發現從地球形成時期殘留至今的痕跡

原始網址：www.sciencedaily.com/releases/2016/05/160512145338.htm

Found: Surviving evidence of Earth's formative years

發現從地球形成時期殘留至今的痕跡

New work from a team including Carnegie's Hanika Rizo and Richard Carlson, as well as Richard Walker from the University of Maryland, has found material in rock formations that dates back to shortly after Earth formed. The discovery will help scientists understand the processes that shaped our planet's formative period and its internal dynamics over the last 4.5 billion years. It is published by Science.

包括卡內基的研究人員Hanika Rizo和Richard Carlson，以及馬里蘭大學的Richard Walker在內的團隊進行的新研究，發現在岩層當中有些物質的形成年代可追溯至地球形成後不久。這項刊登於期刊《科學》(Science)的發現有助於科學家了解地球形成時期受哪些作用影響，以及過去45億年來我們星球內部發生的動力作用。

Earth formed from the accretion of matter surrounding the young Sun. The heat of its formation caused extensive melting of the planet, leading Earth to separate into two layers when the denser iron metal sank inward toward the center, creating the core and leaving the silicate-rich mantle floating above.

地球是由環繞在幼年太陽周遭的物質加積而成。在形成時產生的熱量造成整顆行星幾乎處於融化的狀態，此時密度較高的鐵往中心下沉而形成地核，剩餘下來富含矽酸鹽類的地函就浮在上方，造成地球被分成兩層。

Over the subsequent 4.5 billion years of Earth's evolution, convection in Earth's interior, like water boiling on a stove, caused deep portions of the mantle to rise upwards, melt, and then separate once again by density. The melts, since they were less dense than the unmelted rock, rose to form Earth's crust, while the denser residues of the melting sank back downward, altering the mantle's chemical composition in the process.

在接下來45億年的地球演化過程中，地球內部發生的對流就像鍋爐上的滾水一樣，會造成地函深部的物質上湧、熔化，並再次因為密度差而分離。由於熔化部分的密度較未熔化的岩石低，因此它們會往上浮而形成地球地殼，熔化剩下來較為緻密的物質則往下沉回去，在過程中這些殘留物會改變地函的化學成分。

The mantle residues of crust formation were previously believed to have mixed back into the mantle so thoroughly that evidence of the planet's oldest geochemical events, such as core formation, was lost completely.

過往認為地殼形成過程中殘留下來的地函物質會沉回去並跟地函其他部分徹底混合，而完全抹除像是地核形成這類最古老的地球化學事件留下來的痕跡。

However, the research team--which also included Sujoy Mukhopadhyay and Vicky Manthos of University of California Davis, Don Francis of McGill University, and Matthew Jackson, a Carnegie alumnus now at University of California Santa Barbara--was able find a geochemical signature of material left over from the early melting events that accompanied Earth's formation. They found it in relatively young rocks both from Baffin Island, off the coast of northern Canada, and from the Ontong-Java Plateau in the Pacific Ocean, north of the Solomon Islands.

同時包括加利福尼亞大學戴維斯分校的Sujoy Mukhopadhyay和Vicky Manthos、麥基爾大學的Don Francis以及現任職於加利福尼亞大學聖塔芭芭拉分校，曾是卡內基研究員的Matthew Jackson在內的團隊，卻發現了從地球形成時伴隨發生的早期熔化事件中，殘留下來的物質攜帶的地球化學訊號。他們從加拿大北方外海的巴芬島(Baffin Island)，以及太平洋索羅門群島北方的安通-爪哇海底高原(Ontong-Java Plateau)上，年代相對較年輕的岩石中發現了這些訊號。

These rock formations are called flood basalts because they were created by massive eruptions of lava. The solidified lava itself is only between 60 and 120 million years old, depending on its location. But the team discovered that the molten material from inside Earth that long ago erupted to create these plains of basaltic rock owes its chemical composition to events that occurred over 4.5 billion years in the past.

這些岩層是由大規模的岩漿噴發而形成，因此稱作洪流玄武岩(flood basalt)。經由它們的位置研究團隊推測這些岩漿凝固的年代大約只有6000萬至1億2000萬年。然而，他們發現許久以前從地球內部噴發而形成這些玄武岩平原的熔融物質，其化學成分卻起源自發生在45億年前之久的事件。

Here's how they figured it out:

以下為他們的發現過程：

They measured variations in these rocks of the abundance of an isotope of tungsten--the same element used to make filaments of incandescent light bulbs. Isotopes are versions of an element in which the number of neutrons in each atom differs from the number of protons. (Each element contains a unique number of protons.) These differing neutron numbers mean that each isotope has a slightly different mass.

他們測量了這些岩石當中一種鎢同位素含量的變化。鎢即是用來製作白熾燈泡的燈絲的原料。同位素則是相同一種元素然而中子數不同的版本，這跟質子數並不相同(每一種元素的質子數都是獨一無二的)。這種在中子數上的差異意味著每個同位素之間的質量都有些微差距。

Why tungsten? Tungsten contains one isotope of mass 182 that is created when an isotope of the element hafnium undergoes radioactive decay, meaning its elemental composition changes as it gives off radiation. The time it takes for half of any quantity of hafnium-182 to decay into tungsten-182 is 9 million years. This may sound like a very long time, but is quite rapid when it comes to planetary formation timescales. Rocky planets like Earth or Mars took about 100 million years to form.

為什麼是鎢？當元素發生放射性衰變時，它會在發散出輻射的同時改變其組成。鎢擁有一種質量數為182的同位素，它是由一種鉿(hafnium，讀音同「哈」)同位素發生放射性衰變時產生。要讓任何數量的鉿-182其中一半轉變成鎢-182需要的時間為900萬年。這聽起來或許是段很長的時間，但是跟行星形成花費的時間一比就顯得相當短暫了。要形成一顆像是地球或火星這樣的石質行星，通常需要1億年左右。

The team determined that the basalts from Baffin Island, formed by a 60-million-year-old eruption from the mantle hot-spot currently located beneath Iceland, and the Ontong-Java Plateau, which was formed by an enormous volcanic event about 120 million years ago, contain slightly more tungsten-182 than other young volcanic rocks.

巴芬島的玄武岩是於6000萬年前由現今位在冰島下方的地函熱點(hot spot)噴發而產生，安通爪哇海底高原上的玄武岩則是在1億2000萬年前由一個大型火山噴發事件造成。研究團隊檢驗了這些玄武岩，發現跟其他較年輕的火山岩相比，它們的鎢-182含量多了一些。

Because all the hafnium-182 decayed to tungsten-182 during the first 50 million years of Solar System history, these findings indicate that the mantle material that melted to form the flood basalt rocks that the team studied originally had more hafnium than the rest of the mantle. The likely explanation for this is that the portion of Earth's mantle from which the lava came had experienced a different history of iron separation than other portions of the mantle (since tungsten is normally removed to the core along with the iron.)

由於所有的鉿-182在太陽系歷史上最初的5000萬年就已經全數衰變成鎢-182，因此這項發現意味著研究團隊最初研究的這些洪流玄武岩，熔融而形成它們的地函物質跟地函其餘部分相比之下擁有較多的鉿。可能的解釋是形成這些岩漿的地函跟地函其他部分相較而言，鐵的分離歷程有些許不同。（因為正常情況之下鎢會隨著鐵一起沉到地核中）

It was a surprise to the team that such material still exists in Earth's interior.

地球內部仍存有這樣的物質令研究團隊感到十分震驚。

"This demonstrates that some remnants of the early Earth's interior, the composition of which was determined by the planet's formation processes, still exist today," explained lead author Rizo, now at Université du Québec à Montréal.

「這表示早期地球內部的殘渣仍有一些留存至今日，它們的成分取決於地球形成的過程。」現任職於魁北克大學蒙特婁分校的第一作者 Rizo解釋。

"The survival of this material would not be expected given the degree to which plate tectonics has mixed and homogenized the planet's interior over the past 4.5 billion years, so these findings are a wonderful surprise," added Carlson, Director of Carnegie's Department of Terrestrial Magnetism.

「在經過了過去45億年來板塊運動攪動地球內部並使其成分趨向均勻的情況下，我們並沒有預料到這些物質仍能倖存，因此這項發現確實是個十分美好的驚喜。」卡內基研究所地磁系的主任Carlson補充說。

The team's discovery offers new insight into the chemistry and dynamics that shaped our planet's formative processes. Going forward, scientists will have to hunt for other areas showing outsized amounts of tungsten-182 with the hope of illuminating both the earliest portion of Earth's history as well as the place in Earth's interior where this ancient material is stored.

研究團隊的這項發現讓我們對塑造地球的化學和動力過程有了新的見解。這群科學家的下一步是尋找其他同樣擁有過量鎢-182的地體，他們希望這可以讓他們闡明地球歷史最剛開始時發生的事物，並且找到這些遠古物質究竟儲存在地球內部何處。

引用自：Carnegie Institution for Science. "Found: Surviving evidence of Earth's formative years." ScienceDaily. ScienceDaily, 12 May 2016.