Abstract. Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts.
The Hess Deep Rift, where the Cocos Nazca Ridge propagates into the young, fast-spread East Pacific Rise crust, exposes a dismembered, but nearly complete, lower crustal section. The extensive exposures of the plutonic crust were drilled at 3 sites during ODP Leg 147 (Nov. 1992-Jan. 1993) and IODP Expedition 345 (Dec. 2012-Feb. 2013). We report preliminary results of a bulk rock geochemical study (major and trace elements) carried out on 109 samples representative of the different drilled lithologies. The shallowest gabbroic rocks were sampled at ODP Site 894. They comprise gabbronorite, gabbro, olivine gabbro and gabbronorite. They have evolved compositions with Mg# 39-55, Yb 4-8 x chondrite and Eu/Eu* 1-1.6. Olivine gabbro and troctolite were dominant at IODP Site U1415, with minor gabbro, gabbronorite and clinopyroxene oikocryst-bearing troctolite and gabbro. All U1415 gabbroic rocks have primitive compositions except for one gabbronorite rubble that is similar in composition to the shallow gabbros. Olivine gabbro, gabbro and gabbronorite overlap in composition: they have high Mg# (79-87) and Ni (130-570 ppm), low TiO2 (0.1-0.3 wt.%) and Yb (1.3-2.3 x chondrite) and positive Eu anomaly (Eu/Eu*=1.9-2.7). Troctolite has high Mg# (81-89), Ni (260-1500 ppm) and low TiO2 ( 4). ODP Site 895 recovered sequences of highly depleted harzburgite, dunite and troctolite (Yb down to <0.1xchondrite) that are interpreted as a mantle-crust transition zone. Basalts were recovered at Sites 894 and U1415: they have low Yb (0.5-0.9xN6MORB) and are depleted in the most incompatible elements (Ce/Yb=0.6-0.9xN-MORB). The main geochemical characteristics of Site U1415 and 894 gabbroic rocks are consistent with formation as a cumulate sequence from a common parental MORB melt; troctolites are the most primitive end-member of this sequence. They overlap in composition with the most primitive of slow and fast spread crust gabbroic rocks.
Whole-rock chemical compositions are significant for igneous petrology, especially volcanic rock studies. ICP-MS is widely used as a conventional method to determine the trace-element compositions using solutions prepared by digestion of rocks. In the past decades, LA-ICP-MS technique has been well developed to measure whole-rock compositions by using pressed powdered pellet or fused glass of rock samples. In this study, trace-element concentrations (Li, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, U and REEs) of basaltic and andesitic rocks were determined by using flux-free fused glasses and LA-ICP-MS. The fused glass was prepared by the direct fusion method where the powdered rock sample is melted with an iridium-strip heater. The heating temperature was manually controlled by monitoring the current and the heating lasted less than 30 sec to achieve melting. We evaluated our routine analytical work performed by measurements of 3 random target positions in each fused glass. The data accuracy was evaluated by measurement of 11 fused glasses created from USGS and GSJ international reference materials (BCR-2, BHVO-2, BIR-1a, JB-2, JA-1 and JGb-1). The homogeneity of our fused glasses is represented by less than 5% relative standard deviation (1σ) in most elements. The mean values of most elements are in agreement with the reference values within 10% deviation. However, Pb concentration cannot be reproduced because of strong volatilization during the melting, and Lu, Hf and Ta are occasionally enriched by the elemental contamination/interference from the iridium-strip heater. 9 fused glasses created from selected petrological samples, such as the Ogasawara volcanic rocks and Oman diopsidite, were also measured to examine our routine measurement work and glass preparation method for a wider range of the trace-element abundance, especially the Cr abundance. The analytical results of 20 fused glasses demonstrate that our method is capable of determining the whole-rock trace-element compositions for petrological study.
