更多好文章的沪江博客： 星座下的神迹
地壳很古老，有几十亿岁，老到几乎不可想象。如今，科学家发现了可能是已知最古老的全岩。卡耐基·梅隆大学的地质学家将这一科研成果发表在9月26日的《科学》杂志上。
 
这块岩石来自于魁北克的哈德逊湾。2001年，地质学家认定这个地方可能是古老岩石的贮存处。哈德逊湾由大量的火山岩和称为“变质沉积岩”的岩石组成，并围绕有35亿年的英云闪长岩。

科学家收集了岩石样本，并测定了稀土元素钕和钐的微小的同位素差异。年代测定技术显示，这块岩石可能已经42.8亿岁了。人们很少发现能追溯到地幔形成时期的岩石，那个剧烈变化的时期，大多数岩石被一次又一次地打得粉碎。对这种全岩的进一步研究能帮助我们更好地理解我们的星球形成初期的情况。

（本文由Crux译自《科学美国人》网站的60-Seconds Science节目，欢迎到小组讨论～）

科技词汇：
crust n. 地壳
地球最外层固体部分，基本由一系列火成岩和变质岩组成。大陆地区的地壳主要由花岗岩组成，而海底地壳主要由玄武岩和辉长岩组成。平均而言，地壳从地表向下延伸35千米到达下伏的地幔。
volcanic adj. 火山的
metasedimentary rock n. 变质沉积岩（文中估计是播音员误将"metasedimentary"读成了"metasedentary"）
tonalite n. 英云闪长岩
isotopic adj. 同位素的
rare earth element n. 稀土元素
neodymium n. 【化】钕
samarium n. 【化】钐
mantle n. 地幔
地球地壳以下、地核之上的部分。地幔是从地面向下平均35千米处开始，一直到深约2,900千米处止。主要的岩石是橄榄石、辉石和硅酸盐钙钛矿。

其它词汇整理：
repository : a place, room, or container where something is deposited or stored
tumultuous : involving a lot of change and confusion and/or violence


Rocks from Earth's Origins

The Earth’s crust is old. Billions of years old. So old that it’s nearly impossible to imagine. And now scientists have discovered what may be the oldest whole rocks ever known. Geologists at Carnegie Mellon University published the results of the research in the September 26th issue of the journal Science.

The rocks come from an area of the Hudson Bay in northern Quebec. In 2001 the geologists identified the site as a possible repository of very old rocks. It is made up of folds of volcanic and what are called metasedentary rocks. And it’s surrounded by three-and-a-half-billion-year-old tonalite.

Researchers collected rocks and measured minute isotopic variations in rare earth elements called neodymium and samarium. According to dating techniques, the rocks could be 4.28 billion years old. It’s rare to find rocks that date back to when the Earth’s mantle was forming. It was quite a tumultuous time, and most rocks were smashed up into tiny pieces again and again. Learning more about these whole rocks could help us gain a better understanding of the very beginnings of our planet.

—Cynthia Graber

原文链接：http://www.sciam.com/podcast/episode.cfm?id=rocks-from-earths-origins-08-09-30