Spongy texture in mantle clinopyroxene records decompression-induced melting

Spongy texture in clinopyroxene is found in mantle xenolith suites worldwide while its origin commonly is unclear. Detailed petrologic observations and major- and trace-element analysis were conducted on peridotite xenoliths with spongy clinopyroxene, from the central part of the Great Xing'an Range, in the eastern part of the Central Asian Orogenic Belt, to investigate the origin of the texture. Spongy texture mainly occurs as rims of variable thickness on clean cores and occasionally extends into the inner parts of clinopyroxene or even totally covers the whole grain; there is no obvious relationship between the thickness of the rims and their proximity to the host basalt. The boundaries between spongy-textured clinopyroxenes and surrounding minerals are sharp. Spongy domains consist of secondary clinopyroxenes, glasses, vugs and traces of olivine. Relative to the primary ones, clinopyroxenes in spongy domains have low Na2O, Al2O3 and AlVI/AlIV, and high CaO, Mg# and Cr#. The estimated pressures of spongy domains are 0.34-0.95 Gpa, significantly lower than those of primary peridotite phases (2.10-2.44 Gpa). Glasses in spongy domains are rich in Na2O and Al2O3 and depleted in MgO and FeO, containing a small amount of K2O. Some glasses have low Mg# (down to 43.9), and are not in equilibrium with associated clinopyroxenes and olivines. Based on these observations, we suggest that the spongy texture in the studied peridotites was recently formed by decompression-induced low-degree partial melting. By comparing our petrological and chemical data with those of previous studies, textural and chemical criteria are developed to differentiate various possible origins of spongy texture in mantle clinopyroxene.