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. 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. 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.
The primary purpose of the passage is to
A. discuss an objection to Gurney’s theories about the uses of indicator minerals
B. explore the formation of diamonds and the reasons for their scarcity
C. analyze the importance of kimberlite pipes in the formation of diamonds
D. define the characteristics of indicator minerals under differing conditions
E. explain a method of determining whether kimberlite pipes are likely to contain diamonds
Each of the following is mentioned in the passage as a difference between G10 garnet and other versions of garnet EXCEPT
A. level of oxidation
B. commonness of occurrence
C. chemical signature
D. place of formation
E. appearance in conjunction with diamonds
The passage suggests that the presence of G10 garnet in a kimberlite pipe indicates that
A. the pipe in which the garnet is found has a 90% chance of containing diamonds
B. the levels of calcium and chrome in the pipe are conducive to diamond formation
C. the pipe passed through a diamond-stability field and thus may contain diamonds
D. any diamonds the pipe contains would not have come from the diamond-stability field
E. the pipe’s temperature was so high that it oxidized any diamonds the pipe might have contained
According to the passage, Gurney refined the use of ilmenites in prospecting for diamonds in which of the following ways?
A. He found that ilmenites are brought up from the mantle by kimberlite pipes and erode out into the surrounding land in greater quantities than diamonds.
For many years, theoretical economists characterized humans as rational beings relentlessly bent on maximizing purely selfish reward. Results of an experimental economics study appear to contradict this view, however. In the “Ultimatum Game,” two subjects, who cannot exchange information, are placed in separate rooms. One is randomly chosen to propose how a sum of money, known to both, should be shared between them; only one offer, which must be accepted or rejected without negotiation, is allowed.
If, in fact, people are selfish and rational, then the proposer should offer the smallest possible share, while the responder should accept any offer, no matter how small: after all, even one dollar is better than nothing. In numerous trials, however, two-thirds of the offers made were between 40 and 50 percent; only 4 percent were less than 20 percent. Among responders, more than half who were offered less than 20 percent rejected the offer. Behavior in the game did not appreciably depend on the players’ sex, age, or education. Nor did the amount of money involved play a significant role: for instance, in trials of the game that were conducted in Indonesia, the sum to be shared was as much as three times the subjects’ average monthly income, and still responders refused offers that they deemed too small.
The primary purpose of the passage is to
A. provide evidence in support of the view that human beings are essentially rational and selfish
B. use a particular study to challenge the argument that the economic behavior of human beings may be motivated by factors other than selfishness
C. compare certain views about human nature held by theoretical economists with those held by experimental economists
D. describe a study that apparently challenges theoretical economists’ understanding of human economic behavior
E. suggest that researchers may have failed to take into account the impact of certain noneconomic factors in designing a study of human economic behavior
The passage implies that the results of the Ultimatum Game undermine theoretical economists’ characterization of human beings by
A. demonstrating that most people are inclined to try to maximize their own advantage whenever possible
B. indicating that people who do not have the option of negotiating might behave more generously than do those who have the option of negotiating
C. illustrating how people’s economic behavior depends to some extent on how large a sum of money is involved
D. showing that most people instinctively place their own economic self-interest ahead of the interest of strangers
E. suggesting that people’s economic behavior might in part be motivated by factors other than selfishness
The author refers to the sum of one dollar (line 21) in order to
A. question the notion that the amount of money involved significantly affected players’ behavior
B. provide an example of one of the rare offers made by proposers that was less than 20 percent
C. illustrate the rationality of accepting even a very small offer
D. suggest a reason that responders rejected offers that were less than 20 percent
E. challenge the conclusion that a selfish and rational proposer should offer a responder the smallest possible share
All of the following are expressly mentioned in the passage as factors that did not significantly affect players’ behavior EXCEPT the
A. players’ level of schooling
B. amount of money to be shared
C. ages of the players
D. players’ professions
E. genders of the players
B. He found that since ilmenites do not form in the diamond-stability field, their presence indicates the absence of diamonds.
C. He showed that highly oxidized iron content in ilmenites indicates a low survival rate for diamonds.
D. He found that when the iron in ilmenites is highly oxidized, conditions in the magma were probably conducive to the formation of diamonds.
E. He showed that ilmenites take on more or less oxygen in the kimberlite pipe depending on the concentration of diamonds.