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Started by Recusant, May 02, 2020, 08:30:19 PM
Quote from: Randy on June 10, 2020, 02:28:49 PMNah, eternity sounds boring. Besides, worshiping for eternity does not seem like paradise to me.
QuoteA new study led by the University of Colorado Boulder reveals the complex history behind one of the Grand Canyon's most well-known geologic features: A mysterious and missing gap of time in the canyon's rock record that covers hundreds of millions of years.The research comes closer to solving a puzzle, called the "Great Unconformity," that has perplexed geologists since it was first described nearly 150 years ago.Think of the red bluffs and cliffs of the Grand Canyon as Earth's history textbook, explained Barra Peak, lead author of the new study and a graduate student in geological sciences at CU Boulder. If you scale down the canyon's rock faces, you can jump back almost 2 billion years into the planet's past. But that textbook is also missing pages: In some areas, more than 1 billion years' worth of rocks have disappeared from the Grand Canyon without a trace.Geologists want to know why."The Great Unconformity is one of the first well-documented geologic features in North America," Peak said. "But until recently, we didn't have a lot of constraints on when or how it occurred."Now, she and her colleagues think they may be narrowing in on an answer in a paper published this month in the journal Geology. The team reports that a series of small yet violent faulting events may have rocked the region during the breakup of an ancient supercontinent called Rodinia. The resulting havoc likely tore up the earth around the canyon, causing rocks and sediment to wash away and into the ocean.[Continues . . .]
QuoteAbstract:The Great Unconformity is an iconic geologic feature that coincides with an enigmatic period of Earth's history that spans the assembly and breakup of the supercontinent Rodinia and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history below the Great Unconformity at its classic Grand Canyon locality in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ across both long (∼100 km) and short (tens of kilometers) spatial wavelengths. The spatially variable thermal histories implied by these data are best explained by Proterozoic syndepositional normal faulting that induced differences in exhumation and burial across the region. The data, geologic relationships, and thermal history models suggest Neoproterozoic rock exhumation and the presence of a basement paleo high at the present-day Lower Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Upper Granite Gorge. The paleo high created a topographic barrier that may have limited deposition to restricted marine or nonmarine conditions. This paleotopographic evolution reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that induced multiple events that formed unconformities over hundreds of millions of years, all with claim to the title of a "Great Unconformity."[¶ added. - R]
QuoteThe amount of oxygen in the Earth's atmosphere makes it a habitable planet.Twenty-one per cent of the atmosphere consists of this life-giving element. But in the deep past — as far back as the Neoarchean era 2.8 to 2.5 billion years ago — this oxygen was almost absent.So, how did Earth's atmosphere become oxygenated?Our research, published in Nature Geoscience, adds a tantalizing new possibility: that at least some of the Earth's early oxygen came from a tectonic source via the movement and destruction of the Earth's crust.The Archean eon represents one third of our planet's history, from 2.5 billion years ago to four billion years ago.This alien Earth was a water-world, covered in green oceans, shrouded in a methane haze and completely lacking multi-cellular life. Another alien aspect of this world was the nature of its tectonic activity.One feature of modern subduction zones is their association with oxidized magmas. These magmas are formed when oxidized sediments and bottom waters — cold, dense water near the ocean floor — are introduced into the Earth's mantle. This produces magmas with high oxygen and water contents.On modern Earth, the dominant tectonic activity is called plate tectonics, where oceanic crust — the outermost layer of the Earth under the oceans — sinks into the Earth's mantle (the area between the Earth's crust and its core) at points of convergence called subduction zones. However, there is considerable debate over whether plate tectonics operated back in the Archean era.Our research aimed to test whether the absence of oxidized materials in Archean bottom waters and sediments could prevent the formation of oxidized magmas. The identification of such magmas in Neoarchean magmatic rocks could provide evidence that subduction and plate tectonics occurred 2.7 billion years ago.We collected samples of 2750- to 2670-million-year-old granitoid rocks from across the Abitibi-Wawa subprovince of the Superior Province — the largest preserved Archean continent stretching over 2000 km from Winnipeg, Manitoba to far-eastern Quebec. This allowed us to investigate the level of oxidation of magmas generated across the Neoarchean era.Measuring the oxidation-state of these magmatic rocks — formed through the cooling and crystalization of magma or lava — is challenging. Post-crystallization events may have modified these rocks through later deformation, burial or heating.So, we decided to look at the mineral apatite which is present in the zircon crystals in these rocks. Zircon crystals can withstand the intense temperatures and pressures of the post-crystallization events. They retain clues about the environments in which they were originally formed and provide precise ages for the rocks themselves.Small apatite crystals that are less than 30 microns wide — the size of a human skin cell — are trapped in the zircon crystals. They contain sulfur. By measuring the amount of sulfur in apatite, we can establish whether the apatite grew from an oxidized magma.[Continues . . .]
QuoteAbstract:Oxidized, sulfur-rich arc magmas are ubiquitous in modern subduction-zone environments. These magmas are thought to form when the fluids released during prograde metamorphism of subducting oceanic crust and overlying sediments oxidize and hydrate the asthenospheric mantle. In contrast, Archaean arc-type magmas are thought to be relatively reduced and sulfur poor, owing to the lower concentrations of marine sulfate and limited oxidative seafloor alteration in the anoxic ocean before the Great Oxidation Event some 2.4 billion years ago (Ga). Here we measure the total sulfur concentration and relative abundances of S6+, S4+ and S2− in zircon-hosted apatite grains from sodic and potassic intrusive rocks from the ~2.7 Ga southeastern Superior Province, Canada. We find that, rather than being reduced and sulfur poor, the sulfur budget of the Neoarchaean magmas was dominated by S6+ and abruptly increased to concentrations comparable to Phanerozoic arc magmas following the interpreted onset of subduction at approximately 2.7 Ga, coincident with the first global pulse of crust generation. These findings indicate that oxidized, sulfur-rich magmas formed in subduction zones independent of ocean redox state and could have influenced oceanic–atmospheric and metallogenic evolution in the Neoarchaean.