Geochemical studies revealed two major high-and low-Ti magmatic series composing 22 the Manihiki Plateau in the Western Pacific. Here we report new geochemical data on major 23 and trace element and Sr-Nd-Pb isotope compositions. The rocks belong to the previously 24 rarely sampled high-Ti Manihiki series and represent a section of deep crust of the plateau. 25 The rocks were collected by remotely operated vehicle ROV Kiel6000 during R/V SONNE 26 SO225 expedition from a tectonic block at a stretched and faulted boundary between the 27 Northern and Western Manihiki sub-plateaus. The dataset is accompanied by additional data 28 on samples obtained by dredging during the same cruise. Judging from the age of 29 stratigraphically higher lavas most samples appear ≥125 Ma old. They comprise fully 30 crystalline microdolerites, aphyric and Ol-Px-Pl-phyric basalts and breccias metamorphosed 31 under amphibolite and greenschist facies with peak metamorphic conditions 636-677 o C and 32 2.0-2.7 kbar. A single sample of hornblende gabbro was also recovered and likely represents a 33 late stage intrusion. Despite strong metamorphism, the samples from the ROV profile reveal 34 only minor to moderate chemical alteration and their initial compositions are well preserved. 35 The rocks are relatively primitive with MgO up to 13 wt%, range from enriched to depleted in 36 LREE (La N /Sm N =0.7-1.1), exhibit variable but mostly depleted Nb contents (Nb/Nb*=0.8-37 1.3) and display only a narrow range in isotope compositions with strong EM1 signature 38 (Nd(t)=1.8-3.6, 206 Pb/ 204 Pb (t)=17.9-18.1, 207 Pb/ 204 Pb (t)=15.49-15.53 , 208 Pb/ 204 Pb 39 (t)=38.08-38.42). The parental magmas are interpreted to originate from a thermochemical 40 plume with a potential mantle temperature >1460 o C. The trace element and isotope EM1 41 signature of the high-Ti rocks reflects the presence of recycled lower continental crust 42 material or re-fertilized subcontinental lithospheric mantle in the plume source. A highly 43 refractory mantle was the primary source of the low-Ti basalts and could also contribute to 44 the origin of high-Ti basalts. On average a more depleted mantle source for the Manihiki 45 rocks can explain ~30% lower crustal thickness of this plateau compared to Ontong Java 46 Plateau, which was mainly formed by melting of similarly hot but more fertile mantle. The 47 presently available data suggest that the sources of Ontong Java and Manihiki Plateaus were 48 compositionally different and could represent two large domains of a single plume or two 49 contemporaneous but separate plumes. 50
The Ontong Java and Manihiki oceanic plateaus are believed to have formed through high-degree melting of a mantle plume head. Boninite-like, low-Ti basement rocks at Manihiki, however, imply a more complex magma genesis compared with Ontong Java basement lavas that can be generated by ∼30% melting of a primitive mantle source. Here we show that the trace element and isotope compositions of low-Ti Manihiki rocks can best be explained by re-melting of an ultra-depleted source (possibly a common mantle component in the Ontong Java and Manihiki plume sources) re-enriched by ≤1% of an ocean-island-basalt-like melt component. Unlike boninites formed via hydrous flux melting of refractory mantle at subduction zones, these boninite-like intraplate rocks formed through adiabatic decompression melting of refractory plume material that has been metasomatized by ocean-island-basalt-like melts. Our results suggest that caution is required before assuming all Archaean boninites were formed in association with subduction processes.
Factors controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H 2 O content, pressure, temperature, oxygen fugacity, and oxygen isotope composition of rhyolites from Askja, Öraefajökull, and Hekla volcanoes. All these parameters correlate with tectonic (rift and off-rift) setting of the volcanoes. Askja rift rhyolites originate through extensive assimilation of high-temperature hydrothermally altered crust (δ 18 O < 2‰) at shallow depths (≥1.8 km). These rhyolites are hot (935-1008°C), relatively dry (H 2 O < 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874-902°C), wet (H 2 O = 4-6.3 wt%), and non-oxidized (~QFM to QFM-1) off-rift rhyolites (Öraefajökull, Hekla) originate through differentiation deeper in the crust (≥4 km) with almost no or little assimilation of high-T, altered crust, as reflected by slightly lower to normal δ 18 O values (5.2-6‰). Although off-rift rhyolites predominate during the Holocene, older silicic rocks on Iceland primarily formed in a rift setting possibly analogous to the oldest continental crust on Earth.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.