In the s, scientists used isotopic dating of kimberlitic minerals to determine the first ages of kimberlite eruptions. Using Rb-Sr geochronology of kimberlitic micas, geoscientists at the University of the Witwatersrand determined that kimberlites from the Kimberley area erupted about 86 million years ago Allsopp and Barrett, Around the same time, U-Pb geochronology on kimberlitic zircons of these same kimberlites showed similar results, that they erupted around 90 million years ago Davis et al.
Later analytical work refined these ages e. In the s, Stephen H. Richardson and colleagues at MIT, working on diamonds from the Kimberley mines, found that the diamonds range in age from a billion years to more than three billion years old and that they originated in the lithospheric mantle region below the Kaapvaal craton Richardson et al.
Since the Kimberley kimberlites erupted only 84 million years ago Clement et al. This basic age relationship holds for all other diamondiferous kimberlites. Diamonds are simply the passenger, and kimberlites are their transport.
Another wonderful feature of the way kimberlites transport diamonds from great depth is that the diamonds manage to survive. Rough diamonds are often resorbed from their primary octahedral shapes into secondary shapes called dodecahedrons.
Nearly all other magmas on Earth, such as basalts and andesites, would completely dissolve diamond, so it is a gift of nature that kimberlites allow diamonds to survive. Successful diamond transport and delivery also occurs because kimberlites erupt faster and are less oxidizing than other magmas on Earth. Diamonds may also be shielded in pieces of their host rocks during much of their transport.
Speed is of the essence here: A low-viscosity kimberlite is estimated to travel at speeds around 8 to 40 miles per hour Sparks et al. Chemical composition of the kimberlite and its volatile components are also thought to be important factors. From field observations made at the site of emplaced kimberlites, kimberlites are more explosive than the eruptions we see today in places like Hawaii, Iceland, Indonesia, and Mount St. Evidence for crystal granulation, xenolith rounding, and fragmentation see box A, figure A-1 leads geologists to conclude that kimberlite eruptions are much more violent and breach the surface with the highest velocities of any volcano.
The last known kimberlite eruptions were the circa 10,year-old Igwisi Hills kimberlites Brown et al. Furthermore, these kimberlites are not diamond-bearing. The next youngest African kimberlites are the million-year-old Kundelungu kimberlites in the Democratic Republic of Congo Batumike et al.
The most recent diamond-bearing kimberlite-like eruptions were the West Kimberley lamproites box A , which erupted 24 to 19 million years ago Allsopp et al. Kimberlites have been erupting since at least the Archean, and the oldest ones discovered so far are the Mitzic kimberlites in Gabon West Africa , which erupted around 2.
However, kimberlites have not been continuously erupting since that time, and globally there have been several time periods when kimberlites erupted more frequently Heaman et al. Melt Composition. The primary or original melt composition of kimberlite is poorly known because the rock we see today is such a variable, complicated physical mixture.
At the surface, kimberlite contains fine-grained matrix material and minerals known as phenocrysts, foreign minerals known as xenocrysts diamond being the xenocryst that we want! Xenoliths themselves are very interesting to geologists because they are samples of the rock through which the kimberlite has passed. The predominant mineral in kimberlite is olivine, which could be either phenocrystic from the kimberlite itself or xenocrystic from the mantle and broken off and sampled by the eruption.
Making the distinction between these two populations of olivine is not always clear. Olivine is easily altered to a mineral called serpentine, and this alteration also makes estimation of the original magma composition difficult. There are many different ways to try to determine the primary melt composition: conducting experiments at high pressures and temperatures, looking at melt inclusions found in kimberlite minerals, and performing mass balance calculations where the xenocryst and alteration material are subtracted to arrive at the remaining kimberlite material.
All these different approaches now seem to suggest that kimberlite magmas form as melts that are rich in carbonate in the asthenospheric mantle Stone and Luth, ; Bussweiler et al. Kimberlite magma forms after low amounts of melting of peridotite see Winter Diamonds from the Deep for more information on peridotite , at depths around — km, and contains high amounts of carbon dioxide and water.
The presence of these so-called volatile components in the kimberlite magma is one reason why kimberlite eruptions are thought to be particularly explosive. Why Did Melting Start? We know now roughly where in Earth kimberlite magmas originated, but why did melting actually start? In particular, rifting of continents and supercontinent breakup—with associated fracturing and brittle deformation in the lithosphere—provide the pathways for kimberlite magmas to reach the surface e.
But underlying all these processes of magma generation and the resulting kimberlite eruption is the relationship to the process of plate tectonics. Without plate tectonics to recycle carbonate and volatiles into the mantle, there would be no kimberlites.
The primordial magma is basic, but the incorporation of silicate minerals encountered during its ascent makes the melt more acidic. This leads to the release of carbon dioxide in the form of bubbles, which reduce the density of the melt, essentially causing it to foam.
The net result is an increase in the buoyancy of the magma, which facilitates its continued ascent. Most known kimberlites formed in the period between 70 and million years ago, but some are over million years old. Generally speaking, kimberlites are found only in cratons, the oldest surviving areas of continental crust, which form the nuclei of continental landmasses and have remained virtually unchanged since their formation eons ago.
Kimberlitic magmas form about km below Earth's surface, i. The temperatures and pressures at such depths are so high that carbon can crystallize in the form of diamonds. When kimberlitic magmas are forced through long chimneys of volcanic origin called pipes, like the water in a hose when the nozzle is narrowed, their velocity markedly increases and the emplaced diamonds are transported upwards as if they were in an elevator. This is why kimberlite pipes are the sites of most of the world's diamond mines.
But diamonds are not the only passengers. Kimberlites also carry many other types of rock with them on their long journey into the light. Olivine, the main mineral constituent of the rock, is an olive-green, grayish green, or brown mineral made up of magnesium, iron, and silica. In , the name kimberlite was proposed for this particular rock, based upon the occurrence of these rocks in the vicinity of Kimberley, South Africa. Large volumes of an olivine-rich rock type called peridotite occur at great depths in the earth in a layer called the mantle.
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