重返地表：來自地函的氙有故事要說

原文網址：https://source.wustl.edu/2018/08/there-and-back-again-mantle-xenon-has-a-story-to-tell/

重返地表：來自地函的氙有故事要說

研究界定出揮發性物質從大氣進入地球深處的歷史進程

By Talia Ogliore 

地球在其45億年的歷史中曾出現多次變化，包括開始會將大氣中的揮發性物質帶到地函當中並保留下來，之後藉由火山噴發再次將它們吐回大氣。

根據聖路易斯華盛頓大學在8月9日發表於當期《自然》(Nature)期刊的新研究，這種運輸過程開始的時間不可能早於距今25億年前太多。

「地球上的生命對地表揮發性物質的總量變化相當敏感。」研究第一作者，該大學文理學院地球與行星科學系的地球化學助理教授Rita Parai表示，「而在數十億年的尺度下，地球深部發生的作用和地表的環境變化會互相影響。」

揮發性物質，像是水、二氧化碳和惰性氣體，會經由火山作用從地表噴出，而且也會從大氣注入回地球內部，這兩個作用稱為地函脫氣(mantle degassing)和地函注氣(mantle regassing)。由於這些交換過程決定了對生命來說相當重要的物質，比方說碳、氮和水在地表的多寡，因此也控制了一個星球的適居程度。

由Parai與加州大學戴維斯分校的Sujoy Mukhopadhyay共同發表的新模型建立出一個時間範圍，此時地球的氣體輸送過程從淨值以脫氣為主的狀態(也就是那些不斷冒出氣體的火山)，逐漸傾向淨值為注氣的平衡狀態，這可能是由像是輸送帶一般進行的板塊運動造成的隱沒作用導致。

Parai表示當水進入或離開地函時會讓地函的力學性質改變，因此注氣作用開始運作時會影響地函內部的翻攪運動，也就是地函對流，而地函對流又控制了地表的板塊如何運動。

Parai利用惰性氣體來解決有關行星如何形成並隨時間演化的問題。在這項新研究當中，她以氙同位素為示蹤劑，來模擬揮發性物質在地函內部的運送過程及其宿命。

「對於揮發性物質來說氙是絕佳的示蹤劑，因為所有帶有水的礦物也會攜帶氙。」Parai表示，「因此，如果太古宙時(距今40億至25億年前)氙的注氣作用幾乎沒在進行，那水的脫氣作用在當時必定也是如此。」

研究人員發現大量的注氣作用大概是在距今數百萬年前至25億年前的某個時刻開始進行。

Parai提出如果板塊運動和隱沒作用早於距今25億年前就已經開始進行，那麼25億年前地球內部可能已經冷卻到一定程度，使得揮發性物質可以留在隱沒板塊當中，而不會經由岩漿作用而被釋放或者滲回地表。

「大多數人沒什麼理由去關注待在地球內部的揮發性物質。」Parai表示，「雖然它們在地函中的濃度相當低，但地函的質量相當巨大，因此對於地球揮發性物質的總量來說，地函仍是相當重要的儲集地點。」

在她的計畫中，未來研究重點是把氙同位素在各種地質樣品中的測量精準度更往上推。

她說：「我們可以觀測到的限制條件是越多越好。」

There and back again: Mantle xenon has a story to tell

Study constrains the history of volatile transport from the atmosphere into the deep Earth

The Earth has been through a lot of changes in its 4.5 billion year history, including a shift to start incorporating and retaining volatile compounds from the atmosphere in the mantle before spewing them out again through volcanic eruptions.

This transport could not have begun much before 2.5 billion years ago, according to new research by Washington University in St. Louis, published in the Aug. 9 issue of the journal Nature.

“Life on Earth cares about changes in the volatile budget of the surface,” said Rita Parai, assistant professor of geochemistry in Earth and Planetary Sciences in Arts & Sciences and first author of the study. “And there’s an interplay between what the deep Earth was doing and how the surface environment changed over billion-year timescales.”

Volatiles — such as water, carbon dioxide and the noble gases — come out of the mantle through volcanism and may be injected into the Earth’s interior from the atmosphere, a pair of processes called mantle degassing and regassing. The exchange controls the habitability of the planet, as it determines the surface availability of compounds that are critical to life — such as carbon, nitrogen and water.

The new model presented by Parai and collaborator Sujoy Mukhopadhyay, of the University of California, Davis, also establishes a range of dates during which the Earth shifted from a net degassing regime — again, think about those oozy volcanoes — to one that tilted the balance to net regassing potentially enabled by subduction, the conveyor-belt action of tectonic plates.

Mechanical properties change as water is added or removed from the mantle, so the onset of regassing had an important effect on the internal churning of the mantle, known as convection, which controls plate motions at the surface, Parai said.

Parai uses noble gases to address questions about how planetary bodies form and evolve over time. In this new research, she modeled the fate and transport of volatile compounds into the Earth’s mantle using xenon isotopes as tracers.

“Xenon is an excellent volatile tracer, because all minerals that carry water also carry xenon,” Parai said. “So if xenon regassing was negligible, water regassing must also have been negligible during the Archean (4 billion-2.5 billion years ago).”

Substantial regassing began sometime between a few hundred million to 2.5 billion years ago, the researchers found.

If plate tectonics and subduction began earlier than 2.5 billion years ago, then perhaps by then the Earth’s interior had cooled sufficiently for volatiles to remain in subducting plates, rather than getting released and percolating back to the surface through magmatism, Parai suggests.

“Most people rarely have an occasion to think about volatiles trapped in the Earth’s interior,” Parai said. “They’re present at low concentrations, but the mantle is huge in terms of mass. So for the Earth’s total volatile budget, the mantle is an important reservoir.”

She plans to focus her future research on pushing the limits of precision in xenon isotopic measurements in a variety of geological samples.

“The more observational constraints we have, the better,” she said.

原始論文：Rita Parai, Sujoy Mukhopadhyay. Xenon isotopic constraints on the history of volatile recycling into the mantle. Nature, 2018; 560 (7717): 223 DOI: 10.1038/s41586-018-0388-4

引用自：Washington University in St. Louis. "There and back again: Mantle xenon has a story to tell."