Esprit Rock

When did plate Tectonics Begin

When did plate Tectonics Begin?
ABSTRACT
The Isua complex was calculated to be in operation 0.7-0.8 G. yr. after the formation of the Earth. (Komiya, T., Maruyama, S., Masuda, T., Nohda, S., Hayashi, M. and Okamoto, K., 1999. Plate tectonics at 3.8–3.7 Ga: field evidence from the Isua accretionary complex, southern West Greenland. The Journal of Geology, 107(5), pp.515-554.) Signature Sulphur isotopes were recorded to be more prominent 2.5 Ga ago, indicating subduction (Rollinson, H., 2007. When did plate tectonics begin? Geology Today, 23(5), pp.186-191.) and Blueschists were recorded to be 176.6 Ma of age. (V.B Sisson and T.C. Onstott, Dating blueschist Metamorphism: A combined Argon-40/Argon-39 and electron microprobe approach Pg. 3) These results were obtained by isotopic dating as well as geologic surveys and mass spectroscopy. The different results obtained may infer when the plate tectonics had begun on earth. However, all the tests were carried out in specific locations, and may reflect when plate tectonics started within this location, and not necessarily when tectonics had first started on earth. The differences in results also make it harder to determine a single age at which tectonics started and indicates that answer to the question ‘When did plate tectonics first begin’ is merely an estimate.

Introduction
It is needless to say that The Earth is drastically different to any other planet in our solar system, not only by chemical composition of air and land, but also the fact that plate tectonics are only found here on Earth. This shows that plate tectonics are not automatically generated by silicate planets. (Rollinson, H., 2007. When did plate tectonics begin?. Geology Today, 23(5), pp.186) Google immediately states that plate tectonics took place 3.2 billion years ago but is this value accurate, and if so, what experiments lead to the estimation of this value? The research areas we will be looking at include field evidence, as well as isotopic dating. This will allow us to gain a broad perspective of how the geomorphology of the Earth came to place.

Plate Tectonics Field Evidence from Isua Accretionary Complex, Southern west Greenland by Department of earth and planetary sciences in Tokyo institute of technology
Procedure
The researchers used a 1:5000 scale map in the Isukasia area Isua super crustal belt in southern West Greenland. The mapped area was divided into three units bounded by low angle thrusts: Northern, Middle and southern units. The southern unit which was the best exposed is consisted of 14 subunits with similar lithostratigraphy, bound by layer parallel thrusts. Duplex structures were widespread in the Isua belt and it proceeded from south to north. They reconstructed the lithostratigraphy of each subunit which revealed a simple pattern in increasing order of greenstone, chert and turbidities.

Results
The oceanic geotherm for Early Archaean lithosphere was calculated based on a model of transient half-space cooling of surface and mantle temperatures of 100 degrees and 1450 degrees suggesting that the Archean oceanic lithosphere would have been rigid. These conclusions had helped the team support the idea that modern plate tectonics has been in operation 0.7-0.8 billion G. yr. after the formation of the earth. (Komiya, T., Maruyama, S., Masuda, T., Nohda, S., Hayashi, M. and Okamoto, K., 1999. Plate tectonics at 3.8–3.7 Ga: field evidence from the Isua accretionary complex, southern West Greenland. The Journal of Geology, 107(5), pp.515-554.)
Discussion
The duplex polarity meant that an older accretionary complex should occur to the south of the Isua complex, and the presence of seawater indicates the surface temperature was less than a hundred degrees in the early Archean. This allowed researchers to obtain the field results.

Sulphur isotopes as an indicator of former subduction
Subduction could be recognized through identifying the minerals which we know certainly formed at the Earths surface, but now has a record of deep burial. Sulphur isotopes can also indicate subduction.

Method
Photochemical oxidation of Sulphur in the atmosphere gives it an isotopic signature which is different from isotope fractionations produced by normal geologic processes. In 2002 James Farquhar reported discovery of Sulphur in deep mantle diamonds found in Botswana. (Rollinson, H., 2007. When did plate tectonics begin? Geology Today, 23(5), pp.190.)

Fig.01: General pattern of Sulphur 34 and Sulphur 33 isotopes in the Proterozoic and Archean (Yuichiro Uenoa,b, Matthew S. Johnsonc,1, Sebastian O. Danielacheb,c,d, Carsten Eskebjergc, Antra Pandeyb,d, and Naohiro Yoshidab,d-Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox, volume 106, No.35, Page 4)
Results
This signature is found in Sulphur in the Archean before about 2.5 Ga ago. Farquhar’s discovery shows that there was a process taking place in the Archean which carried materials from earths surface to the mantle to the depth of diamond formation which is over 100 kilometers. One such process is subduction. (Rollinson, H., 2007. When did plate tectonics begin? Geology Today, 23(5), pp.190.)
Discussion
This theory is from stern who used evidence of former subduction as signs of former plate tectonic criteria is required to determine when plate tectonics had first begun. (Rollinson, H., 2007. When did plate tectonics begin? Geology Today, 23(5), pp.191.)
Blueschists analysis: electron microscope approach
Method
The mineral compositions were determined by electron microprobe on an ARL-EMX wavelength dispersive system along with a TN-2000 Tracor Northern energy dispersive system. Isotopic analyses were carried out on a Varian-MAT GD150 mass spectrometer and the measurements were corrected for mass discrimination and sensitivity by using a calibrated argon pipette. (V.B Sisson and T.C. Onstott, Dating blueschist Metamorphism: A combined Argon-40/Argon-39 and electron microprobe approach Pg. 1)Results
The crossite from the Iceberg lake schist yields a date of 176.6 Ma. For slab geometry, an initial age of 108 Ma was calculated. (V.B Sisson and T.C. Onstott, Dating blueschist Metamorphism: A combined Argon-40/Argon-39 and electron microprobe approach Pg. 3)Discussion
The potassium oxide content of the crossite was estimated from the amount of isotope Argon 39 was released, therefore this may have affected the results slightly. (V.B Sisson and T.C. Onstott, Dating blueschist Metamorphism: A combined Argon-40/Argon-39 and electron microprobe approach Pg. 3) As blueschists were formed in low temperature and high-pressure conditions, it is indicative of a time on earth where cool crust was buried to great depth i.e. Indicates when plate tectonics may have begun.

Conclusion
Evidence from the Isua Accretionary complex suggest that plate tectonics had taken place 0.7-0.8 G. yr. after the formation of Earth, signature Sulphur isotopes were more prominent 2.5 Ga ago, Blueschists were recorded to yield at an age of 176.6 Ma. These results are indicative that the beginning of plate tectonics was in fact in the Archean era, however, the ages of the different rocks and complexes differ, making it very difficult to determine a single age at which the tectonics started. Furthermore, it should be reiterated that these results were obtained from the oldest specimen of rock found, not the oldest rock available and uplift and subsidence make it harder to obtain older rock. Moreover, in 2006 the Geologic society of America Penrose had a vote to decide when the scientists thought plate tectonics had begun. It was decided by the second vote that the median answer was that it took place 3.0 to 3.5 Ga ago, however the key point here is that voting is not an accurate representation of the answer to the beginning of tectonics. This is clearly inaccurate and may have some bias which drastically changes the reliability of the answer to the topic at hand. This is indicative of the fact that currently geologists are depending on estimates of when tectonics had begun, rather than factual information, and raises the question: will we ever be sure of the exact age at which tectonics had started?
References:
(Komiya, T., Maruyama, S., Masuda, T., Nohda, S., Hayashi, M. and Okamoto, K., 1999. Plate tectonics at 3.8–3.7 Ga: field evidence from the Isua accretionary complex, southern West Greenland. The Journal of Geology, 107(5), pp.515-554.)
(Rollinson, H., 2007. When did plate tectonics begin? Geology Today, 23(5), pp.190.)
(Rollinson, H., 2007. When did plate tectonics begin? Geology Today, 23(5), pp.191.)
(Stern, R.J., 2007. When and how did plate tectonics begin? Theoretical and empirical considerations. Chinese Science Bulletin, 52(5), pp.578-591.)
(V.B Sisson and T.C. Onstott, Dating blueschist Metamorphism: A combined Argon-40/Argon-39 and electron microprobe approach Pg. 1)
(V.B Sisson and T.C. Onstott, Dating blueschist Metamorphism: A combined Argon-40/Argon-39 and electron microprobe approach Pg. 3)
(Yuichiro Uenoa, b, Matthew S. Johnsonc,1, Sebastian O. Danielacheb, c,d, Carsten Eskebjergc, Antra Pandeyb,d, and Naohiro Yoshidab,d-Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox, volume 106, No.35, Page 4)