新托福阅读材料汇总：Arctic rocks may contain oldest remnants of Earth

北极岩的化学物质：地球初生，成长和故事

Scientists have found Arctic rocks that may preserve the earliest remnants of Earth.

Over billions of years, much of the material that made up the early Earth was modified by processes such as melting and mixing.

But the Arctic rocks seem to contain chemical signatures that date from just after the Earth's violent origin.

If confirmed, the discovery challenges established theories about the formation of our planet.

The results of the study are published today in the leading journal, Nature.

The signatures found in Arctic lavas are more than 4.45 billion years old. By comparison, the Earth is 4.54 billion years old, only slightly older.

The oldest surviving remnants of our planet's turbulent beginnings were unearthed by Dr Matthew Jackson of Boston University, US, and his international team.

They collected the lava samples from Greenland and Baffin Island in the Canadian Arctic. Although they erupted only 60 million years ago, the lavas contain a chemical signature of a far more ancient source.

They show that beneath the Arctic today are small pieces of mantle - the toffee-like layer below the crust - that have survived unchanged since shortly after the formation of the Earth.

The age of this ancient mantle was determined by studying helium gas locked in the lavas. The 4.45 billion-year age means that the samples date from before the Earth's crust developed, but after the core formed.

The search for the oldest remnants of the Earth's mantle has become something of a Holy Grail for planetary scientists in recent times.

Dr Carlson of the Carnegie Institution, a co-author on the study, remarked that, "this was a key phase in the evolution of the Earth. It set the stage for everything that came after".

However, Professor Tim Elliott of the University of Bristol, who was not involved in the latest study, remarked that though the conclusions were interesting, they were not beyond doubt: "A more convincing way to prove the great antiquity of this material would have been to demonstrate an anomalous extinct nuclide signature," he said.

Extinct nuclides are chemicals that were formed in stars before the formation of the Solar System. They subsequently decayed away to nothing but the traces they left behind are good markers of the early Earth.

If proven, the new discovery would challenge our understanding of the early Earth.

The ancient mantle source discovered has a different chemical make-up to that predicted by theory. It suggests that the Earth had a more complicated early history than previously thought.