November 30, 2012
Breaking Through The Crust: Unraveling The Magma Mystery
April Flowers for redOrbit.com - Your Universe Online
Even though two-thirds of the Earth's solid surface is covered with oceanic crust, scientists still do not entirely understood the process by which it is made.
But a recent study from the Carnegie Institution for Science, which analyzed more than 600 samples of oceanic crust, reveals a systemic pattern that alters long-held beliefs about how the process works.
Findings of this study, published in the journal Nature, explain a crucial step in understanding Earth's geological deep processes.
Generated from the melting of the Earth's mantle, magmas rise up below the oceanic crust. These magmas erupt on the Earth's surface at mid-ocean ridge systems, the longest mountain ranges in the world. Basalt, the planet's most common rock and the basis for oceanic crust, forms as the magma cools.
Scientists have long assumed that the composition of magmas erupting out of mid-ocean ridges is altered when minerals that form during cooling sink out of the remaining liquid. This process, called fractional crystallization, in theory, should not affect trace elements that were not included in the crystallizing minerals and their ratios should be the same in the erupting magmas as they were in the original magma before it cooled.
If this theory holds true, trace element ratios in magmas erupting at mid-ocean ridges should represent those of the original parental magma formed in the Earth's mantle. This process, however, does not account for the high abundance of trace elements found in samples of basalt from mid-ocean ridges around the world, making it clear that the reality of the situation is more complicated than the prevailing theories indicate.
The research team, composed of Frances Jenner of the Carnegie Institution and Hugh O'Neill of the Australian National University, used an extensive array of samples and advanced modeling to demonstrate that the concentration of trace elements is due to the process by which magma is cycled through the oceanic crust prior to eruption on the sea floor.
Under the Earth's surface, the magma collects in a pool of liquid rock called a magma chamber. New magma frequently flushes each chamber, mixing with the older magma already present. The newly blended magma erupts out onto the ocean floor. After eruption, the magma remaining in the magma chamber undergoes fractional crystallization, meaning that minerals are separated out from the magma as it cools. These minerals only contain minor amounts of the trace elements, however. These minor amounts build up over time, as the magma chamber is continually replenished with new magma entering the system.
"It's a simple idea, but it fits remarkably well," Jenner said. "These new findings will permit us to explore the conditions of mantle melting and production of the Earth's most-common rock."