Melt inclusions in basaltic and associated volcanic rocks

1. Adam Kent, Oregon State University Melt inclusions in basaltic and associated volcanic rocks

2. 50 µm Melt inclusions: An introduction “Parcels” of melt trapped in igneous crystals  Fluid inclusions Occur in basaltic and related rocks wherever they are found: Arcs, OIB, CFB, MORB, LMI, ET’s Silicic and Plutonic Rocks Xenoliths

3. Saal et al. 1998

4. Scope: Basaltic and related volcanic rocks

5. Scope: Basaltic and related volcanic rocks Nanos gigantum humeris insidentes Bernard of Charles, 1159

6. Why study melt inclusions? Melt inclusions preserve compositions that are different from those of erupted lavas/tephra

7. 9°N Mid Atlantic Ridge Ultra-depleted Sobolev and Shimizu, 1993

8. Why study melt inclusions? Melt inclusions preserve compositions that are different from those of erupted lavas/tephra • More variable than host and associated lavas • Bulk rock, Matrix glass  “Averages” • Provide larger data sets per rock • Preserve low volume or low survivability melts • Primitive Melts • Trap volatile elements • Compare volatile and non volatile behaviour • Provide melt samples in altered rocks

9. But there’s a catch Melt inclusions are NOT a universal panacea! • Specific samples: phyric ± rapidly cooled • More work/time/money per sample • Mineral separation, mounting and polishing • Specialized analysis techniques • Melt inclusions are small! • Typically (trace element and isotope) analyses are less precise • Isotopic data are limited • Require significant additional interpretation

10. Magmatic Crystallization Assimilation Magma mixing Source heterogeneity Degassing Inclusion-specific Boundary layers Post-entrapment crystallization Re-equilibration with host or external melt Non representative trapping Melt Inclusion Variations i.e. things that drive changes in magma compositions i.e. things that are unique to inclusions

11. Re-equilibration between inclusion and host Portnyagin et al. 2007, Spandler et al. 2007, Cottrell et al. 2002, Danyushevsky et al. 2000; Gaetani and Watson, 2000, 2002 Preferential trapping of unusual, non-representative compositions Michael et al. 2002, Danyushevsky et al. 2004, Yaxley et al. 2005 Trapping boundary layers Kohut and Nielsen, 2004; Faure and Schiano, 2005, Baker et al. 2008. Goldstein and Luth, 2007 Alteration of inclusions Nielsen et al. 1998 Inclusion-Specific Processes

12. Analysis *plus speciation

13. How do melt inclusions form? The widespread occurrence of melt inclusions in basaltic rocks shows that their formation is a normal part of the process of crystallization in igneous rocks Melt inclusions form in regions of relatively slow crystal growth

14. How do melt inclusions form? Modified from V.S. Sobolev and Kostyuk 1975; Roedder, 1979, 1984

15. Faure and Schiano 2005 Do melt inclusion formation processes fractionate trapped compositions?

16. Baker et al. 2008

17. Not all experimental studies show boundary layer effects

18. Most natural suites do not show clear indications of boundary layer effects • Perhaps we sample larger inclusions (only significant at < 30 µm) • Longer isothermal times in natural samples • Are boundary layers static? • Kinetic experiments

19. 50 µm Evolution of melt inclusions after trapping Important impact on physical appearance and chemical compositions 25 µm 25 µm

20. Evolution of melt inclusions after trapping Wallace, 2005 Venting/breaching/alteration Post-entrapment crystallization Diffusive exchange

21. Correction for postentrapment crystallization • Experimental • Reheat to (estimated) trapping temperature • Numerical • Based on chemical equilibrium • Olivine: KDFeO*/MgO = 0.33 ± 0.03 Loihi Seamount (Kent et al., 1999)

22. Compatible elements are the least robust after correction for post-entrapment crystallization

23. Equilibration between Host and Inclusion Equilibration more rapid at • Higher Diffusivity • Higher Temperatures • More compatible • Larger inclusion • Smaller host Qin et al. (1992)

24. Danyushevsky et al. 2000 Fe Loss Yaxley et al. 2005 • Negative correlation between measured FeO* and Fohost • Anomalously low FeO* wrt liquid line of descent

25. Trace element re-equilibration • The most robust data sources in melt inclusions are slow diffusing and incompatible elements • Altered only by dilution/concentration • Ratios unchanged Are incompatible trace elements affected by diffusional re-equilibration?

26. Cottrell et al. 2002 REE equilibration with host after 2500 years

27. Cottrell et al 2002

29. Slater et al., 2001

31. Trace element re-equilibration Spandler et al. 2007

33. Baffin Island olivine-hosted n = 103

34. Preserve inter-crystal variations

35. Driving Force?

36. The message from melt inclusions: Variability • In many basaltic systems it is clear that the primary control on melt inclusion compositions is the variability of melts present within the system • These are sampled by erupted lavas as well, but are homogenized • Implies large scale mixing of smaller melt “batches” is extremely widespread • Melt inclusions and host lavas related by mixing

37. Basaltic melt generation and transport systems are variable at scales smaller than individual eruptive units (factors of 10’s) • Phenocrysts • Melt inclusions sample this variation • Some real and apparent homogenization (mixing) occurs prior to eruption • Rates: Transport >> Re-equilibration

38. Comparison between melt inclusions and host lavas Baffin Island olivine-hosted Melt inclusions sample the same population of melts as host lavas Variability in trace element composition is driven by the same processes in inclusions and in lavas

39. Magma Magma Melt Inclusion Kellogg et al. 2002

40. Baffin Island olivine-hosted

41. Borgahraun, Iceland Maclennan et al. 2003

42. Comparison between host and inclusions provides a means to assess relationship between inclusions and magmatic systems

43. 9°N MAR • Anomalous melt inclusions • Low volume melts? • Magma chamber or primary? • Artifacts of trapping? Ultra-depleted Sobolev et al 2000 Sobolev & Shimizu 1993

44. “ There is no necessary connexion between the size of an object and the value of a fact, and…though the objects I have described are minute the conclusions to be derived from the facts are great ” Sorby 1858 Geol. Soc. London. Quart. Jour. 14 453-500 [from Roedder (1979) Bull. Mineral.]