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Clues from some unusual Arizona rocks pointed scientists toward a discovery -- a subtle chemical signature in rocks the world over -- that could answer a longstanding mystery: What stole the iron from Earth's continents? The find has weighty implications. If the iron content of continental rocks was a bit greater, as it is in the rocks beneath Earth's oceans, for example, our atmosphere might look more like that of Mars, a planet so littered with rusty, oxidized rocks that it appears red even from Earth. In a new paper, petrologists make a case that garnet steals the most iron from continents. The hypothesis flies in the face of 40-plus years of geophysical thinking. In 2009, a researcher traveled to central Arizona, where he conducted an initial analysis of the xenoliths, and established that they were formed in a continental arc setting and were rich in garnet. Two years later, another researcher spent a summer characterizing rare earth elements in the xenoliths and found hints that they contained unusual Europium ratios. The oxidation states of the Europium that was found in the Arizona xenoliths suggested they formed in less-oxidized conditions than would be expected in the magnetite scenario, but there was not enough data to confirm this hunch. The quality of data collected in 2016 not only confirmed the low-oxidation Europium ratios but allowed the researchers to develop a new hypothesis that tied everything together: the garnet, the Europium ratios and the fact that thicker continental crusts are more iron-depleted than thinner island arc crusts.