Settling and compaction of olivine in basaltic magmas: an experimental study on the time scales of cumulate formation

Abstract

A series of centrifuge-assisted settling experiments of 30 vol % olivine in 70 vol % basaltic melt was conducted to elucidate the formation mechanisms and time scales of gravitational cumulates. The settling experiments were performed in a centrifuging piston cylinder at 200–1,500g, 1,270–1,280 °C, and 0.8–1.1 GPa on previously annealed and texturally equilibrated samples. The mechanical settling of the dense olivine suspension occurs at about 1/6 the speed of simple Stokes settling, resulting in a sedimentation exponent n = 4.1(6) in agreement with predictions from analogue systems. The porosity (φ m ) of the orthocumulate resulting from gravitational settling of crystals is about 54 % and formation times of olivine orthocumulates result to 0.1–10 m day−1 (for an initial crystal content of the melt of 1–5 % and grain sizes of 2–10 mm). After mechanical settling, olivine grains rest on each other, and further compaction occurs through pressure dissolution at grain contacts, olivine reprecipitation where olivine is in contact with melt, and concomitant expulsion of excess liquid from the cumulate layer. With centrifugation at 400g for 50 h, porosities as low as 30.3 vol % were achieved. The olivine content at the bottom of the gravitational cumulate is 1 − φm ~ log(Δρ · h · a · t), where Δρ is the density difference between crystals and melt, h the crystal layer thickness, a the acceleration, and t the time of centrifuging. Compaction is hence proportional to effective stress integrated over time indicating that pressure dissolution is the dominant mechanism for chemical compaction. The compaction limit, that is the lowermost porosity to be reached by this mechanism, is calculated by equating the lithostatic and hydraulic pressure gradients in the cumulate and results to 3–5 % porosity for the experiments. Crystal size distribution curves and a growth exponent n of 3.1(3) indicate that diffusion-controlled Ostwald ripening is the dominant crystal growth mechanism. The above relationship, combined with a linear scaling for grain size as appropriate for reaction-controlled pressure solution creep, allows calculation of formation times of adcumulates. If chemical compaction is dissolution–reprecipitation limited, then single layers of natural olivine adcumulates of ½ m thickness with 70–75 vol % olivine at the base (as observed in the Rhum layered intrusion) would have typical formation times of 0.4–3 years for grain sizes of 2–10 mm. This time scale compares favourably with characteristic cooling times of sills. If a greater than 20-m-thick series of cumulate layers pressurizes a base layer with the porosity still filled by a melt, then compaction proceeds to the compaction limit within a few years. It can thus be expected that in layered mafic intrusions where cumulates are continuously deposited from a large magma chamber and which characteristic cooling times of more than decades, a compaction zone of several tens of metres forms with adcumulates only maintaining porosities in the order of 5 %. In conclusion, gravitational settling and gravitation-driven chemical compaction are feasible cumulate-forming processes for dense mafic minerals in basaltic magmas and in particular in large layered intrusions.