Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.v11b0586l&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V11B-0586
Physics
1032 Mid-Oceanic Ridge Processes (3614, 8416), 1037 Magma Genesis And Partial Melting (3619), 1038 Mantle Processes (3621), 1043 Fluid And Melt Inclusion Geochemistry
Scientific paper
Melt inclusions in olivine have been thought to be high fidelity recorders of equilibrium melt compositions for incompatible trace elements because partition coefficients are very low, and incompatible elements are very large atoms that do not substitute in the olivine structure. This view has recently been shaken by the Spandler et al. (Nature, 2007) results on very fast REE diffusion in olivine: heavy REE in olivine would entirely re-equilibrate in less than a decade, and the light REE in a century. Spandler et al. conclude that all olivine-hosted melt inclusions record only shallow and very recent processes. If this conclusion holds up, all previous conclusions about mantle processes from olivine-hosted inclusions are in doubt. The new diffusion data also provide predictions for observations in a well constrained set of olivine-hosted melt inclusions: for time scales >100 years, all inclusions should be similar, and for <100 years there should be systematic changes with partition coefficient and inclusion size. These predictions can be addressed using samples along the FAMOUS segment (Mid- Atlantic Ridge, MAR). Lava samples from this region were the original data set from which dynamic melting was proposed (Langmuir et al., EPSL, 1977), and major elements have a very well developed "local vector", that has been ascribed to melting column effects (Langmuir et al., Geophys. Mon., 1992; Asimow et al., G3, 2004). Such effects have been subsequently inferred elsewhere from a single sample on the MAR from melt inclusion data (Sobolev, Petrology, 1996). Ni-zoning profiles (Nabelek and Langmuir, Contrib. Mineral. Petrol., 1986) provide an independent assessment of time scales for the olivines; melt inclusions provide REE data. Would a large set of inclusion data support melting column, crustal processes, or diffusive re-equilibration effects for the REE? Existing data on olivine-hosted melt inclusions (Fo87-92) from the FAMOUS area display a wide range of trace element compositions (Kamenetsky, EPSL, 1996; Shimizu, Physics Earth Planet. Int., 1998; Laubier et al., Chem. Geol., 2007; and new data). Ni-zoning profiles imply time scales of <10 years. While data quantity for all published melt inclusion data sets is limited, melt inclusions analyzed thus far do not show clear evidence for diffusive re-equilibration for the REE. The ratio of the variability in LREE and in HREE in the melt inclusions from a single sample is large and consistent with that observed in the suite of lavas and with mantle melting partition coefficients. Furthermore no correlation between the size of the melt inclusions and the trace element variability is observed. Based on the recent diffusion rates, these results would suggest that these melt inclusions have been formed at shallow depth and record late-stage processes. However the calculation of the liquid lines of descent from the melt inclusions is consistent with the major element variability and trends of the FAMOUS glasses, and the behavior of Sr does not suggest plagioclase, a mineral that would be present at shallow levels. Some lavas suggest high-pressure fractionation (5 kbar) which implies that the formation of the melt inclusions occurs at high pressures, and that time scales from depth to the surface are fast. Therefore the major and trace elements compositions of the melt inclusions may be interpreted in terms of deep petrogenetic processes and can be used to constrain the melting column models as well as transport rates. This may not be the case for liquids with longer crustal residence times, likely also to be found at FAMOUS and elsewhere.
Langmuir Charles H.
Laubier M.
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