The oldest surface layer that forms the continent, called the crust, is approximately 4 billion years old and consists of 25-50 km thick volcanic rocks called basalt. Scientists initially thought that the entire planet was covered by a single lithospheric crust, whereas the individual plates we see today only began to form after 1 billion years. However, the attitude towards this hypothesis is controversial.
The mechanism by which continental crust forms is somewhat mysterious: scientists now suggest that it may be due to plate tectonics, the movement of Earth’s major surface plates around the globe over billions of years, forming land masses and topographic features. we see today.
One theory focuses on how plates coalesce, often causing one to sink beneath the other, leading to partial melting and changes in magma composition, while the other examines the mechanisms that occur in the crust itself (less than 50 km deep). . It is caused by plate boundaries, but also causes partial melting.
A new study published in the journal Nature Geoscience reports experimental studies of an analogue of oceanic plateaus, large, flat, steep-sided elevations that are early basaltic crust that first formed in the Eoarchean (3.6 to 4 billion years ago). From the University of Edinburgh, Dr. Alan Hastie and his colleagues subjected primitive oceanic plateau basalts from the southwestern part of the Ontong Java Pacific Plateau to high temperature and pressure melting experiments.
Exposed continental crust cannot form at pressures <1.4 gigapascals (GPa) at depths below 50 km; this indicates that this type of magma was formed during convergent subduction zones. Therefore, they assume that plate tectonics, even in its primitive form, existed 4 billion years ago.
This information is powerful because plate tectonics is responsible for erosion, sedimentation, orogenesis, and volcanic activity, which play various roles in shaping continental crust. The research team suggests that gases released from volcanism, particularly carbon monoxide and methane, may have aided the origin of life on Earth by providing prebiotic molecules that led to the emergence of the first microbial organisms.
Outside of Earth, smaller amounts of silica-rich continental crust are also found on Mars and Venus, providing insight into the role of plate tectonics in the wider Solar System.
Dr. Hastie and colleagues examined the stability of a series of minerals at different pressures (1.2-1.4 GPa, equivalent to a depth of about 40-50 km) to determine when they transformed, with potential mantle temperatures reaching 1500-1650. °C. The key minerals of the study were garnet (known to be stable at pressure >1 GPa, corresponding to a depth of ~30 km) and plagioclase feldspar (stable to a depth of ~1.8 GPa, stable to a depth of ~60 km), rutile (corresponding to a depth of ~30 km). ) was. pressure 0.7-1.6 GPa, depth ~25-55 km) and amphibole (controls dehydration and melting reactions).
The results of the experiments showed that garnet and rutile were not stabilized at temperatures <1.4 GPa (~45-50 km depth); This is higher than what was found in previous studies, but the team explains that their source is that the ocean crust has a higher magnesium content. this is more consistent with the expected composition of the Eoarchean parent crust (rich in iron and magnesium).
They also performed the reverse experiment, where they grew garnet crystals at a higher pressure (2 GPa) and then subjected them to a lower pressure (1.4 GPa) and found that the garnet crystals began to break down. They then found that pressures of ~1.6 GPa (depth > 50–55 km) were stable for garnet, reinforcing the previously existing idea of stability at 1 GPa pressure and therefore increasing the depth of formation. Therefore, subduction is a more appropriate mechanism to explain this reaction.
Simulations also suggest that primary magmas undergo fractional crystallization as they rise through the Earth’s crust, causing crystals to separate from the liquid magma, and the remaining magma pool becomes deprived of certain elements used in parent crystals, so the composition constantly changes accordingly. to the extent of formation of new crystals.
This led the research team to identify amphibole crystallization as the primary cause of partial melting; because amphibole is a hydrous mineral that may have been overturned and buried and mixed with the crust. This corresponds to features of known Eoarchean volcanic rocks such as tonalite and trondemite.
The Isua Greenstone Belt in Greenland and the Archaean Slave Craton in Canada are considered two remnants of convergent plate edges above former subduction zones. In such regions, metabasic (metamorphosed basalt and related rocks) magmas mixed with fluids from the melting crust to form new silica-rich magmas, beginning the cycle of continental destruction and rebirth that shaped the world we see today. Source
Source: Port Altele
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