[求助]请帮助解答GWD6-Q23阅读题

GWD6-Q23 to Q26:

      Diamonds are almost impos-

       sible to detect directly because they

       are so rare:  very rich kimberlite

Line       pipes, the routes through which

  (5)      diamonds rise, may contain only

three carats of diamonds per ton

of kimberlite.  Kimberlite begins as

magma in Earth’s mantle (the layer

between the crust and the core).  As

 (10)      the magma smashes through layers

of rock, it rips out debris, creating

a mix of liquid and solid material.

Some of the solid material it brings

up may come from a so-called

 (15)      diamond-stability field, where condi-

tions of pressure and temperature

are conducive to the formation of

diamonds.  If diamonds are to sur-

vive, though, they must shoot toward

 (20)      Earth’s surface quickly.  Otherwise,

       they revert to graphite or burn.

       Explorers seeking diamonds look

for specks of “indicator minerals”

peculiar to the mantle but carried up

 (25)      in greater quantities than diamonds

and eroded out of kimberlite pipes

into the surrounding land.  The stan-

dard ones are garnets, chromites,

and ilmenites.  One can spend years

 (30)      searching for indicators and tracing

them back to the pipes that are their

source; however, 90 percent of

       kimberlite pipes found this way are

barren of diamonds, and the rest

 (35)
                       are usually too sparse to mine.

      In the 1970’s the process of

locating profitable pipes was refined

by focusing on the subtle differ-

ences between the chemical

 (40)     signatures of indicator minerals

found in diamond-rich pipes as

opposed to those found in barren

pipes.  For example, G10 garnets,

a type of garnet typically found in

 (45)      diamond-rich pipes, are lower in

calcium and higher in chrome than

garnets from barren pipes.  Geo-

chemists John Gurney showed that

garnets with this composition were

 (50)      formed only in the diamond-stability

field; more commonly found ver-

sions came from elsewhere in the

mantle.  Gurney also found that

though ilmenites did not form in the

 (55)      diamond-stability field, there was a

link useful for prospectors:  when

the iron in ilmenite was highly

oxidized, its source pipe rarely

contained any diamonds.  He rea-

 (60)      soned that iron took on more or less

oxygen in response to conditions in

the kimberlitic magma itself—mainly

in response to heat and the avail-

able oxygen.  When iron became

 (65)      highly oxidized, so did diamonds;

that is, they vaporized into carbon

dioxide.

--------------------------------------------------------------------------------

GWD6-Q23:

The primary purpose of the passage is to

1. discuss an objection to Gurney’s theories about the uses of indicator minerals
2. explore the formation of diamonds and the reasons for their scarcity
3. analyze the importance of kimberlite pipes in the formation of diamonds
4. define the characteristics of indicator minerals under differing conditions
5. explain a method of determining whether kimberlite pipes are likely to contain diamonds

Answer: E   我选了B，请帮助解答，谢谢，这个好像没有人提过，着急

我也想问为什么不是B...NN请帮帮忙吧...

        GWD6-Q23
to Q26:

        Diamonds
are almost impossible to detect directly because they are so rare:
very rich kimberlite pipes, the routes through which diamonds rise,
may contain only three carats of diamonds per ton of kimberlite.(這句是main idea,之後皆是在描述怎樣找尋鑽石的方法,文章內很多描述找尋的字眼,我用紅色標示,幾乎都是用換字的方式去描述主題怎樣偵測鑽石.)
Kimberlite begins as magma in Earth’s mantle (the layer between the
crust and the core).  As the magma smashes through layers of rock, it
rips out debris, creating a mix of liquid and solid material. Some of
the solid material it brings up may come from a so-called
diamond-stability field, where conditions of pressure and temperature
are conducive to the formation of diamonds.  If diamonds are to
survive, though, they must shoot toward Earth’s surface quickly.
Otherwise, they revert to graphite or burn. Explorers seeking
diamonds look for specks of “indicator minerals” peculiar to the
mantle but carried up in greater quantities than diamonds and eroded
out of kimberlite pipes into the surrounding land.  The standard ones
are garnets, chromites, and ilmenites.  One can spend years searching
for indicators and tracing them back to the pipes that are their
source; however, 90 percent of kimberlite pipes found this way are
barren of diamonds, and the rest are usually too sparse to mine.

        In
the 1970’s the process of locating profitable pipes was refined by
                        focusing on the subtle differences between the chemical  signatures
of indicator minerals found in diamond-rich pipes as opposed to those
found in barren pipes.(這段重點在講怎樣判定含有鑽石的礦脈的過程,講的是如何判斷跟方法,)  For example, G10 garnets, a type of garnet
typically found in diamond-rich pipes, are lower in calcium and
higher in chrome than garnets from barren pipes.  Geochemists John
Gurney showed that garnets with this composition were formed only in
the diamond-stability field; more commonly found versions came from
elsewhere in the mantle.  Gurney also found that though ilmenites did
not form in the diamond-stability field, there was a link useful for
prospectors:  when the iron in ilmenite was highly oxidized, its
source pipe rarely contained any diamonds.  He reasoned that iron
took on more or less oxygen in response to conditions in the
kimberlitic magma itself—mainly in response to heat and the
available oxygen.  When iron became highly oxidized, so did diamonds;
that is, they vaporized into carbon dioxide.

GWD6-Q23:

The
primary purpose of the passage is to

1. discuss
       an objection to Gurney’s theories about the uses of indicator
       minerals

   
               

2. explore
       the formation of diamonds and the reasons for their scarcity

   
               

3. analyze
       the importance of kimberlite pipes in the formation of diamonds

   
               

4. define
       the characteristics of indicator minerals under differing conditions

   
               

5. explain
       a method of determining whether kimberlite pipes are likely to
       contain diamonds

   
               

掌握上述兩段的重點,你會發現這篇文章描述的主題是解釋,並告知判斷鑽石礦脈是否含有鑽石的方法.因此答案選E

選項B僅涵蓋一部分文章的內容,並沒有充分掌握整篇文章的重點.

谢谢ashleyyuan这么详细的解释... 很清楚... 赞!

哈 谢谢谢谢！