U-series isotopes: An introduction to principles and petrogenetic applications

1. U-series isotopes: An introduction to principles and petrogenetic applications Bruce F Schaefer Monash University

2. U-series in general • Generally we coinsider the decay from U and Th to Pb to take place in a single step • However we all know that there are a large number of intermediate isotopes of different elements produced along the way • Many have very short half lives, and so have no geological applications… • We’ll investigate the rest!!!!!

3. U,Th - Pb • 238U  206Pb; 1 =0.155x10-9 a-1 • 235U  207Pb; 2 = 0.985x10-9 a-1 • 232Th  208Pb; 3 = 0.049x10-9 a-1 • Generally this is not an oversimplification because, in a geological context, in each case the longest step is that of the decay of the U or Th. Once these elements have decayed, the whole procedure takes < 1 Ma…. • ….BUT!!!!!!

4. Aims • Introduce the key elements (and corresponding isotopes) of use in petrogenesis • Discuss the concept of disequilibria, and indeed the notion of excess, deficit, activity and secular equilibrium • Investigate how we can investigate (the very complex) subduction zone magmatism using these concepts to gain insights into rates and timescales of magmatic processes

5. Other applications • U-series also enjoy widespread applications in: • • recent carbonate geochronology • • rates and timescales of weathering, erosion and groundwater studies • Today however, we will focus on magmatic applications, although the approach will be generic, and hence if you can get your head around the principles, the rest is straight forward….!

6. Concept I • Both isotopes of U, and the only isotope of Th, decay to produce different isotopes of Pb • These are decay chains, involving many short lived intermediaries

7. 231Pa = ~32 000 yrs 230Th = 75 000 yrs 226Ra = ~1600 yrs 210Pb = ~30 yrs Concept I • Note the range of half lives of the intermediaries varies considerably (as does their chemistry)

8. These are timescales of the order of physical processes in magmatic systems Definition: (activity = N)

9. Significance of half life (and ) • Remember that after ~5 half lives, any isotopic anomaly typically decays beyond our ability to measure its magnitude with any precision (this is a very general rule…!) • Therefore the systems we’ll be dealing with today will be investigating processes that are acting on timescales of: • ~375kyr (230Th); ~165kyr (231Pa); 8000yr (226Ra) and ~100yr (210Pb)

10. Chemistry • Although we have a wide range of half lives, what is most important this whole process is that many of the daughters along the decay chain are different chemical elements to the parent • Hence there are a range of chemical (ie, magmatic) processes that are able to fractionate parent from daughter within the chain • We can to a first order consider this in terms of the relative D values of each of the elements with the decay chain • eg, Rn, a noble gas, is far more incompatible in silicate systems, than Ra, which can be relatively compatible in plagioclase

11. 231Pa = ~32 000 yrs 230Th = 75 000 yrs 226Ra = ~1600 yrs 210Pb = ~30 yrs Concept II • Each intermediary has its own chemistry, and if its half life is of the order of a geological process able to fractionate it from its parent, then we are able to introduce disequilibrium in the decay chain….

13. Geochemistry of U-series elements (in Arcs) • We can therefore consider that: • Th and Pa behave as (relatively) immobile HFSE • Ra and U (when oxidising) behave as LILE and are soluble and mobile in aqueous fluids

14. 231Pa = 32 800 yrs 230Th = 75 000 yrs 226Ra = 1599 yrs 210Pb = 22.6 yrs Concept II • Therefore we are able to either enrich or deplete the amount of the daughter with respect to its parent within a given portion of a magmatic system or phase

15. Concept III: Disequilibria • Hence, if there is more of the daughter in a sample than predicted by the measured amount of the parent, the daughter is said to be in excess; conversely the parent can be in excess

16. Activity • Because the actual amounts we measure are tiny, and in many cases there is far more of the parent than the daughter, and historically we were measuring by  counting we express the isotopic ratios as activity ratios. • These are the ratios of the decay constant multiplied the number of atoms present of each of the radioactive species • Activity = N • Activity ratios are always expressed in parentheses (brackets)

17. Analysis • Initially all measurements were made by counting the number of decays of a given energy (and hence a given isotope) per unit time and using this as an estimate of the amount (abundance) of the isotope in the sample • The amount of U (and Th) were then later measured by conventional means, and hence we could generate ratios of parent to daughter

18. Analysis II • Improvements in mass spectrometry (TIMS and MC-ICPMS) meant that we could turn to ID measurements of both parent and daughter simultaneously, and further, spike using other (radioactive but not naturally occurring) isotopes of the same element • This increased the precision of the measurements by an order of magnitude

19. Analysis III • Now samples are routinely run by MC ICPMS, with the exception of Ra which is measured on TIMS with an RPQ filter to remove organic interferences • Spikes are “milked” from a calibrated Np solution • Wet chemistry

20. Concept III: Disequilibria • Therefore, if the activity of the daughter is the same as the activity of the parent, then that implies that for every atom of parent that has decayed, there is a corresponding atom of daughter present, and hence they are in equilibrium, and the activity ratio will =1 • However, if there are more atoms of daughter present than implied by the amount of parent present, then the activity ratio of the daughter will be greater than that of the parent, and hence the activity ratio will be >1, and the daughter will be in excess. • In the converse situation (ie, we are able to remove the daughter through say fractionation), then the parent will be in excess

21. Huh???

22. 1 When in equlibrium, each decay will produce a single atom of a chemically different element… If a geological process is able to act fast enough, then it is able to chemically remove one element from another before it decays to something else…

23. The equiline diagram • Therefore, if we are able to normalise the isotopes measured to a an independent naturally occurring isotope which is not part of the same decay scheme, then we are able to generate a diagram which can be considered to be conceptually analagous to an isochron diagram • However, in this case, when parent and daughter are in equilibrium, they plot on a line of slope 1. This is the equiline.

24. The equiline diagram • Hence, if we perturb a system (either add or remove parent or daughter), samples will plot in either excess or deficit. • Over time, these points will attempt to reach secular equilibrium, and hence will move towards the equiline • The slope of arrays with respect to the equiline gives some indication of how long ago the disturbance must have happened

25. The equiline diagram • Hence, if we perturb a system (either add or remove parent or daughter), then samples will plot in either excess or deficit. Th excess U excess

26. For example • A magma is in secular equilibrium, however it starts to crystallise a phase in which U is highly compatible, but Th is incompatible • Therefore a sample of parental magma will still be in equilibrium, but the phase will have a very high (238U/232Th) ratio while preserving the original (230Th/232Th) ratio of the bulk magma Primary magma Residual liquid High U/Th accessory

27. For example • A magma is in secular equilibrium, however it starts to crystallise a phase in which U is highly compatible, but Th is incompatible • Over time, the accessory will attempt to re-establish secular equilibrium as the excess 238U decays to produce 230Th • Note that this has a vertical path as these are activity ratios, and not abundance ratios t = t1 Primary magma t = 0

28. For example • Over time, the accessory will attempt to re-establish secular equilibrium as the excess 238U decays to produce 230Th • Note that this has a vertical path as these are activity ratios, and not abundance ratios t = t1 Primary magma t = 0

29. For example • Eventually, after ~5 half lives or ~365kyr, the accessory will have returned to secular equilibrium, and hence will be plotting on the equiline again t = t1 Primary magma t = 0

30. For example • So, the shallower the array, the younger the disturbance was, and hence the further from equilibrium the system is… t = t1 Primary magma t = 0

31. Time information and processes • Hence the very observation that magmas erupted at the surface preserve disequilibria implies that there are processes acting within magmatic systems on time scales within 5 half lives of the diseequilibria…

32. Application to arcs • Although arc systems are highly complex, they are instructive in terms of U-series as there is a wide range of behaviour we can consider. • Remember: • Th and Pa behave as (relatively) immobile HFSE • Ra and U (when oxidising) behave as LILE and are soluble and mobile in aqueous fluids

33. Imposing disequilibria: Arc source melting • They are many ways of perturbing systems: • Some disequilibria can be imposed by the conditions in which melting is taking place: eg Th excess is predicted for non-modal partial melting of subducted eclogitic basaltic crust, whereas U-excess is predicted for melting of fertile (amphibole bearing) mantle wedge compositions • -most arc primary magmas have U-excess

34. Imposing disequilibria: fluid transfer • Furthermore, U and Ra are highly fluid mobile in arcs, however because of the short half life of 226Ra (<1600yr), then there is only a very narrow window of opportunity for Ra disequilibria imposed by extraction of U to be transferred to the melting region in the mantle wedge

35. Imposing disequilibria: fluid transfer • Such fluid addition effectively dilutes the effect of the immobile Th and Pa. Hence, melts that are generated in this scenario generally have low Th/U and Pa/U ratios • Note 231Pa is generated from the 235U decay scheme and so has different activity ratio components.

36. Imposing disequilibria: deep crystallisation Sangeang Api, Indonesia, contains lavas and cumulate enclaves which show no variation in U-Th isotopes despite large bulk compositional variation. Therefore crystal fractionation occurred faster than the half life of 230Th (75 000 years).

37. Imposing disequilibria: deep crystallisation However, (226Ra/230Th) decreases towards 1 (equilibrium) with increasing Th content. This suggests crystal fractionation occurred on time scales which were significant relative to the half life of 226Ra (1600 years). If the original disequilibria are mantle derived, the magmas were emplaced in less than 8000 years.

38. Imposing disequilibria: shallow degassing 210Pb is essentially controlled by 222Rn with 4 four intermediaries (total half lives <40min) Even though 222Rn only has a half life of ~3.5 days, if significant near surface degassing is taking place, then the Rn will escape, producing 210Pb deficits in the crystallising magma

39. Imposing disequilibria: shallow degassing Alternatively, if excess Rn is brought into the magma, then 210Pb excesses can occur Hence it is possible to monitor the input of new magma into an actively fractionating magma chamber beneath a volcano Limited to systems that have erupted in the last 100 years

40. Attempting to put it together: Summary of rates and processes • • magma chamber = 210Pb • • differentiation during ascent = 226Ra, 230Th • • ascent rates = 226Ra • • fluid addition/dilution = 238U, 230Th, 231Pa, 226Ra

41. A real example: White Is, NZ • White Island contains some highly unusual high Mg andesites… • …and also some unusual U-series isotopic signatures

42. High-Mg andesites • rare • slab melts? • important? • residual garnet? • wall-rock interaction?

43. White Island: • just south of ocean-continent boundary • high-Mg andesites erupted since 1977 • comparison with Rumble seamounts

44. Primitive lavas with Fo80-93, Mg# 65-71, MgO up to 10%, Ni 188 ppm, Cr 457 ppm But SiO2 = 55-58% High alkalis (increase SiO2 during melting) Different parental magmas to oceanic counterparts (Rumble seamounts)

45. Fairly typical incompatible trace element patterns - flat REE’s

46. Unradiogenic Nd at elevated SiO2 might be attributed to crustal contamination but more likely to reflect high SiO2 parental magmas since MgO is not correlated. Radiogenic isotope signatures can be modelled by ~ 2% addition of sediment to the mantle wedge

47. U-excesses argue against eclogite melting or significant wall-rock interaction in the mantle wedge Ra deficits could reflect residual amphibole and so, if near primary, melt ascent was rapid also arguing against wall-rock interaction

48. Unusual U-series systematics: • minor U excess • Ra deficits and excesses (high Sr/Th) • no correlation with SiO2 • primary array?

49. White island lavas are young enough for 210Pb measurements 210Pb excesses imply gas fluxing from fresh magma Possible positive correlations through time and with MgO may indicate the system is currently being recharged with primitive magma

50. Rumble seamounts White Island