JC24: Dating volcanoes on the Mid-Atlantic Ridge


 JC24



Cruise diary


Monday 23 June 2008

Today we have been on route to Southampton and the science team have been busy preparing their various sections for the cruise report. The cruise report is a document produced at the end of every cruise as a summary of what the cruise objectives were, what was done, what data was gathered and where it is. Most of the science team are responsible for a chapter, focusing on what they were responsible for during the cruise.

Just because we’re heading home doesn’t mean the science has finished, in fact far from it. Once back on land all the science team will begin working on the various bits of data we have collected. Today Peter van Calsteren, from the Open University tells us about what he’ll be doing on his return.

 

Peter writes:
Many elements such as oxygen (chemical symbol O) and carbon (C) have more than one isotope and these differ in the number of neutrons in the nucleus, 12C has 6 protons and 6 neutrons, 13C has 6 protons and 7 neutrons. Isotopes that have too many neutrons to fit in their nucleus, such as 14C with 8 neutrons, are radioactive.  Uranium, (U) has three naturally occurring radioactive isotopes: 238U, 235U and 234U. Radioactive isotopes transform, or decay at a constant rate and the time that it takes for half the isotopes to decay is the half-life. The half-life is specific for each isotope, billions of years for 238U and 244,500 year for 234U. In the uranium decay series (U-series, for short) daughter isotopes are radioactive themselves and decay ultimately to stable lead (Pb). 

The half-life of some isotopes in the U-series is similar to the time-scales of some of the magma evolution processes that affect the basalts that form the ocean floor. The magma evolution processes that generate ocean floor basalt are 1) mantle melting 2) melt or magma transport from >70 km deep in the Earth’s mantle, 3) further melting at lower depth 4) magma fractionation in the crust under the mid-ocean ridge, possibly in magma chambers, 5) eruption on the floor of the mid-ocean ridge. The half-life time-scales are probably also similar to the life-span of the AVR that we are studying in the Atlantic Ocean at 45°N.

U-series daughters, such as thorium, (Th) protactinium (Pa) and radium (Ra) and many others, are mostly different elements with different geochemical character and they can be separated, or fractionated, from each other in the magma evolution processes. Fractionation processes are sometimes not very efficient because for instance, U and Th are very similar geochemically and the ratio U/Th changes only a few percent and only during the early stages of the melting process, deep in the mantle. Other daughters decay too quickly to be useful geologically. However, over time, a secular equilibrium is reached when all daughters decay at the same rate as they are formed. In secular equilibrium the concentration of daughters is proportional to their decay rate. This means that in secular equilibrium the concentration of an element with a short half-life such as 210Bi is very small, and for instance ratio of the 234U daughter to the 238U parent is the same ratio as their half-lifes, or 234U/238U=0.000054 and thus a known  value.

Isotope ratios such as 234U/238U and 230Th/234U can be measured by mass spectrometry to very high precision and accuracy. The very low concentrations of isotopes with very short half-lifes are best measured with alpha or gamma spectrometry.

The best use of U and Th isotope ratio measurements is in calculating melt generation rates deep in the mantle. As an illustration, and assuming that melting takes place in the presence of garnet between 70 and 60 km deep and at a mantle upwelling rate of 1cm/y, a melt rate of the order of 100mg /m3/year would fit with data that we have for ocean island basalts. 100mg is as much as a few grains of sugar when a teaspoon full is about 5 gram.

During magma chamber processes, new minerals such as feldspar, crystallise and grow. 226Ra, the 230Th daughter is incorporated in feldspar crystals and extracted from the melt. This fractionation process means that both the feldspar and the melt are in U-series disequilibrium and the age of the feldspar can be calculated from 226Ra disequilibrium.

One thing that all isotope dating protocols have in common is that the age that can be calculated is between the measurement, that is now, and the moment that the fractionation process came to a halt, such as when the feldspar crystal stopped growing. However, the feldspar may have stopped growing for a number of reasons. It could be that the magma cooled down in the chamber, or that the lava erupted. In any case, a geological understanding of why the fractionation process stopped is essential for the correct interpretation of the calculated age.

In the case of U-series dating there is an upper limit to the time that can be calculated and that is the time it takes to reach secular equilibrium again. This is usually taken as five times the half-life of an isotope. This means that it is safe to say, as a first approximation, that the presence of 234U/ 230Th disequilibrium means that the sample is <350,000y old, 231Pa/235U disequilibrium <150,000y and 226Ra/230Th disequilibrium <8000y old. In favourable circumstances, when the fractionation processes are well-constrained by the modeling of other trace element data, it should be possible to refine these first-order estimates. 



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May 2008