地球最老的岩石可能是由隕石轟炸而形成

原文網址：https://goldschmidt.info/2018/pressReleasesView

地球最老的岩石可能是由隕石轟炸而形成

在加拿大的阿卡斯塔河某些富含矽的長英質岩石年代有40.2億年，是地球已知最古老的岩層。科學家發現它們可能是在地球的初生地殼內部於高溫下形成，且形成時的深度出乎意料地淺。熔化淺層地殼需要的高溫可能是由地球形成之後五億年左右的隕石轟炸提供，使得富含鐵的地殼熔化而形成我們今天看到的花崗岩。研究結果刊登於同儕審查期刊《自然―地質科學》(Nature Geoscience)，之後於8月14日波士頓舉行的歌德施密特大會中首次發表。

阿卡斯塔片麻岩。圖片來源：Geology In

雖然世上其他地方還有年代更老的礦物晶體*，不過就岩石來說，在加拿大阿卡斯塔河發現的長英質岩石(富含二氧化矽/石英的岩石)是世上最老的。今日我們看到的長英質岩石像是廣泛用在建築或裝飾材料上的花崗岩，而科學家長久以來知道阿卡斯塔河的岩石跟這類長英質岩石的絕大多數都不同。最近，一組澳洲和中國的科學家團隊模擬了阿卡斯塔的長英質岩石中年代最老者的形成過程，結果發現這些岩石只能在非常高的溫度下於低壓環境形成。

科學家相信原始地殼的成分大部分是缺乏二氧化矽的暗色鐵鎂質岩石，因此阿卡斯塔河的長英質岩石究竟如何形成一直以來都是個謎題。

團隊主持人，澳洲伯斯科廷大學的Tim Johnson表示：「原始地球的地殼最上層是由富含鐵的玄武岩質岩石構成，我們的模擬顯示阿卡斯塔河的岩石是源自於這些早已存在的岩石形成的熔融物質。」

「我們運用相平衡和稀有元素的模擬，顯示阿卡斯塔河的岩石產自原本為鐵鎂質的岩石在低壓下發生的部分熔融。要在如此低的壓力下形成這些早期的長英質岩石需要900°C的高溫，這需要某些特殊作用才能產生，意謂著當時或許發生了一起劇烈事件，最有可能的就是隕石轟炸造成的高熱。

我們預估鐵鎂質地殼最上層3公里發生熔化才能形成今日看到的阿卡斯塔河岩石。我們認為這些古老的長英質岩石曾經相當常見，但在經過40億年的時光以及板塊運動的運作之後，幾乎沒有任何東西可以保存下來。

地球歷史的最初六億年是段飽受地外天體轟炸的時光，我們相信這些岩石可能是留存至今的唯一見證。」

阿卡斯塔河是加拿大北方斯拉維穩定地塊(Slave Craton)的一部份岩層，此地塊位在黃刀鎮和大斯拉維湖之間。最初居住在此地的族群為提拉丘人，發現這些岩石的地質學家將其命名為「Idiwhaa」，即為提拉丘語中的「古代」。

愛爾蘭都柏林大學三一學院的Balz Kamber對此研究評論道：「考慮到大型或巨型撞擊事件本身帶有的極大能量，以及其他內太陽系行星和月球年代久遠的表面滿覆坑洞，長英質岩漿是由這類撞擊事件產生的想法確實是可行的。然而，在撞擊的極高溫度而產生的岩漿海下方，此處地殼淺層也能位於他們所指稱的溫度壓力範圍而發生熔融。也就是說，撞擊事件的間接結果也能形成這類岩石。」

*：澳洲傑克丘的岩石含有的鋯石晶體年代可達距今44億年前，但其所在的岩石年代較晚。

Meteorite bombardment likely to have created the Earth's oldest rocks

 Scientists have found that 4.02 billion year old silica-rich felsic rocks from the Acasta River, Canada - the oldest rock formation known on Earth - probably formed at high temperatures and at a surprisingly shallow depth of the planet's nascent crust. The high temperatures needed to melt the shallow crust were likely caused by a meteorite bombardment around half a billion years after the planet formed. This melted the iron-rich crust and formed the granites we see today. These results are presented for the first time at the Goldschmidt conference in Boston tomorrow (14 August), following publication in the peer-reviewed journal Nature Geoscience.

The felsic rocks (rocks rich in silica/quartz) found at the Acasta River in Canada, are the Earth's oldest rocks, although there are older mineral crystals*. Scientists have long known that the Acasta rocks are different to the majority of felsic rocks we see today, such as the granites widely used as a building or decorative material. Now a group of scientists from Australia and China have modelled the formation of the oldest Acasta felsic rocks and found that they could only have been formed at low pressures and very high temperatures.

Scientists believe that the primitive crust largely comprised dark, silica-poor mafic rocks, so there has been a question over how the Acasta River felsic rocks could have formed.

"Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth", said team leader Tim Johnson, from Curtin University, Perth.

"We used phase equilibria and trace element modelling to show that the Acasta River rocks were produced by partial melting of the original mafic rocks at very low pressures. It would have needed something special to produce the 900°C temperatures needed to generate these early felsic rocks at such low pressures, and that probably means a drastic event, most likely the intense heating caused by meteorite bombardment.

We estimate that rocks within the uppermost 3km of mafic crust would have been melted in producing the rocks we see today. We think that these ancient felsic rocks would have been very common, but the passage of 4 billion years, and the development of plate tectonics, means that almost nothing remains.

We believe that these rocks may be the only surviving remnants of a barrage of extraterrestial impacts which characterized the first 600 million years of Earth History".

The Acasta River is part of the Slave Craton formation in Northern Canada, north of Yellowknife and the Great Slave Lake. The area is the homeland of the Tlicho people, which led to the geologists who discovered the rocks giving them the name "Idiwhaa", derived from the Tlicho word for ancient.

Commenting, Dr Balz Kamber (Trinity College Dublin) said that "The idea of making felsic melts by large or giant impacts seems plausible considering the high-energy nature of these events and the pockmarked ancient surfaces of other inner Solar System planets and moons. However, the implied pressure-temperature regime might also permit melting of shallow crust below a super-heated impact melt sea. In other words, an indirect consequence of the impact itself".

* Rocks from Jack Hills in Australia contain zircon crystals from up to 4.4 billion years ago, embedded in younger rocks.

原始論文：Tim E. Johnson, Nicholas J. Gardiner, Katarina Miljković, Christopher J. Spencer, Christopher L. Kirkland, Phil A. Bland, Hugh Smithies. An impact melt origin for Earth’s oldest known evolved rocks. Nature Geoscience, 2018; DOI: 10.1038/s41561-018-0206-5

引用自：Goldschmidt Conference 2018. “Meteorite bombardment likely to have created the Earth's oldest rocks.”