Bulk-rock Major and Trace Element Compositions of Abyssal Peridotites: Implications for Mantle Melting, Melt Extraction and Post-melting Processes Beneath Mid-Ocean Ridges

This paper presents the first comprehensive major and traceelement data for 130 abyssal peridotite samples from the Pacificand Indian ocean ridge–transform systems. The data revealimportant features about the petrogenesis of these rocks, mantlemelting and melt extraction processes beneath ocean ridges,and elemental behaviours. Although abyssal peridotites are serpentinized,and have also experienced seafloor weathering, magmatic signaturesremain well preserved in the bulk-rock compositions. The betterinverse correlation of MgO with progressively heavier rare earthelements (REE) reflects varying amounts of melt depletion. Thismelt depletion may result from recent sub-ridge mantle melting,but could also be inherited from previous melt extraction eventsfrom the fertile mantle source. Light REE (LREE) in bulk-rocksamples are more enriched, not more depleted, than in the constituentclinopyroxenes (cpx) of the same sample suites. If the cpx LREErecord sub-ridge mantle melting processes, then the bulk-rockLREE must reflect post-melting refertilization. The significantcorrelations of LREE (e.g. La, Ce, Pr, Nd) with immobile highfield strength elements (HFSE, e.g. Nb and Zr) suggest thatenrichments of both LREE and HFSE resulted from a common magmaticprocess. The refertilization takes place in the ‘cold’thermal boundary layer (TBL) beneath ridges through which theascending melts migrate and interact with the advanced residues.The refertilization apparently did not affect the cpx relicsanalyzed for trace elements. This observation suggests grain-boundaryporous melt migration in the TBL. The ascending melts may notbe thermally ‘reactive’, and thus may have affectedonly cpx rims, which, together with precipitated olivine, entrappedmelt, and the rest of the rock, were subsequently serpentinized.Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios areobserved, which are unexpected. The correlation between thetwo ratios is consistent with observations on basalts that D_Zr/D_Hf< 1 and D_Nb/D_Ta < 1. Given the identical charges (5⁺ forNb and Ta; 4⁺ for Zr and Hf) and essentially the same ionicradii (R_Nb/R_Ta = 1·000 and R_Zr/R_Hf = 1·006–1·026),yet a factor of 2 mass differences (M_Zr/M_Hf = 0·511 andM_Nb/M_Ta = 0·513), it is hypothesized that mass-dependentD values, or diffusion or mass-transfer rates may be importantin causing elemental fractionations during porous melt migrationin the TBL. It is also possible that some ‘exotic’phases with highly fractionated Zr/Hf and Nb/Ta ratios may existin these rocks, thus having ‘nugget’ effects onthe bulk-rock analyses. All these hypotheses need testing byconstraining the storage and distribution of all the incompatibletrace elements in mantle peridotite. As serpentine containsup to 13 wt % H₂O, and is stable up to 7 GPa before it is transformedto dense hydrous magnesium silicate phases that are stable atpressures of 5–50 GPa, it is possible that the serpentinizedperidotites may survive, at least partly, subduction-zone dehydration,and transport large amounts of H₂O (also Ba, Rb, Cs, K, U, Sr,Pb, etc. with elevated U/Pb ratios) into the deep mantle. Thelatter may contribute to the HIMU component in the source regionsof some oceanic basalts. KEY WORDS: abyssal peridotites; serpentinization; seafloor weathering; bulk-rock major and trace element compositions; mantle melting; melt extraction; melt–residue interaction; porous flows; Nb/Ta and Zr/Hf fractionations; HIMU mantle sources     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

福建新生代碱性超基性火山岩地球化学特征及构造意义

为了解福建新生代地幔性质和大地构造背景,对柳城玻基辉橄岩进行系统的年代学和元素地球化学研究。岩石为碱性超基性岩,形成于12.6Ma,富Mg、Fe和贫碱性质,以富集LILE、HFSE、Nb与Ta正异常为特征,强烈富集LREE,相对亏损HREE,(La/Yb)N＝35.1~36.2,δEu＝0.98~1.00,微量元素特征与洋岛玄武岩(OIB)类似。微量元素地球化学特征表明,该岩石是地幔橄榄岩低度部分熔融的产物,成岩过程没有地壳混染;源区地幔具有HIMU性质,由软流圈地幔、脱水洋壳与残留岩石圈地幔混合组成,表现出富集的特征;柳城玻基辉橄岩形成于大陆裂谷环境。与新生代玄武岩进行对比研究表明,中国东南部地幔组成存在多个地幔端元成分。     

The Origin of HIMU in the SW Pacific: Evidence from Intraplate Volcanism in Southern New Zealand and Subantarctic Islands

This paper presents field, geochemical and isotopic (Sr, Nd,Pb) results on basalts from the Antipodes, Campbell and ChathamIslands, New Zealand. New ⁴⁰Ar/³⁹Ar age determinations alongwith previous K–Ar dates reveal three major episodes ofvolcanic activity on Chatham Island (85–82, 41–35,5 Ma). Chatham and Antipodes samples comprise basanite, alkaliand transitional basalts that have HIMU-like isotopic (²⁰⁶Pb/²⁰⁴Pb>20·3–20·8, ⁸⁷Sr/⁸⁶Sr <0·7033,¹⁴³Nd/¹⁴⁴Nd >0·5128) and trace element affinities(Ce/Pb 28–36, Nb/U 34–66, Ba/Nb 4–7). Thegeochemistry of transitional to Q-normative samples from CampbellIsland is explained by interaction with continental crust. Thevolcanism is part of a long-lived (100 Myr), low-volume, diffusealkaline magmatic province that includes deposits on the Northand South Islands of New Zealand as well as portions of WestAntarctica and SE Australia. All of these continental areaswere juxtaposed on the eastern margin of Gondwanaland at >83Ma. A ubiquitous feature of mafic alkaline rocks from this regionis their depletion in K and Pb relative to other highly incompatibleelements when normalized to primitive mantle values. The inversionof trace element data indicates enriched mantle sources thatcontain variable proportions of hydrous minerals. We proposethat the mantle sources represent continental lithosphere thathost amphibole/phlogopite-rich veins formed by plume- and/orsubduction-related metasomatism between 500 and 100 Ma. Thestrong HIMU signature (²⁰⁶Pb/²⁰⁴Pb >20·5) is consideredto be an in-grown feature generated by partial dehydration andloss of hydrophile elements (Pb, Rb, K) relative to more magmaphileelements (Th, U, Sr) during short-term storage at the base ofthe lithosphere. KEY WORDS: continental alkaline basalts; lithospheric mantle, mantle metasomatism; New Zealand; OIB, HIMU; Sr, Nd and Pb isotopes; West Antarctica     

Petrology and Geochemistry of Intraplate Basalts in the South Auckland Volcanic Field, New Zealand: Evidence for Two Coeval Magma Suites from Distinct Sources

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)_N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)_N (12–47). GroupA has slightly higher ⁸⁷Sr/⁸⁶Sr, similar _Nd, and lower ²⁰⁶Pb/²⁰⁴Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high ²³⁸U/²⁰⁴Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources     

Tectonomagmatic origin of some volcanic and sub-volcanic rocks from the Lower Benue rift, Nigeria

Petrological and geochemical studies on some volcanic and sub-volcanic rocks from the Lower Benue rift indicate that they are basalts, basaltic and doleritic sills, trachybasalt and trachyte which generally belong to the alkali basalt series. The alkaline affinity is clearly evident in both their normative and modal mineral compositions, as well as their chemical compositions. The generally high fractionation indices [(La/Yb)N] are 7.06 to 17.65 for the basaltic rocks and 23.59 to 135. 35 for the trachytic ...     

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia,South America

Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO₂; 4.3–5.9 wt.% Na₂O+K₂O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO₂; 4.0–4.4 wt.% Na₂O+K₂O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.     

HIMU型洋岛玄武岩的地球化学特征

    洋岛玄武岩的元素和同位素地球化学特征可以示踪深部地幔的化学结构和化学演化过程。HIMU(Highμ,μ=238U/204Pbt=0) 型玄武岩是一类元素和同位素组成特殊的洋岛玄武岩,被认为与地幔柱中再循环的洋壳物质直接相关,因此,HIMU型玄 武岩的成因是地幔柱（热点） 研究中长期关注的话题。本文概述了HIMU型玄武岩的地球化学定义,对HIMU洋岛的分布、 火山演化阶段以及岩性变化做了综合阐述,并在对比经典HIMU型玄武岩与其他板内玄武岩元素地球化学特征、放射成因 同位素组成以及惰性气体同位素组成特征的基础上,简要探讨其源区组成和成因上的不同。     

Evolution of Mount Cameroon volcanism: Geochemistry,mineral chemistry and radiogenic isotopes (Pb,Sr, Nd)

Mount Cameroon volcano has erupted several times in the 20th Century with documented eruptions in 1909, 1922, 1954, 1959, 1982, 1999 and 2000. Evidence of historic volcanism is represented by several older lava flows and lahar deposits around the flanks of the volcano. This study aims to assess the evolution of Mount Cameroon volcanism through its eruptive history via interpretation of mineralogical, whole rock geochemical and Pb, Sr, Nd isotope data generated from historic and recent lava flows. In this study, samples were collected from the 1959, 1982, 1999 and 2000 eruptions and from several historic eruption sites with unknown eruption dates.Evaluation of major and trace element data demonstrates that Mount Cameroon is geotectonically associated with within-plate Ocean Island Basalt Settings. More than 90% of the studied historic lavas (n ​= ​29) classify as tephrites and basanites whereas the modern lavas (n ​= ​38) are predominantly trachybasalts, demonstrating evolution from primitive to evolved lavas over time typically in response to fractional crystallization. Petrographically, the lavas are porphyritic with main mineral phases being olivine, clinopyroxene, plagioclase feldspars and Fe–Ti–Cr oxides. The 1982 lavas are predominantly aphyric and dominated by lath-shaped flow-aligned plagioclase in the groundmass. Olivine chemistry shows variable forsterite compositions from Fo_60–89. Clinopyroxenes vary from diopside through augite to titanaugite with chemical composition ranges from Wo₄₅En₃₂Fs₇ to Wo₅₁En₄₇Fs₁₇. Plagioclase feldspars vary from labradorite (An_56–70) to bytownite (An_80–87). For the Fe–Ti–Cr oxides, calculated ulvöspinel component shows a wide variation from ulv_38–87. CIPW-normative classification on the Di-Ol-Hy-Qz-Ne system shows that all Mount Cameroon lavas are nepheline-normative (Ne ranges from 4.20 wt.% to 11.45 ​wt.%).Radiogenic isotope data demonstrate that Mount Cameroon lavas are HIMU (or high μ ​= ​²³⁸U/²⁰⁴Pb), characterized by ²⁰⁶Pb/²⁰⁴Pb ​= ​20.19–20.46, ²⁰⁷Pb/²⁰⁴Pb ​= ​15.63–15.69, ²⁰⁸Pb/²⁰⁴Pb ​= ​40.01–40.30, ⁸⁷Sr/⁸⁶Sr ​= ​0.70322–0.70339 (ε_Sr ​= ​−21.37 to −18.96) and ¹⁴³Nd/¹⁴⁴Nd ​= ​0.51276–0.51285 (ε_Nd ​= ​+2.29 to +4.05). The historic lavas show stronger HIMU signature relative to the modern lavas, suggesting evolution towards less HIMU signatures over time. This study has revealed that Mount Cameroon volcanism has evolved from primitive magmas characterized by stronger HIMU signatures with high ^206/204Pb and ^208/204Pb isotopes, low SiO₂ and high Mg, Ni, Cr content towards lower HIMU signatures with relatively higher SiO₂, lower Mg, Cr and Ni compositions. The geochemical and isotopic changes, which account for the evolution of magmatism on Mount Cameroon occur over long periods of time because all the modern lavas erupted within the last 100 years are isotopically homogeneous, with very limited variation in SiO₂ compositions.     

Tectonomagmatic origin of some volcanic and sub-volcanic rocks from the Lower Benue rift,Nigeria

Petrological and geochemical studies on some volcanic and sub-volcanic rocks from the Lower Benue rift indicate that they are basalts, basaltic and doleritic sills, trachybasalt and trachyte which generally belong to the alkali basalt series. The alkaline affinity is clearly evident in both their normative and modal mineral compositions, as well as their chemical compositions. The generally high fractionation indices [(La/Yb)N] are 7.06 to 17.65 for the basaltic rocks and 23.59 to 135. 35 for the trachytic rocks, against low values commonly seen in subalkaline (tholeiitic) series, with strong enrichments in the incompatible elements. All this strongly supports their alkaline affinity. The basaltic rocks are generally fine-grained and porphyritic, consisting of phenocrysts of clinopyroxene and olivine in the groundmass of the same minerals together with plagioclase. The clinopyroxene is either diopside or clinoenstatite. The trachyte consists of oligoclase, orthoclase, biotite, quartz and exhibits typical trachytic, flow structure. The basaltic and doleritic sills are commonly altered, with calcite and epidote as common alteration prod-ucts. This alteration, which is reflected in the erratic behaviour of K2O, MnO and P2O5 on Harker variation diagrams, high values of LOI, strong depletions in the more mobile LILE (Rb, K, Ba and Sr) and high Th/Ta ratios, is attributed to the effects of an aqueous fluid phase and crustal contamination. On the whole, the mineralogical, as well as major-, trace-elements and REE data suggest that the rocks are co-genetic and most likely derived from differentiation of an alkali olivine-basalt magma, generating through variable low degrees of partial melting of probably an enriched lithospheric (upper) mantle following an asthenospheric uplift (mantle plume or intumescence) with HIMU signa-tures in a within-plate continental rift tectonic setting. This corroborates earlier results obtained for the intrusive rocks in the region.