Postcumulus Processes in Oceanic-Type Olivine-Rich Cumulates: the Role of Melt Entrapment vs. Melt-Rock Interaction

Evaluation of postcumulus processes in cumulate rocks can provide insights on the mechanisms and scales of melt migration and interaction within the crust, thus contributing to define crustal accretion models. Here we present a microstructural-geochemical study on MORB-type primitive olivine-rich cumulates intruded in the Erro-Tobbio (ET) mantle peridotites (Voltri Massif, Ligurian Alps, Italy), an on-land analogue of (ultra-) slow spreading settings. Postcumulus crystallization is indicated by the occurrence of accessory interstitial minerals (Ti- pargasite, opx , Fe-Ti oxides), and by chemical zoning in intercumulus clinopyroxene related to its textural occurence, i.e. marked REE, Ti, Zr enrichment at almost constant Mg-numbers (88-90) and LREE depletion, from core to rim of coarse anhedral clinopyroxene, to thin vermicular cpx grains. Interstitial pargasites have high Nb, Zr, REE contents although preserving "primitive" major element compositions (Mgvalue = 0.86-0.89) and LREE depletion. Significant trace element enrichment at almost constant LREE fractionation in interstitial clinopyroxenes and Ti-rich pargasites argue against the infiltration of exotic evolved melts and indicate that interstitial minerals were mainly related to close-system trapped melt crystallization. Geochemical modeling shows that crystallization of less than 5% trapped melt is sufficient to produce the REE enrichments observed in cpx. The progressive increase in (REE-Zr-Ti) abundances from core to rim of large clinopyroxene grains is accounted by an "in-situ" fractional crystallization process. Anomalous Zr enrichment is observed in thin (< 400 µm) interstitial and vermicular clinopyroxenes and pargasitic amphiboles, representing the very last melt fraction (reasonably < 2%). At this final crystallization stage, the low residual porosity likely inhibited large-scale melt migration. AFC modeling indicates that the high Zr/Nd ratios in cpx could be related to small-scale migration and interaction between residual, evolved, low melt fractions and the olivine cumulus matrix, consistent with textural evidence of lobate contacts between vermicular clinopyroxenes and cumulus olivine, indicative of partial olivine resorption. Our study points that small amount of melt trapped in an olivine-rich matrix can induce significant trace element enrichment in minerals. Such an evidence argue against large-scale residual melt migration, this latter being controlled by porosity and permeability of the crystal mush, and indicates that compaction was very efficient. This could have been favoured by the interplay of different factors, i.e. slow cooling rates and low thermal gradients, and synkinematic deformation (Natland and Dick, 2001, J. V. G. Res, 110, 191-233). The ET cumulates could thus represent the base of a stack of cumulates (3-5 Kb intrusion depth) from which differentiated melts migrated upwards and crystallized at shallower lithospheric environments. Overall, this indicates that different melt transport and interaction processes may act during crust accretion at very slow spreading settings, e.g. low melt fraction entrapment of indigenous melts in compaction-dominated deep-seated gabbroic intrusions vs. migration of exotic evolved melts through uppermost crustal levels.