月球來的新礦物可以解釋地函內部發生的作用
一組歐洲研究團隊在月球隕石裡發現的高壓礦物，有助於解釋當物質處在地函內部的極端壓力之下會發生什麼事。
由維也納自然史博物館的Ludovic Ferrière取得的Oued Awlitis 001隕石碎片。
這種新礦物donwilhelmsite是在隕石發現的高壓礦物中，第一種可以用來探討陸地沉積物被板塊運動拖入地函深處的過程。Donwilhelmsite主要的成分是鈣、鋁、矽和氧原子，發現於2014年在撒哈拉沙漠西部找到的月球隕石Oued
Awlitis(奧利提斯乾)001。
這塊隕石成分上來說類似於組成地球陸地的岩石。從陸地侵蝕出來的沉積物會經由水和風運送到海洋，並且隨著密度較高的海洋地殼一起隱沒至地函。當它們被拖入460到700公里深的地方時，礦物組成會受到此處的高溫高壓作用，轉變成更加緻密的礦物相，其中一類即為這種新發現的礦物donwilhelmsite。因此在陸地的岩石循環中，當大陸地殼來的沉積物要穿過地函的過渡帶(460到700公里深)，donwilhelmsite便是一種相當重要的中介。
阿波羅計畫和月球任務蒐集了大約382公斤的土壤和岩石；除此之外，月球隕石也可以讓我們得到關於月球如何形成和演變的珍貴見解。這些隕石原本是月球表面的岩石，它們受到撞擊，飛濺到太空之後才來到地球，因此其中某些曾經承受過極高的溫度壓力。
曼徹斯特大學的Vera
Assis Fernandes博士測量了月球岩石的氬同位素組成，目的是找出在它們數十億年的複雜歷史當中各個事件的時間點，包括從岩漿中形成、遭受多次的撞擊轟炸、暴露在月球表面的宇宙射線等等。Fernandes博士解釋：「Oued
Awlitis 001這類隕石在撞擊轟炸期間經歷了極端的物理條件，時常造成隕石內部在微觀之下發生局部的衝擊融化，形成礦脈或者熔體。」
「由於這些遭到衝擊的區域呈現的溫度壓力範圍類似於地函內部常見的範圍，因此擁有相當重大的價值。它們就像天然的坩鍋一樣，具有在自然情況下，地球表面透過其他方法都無法產生的礦物。」
這項新發現發表在期刊《美國礦物學家》(American Mineralogist.)。重要的國際合作同仁包括：Jörg
Fritz(德國訥德林根的里斯隕石坑與撞擊研究中心)，以及柏林自然博物館、維也納自然史博物館、捷克科學院物理研究所、德國地球科學研究中心、奧斯陸自然史博物館、曼徹斯特大學、德國航太中心的研究人員。從取得月球隕石、鑑定新礦物、瞭解其科學意義，並且非常精準地測出這些大小只有千分之幾厘米的微小礦物晶體有著什麼樣的結晶構造時，都需要這些歐洲各地的研究人員彼此合作才能完成。
Mariana Klementova運用頂尖的3D電子繞射(3DED)技術，配合專門開發的軟體，首度解析出地外礦物的結晶構造。Vera
Assis Fernandes博士定出了這顆隕石的複雜歷史中各起事件的發生年代，包括新礦物donwilhlemsite是在什麼時候形成。Donwilhlemsite的命名來源是致敬美國的月球地質學家Don
E. Wilhelms，他參與了阿波羅任務登月地點的選擇過程，並且分析了第一批從月球取回地球的岩石樣品。Oued
Awlitis 001隕石的一部份目前正在維也納自然史博物館展覽。
New mineral from the Moon
could explain what happens in the Earth’s mantle
A team of European researchers discovered
a new high-pressure mineral in a lunar meteorite which is helping to explain
what happens to materials within the extreme pressures of the Earth’s mantle.
The new mineral donwilhelmsite is the first
high-pressure mineral found in meteorites with application for terrestrial
sediments dragged deep into the Earth mantle by plate tectonics. Mainly
composed of calcium, aluminum, silicon, and oxygen atoms, donwilhelmsite was
discovered within the lunar meteorite Oued Awlitis 001 found in 2014 in the
Western Sahara.
The meteorite is compositionally similar to rocks
comprising the Earth’s continents. Eroded sediments from these continents are
transported by wind and rivers to the oceans, and subducted into the Earth’s
mantle as part of the dense oceanic crust. Once dragged to depths of about
460-700 km, their constituent minerals transform at high pressures and high
temperatures existing at those depths into denser mineral phases, including the
newly discovered mineral donwilhelmsite. In the terrestrial rock cycle,
donwilhelmsite is therefore an important agent for transporting continental
crustal sediments through the transition zone of the Earth’s mantle (460-700 km
depth).
Around 382 kilograms of rocks and soils have been
collected by the Apollo and Luna missions, lunar meteorites allow valuable
insights into the formation and evolution of the Moon. Ejected by impacts onto
the lunar surface and subsequently delivered to Earth, some of these meteorites
experienced particularly high temperatures and pressures.
Dr Vera Assis Fernandes of The University of
Manchester measured the Argon isotopic composition of lunar rocks to date their
complex history including magmatic formation, multiple impact bombardments, and
the exposure to cosmic rays on the lunar surface, over billions of years. Dr
Fernandes explains: “During impact bombardment rocks like the lunar meteorite
Oued Awlitis 001 experience extreme physical conditions. This often led to
shock melting of microscopic areas forming veins or melt pockets within these
meteorites.
“These shocked areas are of great relevance as they
mirror pressure and temperature regimes similar to those prevailing in the
Earth’s mantle, and therefore are natural crucibles hosting minerals that are
otherwise naturally inaccessible at the Earth’s surface.”
The new discovery is published in the journal American Mineralogist. The international
collaboration responsible includes: Jörg Fritz (Zentrum für Rieskrater und
Impaktforschung Nördlingen, Germany) and colleagues at the Museum für Naturkunde
Berlin, Natural History Museum Vienna, Institute of Physics of the Czech
Academy of Science, Helmholtz-Zentrum GFZ Potsdam, Natural History Museum Oslo,
University of Manchester, and Deutsches Zentrum für Luft und Raumfahrt Berlin.
This pan-European collaboration was essential to obtain the lunar meteorite,
recognise the new mineral, understand its scientific relevance, and to
determine the crystal structure of the tiny, thousands parts of a millimeter
thick, mineral crystal with high accuracy.
Mariana Klementova applied the cutting edge 3D
electron diffraction (3DED) technique, together with a specially developed
software to solve, for the first time, the crystal structure of an
extraterrestrial mineral. Dr. Vera Assis Fernandes determined the ages of
various events in the complex history of this meteorite, including the
formation of the new mineral donwilhlemsite. The new mineral was named in honor
of the lunar geologist Don E. Wilhelms, an American scientist involved in
landing site selection and data analyses of the Apollo space missions that
brought to Earth the first rock samples from the Moon. Part of the meteorite
Oued Awlitis 001 is now on display at the Natural History Museum Vienna.
原始論文：Jörg
Fritz, Ansgar Greshake, Mariana Klementova, Richard Wirth, Lukas Palatinus,
Reidar G. Trønnes, Vera Assis Fernandes, Ute Böttger, Ludovic Ferrière. Donwilhelmsite, [CaAl4Si2O11], a new lunar
high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments.
American Mineralogist, 2020; 105
(11). DOI:
10.2138/am-2020-7393
引用自：The
University of Manchester. “New mineral from the Moon could explain what happens
in the Earth’s mantle.”
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