Xenolith Evidence for Melt-Rock Reaction at the Lithosphereplume Boundary

Abstract

Poikiloblastic harzburgite xenoliths from Borne, French Massif Central have been interpreted as the deepest spinel facies mantle segments sampled by alkali basalts (Mercier, 1977). However, there exists considerable debate about their petrogenesis (selective recrystallisation of the granular peridotites within a diapir vs precipitation from tholeiitic melts). We thus undertook a comprehensive study on these particular rocks with a strategy of understanding the nature of processes in the lower lithosphere which is imparted by an upwelling plume (Granet et al., 1995). Petrography and geochemical characteristics. The Borne xenoliths can be classified into two groups. The G-type samples are mainly spinel lherzolites and are similar to Group I peridotites (Frey and Prinz 1978) with a textural variation from protogranular, porphyroclastic to equigranular. The equilibrium temperatures are 850-1000~ The P-type samples are harzburgites which are divided into two subgroups in terms of the degree of recrystallisation/deformation. The first subgroup, referred to as non-recrystallised P-type, is characterised by large (>1 cm) and essentially unstrained olivines, whereas the second subgroup, the recrystallised P-type, is characterised by the presence of two generations of olivine and orthopyroxene with some samples being mylonitic in texture. The P-type peridotites are equilibrated under a high temperature (majority > 1200~ The olivines in the two groups of peridotites are similar in Mg# and in NiO content. However, distinctive compositions are observed in P-type and G-type spinels. The P-type spinels display inhomogeneous composition at grain-scale and have much higher Cr contents and calculated ferric ion contents than G-type spinels. In Cr#sp-Mg#ol diagram, the Gtype and non-recrystallised P-type samples follow the progressive partial melting trend, whereas the recrystallised P-type samples tend to plot below the trend. The deviation from the progressive partial melting trend is even more obvious for the Ti contents in all P-type samples which are significantly higher than the normal mantle values. Clinopyroxenes in the G-type peridotites display R E E pattems similar to those commonly observed in peridotites from world-wide occurrences. A strong correlation between R E E patterns and texture is observed for the P-type samples. The recrystallised samples have bell shaped R E E patterns with apex at Nd and Pr. In contrast, cpxs in non-recrystallised samples have lower total R E E and Ti contents, and display a relatively flat pattern. The isotopic compositions of cpxs correlate with texture despite a restricted range in isotopes. G-type peridotites have slightly higher ~Na (11.8-14.9) and low S7Sr/S6Sr (0.7022-0.7030) than the P-type p e r i d o t i t e s (ENd = 6 .0--8 .3 ; S7Sr/86Sr = 0.7031-0.7035), defining a depleted mantle reservoir. The P-type peridotites plot between the MORB field and bulk Earth composition. A percolation-reaction model for the origin of Borne P-type harzburgites. The previous cumulative model is inadequate when following lines of evidences are taken into account. (1) In terms of major element chemistry, the 'refractory' characteristics and homogeneous silicate phases of the P-type peridotites are different from those magmatic segregates which are commonly pyroxene-rich and high in CaO and A1203; (2) The relatively constant Mg# and NiO contents of P-type samples are not consistent with the hypothesis of large volume of cumulates; (3) The concentrations of hypothetical melts in equilibrium with P-type samples are highly variable, attesting to a very strong fractionation processes associated with the formation of P-type xenoliths. This is not consistent with the lack of fractionation revealed from the major chemistry; and (4) A small, but significant difference in Sr-Nd