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To understand the oxygen fugacity (fO2) during subduction initiation, we examined proto‐arc boninites and associated mantle peridotites from the Troodos ophiolite, Cyprus. The Troodos ophiolite represents an exhumed piece of oceanic lithosphere that formed during subduction initiation in Neo‐Tethys in Late Cretaceous. Dunites and surrounding harzburgites in the Kokkinorotsos podiform chromite deposit of the ophiolite were formed by reactions between peridotites and ascending boninitic magmas. The olivine‐chromite oxybarometers show that the fO2 values (∆QFM −0.41 ± 0.37) of the dunites are slightly higher than those (∆QFM −0.85 ± 0.52) of the harzburgites under mantle conditions, indicating that the mantle fO2 was raised limitedly by percolation of the boninitic magmas. Meanwhile, the boninitic upper pillow lavas in the Margi area also recorded relatively reduced fO2 values (∆QFM −0.52 ± 0.20) during olivine‐chromite crystallization, comparable to the fO2 of mid‐ocean ridge basalts. Shallow‐level processes (i.e., magma ascent, crystallization, crustal assimilation, and degassing) appear to have limited influence on the estimated fO2 values in Troodos. The similarly low fO2 values of the mantle peridotites and boninitic magmas suggest that the Troodos primary boninitic magmas were perhaps not as oxidized as the Izu‐Bonin‐Mariana (IBM) proto‐arc boninitic magmas, though both mantle sources have captured the slab fluids released during subduction inception. This difference might be attributed to the different nature of the slab components (i.e., a more reduced subduction input for Troodos). Our study thus suggests that subducted slabs might not always release oxidized fluids, and the oxidation processes might be different for each nascent subduction zones. Plain Language Summary: Subduction zones have played a key role in mass exchange between the Earth's surface and its interior, and are one of the main factories for the generation of Earth's magmas. The redox state of arc lavas produced at subduction zones controls the formation of important mineral deposits. In fact, subduction zone lavas are believed to be generally more oxidized than mid‐ocean ridge lavas, but the origin of their oxidized nature and how their redox state change during the birth of a subduction zone remains poorly understood. Boninites are special Mg‐ and Si‐rich but Ti‐poor lavas (SiO2 > 52 wt.%, MgO > 8 wt.%, and TiO2 < 0.5 wt.%), and are typical products of magmatism that formed during subduction initiation. Here, we show that the primary boninitic magmas of the Troodos ophiolite of Cyprus are in fact no more oxidized than mid‐ocean ridge basalts, even though their mantle source interacted with the slab fluids released from the incipient subducted plate. Therefore, we propose that proto‐arc magmas, that formed as the down‐going slab began foundering, might not always be oxidized during the inception of a subduction zone. Key Points: Percolation of boninitic magmas raised the oxygen fugacity of the Troodos mantle peridotites to a limited extentThe primary boninitic magmas of the Troodos ophiolite may not be more oxidized than mid‐ocean ridge basaltsThe oxidation processes of the Troodos proto‐arc mantle may be different from those of the Izu‐Bonin‐Mariana proto‐arc mantle [ABSTRACT FROM AUTHOR]

Olivine‐hosted melt inclusions have the potential to provide direct evidence about the nature of the mantle sources of magmas. In this study, we report the major and trace element and Sr–Nd–Pb–Hf isotope compositions of the Lijiang picrites from the Emeishan large igneous province, as well as major and trace element and Pb isotopic compositions of olivine‐hosted melt inclusions in the picrites. Using these data, we infer the nature of the mantle sources for the Lijiang high‐Ti picrites (Ti/Y = 645–654), and compare them with the Dali low‐Ti picrites (Ti/Y = 351–408). Lijiang melt inclusions have highly heterogeneous Pb isotope compositions (208Pb/206Pb = 2.015–2.201, 207Pb/206Pb = 0.812–0.882, 260 Ma age‐corrected) that overlap the majority of the fields from FOZO to enriched mantle 1 (EM1). The primitive mantle‐normalized trace element patterns of these melt inclusions reveal an enriched composition with depletions in Ba, Nb, Ta, Zr, and Hf. We suggest that an old recycled GLOSS II‐like sediment was probably the main component that imparted the EM1 signature into the sources of the Lijiang picrites. Furthermore, we reverse calculated the mantle potential temperature (Tp) beneath Lijiang and Dali using the Al‐in‐ol temperature. The estimated mantle Tp beneath Lijiang is lower than Dali. We infer that the Lijiang picrites might have originated from the mantle plume with lower thermal energy, corresponding to the waning stage of the axis of the Emeishan mantle plume. Our study suggest that high‐Ti magmas of the Emeishan large igneous province were derived from those parts or stages of the mantle plume with lower thermal energy. Key Points: Highly heterogeneous Pb isotope compositions of olivine‐hosted melt inclusions are found in the Lijiang picritesGLOSS II‐like subduction sediments are responsible for the enriched mantle 1‐like component in the source of the Emeishan large igneous province (LIP)Thermal variation of the mantle plume controlled the types and distribution of basaltic magmas in the Emeishan LIP [ABSTRACT FROM AUTHOR]

Picrites are potentially near‐primary melts that offer rare insights into the earliest stages of magmatic evolution. However, without detailed investigation of magmatic processes, robust records of early melt evolution and mantle melting behavior cannot be acquired. Here petrological and geochemical interrogations of the Dali picrites were conducted to investigate the magmatic processes. Compositional variation observed in the Dali picrites is dominated by variable accumulation of a wehrlitic assemblage, transported by a low MgO carrier melt. Groundmass clinopyroxenes were produced by direct crystallization of the carrier melt. In contrast, compositions of olivine‐hosted melt inclusions and clinopyroxene macrocrysts indicate that olivine and clinopyroxene macrocrysts crystallized from diverse melt batches that are genetically related to the carrier melt, which formed as a consequence of magma mixing. The compositional range of melts in equilibrium with clinopyroxene macrocrysts with Mg# < 88 (> 88) is similar to (notably smaller than) the range of melt inclusions hosted in olivine macrocrysts with equivalent forsterite values. This observation can be explained by coupling between major and trace elements during partial melting in the mantle. Overall, a process involving periodic magma replenishment, tapping, and fractional crystallization in deep magma chamber(s) can explain the compositional variations recorded in the Dali macrocryst assemblage. Close genetic relationships between macrocrysts and carrier melts appear to be common to many primitive mafic rocks, which may reflect common deep magma chamber processes whereby diverse mantle‐derived melts are injected and evolved, and the primitive macrocrysts thus formed are subsequently transported by mixed liquids, producing macrocryst‐bearing primitive mafic rocks. Key Points: Compositional variation in the Dali picrites is dominated by variable accumulation of olivine and clinopyroxene macrocrystsThe macrocrysts crystallized from multiple parental magmas and are genetically related to the carrier meltPeriodic RTX processes in the magma chamber(s) dominate the compositional evolution of the diverse parental melts [ABSTRACT FROM AUTHOR]