GWD6-Q25:请ＮＮ们，指点一下

云的足迹

     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.

The passage suggests that the presence of G10 garnet in a kimberlite pipe indicates that

             

1. the pipe in which the garnet is found has a 90% chance of containing diamonds
2. the levels of calcium and chrome in the pipe are conducive to diamond formation
3. the pipe passed through a diamond-stability field and thus may contain diamonds
4. any diamonds the pipe contains would not have come from the diamond-stability field
5. the pipe’s temperature was so high that it oxidized any diamonds the pipe might have contained

看答案上争议比较搞不懂，不同版本答案不同（Ｂ＆Ｃ），到底是哪个呢，请ＮＮ指点一下．谢谢

PrinceD

个人认为是C对一点。
Line 47 - Line 51 说，G10 garnet只能在孕育钻石的 diamond-stability field 中形成。
G10 garnet的存在，表明了kimberlite pipe penetrated that field and brought up to the surface G10 garnet and probably diamonds as well.

barila

同请教一下，我也觉得是C啊~~~答案错了嘛？

summerymiao

我选的是B，可是我觉得应该是C。B错的原因是：

A.      the levels of calcium and chrome in the pipe are conducive to diamond formation， 应该是in the garnet而不是in the pipe 所以应该是C 而不是B

swimswan

我认为B时这样错的：选项说这些化学元素的level有益于diamond formation，但是diamond不是在这些pipe里面行程的，说以该项可以认为是无关吧

swimswan

我在楼上说的不对，又看了一下文章，文中谈到diamond的形成时说：
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.
而并没有提到the levels of calcium and chrome和diamond formation有关
这里the levels of calcium and chrome只可以作为diamond存在的特征，不可以作为diamond形成的条件吧