地球化學偵探利用實驗室模擬來回推過去

原文網址：www.sciencedaily.com/releases/2016/04/160428151836.htm

Geochemical detectives use lab mimicry to look back in time

地球化學偵探利用實驗室模擬來回推過去

New work from a research team led by Carnegie's Anat Shahar contains some unexpected findings about iron chemistry under high-pressure conditions, such as those likely found in the Earth's core, where iron predominates and creates our planet's life-shielding magnetic field. Their results, published in Science, could shed light on Earth's early days when the core was formed through a process called differentiation--when the denser materials, like iron, sunk inward toward the center, creating the layered composition the planet has today.

由卡內基實驗室的Anat Shahar領導的研究團隊最新發表的工作成果中，包含了某些意料之外的發現：有關於鐵的化學性質在高壓環境之下的行為。地核可能即屬這種環境，此處的成分以鐵為主，並由此產生了保護地球生命的磁場。他們刊登在期刊《科學》(Science)的成果，或許可以闡明當地球年幼時，地核經由「分異作用」(differentiation)而形成時的細節。當這種作用發生時，像鐵這類密度較高的物質會往下沉至地球中心，使得今日的地球成分呈現層狀分布。

Earth formed from accreted matter surrounding the young Sun. Over time, the iron in this early planetary material moved inward, separating from the surrounding silicate. This process created the planet's iron core and silicate upper mantle. But much about this how this differentiation process occurred is still poorly understood, due to the technological impossibility of taking samples from the Earth's core to see which compounds exist there.

地球是由環繞在幼年太陽周遭的物質聚積而成。隨著時間流逝，這些初生行星物質中的鐵會逐漸往內部移動，而跟周圍的矽酸鹽分離開來。這種作用使得地球擁有鐵質核心以及位於上方的矽酸鹽質地函。但有關於這種分異作用的更多細節究竟是如何進行，卻還是所知甚少。這是因為技術上的限制使我們不可能從地核取得樣品，觀察存在於此的到底是什麼物質。

Seismic data show that in addition to iron, there are "lighter" elements present in the core, but which elements and in what concentrations they exist has been a matter of great debate. This is because as the iron moved inward toward the core, it interacted with various lighter elements to form different alloyed compounds, which were then carried along with the iron into the planet's depths.

地震波資料顯示除了鐵之外，地核內部還有其他較「輕」的元素，但是這些元素的真實身分以及濃度多寡仍然眾說紛紜。這是因為當鐵往下沉至地核的旅途中，它會跟周遭各式各樣的較輕元素交互作用，而產生許多種不同的合金，接著它們會夥同鐵一起進入地球深處。

Which elements iron bonded with during this time would have been determined by the surrounding conditions, including pressure and temperature. As a result, working backward and determining which iron alloy compounds were created during differentiation could tell scientists about the conditions on early Earth and about the planet's geochemical evolution.

在這段時期鐵會跟什麼樣的元素結合跟它們所處的環境條件有關，像是溫度和壓力。因此，反過來研究並找出在分異作用進行時形成的合金種類，可以告訴科學家早期地球的環境條件，以及地球的化學性質如何演變。

The team--including Carnegie's Jinfu Shu and Yuming Xiao--decided to investigate this subject by researching how pressures mimicking the Earth's core would affect the composition of iron isotopes in various alloys of iron and light elements. Isotopes are versions of an element where the number of neutrons differs from the number of protons. (Each element contains a unique number of protons.)

包括Jinfu Shu和Yuming Xiao這兩位卡內基科學家的研究團隊決定研究這道難題。他們用的方法是模擬鐵和輕元素的不同合金中，鐵的同位素成分會如何受地核內部壓力高低影響。同位素是隸屬同一種元素但中子數不同的版本，這點與質子數並不相同(每一種元素都有各自獨一無二的質子數)。

Because of this accounting difference, isotopes' masses are not the same, which can sometimes cause small variations in how different isotopes of the same element are partitioned in, or are "picked up" by, either silicate or iron metal. Some isotopes are preferred by certain reactions, which results in an imbalance in the proportion of each isotope incorporated into the end products of these reactions--a process that can leave behind trace isotopic signatures in rocks. This phenomenon is called isotope fractionation and is crucial to the team's research.

由於在中子數目上的差異，同位素的質量也各不相同，這種微小的差異有時會導致矽酸鹽或金屬鐵在分配(partition)，也就是「揀選」同一元素的同位素時會有些許的不同。特定反應會偏好某些種類的同位素，這會導致每一種同位素的比例在各個反應終產物中的分配並不均衡。這種過程可以在岩石當中留下可供追循的同位素訊號。對此團隊的研究來說，這種稱作同位素分餾(isotope fractionation)的現象相當重要。

Before now, pressure was not considered a critical variable affecting isotope fractionation. But Shahar and her team's research demonstrated that for iron, extreme pressure conditions do affect isotope fractionation.

在此之前，科學家並不認為壓力是影響同位素分餾的重要因素。但Shahar 和她的團隊在研究中顯示，對鐵而言，極端的壓力環境確實會影響同位素分餾。

More importantly, the team discovered that due to this high-pressure fractionation, reactions between iron and two of the light elements often considered likely to be present in the core--hydrogen and carbon--would have left behind an isotopic signature in the mantle silicate as they reacted with iron and sunk to the core. But this isotopic signature has not been found in samples of mantle rock, so scientists can exclude them from the list of potential light elements in the core.

更重要的是，研究團隊發現由於高壓分餾作用的進行，兩種常被視為可能出現在地核中的輕元素—氫和碳，在跟鐵產生反應並往下沉至地核的過程中，會在地函矽酸鹽內留下某種同位素訊號。然而目前為止還尚未在地函岩石的樣品當中找到這種同位素訊號，因此科學家可以將氫和碳從可能出現在地函的輕元素名單上剔除。

Oxygen, on the other hand, would not have left an isotopic signature behind in the mantle, so it is still on the table. Likewise, other potential core light elements still need to be investigated, including silicon and sulfur.

另一方面，氧並不會在地函留下同位素訊號，因此它仍然列於名單上。類似地，其他可能出現在地函，像是矽和硫之類的輕元素還有待調查。

"What does this mean? It means we are gaining a better understanding of our planet's chemical and physical history," Shahar explained. "Although Earth is our home, there is still so much about its interior that we don't understand. But evidence that extreme pressures affect how isotopes partition, in ways that we can see traces of in rock samples, is a huge step forward in learning about our planet's geochemical evolution."

「這意味著什麼？這代表說我們對我們星球的化學和物理發展史有了更深一層的認識。」Shahar如此解釋。「雖然地球是我們的家園，但我們對它的內部仍有諸多不瞭解之處。然而，現在我們有了證據顯示極度的高壓會影響同位素分餾作用，而我們可以從岩石樣本中看到這種作用留下來的痕跡。對了解我們星球的地球化學是如何演變來說，這是相當重大的進展。」

引用自：Carnegie Institution for Science. "Geochemical detectives use lab mimicry to look back in time." ScienceDaily. ScienceDaily, 28 April 2016.