原文網址：www.sciencedaily.com/releases/2017/05/170523083216.htm
發現過去25億年來噴發過的最熱岩漿
研究為地球深部的熱演化過程帶來前所未見的新證據
這篇研究由維吉尼亞理工學院地質科學系的助理教授Esteban
Gazel和他的博士生Jarek
Trela主持，發表於最近一期的《自然―地質科學》(Nature Geoscience)。Gazel表示此研究為地球深部過去25億年來的熱演化帶來了前所未有的新證據。
Gazel表示涵蓋25億至40億年前的太古宙(Archean
Eon)是地球演化史上最有活力的時期之一。地球的地函是位在地殼和外核之間的矽酸鹽質區塊。在此時期，地函的溫度比現今高上許多。這要歸因於當時的放射性元素，像是鉀、釷和鈾，衰變產生的熱量比現在高。由於地球溫度比較高的緣故，這段地質時期的特色是有一種稱為科馬提岩(komatiite)的特殊岩石廣泛形成。
Gazel表示：「科馬提岩基本上是夏威夷類型熔岩流的超高溫版本。你可以把科馬提岩的熔岩流想像成夏威夷的熔岩流，但因為溫度極高的關係，它們發出的不是紅光而是白光。它們在地表流動時大氣的組成也跟我們今日的生活環境相當不同，當時的大氣組成比較像金星。」
Gazel表示地球過去45億年來由於地函對流造成的冷卻效應和放射性元素衰變產生的熱能減少，造成地函溫度下降，使得太古宙之後地球基本上就不再產生大量的高溫科馬提岩。
然而，Gazel和團隊研究位於今日的中美洲，跟加拉巴哥群島有關的古代熔岩流的地球化學性質時，找到了他們所謂的「驚人發現」：此地有組熔岩流熔化和結晶時的特性，跟神秘的太古宙科馬提岩十分相似。
Gazel和合作人員研究了哥斯大黎加年代為9000萬年的Tortugal組岩石，發現它們的鎂含量跟太古宙的科馬提岩一樣高，且組織結構呈現的證據也顯示其熔岩流的溫度極高。
Gazel表示：「實驗研究告訴我們玄武岩和科馬提岩的鎂含量跟最初熔化時的溫度有關。溫度越高，玄武岩的鎂含量就越多。」
團隊也研究了這類岩漿結晶時最先形成的礦物――橄欖石的成分。Gazel專注於探索許多火山或火成岩地區來尋找橄欖石，因為這種淡綠色礦物是地函熔化形成的岩漿冷卻時最先結晶出來的礦物相，因此可以用來探討關於熔岩流形成時的許多環境因子。橄欖石也會攜帶由岩漿形成的玻璃質包裹體以及其他微小礦物，而有助於解開地球深處隱藏的秘密。
Gazel表示：「我們把橄欖石的成分當作另一種溫度計，以佐證這些岩漿開始冷卻時溫度有多高。分析橄欖石的成分和另一種稱為尖晶石的礦物包裹體成分，可以測出玄武岩質岩漿開始結晶時的溫度。在溫度比較高時，橄欖石會將更多的鋁加進它的結晶構造，而尖晶石則會加入更多的鉻。如果可以知道這兩種礦物中個別的元素含量，就可以知道它們結晶時的溫度高低。」
團隊發現Tortugal的橄欖石結晶時的溫度將近攝氏1600度(華氏2900度)，跟科馬提岩的橄欖石紀錄的溫度一樣高，使得Tortugal的熔岩成為過去25億年岩漿溫度的新紀錄。
Gazel和合作人員在他們的研究中提出地球或許仍有能力製造出科馬提岩類型的熔岩。他們的研究認為Tortugal的熔岩最有可能是起源於加拉巴哥地函柱的高熱核心地帶，該地函柱在大約9000萬年前開始形成岩漿，從那時開始到至今仍然持續活動著。
地函柱是一種可能起源於地球核幔邊界的深層構造。當它靠近地表並開始熔化時，就會形成稱為熱點的特殊構造，如同在夏威夷或加拉巴哥群島可看到的。於是地質學家可以研究這些熱點產生的熔岩流，並利用它們的地球化學訊息來當作探究地球深處的管道。
Gazel表示：「這項研究的迷人之處在於我們顯示出地球仍有能力產生跟太古宙時代一樣高溫的岩漿。根據從Tortugal熔岩得到的結果，我們認為地函柱『連通』了地函自太古宙以來冷卻程度並不多的高溫深層區域。我們推測這些區域可以維持高溫的原因，可能是接收了地核凝固時產生的熱量。」
本研究的第一作者，Gazel的博士生Trela表示：「這項研究的成果真得令人深感興趣，而我們計劃要繼續深究Tortugal的岩石。雖然Tortugal地層組在超過20年以前就被首度發現並有文獻紀錄，但直到現在我們才有技術和實驗經費，來深入瞭解這個區域對整個地球來說有何啟發。」
Trela補充：「我們的新數據顯示這組岩石提供了重要契機來回答某些重要問題：關於地球的加積過程、地球的熱演化歷史，以及地函柱帶到地表的地球化學訊息意義為何。」
Hottest lavas that erupted in past 2.5 billion
years revealed
The study
brings new, unprecedented evidence on the thermal evolution of the deep Earth
An international team of researchers led
by geoscientists with the Virginia Tech College of Science recently discovered
that deep portions of Earth's mantle might be as hot as it was more than 2.5
billion years ago.
The study, led by Esteban Gazel, an assistant
professor with Virginia Tech's Department of Geosciences, and his doctoral
student Jarek Trela of Deer Park, Illinois, is published in the latest issue of
Nature Geoscience. The study brings
new, unprecedented evidence on the thermal evolution of the deep Earth during
the past 2.5 billion years, Gazel said.
The Archean Eon -- covering from 2.5 to 4 billion
years ago -- is one of the most enigmatic times in the evolution of our planet,
Gazel said. During this time period, the temperature of Earth's mantle -- the
silicate region between the crust and the outer core -- was hotter than it is
today, owing to a higher amount of radioactive heat produced from the decay of
elements such as potassium, thorium, and uranium. Because Earth was hotter
during this period, this interval of geologic time is marked by the widespread
of occurrence of a unique rock known as komatiite.
"Komatiites are basically superhot versions of
Hawaiian style lava flows," Gazel said. "You can imagine a Hawaiian
lava flow, only komatiites were so hot that they glowed white instead of red,
and they flowed on a planetary surface with very different atmospheric
conditions, more similar to Venus than the planet we live on today."
Earth essentially stopped producing abundant hot
komatiites after the Archean era because the mantle has cooled during the past
4.5 billion years due to convective cooling and a decrease in radioactive heat
production, Gazel said.
However, Gazel and a team made what they call an
astonishing discovery while studying the chemistry of ancient Galapagos-related
lava flows, preserved today in Central America: a suite of lavas that shows
conditions of melting and crystallization similar to the mysterious Archean
komatiites.
Gazel and collaborators studied a set of rocks from
the 90 million-year-old Tortugal Suite in Costa Rica and found that they had
magnesium concentrations as high as Archean komatiites, as well as textural
evidence for extremely hot lava flow temperatures.
"Experimental studies tell us that that the
magnesium concentration of basalts and komatiites is related to the initial
temperature of the melt," Gazel said. "They higher the temperature,
the higher the magnesium content of a basalt."
The team also studied the composition olivine, the
first mineral that crystallized from these lavas. Olivine -- a light green
mineral that Gazel has obsessively explored many volcanoes and magmatic regions
to search for -- is an extremely useful tool to study a number of conditions
related to origin of a lava flow because it is the first mineral phase that
crystallizes when a mantle melt cools. Olivines also carry inclusions of glass
-- that once was melt -- and other smaller minerals that are helpful to
decipher the secrets of the deep Earth.
"We used the composition of olivine as another
thermometer to corroborate how hot these lavas were when they began to
cool," Gazel said. "You can determine the temperature that basaltic
lava began crystallizing by analyzing the composition of olivine and inclusions
of another mineral called spinel. At higher temperatures, olivine will
incorporate more aluminum into its structure and spinel will incorporate more
chromium. If you know how much of these elements are present in each mineral,
then you know the temperature at which they crystallized."
The team found that Tortugal olivines crystallized at
temperature nearing 2,900 degrees Fahrenheit (1,600 degrees Celsius) -- as high
as temperatures recorded by olivines from komatiites -- making this a new
record on lava temperatures in the past 2.5 billion years.
Gazel and collaborators suggest in their study that
Earth may still be capable of producing komatiite-like melts. Their results
suggest that Tortugal lavas most likely originated from the hot core of the Galapagos
mantle plume that started producing melts nearly 90 million years ago and has
remained active ever since.
A mantle plume is a deep-earth structure that likely
originates at the core-mantle boundary of the planet. When it nears the surface
of the planet it begins to melt, forming features known as hotspots such as
those found in Hawaii or Galapagos. Geologists can then study these hotspot
lava flows and use their geochemical information as a window into the deep
Earth.
"What is really fascinating about this study is
that we show that the planet is still capable of producing lavas as hot as
during Archean time period," Gazel said. "Based on our results from
Tortugal lavas, we think that mantle plumes are 'tapping' a deep, hot region of
the mantle that hasn't cooled very much since the Archean. We think that this
region is probably being sustained by heat from the crystallizing core of the
planet."
"This is a really interesting discovery and we
are going to keep investigating Tortugal," said Trela, a doctoral student
and the first author of the paper. "Although the Tortugal Suite was first
discovered and documented more than 20 years ago, it wasn't until now that we
have the technology and experimental support to better understand the global
implications of this location."
Trela added, "Our new data suggest that this
suite of rocks offers tremendous opportunity to answer key questions regarding
the accretion of Earth, its thermal evolution, and the geochemical messages
that mantle plumes bring to the surface of the planet."
原始論文：Jarek Trela,
Esteban Gazel, Alexander V. Sobolev, Lowell Moore, Michael Bizimis, Brian
Jicha, Valentina G. Batanova. The hottest lavas of the Phanerozoic and
the survival of deep Archaean reservoirs. Nature Geoscience,
2017; DOI: 10.1038/ngeo2954
引用自：Virginia
Tech. "Hottest lavas that erupted in past 2.5 billion years revealed: The
study brings new, unprecedented evidence on the thermal evolution of the deep
Earth." ScienceDaily. ScienceDaily, 23 May 2017.
沒有留言:
張貼留言