中印度洋海盆表层沉积物稀土元素分布特征及富集规律

对中印度洋海盆14个站位的表层沉积物进行了稀土元素(REE+Y,简称REY)分布特征和富集规律研究.结果表明,样品中REY主要富集于沸石黏土和远洋黏土中(稀土元素总量最高为1239×10?6),且明显富集钇(Y)等重稀土元素(Y富集系数高达14.1,重稀土元素和Y富集系数最高为11.6);富稀土沉积物呈明显Ce亏损,发...     

Comparative study on the geochemical characteristics of rare earth elements in deep-sea sediments from different regions of the Pacific OceanSCIEI北大核心CSCD

太平洋深海沉积物中富集稀土元素(REY,包括钇),被认为是富有潜力的新型稀土资源。(含)沸石粘土、深海粘土和多金属软泥是主要的富含REY的沉积物类型,其中(含)沸石粘土和深海粘土在中、西北太平洋海盆大面积分布,而多金属软泥则多分布于靠近东太平洋洋脊热液活动的深海盆地中。目前关于中、西北太平洋海盆的深海粘土和(含)沸石粘土已有较多的研究,但关于多金属软泥中REY的研究较少。不同区域、不同类型深海沉积物中的稀土元素赋存状态有何差异?影响稀土富集的机制又是什么目前尚不清楚,也就进一步影响了对深海沉积物稀土资源的勘查和开发工作。本文分析对比了太平洋不同区域不同类型深海沉积物的地球化学特征及矿物学特征。结果表明,总体上,中、西北太平洋海盆深海沉积物中,尤其是(含)沸石粘土中的REY含量明显高于东太平洋海盆多金属软泥REY含量,其REY的富集主要与磷酸盐有关。超常富集REY(∑REY>2000×10^(-6))的沉积物中的CaO/P_(2)O_(5)比值趋向于一致(~1.4),几乎接近于磷灰石CaO/P2O5比值(~1.3),因此REY主要赋存载体为磷灰石,该区沉积物中REY的富集可能受到磷酸盐化的影响;东太平洋海盆多金属软泥明显受到热液影响,铁和锰的含量明显增加,但其∑REY含量集中于500×10^(-6)~800×10^(-6),不随铁和锰的增加而变化,REY的富集仍与磷酸盐关系密切,而与铁锰物质和铝硅酸盐关系不大。中、西北太平洋海盆富稀土的深海沉积物形成时处于较强的氧化环境,同时又有充足的含磷物质补给,才造成REY在该区沉积物中的超常富集;而东太平洋海盆多金属软泥虽然处于氧化环境,但缺少足够的磷补给,所以其∑REY含量通常低于中、西北太平洋海盆沸石粘土中∑REY含量。     

深海沉积物中的稀土矿产资源研究进展

近年发现,太平洋和印度洋的深海盆地中存在大量富含稀土的深海沉积物。主要类型为多金属软泥、沸石黏土和远洋黏土,其中的全稀土含量(∑REY,∑REE+Y)为400×10-6~2000×10-6,最高可达6600×10-6,重稀土含量(HREE)已达到或超过中国南方离子吸附型矿床的重稀土品位两倍以上,是潜在的新型稀土资源,具有重要的经济价值。目前不少学者对富稀土的深海沉积物进行了大量地球化学及部分矿物学的工作,认为多金属软泥中的稀土元素多赋存于与海底热液作用有关的铁锰氧化物和氢氧化物中,而沸石黏土和远洋黏土中稀土元素的富集则与磷酸盐的混入密切相关,其稀土元素主要存在于与磷灰石成分相当的生物鱼骨屑中。深海黏土的北美页岩标准化稀土配分模式与海水相似,表明其中的稀土元素主要来自于海水,REY富集成矿可能主要受控于磷灰石早期成岩阶段,期间稀土元素未发生分异。尽管近些年对深海沉积物中的稀土元素研究取得了不少成果,但是,对于沉积物中的稀土富集机制及影响因素等问题仍然需要更加深入的研究。作为稀土资源大国,为了争取我国在国际海底稀土资源竞争中的话语权,维护中国的稀土利益,中国应加紧开展相关的稀土资源勘查和潜力评价。     

Geochemical Characteristics of Apatite in Heavy REE‐rich Deep‐Sea Mud from Minami‐Torishima Area,Southeastern Japan

We have conducted geochemical and mineralogical investigations of the rare earth and yttrium (REY)‐rich mud from the Minami‐Torishima area in the Pacific in order to clarify the concentration of REY and their host‐phase in the mud. X‐ray diffraction analysis shows that the mud is mainly composed of phillipsite, fluorapatite, quartz, albite, illite and montmorillonite. Whole‐rock CaO, P₂O₅ and total REY contents of the mud are positively correlated. Relative abundance of apatite is also positively correlated to P₂O₅ and total REY contents. These correlations suggest that apatite is the main host of the P₂O₅ and REY in the mud. We make in situ compositional analyses of constituent minerals in the REY mud. The results show that the apatite is abundant in REY (9300–32,000 ppm) and is characterized by a negative Ce anomaly and enrichment in heavy rare‐earth elements. This abundance and composition of REY of the mud is similar those of fish debris apatites. In contrast, phillipsite is less abundant in REY (60–170 ppm). Therefore we conclude that the main REY host phase of the mud is apatite.     

太平洋中部富REY深海粘土的地球化学特征及REY富集机制

深海沉积物中的稀土资源是一种新发现的、潜在的海底稀土资源.对太平洋中部重力活塞取样获得的90个深海粘土样品的矿物组分、常量和稀土化学分析结果进行了系统分析,并与中北太平洋以及西北太平洋南鸟岛附近海域深海沉积物稀土元素地球化学特征进行了对比.研究结果表明:太平洋中部深海粘土以富含沸石、富P及富REY为特征,其碎屑矿物中含有较多的鱼牙骨,其P₂O₅与CaO之间、P₂O₅、CaO与∑REY之间呈良好的正相关关系;其稀土分布模式表现为明显的Ce负异常、一定程度的重稀土元素富集和Y正异常.太平洋中部深海粘土REY富集的主要原因是深海粘土中含有过量的磷酸盐组分,推测过量的磷酸盐组分是由于深海粘土中鱼牙骨碎屑的加入引起的.在北太平洋海域,未受到热液活动影响的条件下,富REY的深海沉积物的稀土元素富集机制具有统一性和普遍性,可以归纳为深海沉积物中高REY磷酸盐的混入作用.      

Controls on the distribution of rare earth elements in deep-sea sediments in the North Atlantic Ocean

Deep-sea sediments can contain relatively high concentrations of rare earth elements and yttrium (REY), with a growing interest in their exploitation as an alternative to land-based REY resources. To understand the processes that lead to enrichment of the REY in deep-sea sediments, we have undertaken a detailed geochemical study of sediments recovered from the Atlantic Ocean, on a transect along ~ 24°N that includes the deep Nares Abyssal Plain and the Canary and North America Basins.Total REY concentrations (ΣREY) range from 7.99 to 513 ppm, and total concentrations of the heavy REY (Eu - Lu) range from 0.993 to 56.3 ppm. REY concentrations are highest in slowly accumulating pelagic red clays, especially in samples that contain ferromanganese micronodules. Factor analysis reveals that hydrogenous Fe- and Mn-(oxyhydr)oxides are the primary REY carrier phase in the red clays. In situ analysis of individual micronodules confirms that they have high ΣREY (up to 3620 ppm). REY concentrations are higher in micronodules that have a hydrogenous source, characterised by higher Fe/Mn, compared to micronodules that have a diagenetic source.The ΣREY content of North Atlantic deep-sea sediments is ~ 4 times lower than in Pacific deep-sea sediments. We calculate that the area of seafloor required to extract ~ 10% of the global annual REY demand is ~ 100 km², assuming removal of the upper 1 m of sediment.     

我国深海矿产研究:进展与发现(2011-2020)

深海矿产是地球上尚未被人类充分认识和利用的最大潜在战略矿产资源,近十年我国在该领域的研究取得了重要进展.在太平洋国际海底区域申请到2块多金属结核勘探区、1块富钴结壳勘探区,在西南印度洋中脊申请到1块多金属硫化物勘探区.研究阐明了我国多金属结核和富钴结壳勘探区小尺度成矿规律,揭示了其成矿作用过程及古海洋古气候记录,探讨了...     

Peculiarities of the distribution of rare-earth elements and yttrium in mineral phases of the ferromanganese crusts from the Detroit guyot (Pacific Ocean)

The concentrations of rare-earth elements and yttrium (REY) were first determined in four major mineral fractions of cobalt-rich ferromanganese crusts (CMC) from the Detroit guyot at the northern latitudes of the Pacific Ocean. It was shown that REY in the CMC from these latitudes are mainly adsorbed by the manganese phase unlike the crusts of the equatorial part of the ocean where iron hydroxides are the key REY sorbents from seawater. This is caused by the variations in the hydrochemical characteristics of seawater with the latitude of the CMC formation.     

Subduction-modified pelagic sediments as the enriched component in back-arc basalts from the Japan Sea: Ocean Drilling Program Sites 797 and 794

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Japan Sea, a Miocene-age back-arc basin in the western Pacific Ocean, recovered incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a significant sedimentary component in their mantle source. Isotopically, the volcanic rocks cover a wide range of compositions (e.g., ⁸⁷Sr/⁸⁶Sr=0.70369–0.70503, ²⁰⁴Pb/²⁰⁴Pb=17.65–18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched component with the composition of EM II. At Site 797, the combined isotope and trace element systematics support a model of two component mixing between depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been determined by analyzing samples of differing lithology from DSDP Sites 579 and 581 in the western Pacific, east of the Japan arc. The sediments have large depletions in the high field strength elements and are relatively enriched in the large-ion-lithophile elements, including Pb. These characteristics are mirrored, with reduced amplitudes, in Japan Sea enriched tholeiites and northeast Japan arc lavas, which strengthens the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than the enriched component inferred from mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/REE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-arc basalt source. Samples from other potential sources of an enriched. EM II-like component beneath Japan, such as the subcontinental lithosphere or crust, have isotopic compositions which overlap those of the Japan Sea tholeiites and are not enriched enough to be the EM II end-member.     

Evidence for heterogenes primary MORB and mantle sources,NW Indian Ocean

Basalts from 5 Deep Sea Drilling Project (DSDP) sites in the northwest Indian Ocean (Somali Basin and Arabian Sea) have general geochemical features consistent with a spreading origin at the ancient Carlsberg Ridge. However, compared to most MORBS from other oceans they have low normative olivine, TiO₂, and Zr contents. There is no evidence that the mantle source of these northwest Indian Ocean basalts was enriched in incompatible elements relative to the Atlantic and Pacific ocean mantles. In detail, incompatible element abundances in these DSDP basalts establish that they evolved from several compositionally distinct parental magmas. In particular, basalts from site 236 in the Somali Basin have relatively high SiO₂ and low Na, P, Ti, and Zr contents. These compositional features along with low normative olivine contents are similar to those proposed for melts derived by two-stage (or dynamic) melting. Published data also indicate there is no enrichment in incompatible elements at the southwest Indian Ocean triple junction, although southwest Indian Ocean basalts have slightly higher ⁸⁷Sr/⁸⁶Sr than normal Atlantic MORB. The data suggest that there are significant subtle geochemical variations in the Indian Ocean mantle sources, but are insufficient to show whether these variations have a systematic temporal or geographic distribution.     

Mercury Anomaly in the Okinawa Trough Sediments—An Indicator of Modern Seafloor Hydrothermal Activity

The Okinawa Trough is located between the shelf-sea area of the East China Sea and the deep-sea area of western Pacific Ocean. More than 60 chemical elements in the sediments from the shelf area of the East China Sea, the Okinawa Trough and western Pacific Ocean were determined by advanced techniques including neutron activation analysis, X-ray fluorescence spectrometry, atomic fluorescence spectrometry and atomic absorption spectrometry. Quantitative comparisons of the element abundances of the sediments were made in terms of the enrichment coefficients (K) of the elements.K > 1.5 indicates enrichment (K = 1.5–2, weak enrichment; K = 2–4, strong enrichment) andK > 4, anomalous enrichment. The results show that the Okinawa Trough sediments are characterized by Hg anomaly and the enrichment of such elements as Au, Ag, Se, Te, Sb, Cd, Mn, Mo, etc. Detailed studies show that the excess Hg comes from hydrothermal solutions rather than from the continent, sea water, marine organisms, cosmic dust or volcanic rocks. Attributed to modern hydrothermal activities on the sea floor, Hg anomaly can be used as a geochemical indicator of modern seafloor hydrothermal activity. This project was granted by the National Natural Science Foundation of China.     

High-resolution LA-ICP-MS mapping of deep-sea polymetallic micronodules and its implications on element mobility

Enrichments of REY (rare earth + yttrium) and other trace metals (Co and Ni) in deep-sea ferromanganese (FeMn) micronodules have received increasing attention in both deep-sea research and mineral exploration. Due to the presence of multiple, easily-crushed and poorly-crystallized phases in micronodules, the genesis of micronodules and their adsorption of various trace elements are poorly understood. To address this gap, we examined the spatial distributions of elements in cross-sections of micronodules from the western tropical North Pacific Ocean using high-resolution (HR) LA-ICP-MS raster mapping coupled with laser Raman and X-ray photoelectron spectroscopy (XPS). The ferromanganese micronodules we studied are dominated by Fe and Mn oxides with minor carbonate minerals, such as siderite, rhodochrosite and calcite. LA-ICP-MS maps show that these micronodules consist of a Mn-rich core and a Fe-rich rim. The Fe-enriched rim is enriched in As and surrounds a Mg, Mn, Cu, Co and Ni concreted core. Laser Raman maps show that the micronodule core contains more birnessite, an important scavenger of trace metals in deep sea sediments, than the rim. The birnessite filled core of these micronodules does not have elevated REY. Indeed, birnessite line channels may feed metal-rich fluid containing REY to adjacent minerals, including well-crystallized bio-apatite and zeolite, as high Ce and Y levels are spatially correlated with these minerals. The observed element profiles and XPS observations showing the coexistence of multiple oxidation states of Mn (+2, +3 and +4), Fe (+2 and +3) and Ce (+3, +4) demonstrate that the FeMn phases of these micronodules are of a diagenetic origin and that they are sites of redox-driven metal enrichment in deep-sea sediment.     

贵州渣拉沟剖面下寒武统黑色硅质岩微量元素富集机制

华南早寒武世发育了一套富有机质黑色硅质岩,其成因尚有较大争议。选择贵州省三都县渣拉沟剖面早寒武世牛蹄塘组底部硅质岩段进行研究,发现该硅质岩具有微量元素富集的特征。其可能的原因包括,静海环境、上升流和热液活动。Ce/Ce*比值显示硅质岩沉积时水体为次氧化条件,不支持静海环境;而Ba、Zn、Cu、Ni、Cd、P与有机碳含量不存在相关关系,也不支持上升流为微量元素富集的主要原因。Al-Fe-Mn三角图、Al2O3/(Fe2O3+Al2O3)、REY配分模式等指标则表明存在热液活动。上述认识与现代上升流、静海环境的微量元素富集系数对比结果相吻合。与华南地区早寒武世初期不同沉积环境硅质岩对比,发现:深海环境并不富集Ba、Zn、Cu、Ni等氧化还原敏感元素,而沿着斜坡相带同沉积断层分布的热液活动,更有可能造成这些微量元素的富集。     

Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments

Analyses have been made of REE contents of a well-characterized suite of deep-sea (> 4000 m.) principally todorokite-bearing ferromanganese nodules and associated sediments from the Pacific Ocean. REE in nodules and their sediments are closely related: nodules with the largest positive Ce anomalies are found on sediments with the smallest negative Ce anomalies; in contrast, nodules with the highest contents of other rare earths (3 + REE) are found on sediments with the lowest 3 + REE contents and vice versa. ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios in the nodules (～0.51244) point to an original seawater source but an identical ratio for sediments in combination with the REE patterns suggests that diagenetic reactions may transfer elements into the nodules. Analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to REE contents of nodules and sediments is negligible. Inter-element relationships and leaching tests suggest that REE contents are controlled by a P-rich phase with a REE pattern similar to that for biogenous apatite and an Fe-rich phase with a pattern the mirror image of that for sea water. It is proposed that 3 + REE concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate. Processes which favour the enrichment of transition metals in equatorial Pacific nodules favour the depletion of 3 + REE in nodules and enrichment of 3 + REE in associated sediments. In contrast, Ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions.     

A study on pre- and post-tsunami shallow deposits off SE coast of India from the 2004 Indian Ocean tsunami: a geochemical approach

A study was made on samples from one core collected immediately after the December 2004 Asian tsunami to know the geochemical nature of the offshore tsunami sediments. The core sample was analyzed for sediment grain size, CaCO₃, organic carbon (OC) and major elements (SiO₂, TiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O, K₂O, P₂O₅, MnO). The results indicate that the core sample can be divided into two parts: (1) upper 0–25 cm, interpreted to be deposited after tsunami (AT), and (2) lower 25–45 cm, interpreted as before tsunami (BT) as evidenced by the sandy nature AT with fluctuating CaCO₃ contents. The AT part is devoid of OC suggesting that the sediment could have been transported to deeper regions along with the finer particles. Major elements such as SiO₂, TiO₂, CaO indicate high values than the other elements in AT part than in the BT part. The BT part contains Al rich alluvium mud associated with finer mud and organic particles. An analysis of the correlation matrix indicates the possible source of elements and transport of heavy minerals in the AT part than the BT part. The overall results suggest that the sediments could possibly have two different origins.     

Rare earth elements in ferromanganese nodules and other marine phases

The concentrations of rare-earth elements (REE) have been measured in 31 ferromanganese nodules from the Pacific and Indian Oceans and vary by almost a factor of 5. Too few nodules have been analyzed to define possible regional trends. The shale-normalized patterns, however, permit division of nodules into two groups: those from depth greater than 3000–3500 m and those from less depth. The factors that determine this change in the relative concentration of REE may be related to the mineralogy of manganese phases and/or the transport of REE to the deep ocean by particulate matter.Comparison of the REE patterns of nodules with those of phillipsite, phosphorite, clays, CaCO₃ and seawater suggests that the patterns of these phases reflect fractionation from an initial pattern closely resembling that of shale. By assuming that the accumulation rate of REE in clays, CaCO₃ and nodules is represented by that for surface sediments, it has been possible to estimate an accumulation rate of phillipsite in pelagic sediments of the Pacific of 0.02 mg/cm²/yr.     

The Fe/ΣFe ratios of MORB glasses and their implications for mantle melting

The Fe³⁺/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe²⁺ analyses and electron microprobe FeO_total measurements. The data provide a new estimate for the MORB oxygen fugacity (fO₂) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe³⁺/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO₂ estimate is 0.8 log units more oxidized than the average fO₂ proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe³⁺/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe²⁺ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe³⁺/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe³⁺/ΣFe relationships with partial melting proxies such as Na_8.0.Based on the Fe³⁺ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe³⁺ during partial melting, and in the relatively constant Fe³⁺/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe³⁺/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe₂O₃ systematics can be accounted for by using a fO₂ of FMQ-1 that is equivalent to the average fO₂ reported for abyssal peridotites.     

Environmental assessment of Dhaka City (Bangladesh) based on trace metal contents in road dusts

Geochemical analysis of street dusts was conducted to evaluate the environment of Dhaka City, Bangladesh. Dust samples were collected from different areas (industrial, commercial, and residential) of Dhaka City, and their major, trace and rare earth elements (REE) were determined. Samples from the commercial area had Pb concentrations double those of the industrial and residential areas. Contents of Zn, Cu, Ni, and Cr in the industrial areas were greater than those in the commercial and residential areas. The REE patterns of all dusts in Dhaka are similar and are comparable to the average upper continental crust. The condition of the Dhaka environment was compared to that in Japan and other baseline sediments using Zn–Fe₂O₃ and Pb–Fe₂O₃ diagrams. Zn–Fe₂O₃ trends for the dusts show steep inclination compared to the baseline sediment and the Japanese urban sediment trends. Dhaka lake data show enrichment of Zn over the dusts, suggestive of Zn pollution from poorly controlled industrial sources. In contrast, on the Pb–Fe₂O₃ diagram, Dhaka dusts have greater Pb contents than lake sediments, probably due to the higher traffic density in the commercial area compared to the residential area including the area around the lake. The results suggest that higher levels of Pb and Zn in street dusts in Dhaka can most likely be attributed to the anthropogenic sources like vehicle emissions to the atmosphere and a rapid development.     

冲绳海槽海底沉积物汞异常──现代海底热水效应的“指示剂”

基于６０余种化学元素丰度的比较，发现冲绳海槽海底存在汞异常，论证了“过剩汞”系源自海底热水，指出汞异常可作为现代海底热水效应的一个地球化学指示剂。     

Source characteristics of the basement rocks from the Sulu and Celebes Basins (Western Pacific): chemical and isotopic evidence

 New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacific, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd show that the seafloor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORB), and distinct from East Pacific Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics are different, in that they have distinctly lower ɛ_Nd(T) for a given ɛ_Sr(T), compared to Sulu and Celebes basalts. In the ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high Δ7/4 Pb (5.3–9.3) and D8/4 Pb (46.3–68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type (“normal”) mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low _Nd(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western Pacific region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources. Received: 23 May 1995/Accepted: 12 October 1995