Minerals offer evidence
MY COLLEAGUES and i set out to test these ideas by exploring how the topography along the Mid-Atlantic Ridge relates to the temperature, structure and composition of the underlying mantle. One way to collect such information is to examine the velocities of seismic waves passing through the mantle under the ridge. Another approach involves searching for local variations in the chemistry of basalts that erupted along the axis of the ridge. Those variations can be used to infer the extent of melting and the physical nature of the mantle source from which they derived.
We followed a third approach by attempting to collect rock samples of mantle peridotite. Peridotite is left as a solid residue after the basaltic magma component melts out of the upper mantle rocks. Mantle rocks usually lie buried under several kilometers of ocean crust, but in some cases blocks of upper mantle peridotite are accessible. They are typically exposed where the axis of the mid-ocean ridge is faulted or where it is offset laterally by transform faults; these rocks can be sampled by drilling or dredging or retrieved directly through the use of a submersible.
To analyze the mantle minerals in the Atlantic peridotite samples, we used an electron microprobe. This instrument focuses a beam of electrons only a few microns in diameter onto a slice of rock. In response, the mineral emits x-rays of characteristic wavelengths. An analysis of the wavelengths and intensities of these x-rays allows a determination of the chemical composition of the mineral. Collaborating with Nobumichi Shimizu of the Woods Hole Oceanographic Institution and Luisa Ottolini of the Italian Research Council in Pavia, we also used a different instrument--an ion microprobe--to determine the concentration of trace elements such as titanium, zirconium and rare-earth elements. The ion probe focuses a beam of ions onto a sample, which dislodges other ions in the sample for measurement. The method enabled us to determine the concentrations of trace elements down to a few parts per million.
Such analyses reveal much about the conditions in the mantle where the sample rocks formed, because the temperatures and pressures there produce distinct compositions in the peridotites. Petrologists, including Green and A. Lynton Jaques of Geoscience Australia, have shown that partial melting modifies the relative abundances of the original minerals in the peridotite. Some minerals, such as clinopyroxene, melt more easily than do others and hence decrease in abundance during the melting. Moreover, the partial melting process changes the composition of the original minerals: certain elements in them, such as aluminum and iron, tend to follow the melt. Their concentration in the minerals decreases as melting proceeds. Other elements, such as magnesium and chromium, tend to stay behind, so that the solid residue becomes enriched with them. Thus, as a result of partial melting, olivine becomes more magnesium-rich and iron-poor; the ratio of chromium to aluminum in spinel increases; and so on.
The composition of these minerals, calibrated by laboratory experiments, allows us to estimate the degree of melting that mantle peridotites undergo during their ascent below the ridge. Our data showed that substantial regional variations exist in the composition of the mantle. For instance, the chromium-to-aluminum ratio of orthopyroxene and spinel is highest in peridotites sampled from a broad area between about 35 degrees and 45 degrees north latitude. The ratio suggests that the average degree of melting of the upper mantle lying below this region may reach as high as 15 percent. In most parts, about 10 to 12 percent of the mantle melts during the trip upward. This area of above-average melting corresponds to the Azores hot-spot region, lending credibility to the theory that hot spots result from unusually hot mantle plumes upwelling deep within Earth. Other findings support that idea, including work by Emily M. Klein, along with Charles H. Langmuir of Columbia University's Lamont-Doherty Earth Observatory, who independently examined the chemistry of basalts along the Mid-Atlantic Ridge.