- Southern University of Science and Technology, College of engineering, ocean science and engineering, Shenzhen, China (jiashao2019@gmail.com)
Oceanic island basalts (OIBs) and mid-ocean ridge basalts (MORBs) exhibit distinct chemical compositions in both major and trace elements, reflecting differences in their mantle sources and melting processes. Mantle plumes are now understood to also contain denser, recycled materials derived from subducted sediments and oceanic basalts — pyroxenites. Compared to peridotites, pyroxenites form a relatively small volume fraction (~20%) of the mantle. They are denser, enriched in pyroxene and aluminum-phase minerals, and thought to contribute a higher concentration of incompatible elements to mantle melts. While typical MORBs are depleted in incompatible elements, some MORBs (EMORBs) located far from hotspots show both trace element and isotopic enrichment, implying that the upper mantle is chemically and isotopically heterogeneous, although MORB sources typically contain a smaller pyroxenite fraction than OIB sources. Pyroxenite lithologies appear to be widespread throughout the mantle.
In this study, we explore the melting processes and variations in the density and viscosity of a bi-lithologic mantle composed of peridotite and pyroxenite, using 1D numerical simulations. The experiments cover a temperature range of 1300–1700°C and pyroxenite contents ranging from 1% to 30%, encompassing conditions representative of both mantle plumes and mid-ocean ridges. Our findings indicate that the melting of upwelling bi-lithologic mantle typically occurs in three stages: (1) preferential melting of pyroxenite at greater depths, (2) simultaneous melting of pyroxenite and peridotite as peridotite starts to melt — especially when the peridotite is damp, and (3) only peridotite melting even when pyroxenite still remains in the source. If the pyroxenite content in the mantle is low and/or the temperature is sufficiently high, pyroxenite can be completely exhausted before peridotite begins to melt, resulting in the absence of stage 2. Pyroxenite melting plays a significant role in reducing mantle density due to the decrease in the volume fraction of the relatively denser pyroxenite. For instance, 56% partial melting of a 20% pyroxenite fraction leads to a mantle density reduction of 0.54%, comparable to the thermal buoyancy effect of a 180°C increase in temperature, assuming a mantle thermal expansivity of 3×10-5 1/K. The effect of pyroxenite melting on mantle viscosity is relatively small (< 0.5 orders of magnitude) due to the small volume fraction of pyroxenite, even though pyroxenite becomes significantly stronger by melt-induced dehydration. Confirming previous findings, these experiments also show that once peridotite begins to melt, a notable increase in mantle viscosity (1–2 orders of magnitude) occurs due to its dehydration, while partial melt-extraction from peridotite will further reduce the compositional density of the mantle.
How to cite: Shao, J. and Morgan, J.: Decompression melting of a wet bi-lithologic mantle and its effects on density and viscosity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10227, https://doi.org/10.5194/egusphere-egu25-10227, 2025.