal melts and cools on the way.
But that is hard when it is such dense stuff. The ancient komatiite was mainly an oceanic lava. The most likley to appear when Kilauea reaches here peak shield stage boost And magmas rise fastest from the C plume source. Potential Swift Tuttle impact will be very similar to the large asteroid in the Article Swift Tuttle also goes ridicusloy fast adding more energy. Maybe one day even basalt will stop erupting?
In other volcano news: absolutely nothing apparently- very quiet at the moment! As the mantle cools, the melt under the mid-oceanic ridges will become more shallow and less volume. Subduction will still continue though. Back to the magnetic field at those times. The planet rotation was faster and the interior very hot.
Weaker or stronger? More frequent reversal? Perhaps more importantly, the core was still completely liquid. That would have made it easier to maintain a stable magnetic field. I would expect a stronger field and fewer or no reversals. Komatite eruptions resembles eruptions of liquid iron or liquid iron slag… These prehistoric lava flows where white hot at C Glowing intensely and pouring like water. Flows where extremely thin and narrow and filled small spaces and formed small channel systems at low eruptive rates. And large and sheetlike floodlike when eruptive rates where high poured like a river in flood.
A Komatite flow where completely white hot with a orange yellow glowing crust or scum floating on the quickly moving lava stream.. Komatite pahoehoe must have been much less than a centimeter thick and dark and almost steelish and very dense when its erupted at low eruptive rates. Been busy making this, a map of all the individual fissures on kilauea since Vents within the summit caldera fault I have coded in blue while vents outside the caldera are red.
But it looks almost like the caldera part is on a kind of junction, or twist. Good map. Up until only a few hundred years ago the main summit center was even further north than it is now with the main vent being next to HVO, and called the observatory vent. This is what became the original caldera that has been modified since then, and even comparing to now the main activity is much further south and with the shallow system of halemaumau now gone I expect activity to migrate again, the next eruptions might happen from the south caldera ring fault, or even within parts of the volcano that have never erupted before like the koae faults that are in the blank area below the summit.
Last years eruption was likely so big because the quake opened up enough and at a sufficient depth that magma was able to drain out at the base of the east rift, making todays east rift now a lot deeper than this time last year which might be significant down the road. The eruption before fissure 8 started was pretty typical of a LERZ eruption that was beginning to enter its final stage, but then fissure 8 stepped into an entirely different league that was almost like an entirely different eruption.
If I was to guess, in a few hundred years the summit will be completely south of where it is now, making a smooth connection to the east rift, which might result in eruption styles more similar to mauna loa. The long protected and old surfaces of the hilina pali will likely get extensively overflowed in this activity too, while the currently relatively young lava north of the east rift will probably be largely untouched for a long time until the new summit is able to get high enough to flow that way.
It would be interesting to be around to see that. A fascinating system! I appreciate your thorough knowledge of it all — the last eruption has been amazing to watch.
komatiite, Alexo (Canada)
A komatite volcano on land woud either form a flat fissure flood plain if eruptions at high eruptive rates. Or a very very very very flat low shield volcano at slow eruptive rates. Flows and hornitoes and flow lobes gets very small and delicate at this molten metal fluidity.
A komtatite flow is much more fluid than Kilauea thoelite basalts or Nyiragongo nephelinite lavas. Waveform is clearly visible on the DYN drumplot. A couple of times the two have been close in time, but most of the time they are not. But at some timepoint the inflation should reach the pre-eruption level. When, and will there be a new Holohraun?
Greip may be a forming magma chamber Being close to the plume source and at the spreading axis its high chance for that. These two are for now in orbital resonace. Its going to be ugly..