Ken Rubin/SOEST Isotope Lab Research
210Pb-226Ra Disequilibria Very Young Submarine Basalts
A presentation from the 2005 Goldschmidt Conference on Geochemistry
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My group's research with this isotope system aims to look at volcanic processes occuring the past century. Our work in this area uses radioactive disequilibria in naturally-occuring isotopes of Th, Ra and Pb in very young sea floor lava.
Recently, we have discovered 210Pb-226Ra-230Th radioactive disequilibria in <20-year-old oceanic basalts. We have used this and relationships to other geochemical attributes of the lavas to argue that melts of Earth's mantle can be transported, accumulated and erupted in a few decades, implying that magmatic conditions can fluctuate rapidly at ridge volcanoes. These very fast magmatic timescales imply that eruptions are likely much more frequent than we previously imagined, and that geologic, biologic and chemical conditions can fluctuate rapidly in this setting.
Figure 1: (at left) U-series data for young lava flows. These plots show
interrelationships between 210Pb-226Ra-230Th activity ratios in submarine lavas
(diamonds=Juan de Fuca Ridge,
down-triangles=Southern East Pacific Rise *EPR), up-triangles=Northern EPR,
circles=Loihi seamount (Hawaii), squares=Axial seamount (North East Pacific);
black, grey and open symbols are n-MORB, seamount, and older n-MORB lava flows,
respectively). Stippled fields after Sims, et al. (2002) Geochim. Cosmochim.
210Pb-deficits of <15% relative to 226Ra occur in normal
mid-ocean ridge basalts (n-MORB), with the largest in the most magnesian lavas.
The 22-year 210Pb t½ requires very recent fractionation of these two
U-series nuclides. Relationships between 210Pb-deficits, (226Ra/230Th)
activity ratios, and compatible trace element ratios preclude crustal magma
differentiation or daughter isotope degassing as primary causes. A
common mantle melting model can simulate observed disequilibria but
preservation requires a subsequent rapid-flow melt transport mechanism.
The likelihood of magmatic disequilibria before melts enter shallow crustal
magma bodies also limits differentiation and heat replenishment timescales
to decades at the localities studied.
Figure 2. (below).
210Pb-226Ra-230Th activity ratios vs. Mg# and U abundance. N-MORB data are strongly correlated (linear regressions exclude seamount data) in all plots. The largest 210Pb-226Ra-230Th disequilibria occur in the most magnesian and lowest U-abundance
lavas. Error bars reflect 2s statistics for all nuclides except 210Pb (1s) and
are smaller than the data symbols when not shown. Symbols and stippled fields
same as Fig. 1.
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last page update on 2 Aug 2005