It is now widely accepted that globular cluster red giant branch stars owetheir strange abundance patterns to a combination of pollution from progenitorstars and in situ extra mixing. In this hybrid theory a first generation ofstars imprint abundance patterns into the gas from which a second generationforms. The hybrid theory suggests that extra mixing is operating in bothpopulations and we use the variation of [C/Fe] with luminosity to examine howefficient this mixing is. We investigate the observed red giant branches of M3,M13, M92, M15 and NGC 5466 as a means to test a theory of thermohaline mixing.The second parameter pair M3 and M13 are of intermediate metallicity and ourmodels are able to account for the evolution of carbon along the RGB in bothclusters. Although, in order to fit the most carbon-depleted main-sequencestars in M13 we require a model whose initial [C/Fe] abundance leads to acarbon abundance lower than is observed. Furthermore our results suggest thatstars in M13 formed with some primary nitrogen (higher C+N+O than stars in M3).In the metal-poor regime only NGC 5466 can be tentatively explained bythermohaline mixing operating in multiple populations. We find thermohalinemixing unable to model the depletion of [C/Fe] with magnitude in M92 and M15.It appears as if extra mixing is occurring before the luminosity function bumpin these clusters. To reconcile the data with the models would require firstdredge-up to be deeper than found in extant models.
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