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The compositions of chromitites and dunites from Moho transition zone (MTZ) of the Coto block of the Zambales ophiolite, Philippines, are used to investigate the geodynamic transition from anhydrous to hydrous magmatism during subduction initiation (SI). Chromite grains in the chromitites have Cr# values [100 × Cr/(Cr + Al)] and TiO2 contents ∼35–50 and 0.05–0.30 wt.%, respectively, intermediate between those of chromite in typical MORB‐like lavas (Cr#, ∼20–60; TiO2, ∼0.6–1.7 wt.%) and boninites (Cr#, ∼70–85; TiO2, <0.4 wt.%). Olivine grains in the dunites have δ7Li values varying from ∼−2‰ to +21‰ with most between +10‰ and +15‰, beyond that of normal mantle (+4 ± 2‰) but comparable to those of some arc lavas (up to +12‰). The data set indicates that parental magmas of the high‐Al chromitites originated from hydrated harzburgitic mantle sources and formed temporally between MORB‐like and boninitic magmatism during SI, resulting from the early stage of flux melting in the Zambales proto‐forearc mantle. Modeling of Li diffusion reveals that the MTZ cooled down at a minimum rate of 0.1°C/yr in order to preserve the large δ7Li variation of olivine in the dunites, comparable to the thermal conditions below ultra‐slow to slow spreading ridges. Such a stage of transitional magmatism, although displaying notable slab contributions, took place at a sluggish period of slab rollback and asthenospheric upwelling, leading to a trough level of heat flow and magma production during the entire course of SI. Plain Language Summary: Subduction initiation (SI) is a prerequisite for starting plate movement. It is featured by rollback of subducted oceanic slabs in the mantle and upward flow of deep mantle materials into the shallow mantle wedges, resulting in mid‐ocean ridge‐like spreading settings above the retreating slabs and mid‐ocean ridge basalt‐like (MORB‐like) lavas. Heated by surrounding hot mantle, slabs gradually release fluids into the overlying mantle, inducing H2O‐rich magmatism such as boninitic ones. Although MORB‐like and boninitic lavas are accepted to originate from H2O‐poor and H2O‐rich mantle sources, respectively, few details were revealed on how transition between the two contrasting types of magmatism is achieved. This work did chemical analyses and modeling on some SI‐related rocks that show affinity to transitional MORB‐boninitic magmatism. Our results show the transitional magmas originated from mantle sources chemically similar to those of boninites, but formed under cooler conditions than both MORB‐like and boninitic lavas. Development of such transitional magmatism reveals that the sources of SI‐related magmas became increasingly rich in fluids, and there was a cooling period between the MORB and boninitic magmatism, possibly due to slow slab rollback and sluggish upwelling of deep hot mantle below the SI‐induced spreading centers during the transitional period. Key Points: Parental magmas of high‐Al chromitites were derived from hydrous harzburgitic mantleHigh‐Al chromitites formed at the early stage of flux melting during subduction initiationThe δ7Li variation of olivine in dunites reveals a period of sluggish slab rollback [ABSTRACT FROM AUTHOR]

The Acoje block of the Zambales ophiolite in the Northwest Philippines is a remnant of forearc lithosphere that formed during an Eocene subduction initiation event in the Western Pacific Ocean. It hosts abundant high-Cr chromitites in the uppermost part of the mantle sequence and the overlying dunitic Moho transition zone. The host harzburgites of the uppermost mantle sequence vary upward from Cpx-rich to Cpx-poor varieties. Clinopyroxene grains in both types of harzburgites and the dunites have prevalent finger-like protrusions and diopsidic compositions, indicative of melt impregnation. From the Cpx-rich harzburgites to Cpx-poor harzburgites and dunites, the clinopyroxene grains display increasingly depleted major element and rare earth element (REE) compositions, e.g., Al 2 O 3 contents from 5.15 wt% to 0.84 wt% and Yb concentrations from ~ 0.2 ppm to 1.5 ppm. These changes are compatible with a transition from MORB to boninitic affinity, Olivine grains in the dunites and sparsely disseminated chromitites vary widely in δ7Li values, primarily from ~ +1 to +10. Such a δ7Li range implies that the parental magmas of the high-Cr chromitites were derived from mantle sources that were modified by slab fluids and had Li isotopic signatures either similar to, or heavier than, the normal mantle range (+4 ± 2). Partial melting in the mantle sources was triggered by heating of previously hydrated Cpx-rich harzburgites during slab rollback and subsequent asthenospheric upwelling. The upwelling brought asthenospheric materials and high heat flux into the lithospheric mantle, facilitating formation of high-Ca boninitic magmas and high-Cr chromitites in the nascent mantle wedge. The different Li isotopic domains in the mantle sources of the high-Cr chromitites indicate that the mantle temperatures were elevated rapidly after the hydration event in order to generate the boninitic magmas. Unlabelled Image • Olivine grains in the Acoje chromite deposit have large Li isotopic variation. • Mantle sources of the high-Cr chromitites were variably modified by slab fluids. • Asthenospheric upwelling facilitated the high-Cr chromite mineralization. [ABSTRACT FROM AUTHOR]

• Parental magmas of all Acoje chromitites originated from harzburgitic sources. • The Acoje peridotites had reacted with parental magmas of different chromitites. • The Acoje mantle wedge was infiltrated by tiny amounts of slab-derived fluids. • Coexistence of high-Al and high-Cr chromitites evidences subduction initiation. The Acoje block of the Eocene Zambales ophiolite (NW, Philippines) is a fragment of forearc lithosphere in the Western Pacific and hosts a world-class chromite deposit. The compositions of chromitites, dunites and harzburgites in the block are used to explore origin of ophiolitic chromite deposits during subduction initiation. Chromite grains in these rocks have a large range of Cr# [100 × Cr/(Cr + Al)]. Those in the cpx-rich harzburgites have the lowest Cr# range (15–35). Those in the chromitites have a bimodal Cr# distribution, ~45–48 and 73–76, confirming presence of both high-Al and high-Cr chromitites in the block. Those in the cpx-poor harzburgites and dunites have similar Cr#s, ranging from ~35 to 85. The TiO 2 contents of chromite grains are generally <0.25 wt% and increase from the cpx-rich harzburgites to cpx-poor harzburgites, dunite and chromitites. Olivine grains in all the rocks have forsteritic compositions (Fo, 90–93) and δ18O values consistent with that of the upper mantle (+5.18 ± 0.20). The oxygen fugacities (f O 2) of the harzburgites, dunites and chromitites range from ~−0.5 log unit below the fayalite-magnetite-quartz (FMQ) buffer to ~1.5 log unit above the buffer, generally comparable to or slightly higher than the range of abyssal peridotites. The dataset reveals that parental magmas of the high-Cr chromitites have boninitic affinities, whereas those of the high-Al chromitites have compositions intermediate between MORB and boninitic lavas. Both types of chromitites originated from mantle sources that may have moderately refractory compositions equivalent to residues after ~10% melting of fertile MORB mantle, accounting for the low TiO 2 contents of chromite in the chromitites. The Acoje mantle wedge was infiltrated by small amounts of slab-derived fluids, which were insufficient to elevate the f O 2 values of peridotites but notably facilitated partial melting in mantle sources of the chromitites. The Acoje mantle sequence was modified by parental magmas of both the high-Al and high-Cr chromitites, causing two different chemical variation trends of peridotites. Peridotites and chromitites in the Acoje mantle sequence record evolving magmatism changing from early MORB-like to later boninitic features, comparable to those developed in the infant IBM arc system, and relevant information can be used for reconstructing the geodynamic evolution of subduction initiation. [ABSTRACT FROM AUTHOR]

Sea level variations in the tropical Pacific Ocean (TPO) exhibit the prominent periods of 30 and 52months on interannual timescale, and the interannual variance illustrates the geographical distribution over the 1993–2010 period. The quasi-biennial amplitude is comparable to the seasonal variation in the east off the Philippine coast (EOP). The Central Pacific El Niño (CP El Niño) events have occurred frequently and comparably in the satellite altimetry era compared with the Eastern Pacific El Niño (EP El Niño), and the sea level varies closely related to the two types of El Niño events. A see–saw mode of sea level is dominant during the EP El Niño event, but a “shuttle” pattern towers mainly in the central equatorial Pacific during the CP El Niño events. The oceanic and atmospheric processes interpret the sea level evolution during the CP El Niño and the EP El Niño events. Sea level variations in the EOP are impacted by the local and remote responses of atmospheric wind fields. The remote effect caused by the westward Rossby wave dominates in the atmospheric contribution which is nearly twice the magnitude of the local Ekman pumping effect. The local wind contributes to sea level evolution primarily in the beginning of the El Niño events. The remote process of the Rossby wave persists in 35% of the three CP El Niño phases, while the predicted skill distribution, about 43—53%, maps geographical variability in the EP El Niño event. [ABSTRACT FROM AUTHOR]