Origin of mid-ocean ridge basalts

April 19, 2001

Scientists at the Max Planck Institute for Chemistry in Mainz have shown that certain major and trace elements in minerals from the Earth's mantle exhibit very similar geochemical behavior during mantle melting (Nature, Vol. 410, 5. April 2001). The newly discovered correlation now makes it possible to determine the degree of mantle partial melting along ocean ridges with relatively simple technical means. In this way, the thermal structure of the mid-ocean-ridge mantle can now be better understood.

The formation of mid-ocean-ridge basalts is one of the most important mass transfer processes on Earth. Every year along the 75,000 kilometers of mid-ocean-ridge more than 20 cubic kilometers of magma is erupted, about 90% of the total global magma production. Although ocean ridges and mid-ocean-ridge basalts are among the most intensively studied thematic areas in Earth Sciences, there are still many unanswered questions about their formation. One of the most important of these is the absolute degree of partial melting experienced by the oceanic mantle, and the factors that determine it. One can understand this process as being similar to squeezing out a sponge, the "water" in this case rising up as basaltic magma to form the ocean crust, and the dry sponge (a magnesium-rich rock called "harzburgite", first described in Bad Harzburg in the 19th century) remaining in the oceanic mantle. This process preferentially removes from the harzburgite some major constituents, such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, they prefer to enter the basaltic melt), such as the rare-earth elements. In certain places, this rock is transported to the ocean floor, where it has been sampled and studied over the past few decades. This has resulted in a wealth of additional information about mantle melting that would be unavailable from the study of basalts alone. Up until now, studies have shown that the degree of partial melting (the "dryness" of the sponge) is not uniform, but instead is dependent on the mantle temperature, the speed of plate motions, and other factors. The compositions of mantle minerals from mid-ocean ridges contain information about the melting process because their compositions change during the melting. For example in spinel the major elements chromium (Cr) and aluminium (Al) - measured with an electron microprobe - provide a good qualitative indicator for partial melting because one (Cr) is retained in the mantle mineral and the other (Al) is extracted into the melt. Trace elements with lower concentrations (e.g. the rare-earth elements) provide a more quantitative measure of the degree of partial melting, but require the more expensive and less easily available ion probe for their determination. The key hurdle until now has been that these two types of information did not seem to agree with one another, one reason why the information from abyssal peridotites has not yet been widely utilized as a component of geodynamic models of melting and the formation of the ocean crust.

In connection with his Ph.D. thesis at the Max Planck Institute for Chemistry and the University of Mainz, Eric Hellebrand studied deep-sea harzburgites from the Indian Ocean. In so doing, he noticed that the major and trace elements in such rocks are only apparently poorly correlated. In fact, combined with data from the literature, very good correlations were found between incompatible trace elements (e.g. the heavy rare-earth elements dysprosium, erbium and ytterbium) and the major element chemistry (e.g., the so called chromium number, Cr/(Cr + Al, in spinel) of the rocks. The light rare-earth elements lanthanum and cerium that had previously received the lion's share of scientific attention, however, do not correlate with the major elements, being affected by other processes than just melting. This means that both major and incompatible trace elements present a coherent, unified indicator for the degree of mantle melting that can be quantified relatively simply, as now only the chromium number is required to do so. In this way, the above mentioned geodynamic processes (seafloor spreading, mantle thermal structure) can now be studied from a new, more quantitative perspective.
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