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  3. Rubidium, potassium and carbon are three common elements used to date the history of Earth. The rates of radioactive decay of t

Rubidium, potassium and carbon are three common elements used to date the history of Earth. The rates of radioactive decay of t

admin2013-11-29  61
问题    Rubidium, potassium and carbon are three common elements used to date the history of Earth.  The rates of radioactive decay of these elements are absolutely regular when averaged out over a period of time; nothing is known to change them. To be useful as clocks, the elements have to be fairly common in natural minerals, unstable but decay slowly over millions of years to form recognizable "daughter" products which are preserved minerals.
   For example, an atom of radioactive rubidium decays to form an atom of strontium (another element) by converting a neutron in its nucleus to a proton and releasing an electron, generating energy in the process.  The radiogenic daughter products of the decay-in this case strontium atoms--diffuse away and are lost above a certain very high temperature. So by measuring the exact proportions of rubidium and strontium atoms that are present in a mineral, researchers can work out how long it has been since the mineral cooled below that critical "blocking" temperature. The main problems with this dating method are the difficulty in finding minerals containing rubidium, the accuracy with which the proportions of rubidium and strontium are measured, and the fact that the method gives only the date when the mineral last cooled below the blocking temperature. Because the blocking temperature is very high, the method is used, mainly for recrystallized (igneous or metamorphic) rocks, not for sediments--rubidium-bearing minerals in sediments simply record the age of cooling of the rocks which were eroded to form the sediments, not the age of deposition of the sediments themselves.
   Potassium decays to form (a gas) which is sometimes lost from its host mineral by escaping through pores.  Although potassium-argon dating is therefore rather unreliable, it can sometimes be useful in dating sedimentary rocks because potassium is common in some minerals which form in sediments at low temperatures. Assuming no argon has escaped, the potassium-argon date records the age of the sediments themselves.
   Carbon dating is mainly used in archaeology.  Most carbon atoms (carbon-12) are stable and do not change over time. However, cosmic radiation bombarding the upper atmospheres constantly interacting with nitrogen in the atmosphere to create an unstable form of carbon, carbon-14.
What is the limitation of the rubidium method?
选项 A、Rubidium is everywhere in the rock.
B、Strontium atoms are hard to detect at the normal temperature.
C、It cannot date sediments.
D、It is time-consuming.
答案C
解析 文章第2段最后指出由于the blocking temperature is very high，因此这种方法只用于recrystallized (igneous or metamorphic) rocks，not for sediments，段尾也提到它只能判定岩石被侵蚀形成沉积的日期，not the age of deposition of the sediments themselves(不能判定沉积物的日期)，故选项C正确。
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