研究對於地球陸塊的出現原因提供了新的線索

 原文網址：https://www.si.edu/newsdesk/releases/study-presents-new-clues-about-rise-earths-continents

造成陸地出現的性質源自何方？新實驗顯示一種廣為接受的解釋其實不太可能

地球在太陽系的行星中是唯一適合居住的，原因之一是地球擁有陸塊。但驚人的是，對於地球表層這些巨大的裂片以及它們的特殊性質是如何產生這方面，我們卻所知不多。相較於海洋地殼，大陸地殼的鐵含量較低而氧化程度較高的問題，由Elizabeth Cottrell(史密森尼美國國立自然史博物館的岩石研究員暨地質學家)及主要作者Megan Holycross(之前為該博物館彼得•巴克獎學金與美國國家自然基金會獎學金補助的研究員，現為康乃爾大學的助理教授)進行的新研究測試了一項廣為接受的假說之後將其否決，也讓我們對於地殼有更深的理解。大陸地殼的成分中鐵含量稀少是主要的原因，造成地球表面許多地方可以高出海平面而形成陸地，使得今日所見的陸生生物得以出現。

本研究中一次實驗中的顯微鏡照片，裡面包含了玻璃質(橘色)、石榴子石(粉紅色)以及其他微小的礦物晶體。視野大小為410微米寬，跟一粒糖晶差不多。圖片來源：G. Macpherson and E. Cottrell, Smithsonian

今日發表在《科學》(Science)的這項研究，利用實驗室試驗得出地球大陸地殼的典型成分——缺乏鐵而氧化程度較高——可能並非由石榴子石礦物的結晶作用所造成，也就是2018年提出的一項廣為接受的解釋。

在海洋板塊俯衝至大陸板塊底下的隱沒帶可以看到陸弧火山，其下方深處便是新的大陸地殼元件產生的地方。在用石榴子石來解釋大陸地殼缺鐵且為氧化態的假說中，陸弧火山下方的岩漿結晶出石榴子石的過程，會把大陸板塊裡的非氧化鐵(科學家稱為還原鐵或亞鐵)移除，同時造成熔岩裡的鐵變少、氧化程度變高。

大陸地殼的鐵含量比海洋地殼還低的一項重要結果便是陸地的密度較低而浮力較強，使得大陸板塊在地函當中可以立得比海洋板塊還高。密度與浮力的差異是主要的原因，造成大陸地殼的特徵為形成陸地，海洋地殼則被水淹沒；也造成兩者在隱沒帶相遇時，大陸板塊總是會爬到海洋板塊上方。

利用石榴子石來解釋陸弧岩漿缺鐵並氧化的假說很有說服力，但Cottrell說其中一點卻令她無法接受。

「要讓石榴子石穩定需要很高的壓力，而鐵含量低的岩漿卻出現在地殼沒那麼厚的地方，因此壓力也不會到非常高，」她說。

2018年，Cottrell和同事開始尋找方法來驗證陸弧火山下方的石榴子石結晶作用，確實對產生我們理解中的大陸地殼來說是必須的。為了達成這項目標，Cottrell和Holycross必須找到方法在實驗室重現地殼內部的高溫高壓，並研發足夠靈敏的技術來測量裡面的鐵有多少，還能分辨這些鐵是否受到氧化。

為了重現陸弧火山下方的高溫高壓，團隊利用了自然史博物館高壓實驗室以及康乃爾大學的活塞鋼圈高壓儀(piston-cylinder press)。這種液壓驅動的儀器跟一台小冰箱差不多大，多數是由厚度跟強度都很高的鋼和碳化鎢製成。透過大型油壓缸產生的力量可以對約莫只有一立方公釐的微小岩石樣品施加非常高的壓力。將活塞鋼圈高壓儀和加溫裝置組合起來進行實驗，就能達到在火山下方出現的極高溫度壓力。

Cottrell和 Holycross設計了不同的溫度壓力條件來模擬地殼深部岩漿庫的環境。他們進行了13次實驗，在活塞鋼圈高壓儀內的熔岩培養石榴子石樣品。實驗用的壓力範圍從1.5到3 GPa (1GPa=10億帕斯卡)，大約為地球大氣壓力的15,000到30,000倍，或者是汽水罐內部壓力的8000多倍。溫度則從950到1,230℃，足以把岩石熔化。

接著團隊從史密森尼國家岩石館藏以及世界各地的其他研究人員手中蒐羅石榴子石。重要的是，這些石榴子石已經被分析過，因此它們的氧化鐵和非氧化鐵濃度都是已知的。

最後，研究作者將實驗生成的樣品以及蒐集來的礦物，帶到美國能源部在伊利諾州的阿貢國家實驗室中的先進光子源。團隊在此利用高能X光射線來進行X光吸收光譜法，這種分析技術可以讓科學家根據物質如何吸收X光來得出其成分與結構。研究人員此次探討的對象便是氧化鐵與非氧化鐵的濃度。

氧化鐵與非氧化鐵之間的比例已經測出來的樣品，可以讓團隊檢查及校正X光吸收光譜的測量結果，也能幫助比較不同次實驗產生出來的石榴子石。

試驗的結果顯示石榴子石從岩石樣品中吸收的非氧化鐵並不夠多，無法用來解釋做為大陸地殼原料的岩漿呈現出來的缺鐵與氧化程度。

「這些結果讓石榴子石結晶模型在解釋為什麼陸弧火山的岩漿較為氧化而缺鐵時，變成可能性相當低的假說，」Cottrell表示。「更有可能的是大陸地殼下方的地函條件，決定了這種氧化環境。」

就像為數眾多的科研結果一樣，這項發現引出了更多問題：「造成氧化或缺鐵的原因到底是什麼？」Cottrell問道。「如果不是地殼中的石榴子石結晶作用，而是和岩漿從地函升上來的過程有關，那麼在地函中發生了什麼事？岩漿的成分是怎麼被改變的？」

Cottrell表示這些問題並不好回答，但目前的主流理論認為氧化硫或許可以把鐵氧化。目前一位研究員正在她的指導之下，於自然史博物館進行相關研究。

本研究是絕佳的範例，顯示自然史博物館的科學家在該館的新計畫「獨一無二的地球」之下進行的研究類型。由公私部門合夥的「獨一無二的地球」資助的研究類型，目標為探討讓地球獨一無二的問題中最重要且懸而未解的。其他研究將探討地球液態海洋的來源，以及礦物可能透過什麼方式成為生命的模板。

研究經費來自史密森尼學會、美國國家科學基金會、美國能源局以及萊達•希爾基金會。

 

Study presents new clues about the rise of Earth’s continents

One popular explanation for properties that result in dry land is unlikely according to new experiments

Continents are part of what makes Earth uniquely habitable for life among the planets of the solar system, yet surprisingly little is understood about what gave rise to these huge pieces of the planet’s crust and their special properties. New research from Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian’s National Museum of Natural History, and lead study author Megan Holycross, formerly a Peter Buck Fellow and National Science Foundation Fellow at the museum and now an assistant professor at Cornell University, deepens the understanding of Earth’s crust by testing and ultimately eliminating one popular hypothesis about why continental crust is lower in iron and more oxidized compared to oceanic crust. The iron-poor composition of continental crust is a major reason why vast portions of the Earth’s surface stand above sea level as dry land, making terrestrial life possible today.

The study, published today in Science, uses laboratory experiments to show that the iron-depleted, oxidized chemistry typical of Earth’s continental crust likely did not come from crystallization of the mineral garnet, as a popular explanation proposed in 2018.

The building blocks of new continental crust issue forth from the depths of the Earth at what are known as continental arc volcanoes, which are found at subduction zones where an oceanic plate dives beneath a continental plate. In the garnet explanation for continental crust’s iron-depleted and oxidized state, the crystallization of garnet in the magmas beneath these continental arc volcanoes removes non-oxidized (reduced or ferrous, as it is known among scientists) iron from the terrestrial plates, simultaneously depleting the molten magma of iron and leaving it more oxidized.

One of the key consequences of Earth’s continental crust’s low iron content relative to oceanic crust is that it makes the continents less dense and more buoyant, causing the continental plates to sit higher atop the planet’s mantle than oceanic plates. This discrepancy in density and buoyancy is a major reason that the continents feature dry land while oceanic crusts are underwater, as well as why continental plates always come out on top when they meet oceanic plates at subduction zones.

The garnet explanation for the iron depletion and oxidation in continental arc magmas was compelling, but Cottrell said one aspect of it did not sit right with her.

“You need high pressures to make garnet stable, and you find this low-iron magma at places where crust isn’t that thick and so the pressure isn’t super high,” she said.

In 2018, Cottrell and her colleagues set about finding a way to test whether the crystallization of garnet deep beneath these arc volcanoes is indeed essential to the process of creating continental crust as is understood. To accomplish this, Cottrell and Holycross had to find ways to replicate the intense heat and pressure of the Earth’s crust in the lab, and then develop techniques sensitive enough to measure not just how much iron was present, but to differentiate whether that iron was oxidized.

To recreate the massive pressure and heat found beneath continental arc volcanoes, the team used what are called piston-cylinder presses in the museum’s High-Pressure Laboratory and at Cornell. A hydraulic piston-cylinder press is about the size of a mini fridge and is mostly made of incredibly thick and strong steel and tungsten carbide. Force applied by a large hydraulic ram results in very high pressures on tiny rock samples, about a cubic millimeter in size. The assembly consists of electrical and thermal insulators surrounding the rock sample, as well as a cylindrical furnace. The combination of the piston-cylinder press and heating assembly allows for experiments that can attain the very high pressures and temperatures found under volcanoes.

In 13 different experiments, Cottrell and Holycross grew samples of garnet from molten rock inside the piston-cylinder press under pressures and temperatures designed to simulate conditions inside magma chambers deep in Earth’s crust. The pressures used in the experiments ranged from 1.5 to 3 gigapascals—that is roughly 15,000 to 30,000 Earth atmospheres of pressure or 8,000 times more pressure than inside a can of soda. Temperatures ranged from 950 to 1,230 degrees Celsius, which is hot enough to melt rock.

Next, the team collected garnets from Smithsonian’s National Rock Collection and from other researchers around the world. Crucially, this group of garnets had already been analyzed so their concentrations of oxidized and unoxidized iron were known.

Finally, the study authors took the materials from their experiments and those gathered from collections to the Advanced Photon Source at the U.S. Department of Energy’s Argonne National Laboratory in Illinois. There the team used high-energy X-ray beams to conduct X-ray absorption spectroscopy, a technique that can tell scientists about the structure and composition of materials based on how they absorb X-rays. In this case, the researchers were looking into the concentrations of oxidized and unoxidized iron.

The samples with known ratios of oxidized and unoxidized iron provided a way to check and calibrate the team’s X-ray absorption spectroscopy measurements and facilitated a comparison with the materials from their experiments.

The results of these tests revealed that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron-depletion and oxidation present in the magmas that are the building blocks of Earth’s continental crust.

“These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron depleted,” Cottrell said. “It’s more likely that conditions in Earth’s mantle below continental crust are setting these oxidized conditions.”

Like so many results in science, the findings lead to more questions: “What is doing the oxidizing or iron depleting?” Cottrell asked. “If it’s not garnet crystallization in the crust and it’s something about how the magmas arrive from the mantle, then what is happening in the mantle? How did their compositions get modified?”

Cottrell said that these questions are hard to answer but that now the leading theory is that oxidized sulfur could be oxidizing the iron, something a current Peter Buck Fellow is investigating under her mentorship at the museum.

This study is an example of the kind of research that museum scientists will tackle under the museum’s new Our Unique Planet initiative, a public–private partnership, which supports research into some of the most enduring and significant questions about what makes Earth special. Other research will investigate the source of Earth’s liquid oceans and how minerals may have served as templates for life.

This research was supported by funding from the Smithsonian, the National Science Foundation, the Department of Energy and the Lyda Hill Foundation.

原始論文：Megan Holycross, Elizabeth Cottrell. Garnet crystallization does not drive oxidation at arcs. Science, 2023; 380 (6644): 506 DOI: 10.1126/science.ade3418

引用自：Smithsonian. "New clues about the rise of Earth's continents: One popular explanation for properties that result in dry land is unlikely according to new experiments."