PDA

View Full Version : Subduction is 3.8 billion years old



Swift
2013-Jan-21, 04:13 PM
From Laboratory Equipment magazine (http://www.laboratoryequipment.com/news/2013/01/researchers-study-earth%E2%80%99s-ancient-geochemistry?et_cid=3050675&et_rid=54636800&linkid=http%3a%2f%2fwww.laboratoryequipment.com%2f news%2f2013%2f01%2fresearchers-study-earth%25E2%2580%2599s-ancient-geochemistry)

New evidence from a team led by Carnegie’s Frances Jenner demonstrates that some of the tectonic processes driving volcanic activity, such as those taking place today, were occurring as early as 3.8 billion years ago. Their work is published in Geology.

Upwelling and melting of the Earth’s mantle at mid-ocean ridges, as well as the eruption of new magmas on the seafloor, drive the continual production of the oceanic crust. As the oceanic crust moves away from the mid-ocean ridges and cools it becomes denser than the underlying mantle. Over time the majority of this oceanic crust sinks back into the mantle, which can trigger further volcanic eruptions. This process is known as subduction and it takes place at plate boundaries.

Volcanic eruptions that are triggered by subduction of oceanic crust are chemically distinct from those erupting at mid-ocean ridges and oceanic island chains, such as Hawaii. The differences between the chemistry of magmas produced at each of these tectonic settings provide “geochemical fingerprints” that can be used to try to identify the types of tectonic activity taking place early in the Earth’s history.

Previous geochemical studies have used similarities between modern subduction zone magmas and those erupted about 3.8 billion years ago, during the Eoarchean era, to argue that subduction-style tectonic activity was taking place early in the Earth’s history. But no one was able to locate any suites of volcanic rocks with compositions comparable to modern mid-ocean ridge or oceanic island magmas that were older than 3 billion years and were also free from contamination by continental crust.

Because of this missing piece of the puzzle, it has been ambiguous whether the subduction-like compositions of volcanic rocks erupted 3.8 billion years ago really were generated at subduction zones, or whether this magmatism should be attributed to other processes taking place early in the Earth’s history. Consequently, evidence for subduction-related tectonics earlier than 3 billion years ago has been highly debated in scientific literature.

Jenner and her team collected 3.8 billion-year-old volcanic rocks from Innersuartuut, an island in southwest Greenland, and found the samples have compositions comparable to modern oceanic islands, such as Hawaii.

John Mendenhall
2013-Jan-21, 06:46 PM
Whoa, it's going to be fun to sample lavas from the Martian volcanoes!

Ara Pacis
2013-Jan-22, 12:57 PM
Dry, with a hint of earthiness.

John Mendenhall
2013-Jan-22, 10:19 PM
Dry, with a hint of earthiness.

The vintages after the late heavy bombardment are a little sweeter.

NaturalistG
2013-Jan-23, 09:12 AM
"Upwelling and melting of the Earth’s mantle at mid-ocean ridges, as well as the eruption of new magmas on the seafloor, drive the continual production of the oceanic crust."

Slightly misleading, the crust is pulled apart at the mid oceanic ridge and not due, generally to any upwelling (on occasions of rift initiation it can be argued upwelling or hotspots can trigger the rift, as is supposed with Iceland and the Atlantic, but are not the general cause of new oceanic crust creation on say, the mid atlantic ridge). As the old oceanic crust becomes older, cools and becomes dense enough to 'sink' it 'pulls' the less dense, more recent bit of crust attached to it away at its boundaries, releasing the mantle below and forming new crust at the mid ocean ridges. At least, that's what i read back in Uni.

Githyanki
2013-Jan-27, 04:29 AM
Couldn't the late-heavy bombardment cause the crust to melt causing any earlier evidence of subduction to be erased?

Icefox
2013-Feb-03, 05:43 PM
"Upwelling and melting of the Earth’s mantle at mid-ocean ridges, as well as the eruption of new magmas on the seafloor, drive the continual production of the oceanic crust."

Slightly misleading, the crust is pulled apart at the mid oceanic ridge and not due, generally to any upwelling (on occasions of rift initiation it can be argued upwelling or hotspots can trigger the rift, as is supposed with Iceland and the Atlantic, but are not the general cause of new oceanic crust creation on say, the mid atlantic ridge). As the old oceanic crust becomes older, cools and becomes dense enough to 'sink' it 'pulls' the less dense, more recent bit of crust attached to it away at its boundaries, releasing the mantle below and forming new crust at the mid ocean ridges. At least, that's what i read back in Uni.

True, but when the crust is pulled apart at the mid-ocean ridge, that releases pressure on the rock below it and that rock tends to flow upwards as a result... an upwelling. This is more of an effect than I cause, I think, but separating the two when it comes to plate tectonics can be tricky.


Couldn't the late-heavy bombardment cause the crust to melt causing any earlier evidence of subduction to be erased?

The late heavy bombardment was nowhere near heavy enough to cause the entire crust to melt, at least not all at once. But yes, the LHB and the last 3.8 billion years of history has resulted in almost all evidence being destroyed. "Almost" is the key here. :)

Eclogite
2013-Feb-08, 12:45 PM
True, but when the crust is pulled apart at the mid-ocean ridge, that releases pressure on the rock below it and that rock tends to flow upwards as a result... an upwelling. This is more of an effect than I cause, I think, but separating the two when it comes to plate tectonics can be tricky.Slab pull seems generally to be viewed as the dominant element. For example:

Slab pull, mantle convection, and Pangaean assembly and dispersal (http://www.soest.hawaii.edu/GG/FACULTY/conrad/classes/GG672_S09/topic13/Collins_EPSL2003.pdf)
The temporal evolution of plate driving forces: Importance of ‘‘slab suction’’ versus ‘‘slab pull’’ during the Cenozoic (ftp://ftp.es.usyd.edu.au/pub/dietmar/subduction_papers/Conrad_LithgowB_Evolution_plate_forces_JGR_2004.pd f)
The importance of slab pull and a global asthenosphere to plate motions (http://www.soest.hawaii.edu/GG/POSTDOC/summeren/Documents/articles/vanSummeren_etal2012_GCubed.pdf)

IreneAnt
2013-Feb-12, 06:18 PM
It is true that slab pull is believed to drive plate motion today, when the plate tectonic cycle is well established.

However, we still have no idea how to start either subduction or the entire plate tectonic system from scratch (e.g. The generation of plate tectonics from mantle convection (http://www.sciencedirect.com/science/article/pii/S0012821X02010099). This is why looking at potential plate tectonics in the Archean (3.8-2.5 b.y ago) and Hadean (4.6-3.8 b.y. ago) is so exciting. It may give us some insight into plate tectonic's beginnings.

Ara Pacis
2013-Feb-16, 05:37 AM
Could it the plates perhaps have used/needed some sort of impact-produced destabilization, or was that before water was present in enough quantities to serve as lubricant/ballast?