Yan Liang

and 2 more

Clinopyroxene and orthopyroxene are the two major repositories of rare earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis has made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between cpx and opx at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith, as HREE in clinopyroxene in the xenolith are reset by subsolidus reequilibration. In general, LREE in orthopyroxene and HREE in clinopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, by using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.

Chunguang Wang

and 2 more

Amphibole is a common hydrous mineral in mantle rocks. To better understand the processes leading to the formation of amphibole-bearing peridotites and pyroxenites in mantle rocks, we have undertaken an experimental study reacting lherzolite with hydrous basaltic melts in Au-Pd capsules using the reaction couple method. Two melts were examined, a basaltic andesite and a basalt, each containing 4 wt% of water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C and 1 GPa produced a melt-bearing orthopyroxenite-dunite sequence. The cooling stimulates crystallization of orthopyroxene, clinopyroxene, amphibole, and plagioclase, leading to the formation of an amphibole-bearing gabbronorite–orthopyroxenite–peridotite sequence. Compositional variations of minerals in the experiments are controlled by temperature, pressure, and reacting melt composition. Texture, mineralogy, and mineral compositional variation trends obtained from the experiments are similar to those from mantle xenoliths and peridotite massif from the field including amphibole-bearing peridotites and amphibole-bearing pyroxenite and amphibolite that are spatially associated with peridotites, underscoring the importance of hydrous melt-peridotite reaction in the formation of these amphibole-bearing rocks in the upper mantle. Amphiboles in some field samples have distinct textual and mineralogical features and their compositional variation trends are different from that defined by the melt-peridotite reaction experiments. These amphiboles are either crystallized from the host magma that entrained the xenoliths or product of hydrothermal alterations at shallow depths.