Geologic column dating
Terms listed in the glossary are italicized on their first use in the text. Carbon is widely dissolved in the earth’s silicate minerals at part-per-million levels and lower.
But whenever carbon occurs as a free species, diamonds have the potential to form.
Research into natural diamonds (figure 1) has emerged over the last two decades as one of the keys to understanding the deep earth.
Analytical advances, improved geologic knowledge, and the emergence of new diamond-producing regions (such as the Slave craton of Canada) have all contributed to this change.
It is hoped that this article will give the gemologist a ready way to convey how nature first created the rough diamonds.
But it is also of importance to the practicing gemologist, since these are fundamental questions that a wearer of a beautiful diamond might ask.By comparison, oceanic crust is much younger and progresses regularly in age from zero (formation today) to the oldest known ocean floor, which is about 0.2 billion years old.This basic age distribution of rocks at the earth’s surface (Hurley and Rand, 1969) became widely known within five years of the acceptance of plate tectonics theory in the mid-1960s, as naturally decaying radioactive elements (uranium, thorium, and rubidium) provided a quantitative way to measure the geologic age of exposed crustal rocks. World diamond localities are shown here in relation to Archean cratons and classified as either kimberlite-hosted and from mantle keels (lithospheric), kimberlite-hosted and from the convecting mantle (superdeep), of surface origin (alluvial), from ultra-high-pressure crustal terranes (UHP crustal), or formed by the shock of meteorite impact (impact).This graph depicts the rise of temperature with depth (the geothermal gradient) in the lithosphere.
Diamonds are stable under the high pressure and temperature conditions that are only met at great depth in the earth’s mantle.
Yet diamonds are very rare because the mantle has a relatively low abundance of carbon. Earth is special among the planets in that it has two crustal types, continental and oceanic, that sit at two very different heights, approximately 840 meters above and 3,840 meters below sea level on average.