LBL Geologists date ancient magma chambers

May 22, 1992

By Lynn Yarris,

A new way of determining the lifetime of ancient magma chambers has been reported by researchers at LBL's Center for Isotope Geochemistry.

Geologist and center director Donald DePaolo, working with Brian Stewart, a geologist now at the California Institute of Technology, found that isotopes are exchanged between different layers of magma as the magma is crystallized, and that this diffusion takes place on a time-scale of 1,000 to 10,000 years. The diffusion can be measured by shifts in the ratios between radioactive and stable isotopes of strontium and neodymium.

The Center for Isotope Geochemistry was established two years ago by LBL's Earth Sciences Division -- in collaboration with UC Berkeley and the National Science Foundation -- to learn about the geological processes that make and shape our planet. This is done primarily through the measurement of select radioisotopes and the stable isotopes to which they decay. From the resulting ratios, scientists can calculate when a sample of rocks or minerals was formed and what was taking place chemically at the time.

In a study reported in the Feb. 7 issue of Science, DePaolo and Stewart examined isotopic diffusion in the "Muskox intrusion," an underground stretch of crystallized magma some 2,000 meters thick along the Arctic Circle in Canada's Northwest Territories.

Says DePaolo, "Chemical diffusion is generally thought not to be important in magma evolution because the effective diffusivities of most elements are small in comparison to the size of magma chambers."

New evidence, however, indicates that low-density silica-rich magma can be maintained for a long time as a stable layer over denser mafic magma (magma containing ample amounts of iron and magnesium) with a sharply defined interface and little mixing between the two. "In this situation, diffusion need operate only over the short distances of the compositional boundary layer between the two magma layers," DePaolo says. Although chemical diffusion will still be minimal and the concentrations of the major components in one magma layer will remain relatively unaffected by those in the other, isotopic ratios will be strongly affected by diffusion across the interface.

DePaolo explains: "The net result is that diffusion of isotopic ratios takes place much more rapidly than the diffusion of chemical concentrations. This indicates that the two processes are independent."

The Muskox intrusion was crystallized from a series of 25 injections of magma into a shallow chamber. As the magma filled the chamber, rocks in the surrounding walls melted to form a layer of silicic liquid on top. When the mafic magma crystallized, the overlying silicic magma was preserved as a granite roof. DePaolo and Stewart collected rock samples from the most recent mafic magma injection as well as from the granite roof and the chamber walls of the Muskox intrusion. They then measured these samples for isotopic ratios of strontium-87 to strontium-86, and neodymium-143 to neodymium-144, and for concentrations of rubidium, strontium, samarium, and neodymium.

Their data confirmed that isotopic diffusion was "decoupled" from chemical diffusion across the interface between the silicic and mafic magma layers. From their measurements of the isotopic diffusion that occurred, they determined the crystallization time of the most recent Muskox magma injection to be approximately 8,000 years. Based on this figure, they concluded that the total crystallization time of the magma chamber must be 50,000 to 100,000 years. "Improved understanding of boundary layer conditions should lead to more accurate estimates of crystallization times," DePaolo says. "At deeper crustal levels where crystallization times are likely to be much greater, the effects of diffusional isotopic exchange may be even larger than those observed in the Muskox intrusion and could help explain a number of heretofore mysterious observations in igneous rocks."

DePaolo also believes that the rate differences between isotopic and chemical diffusion can help geologists study the trace components in materials such as metals and glasses.

"Our mass spectrometric techniques are sensitive enough to measure concentrations and isotope ratios very precisely even when the elements are present in extremely low concentrations," he says.