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We attempt to constrain the efficiency of additional transport or mixingprocesses that reduce the effect of atomic diffusion in stellar atmospheres.  We apply spectral synthesis methods to spectra observed with the GIRAFFEspectrograph on the VLT to estimate abundances of Mg, Ti, Fe, and Ba in starsin the metal-poor globular cluster M30. To the abundances we fit trends ofabundances predicted by stellar evolution models assuming differentefficiencies of additional transport or mixing processes. The fitting procedureattempts to take into account the effects of parameter-dependent systematicerrors in the derived abundances.  We find that the parameter $T_0$, which describes the efficiency ofadditional transport or mixing processes, can almost certainly be constrainedto the narrow range $\log_{10}{\left( T_0 / \left[ \mathrm{K} \right] \right)}$between $6.09$ and $6.2$. This corresponds to decreased abundances for stars atthe main sequence turn-off point compared to the red giant branch by$0.2\,\mathrm{dex}$ for Mg, $0.1\,\mathrm{dex}$ for Fe, and$0.07\,\mathrm{dex}$ for Ti. We also find that while our estimates do havenon-negligible systematic errors stemming from the continuum placement and theassumed microturbulence, our method can take them into account.  Our results partly amend the results of an earlier paper in this articleseries, that tentatively used a value of $\log_{10}{\left( T_0 / \left[\mathrm{K} \right] \right)} = 6.0$ when modelling the Spite plateau of lithium.To more easily distinguish physical effects from systematic errors, werecommend that studies of this kind focus on elements for which the expectedsurface abundances as functions of effective temperature have a distinctstructure and cover a wide range.

Atomic diffusion and mixing in old stars VII. Abundances of Mg, Ti, and Fe in M30