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.同义改写
here 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. 氧化程度判定钻石含量是他们在挖掘中发现的另一个有用途径，而不是G10和其他garnet的差别
A中我认为最大的错误是最后一个部分 the region of space under the invisible influence of Earth`s magnetic field修饰的对象是紧邻的名词 magnetosphere，而这个名词所在的part前后被逗号隔开，是nonessential modifier，删去之后应该不会影响句子整体意思；但是这里如果删去了最后一个修饰成分就没有了修饰对象，句子也说不通了
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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.同义改写