Everybody,
As we all know, the prograde metamorphism of mafic rocks generally follows the common path from amphibolite through granulite to, finally, eclogite. Garnet is an important phase in the high-graded metamorphic rocks such as garnet-granulite and eclogite. Suitable P-T conditions are necessary for the occurrence of garnet. However, in addition to pressure and temperature, water maybe is another key element that effects the occurrence of garnet. So I want to ask whether occurrence of garnet need sufficient water during metamorphism or partial melting of mafic rocks? Under dry conditions, whether the occurrence would need higher pressure? This also leads to the classical question that whether the partial melting of dry mafic lower continental crust can successfully produce adakites?
Thanks.
Lu
Dear Lu,
During subsolidus metamorphism and eclogitisation in particular H2O is a fundamental catalyst that is required to trigger reactions. Whereas very small amounts are sufficient for this, eclogitisation might fail and the igneous assemblage may persist if H2O is absent, even well inside eclogite facies conditions. For mafic rocks it must be introduced from the outside as the protoliths are typically dry. On the other hand, if fluid is present in abundance at high pressures, the resulting rock might be dominated by hydrous phases such as amphiboles, epidote, white mica, lawsonite or talc (depending on P-T conditions and bulk composition) rather than producing much garnet. In the case of partial melting pressure is key, and depending on bulk composition (especially MnO) garnet should be stable at P>8-10 kbar. At lower pressure garnet is replaced by orthopyroxene, but both phases typically grow during the peritectic breakdown of hornblende, hence H2O is key as it required for the formation of amphibole prior to melting. This is however a simplification. I recommend Palin et al., 2016 or Stuck & Diener, 2017 (both JMG) for info on melting in mafic rocks.
At Broken Hill Australia hornblende and garnet coexist in amphibolite at lower sillimanite grade. This might relate to relatively high Fe/Mg ratio.
Hi Lu,
water actually has the opposite effect on the stability of garnet. A decrease in water content due to melt loss increases the stability of garnet in mafic rocks. See Dziggel et al. (2012) JMG for further details.
take a look at this paper:
Powell, R., & Downes, J. (1990). Garnet porphyroblast-bearing leucosomes in metapelites: mechanisms, phase diagrams, and an example from Broken Hill, Australia. In High-temperature metamorphism and crustal anatexis (pp. 105-123). Springer, Dordrecht.
although the context is different (pelites versus mafic rocks in your case), I believe that the problem may be similar.
We also recently reported large garnetite bands and 20cm-large garnet porphyroblasts in metabasalts from the Diego de Almagro island.
Angiboust, S., Hyppolito, T., Glodny, J., Cambeses, A., Garcia-Casco, A., Calderón, M., & Juliani, C. (2017). Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia. Gondwana Research, 42, 104-125.
These two contributions show examples on how fluid availability and melt extraction may control garnet formation in very high grade rocks.
good luck with your investigations on this not-so-easy question.
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