minerals Article Geochemical Constraints on Mantle Melting and Magma Genesis at Pohnpei Island, Micronesia Tong Zong 1 , Zheng-Gang Li 2,* , Yan-Hui Dong 2, Xu-Ping Li 1 , Ji-Hao Zhu 2, Ling Chen 2 and Ji-Qiang Liu 2 1 Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China; [email protected] (T.Z.); [email protected] (X.-P.L.) 2 Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China; [email protected] (Y.-H.D.); [email protected] (J.-H.Z.); [email protected] (L.C.); [email protected] (J.-Q.L.) * Correspondence: [email protected] Received: 30 June 2020; Accepted: 13 September 2020; Published: 16 September 2020 Abstract: The lithospheric mantle is of paramount importance in controlling the chemical composition of ocean island basalts (OIBs), influencing partial melting and magma evolution processes. To improve the understanding of these processes, the pressure–temperature conditions of mantle melting were investigated, and liquid lines of descent were modelled for OIBs on Pohnpei Island. The studied basaltic samples are alkalic, and can be classified as SiO2-undersaturated or SiO2-saturated series T rocks, with the former having higher TiO2 and FeO contents but with no distinct trace-element composition, suggesting melting of a compositionally homogenous mantle source at varying depths. Both series underwent sequential crystallization of olivine, clinopyroxene, Fe–Ti oxides, and minor plagioclase and alkali feldspar. Early magnetite crystallization resulted from initially high FeOT contents and oxygen fugacity, and late feldspar crystallization was due to initially low Al2O3 contents and alkali enrichment of the evolved magma. The Pohnpei lavas formed at estimated mantle-melting temperatures of 1486–1626 C (average 1557 43 C, 1σ), and pressures of 2.9–5.1 GPa ◦ ± ◦ (average 3.8 0.7 GPa), with the SiO -undersaturated series forming at higher melting temperatures ± 2 and pressures. Trace-element compositions further suggest that garnet rather than spinel was a residual phase in the mantle source during the melting process. Compared with the Hawaiian and Louisville seamount chains, Pohnpei Island underwent much lower degrees of mantle melting at greater depth, possibly due to a thicker lithosphere. Keywords: lithospheric mantle; mantle melting; fractional crystallization; OIB-type lavas; Pohnpei Island 1. Introduction Ocean island basalts (OIBs) are widely considered to be produced by deep-seated thermal mantle plumes from the core–mantle boundary [1] or a shallower mantle transition zone [2], and their chemical compositions have commonly been used to constrain the chemistry of mantle sources. However, determination of the chemical composition of the mantle source is not straightforward, due to overprinting by mantle melting [3], melt–lithosphere interaction [4–6], and fractional crystallization [7]. Variations in lithospheric thickness are thus of paramount importance in controlling the chemical composition of OIBs; although, this is not well understood. The Caroline Islands in Micronesia were formed on a thick, ancient Pacific lithosphere at 162–153 Ma [8], and are associated with the Caroline hotspot [9–13]. Three of these volcanic Minerals 2020, 10, 816; doi:10.3390/min10090816 www.mdpi.com/journal/minerals Minerals 2020,, 10,, 816x FOR PEER REVIEW 2 of 18 to the Caroline Plateau [13]. Jackson et al. [10] suggested that the Caroline Islands were formed by islands—Chuuk,partial melting of Pohnpei, a compositionally and Kosrae—display homogeneous progressive mantle age source, variations, with anda small are spatiotemporally contribution of relatedrecycled to oceanic the Caroline crust. Plateau However, [13 ].the Jackson effect etof al.thic [10k ]lithospheric suggested that mantle the Carolineon the chemical Islands werecomposition formed byof Caroline partial melting Island lavas of a compositionally has not yet been homogeneous explored. In this mantle study, source, we undertook with a small whole-rock contribution major- of recycledand trace-element, oceanic crust. and However,in situ chemical the eff analysesect of thick of lithosphericvolcanic lavas mantle from Pohnpei on the chemical Island, with composition the aim of Carolineexploring Island the lavaseffects has of not fractional yet been crystallization explored. In this and study, mantle we undertookmelting on whole-rock their chemical major- andcompositions. trace-element, and in situ chemical analyses of volcanic lavas from Pohnpei Island, with the aim of exploring the effects of fractional crystallization and mantle melting on their chemical compositions. 2. Geological Background 2. Geological Background The threethree volcanic volcanic islands, islands, Pohnpei, Pohnpei, Chuuk, Chuuk, and Kosrae,and Kosrae, are associated are associated with the Carolinewith the Seamount Caroline ChainSeamount connected Chain toconnected the western to the Caroline western Plateau Caroline (Figure Plateau1), which (Figure developed 1), which at developed 23.9–8.13 Maat 23.9–8.13 [ 13–15] afterMa [13–15] formation after of formation the Caroline of the Plateau, Caroline and Plateau, their ages and decrease their ages from decrease west to from east west (since to theeast middle (since Cenozoic):the middle Chuuk,Cenozoic): 14.8–4.3 Chuuk, Ma; 14.8–4.3 Pohnpei, Ma; 8.7–0.92 Pohnpei, Ma; and8.7–0.92 Kosrea, Ma; 2–1 and Ma Kosrea, [16]. They 2–1 Ma are [16]. considered They are to beconsidered derived fromto be aderived common from mantle a common source mantle related source to the deep-rootedrelated to the Caroline deep-rooted hotspot, Caroline which hotspot erupted, throughwhich erupted an old (162–153through Ma)an old portion (162–153 of the Ma) Pacific portion Ocean of the crust Pacific with aOcean relatively crust thick with lithosphere a relatively [ 8thick–11]. Thelithosphere estimated [8–11]. lithosphere–asthenosphere The estimated lithosphere–asthen boundary depthosphere for theseboundary islands depth is ~96 for km these [17 ],islands thicker is than ~96 thosekm [17], of the thicker Caroline than Plateau those [of13 the] and Caroline Hawaiian Plateau Islands [13] (88–90 and km) Hawaiian [17]. The Islands3He/4 He(88–90 ratios km) (7.6–12.8 [17]. The Ra) of3He/ basalts4He ratios from (7.6–12.8 these islands Ra) of indicate basalts afrom deep these mantle islands source indicate [10,13 ,a16 deep], possibly mantle related source to [10,13,16], recycled oceanicpossibly componentsrelated to recycled [10]. However,oceanic components Caroline lavas [10]. However, do not have Caroline the high lavas3He do/4 notHe have compositions the high that3He/4 areHe characteristiccompositions of that Hawaii, are characteristic Iceland, and Samoaof Hawaii, lavas Iceland, among others,and Samoa possibly lavas because among they others, are morepossibly degassed, because or they higher are U–Th more concentrationsdegassed, or higher were maintainedU–Th concentrations in their mantle were source maintained over geologic in their timemantle [10 source]. The absenceover geologic of newer time volcanism [10]. The near absence Kosrae, of newer the youngest volcanism island, near suggests Kosrae, thethe activityyoungest of theisland, Caroline suggests hotspot the activity has decreased of the Caroline or, more hots likely,pot ceased has decreased [13,16]. or, more likely, ceased [13,16]. Figure 1. ((a)) Bathymetric Bathymetric map of the the Caroline Caroline Seamount Ch Chainain showing the major tectonic features of the seamount chain, CarolineCaroline Plateau,Plateau, and adjacentadjacent areas.areas. (b) The sampling locations on the Pohnpei Island. TheThe abbreviationsabbreviations P, P, S, AWAK,S, AWAK, OHWA, OHWA, WF, and WF, NAN and representNAN represent sampling sampling stations, asstations, described as indescribed Table1. in Table 1. Pohnpei Island (6°54’ (6◦540 NN lat,lat, 158158°14’◦140 EE long)long) isis roughlyroughly circularcircular inin shape,shape, almostalmost completely surrounded by by a abarrier barrier reef, reef, and and is characterized is characterized by extensive by extensive dike systems dike systems and columnar and columnar jointing jointing[14–16]. [Lavas14–16 ].exposed Lavas exposed on Pohnpei on Pohnpei comprise comprise alkali alkalibasalts, basalts, trachytes, trachytes, hawaiites, hawaiites, ankramites, ankramites, and andnephlinites/basanites nephlinites/basanites [16]. They [16]. can They be subdivided can be subdivided into three intostages three based stages on chemical based composition: on chemical composition:the main lava, the transitional main lava, lava, transitional and nephelinite/ba lava, and nephelinitesanite series/basanite [14,18]. series The [14 former,18]. The two former represent two shield-building stages of volcanism, and the nephelinite/basanite series represents overlying post- Minerals 2020, 10, 816 3 of 18 represent shield-building stages of volcanism, and the nephelinite/basanite series represents overlying post-shield deposits [14]. The main lava series comprises basalt, hawaiite, mugearite, and trachyte, with hawaiite being the most abundant and containing phenocrysts of olivine, clinopyroxene, and rare magnetite [16]. The transitional lava series contains olivine, clinopyroxene, and plagioclase phenocrysts. The nephelinite/basanite series is SiO2-undersaturated and distinguished from the main lava series by a narrow range of higher more- to less-incompatible