Solidification of Trapped Liquid in Rocks and Crystals

Trapped liquid in an igneous cumulate solidifies over a range of time and temperature that can be retrieved by use of the lever rule in binary solutions applied to plagioclase using the range in the An content found for the individual rock studied. The resident crystals in the cumulate count in the phase equilibria as though deposited by the resident liquid in pure fractional crystallization at the moment of trapping. The An range (Morse JPet 53:891, 2012) when measured in sufficient detail, defines the solidification history. The instantaneous solid composition along the solidus defines the zoning of the plagioclase as it follows the trapped liquid on the liquidus. The reference bulk composition of the trapped liquid is given by an intercept on the initial solid-liquid lever, defined by the fraction of plagioclase in the trapped parent magma times the residual porosity. The mafic fraction is assumed to solidify by reaction independently of the plagioclase zoning. The residual porosity is calculated from the An range when that is calibrated to a value independently determined from the evolved components. Examples from a recent treatment of residual porosity (cited above) will be given for the solidification of selected rock compositions from the Kiglapait and Skaergaard intrusions. The same principles apply to the solidification of melt inclusions, with the difference that the latter tend to sample an evolved sheath by capture, rather than a parent magma trapped by closure of a cumulate. Melt inclusions are evolved from birth, and then are likely to evolve further with continued growth and re-equilibration of the container. The cumulate, by contrast, given any small degree of adcumulus growth, has had time to exchange the evolved rejected solute owing to its slow solidification, so its trapped liquid is the contemporaneous magma at the cumulate interface. Experimental results on melt inclusions in mafic magma demonstrate their intrinsic evolved nature. For example, one near-liquidus experiment in an olivine-enhanced Kiglapait bulk composition at 13 kbar, 1375 deg C, 3.5 hr gave a melt inclusion in olivine that was fractionated 23%, with normative FSP from 52 to 68 %, and XMg from 0.65 to 0.42, demonstrating the principle that the trapping process itself makes the inclusion evolved (electron probe analyses by Prof. M.J.Jercinovic). The application of the solidification principle to presumed liquid immiscibility in the Skaergaard intrusion suggests that the melt inclusions in plagioclase are evolved during the capture process and then solidified as given by the An range in the plagioclase. The hypothesis of liquid immiscibility is not needed in this analysis. The very considerable difference in the end-stage feldspar histories of the two intrusions can be attributed to the sequestering of low-melting feldspar components in the abundant trapped liquids of the Skaergaard cumulates, compared to the lesser amounts trapped in the Kiglapait cumulates.