Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

B (Biogeosciences) » B-CG Complex & General

[B-CG09] Decoding the history of Earth: From Hadean to the present

Tue. May 22, 2018 1:45 PM - 3:15 PM 101 (1F International Conference Hall, Makuhari Messe)

convener:Tsuyoshi Komiya(Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo), Yasuhiro Kato(Department of Systems Innovation, Graduate School of Engineering, University of Tokyo), Katsuhiko Suzuki(国立研究開発法人海洋研究開発機構・海底資源研究開発センター), Chairperson:Fujisaki Wataru(Japan Agency for Marine-Earth Science and Technology)

2:15 PM - 2:30 PM

[BCG09-09] Origin of Archean quartzose sandstones

*Hikaru Sawada1, Akira Ishikawa1, Yukio Isozaki1 (1.Department of General Systems Studies, the University of Tokyo)

Keywords:Archean, continental growth, sandstone, trace elements

Granitic continental crust is produced in plate subduction zone. Continental growth through the history of the Earth is a significant matter to understand evolving planetary interior, especially secular change of plate tectonics. Most of the previous geochemical studies have treated continental crust as a single reservoir separated from the mantle and oceanic crust, and not focused on a form of each continent. In particular, the form of continents in the early half of the Earth’s history have not determined yet.

The chemical composition of (meta-)sandstone is useful to identify sorting of detrital grains and difference in form of continents. Sandstones composed of not well-sorted grains have a composition similar to rocks in its provenance, whereas those composed of well-sorted grains have more quartzose composition. Quartzose sandstones have been found widely from even in the Archean greenstone belts after 3.5 Ga. These Archean quartzose sandstones could be interpreted as below; i.e. 1) large exposure of granitic continental crust in the Archean, or 2) extensive occurrence of silicified mafic/ultramafic rocks and/or hydrothermal chert.

The present study measured bulk trace and rare earth element concentration of Archean and early Proterozoic (3.5-2 Ga) sandstone sampled from S. Africa, N. America, and W. Australia. All measured samples show Zr content as high as granitoid, which indicates that contribution of silicified rocks and chert had been minor through 3.5-2.0 Ga. Data normalized by primitive mantle values shows positive Zr anomaly (=Zr/(Nd*Sm)^0.5) in approximately 70 % of the analyzed samples. The Zr anomaly indicates zircon-enriched sandstone by sedimentary sorting. The Zr anomaly in the middle Archean (3.5-3.0 Ga) and the early Proterozoic (2.5-2.0 Ga) is 10 times larger than those in the late Archean (3.0-2.5 Ga). The size of each continent was smaller in the late Archean (3.0-2.5 Ga) than middle Archean (3.5-3.0 Ga), and re-increased after 2.5 Ga. In other words, there were continents between 3.5 and 3.0 Ga as large as those between 2.5- 2.0 Ga Paleoproterozoic.