Cr-Spinel as an Indicator of Cumulates Partial Melting and Liquid Hybridization

Abstract:

The Rum Layered Intrusion (Scotland) was emplaced 60.53 ± 0.08 Ma ago [1], in response to the proto-Iceland plume [2]. The Unit 9 gabbro cumulates were successively intruded by olivine-phyric picritic sills. Reactive liquid flow produced clinopyroxene-poor gabbro, troctolite and dunite restite, and expelled melt crystallized to form gabbro with poikilitic clinopyroxene and Cr-spinel-rich anorthosite [3]. The Cr-spinel origin is strongly debated (e.g. [4]).

We have run one atmosphere, fO₂-controlled equilibrium experiments of the Rum parental picritic parental liquid [5]. At NNO-0.8 conditions, Cr- spinel saturates from 1360°C, olivine from ~1330°C, plagioclase from 1240°C and clinopyroxene from 1220°C, systematically ~40°C above MELTS calculations. Natural Cr- spinel grains have a higher Cr/(Cr+Al+Fe³⁺) ratio (~0.51 to 0.03) than grains crystallized along the picrite NNO-0.8 liquid line of descent (0.38 to 0.06). Fe³⁺-rich spinel is abundant (~1 vol%) at NNO+1, and Al-rich spinel occurs as trace at NNO-3. In picrite-troctolite hybrid experiments, plagioclase (~An86) saturates from 1280°C and clinopyroxene from 1200°C. Al- spinel crystallizes at high temperature, and gets more Cr-rich upon cooling, reaching the highest measured Cr/(Cr+Al+Fe³⁺) ratio of ~0.44 at 1225°C, falling to 0.09 at 1200°C. Plagioclase and clinopyroxene stability plays a major role in spinel composition. Increasing the pressure or the parental magma water content would delay plagioclase saturation and result in spinel with lower Cr/Al ratio.

We deduce that Unit 9 peridotite, troctolite, anorthosite and also gabbro and poikilitic gabbro Cr-rich spinel did not crystallize from the Rum parental picritic liquid. Instead, they crystallized from a hybrid liquid produced by the mixing of invading picritic liquid with a partially molten plagioclase ± clinopyroxene -rich cumulate, leaving an olivine-rich residue/cumulate.

[1] Hamilton et al. (1998) Nature 394: 260-263

[2] Saunders et al. (1997) AGU 45-93

[3] Leuthold et al. (2014) Contrib Mineral Petrol 167:1021

[4] O’Driscoll et al. (2009) Lithos 111: 6-20

[5] Upton et al. (2002) Geol Mag 139: 437-452