Science Opportunities - Basic Research Opportunities in Earth Science - page 69

the physical state and dynamics of the solid Earth and the near-Earth space
environment. Advances in theory and computational capability, together with
modern satellite data, provide exciting new possibilities for extending the
knowledge (and use) of geomagnetism on a number of fronts. However,
achieving this promise will require a global array of geomagnetic observatories,
both on the land surface and on the seafloor, in conjunction with ongoing satellite
measurements. The need for better distribution of geomagnetic observatories with
modern digital equipment is a well-known problem. 11
Geochemistry Studies
Isotope and trace element geochemistry of mantle-derived rocks, including
ultramafic xenoliths and lavas erupted at midocean ridges and ocean islands,
documents the chemical composition of the deep Earth, as well as the time-
integrated effects of four and a half billion years of planetary differentiation. The
geochemistry of such samples provides unique insights to the formation of the
Earth from the solar nebula, the processes and time scales of crust and
atmosphere formation, the style of mantle convection and the origin of hotspots,
and the fate of slabs subducted back into the mantle. New geochemical
techniques have frequently arisen from the need to analyze mantle samples for
previously inaccessible elements or with enhanced precision, speed, and spatial
resolution. The results of these advances have shed new light on the deep
interior, and the coming decade will continue this evolution. The new generation
of thermal ionization mass spectrometers, offering enhanced precision and
sensitivity for radiogenic isotope analyses, has been complemented with
inductively coupled plasma mass spectrometry (ICPMS), which permits routine
analyses of nearly every element in the periodic table on very small samples. In
the past few years, results from the first multiple-collector ICPMS instruments
have also begun to appear; this new technique permits rapid isotopic analyses
with minimal chemical purification as well as analyses of elements not
appropriate for thermal ionization. Advances have also occurred in the ability to
map small-scale (tens of microns down to submicron) chemical and isotopic
variability in crystals using both laser ablation ICPMS and secondary ion mass
spectrometry. Of particular interest is the ability to analyze melt inclusions, which
richly document previously unknown small-scale variability in the chemical and
isotopic composition of the mantle.
11 For example, see The National Geomagnetic Initiative , National Academy Press,
Washington, D.C., 261 pp., 1993.