Effects of melt-mush reaction on the lower oceanic crust and MORB chemistry: constraints from 53° E Southwest Indian Ridge

Mid-ocean ridge basalt (MORB), as Earth’s most abundant magma type, has long served as a cornerstone for studying mantle compositions. However, recent studies suggest that melt migration through a crystal mush can strongly fractionate incompatible element ratios in cumulates, which would subsequently contribute to MORB, challenging traditional approaches to interpreting mantle characteristics. To test this hypothesis, we analyzed the geochemical compositions of primitive to evolved intrusive rocks and MORB from the 53° E segment of the ultraslow-spreading Southwest Indian Ridge, a region with poor magma supply. Our results reveal that the olivine-rich troctolites experienced two stages of melt-mush reaction: an initial stage recorded in interstitial clinopyroxene (high Mg# and 4- to 5-fold variations in Zr/REE), and a second stage documented in olivine (uniquely high Ni at low Fo), linked to a gabbro vein with felsic-like trace element signatures (high Yb, low Ti/Ti* and Sr/Sr*). This study confirms that melt-mush reactions strongly influence mineral chemistry of intrusive rocks in low-magma-supply ridges, particularly by enhancing Zr enrichment relative to REE. However, minimal Zr/REE variation in both local and global MORB indicates that highly fractionated reactive melt has limited impact on MORB trace element ratios. Instead, large variations in more-to-less incompatible element ratios (e.g., La/Sm) of MORB, particularly at high-Mg# values, underscore a mantle control.