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Yazdani, S, Castillo PR, Hassanzadeh J.  2018.  Crust-mantle interaction inferred from the petrology and Sr-Nd-Pb isotope geochemistry of Eocene arc lavas from the Kahrizak Mountains, north-central Iran. Lithos. 318:299-313.   10.1016/j.lithos.2018.08.018   AbstractWebsite

The Eocene volcanic rocks from the Kahrizak Mountains in north-central Iran are part of the Urumieh-Dokhtar magmatic arc, which runs parallel to the Main Zagros Thrust segment of the Neo-Tethys suture. These volcanic rocks, similar to those from eastern Pontides and northern Anatolia, Turkey, were mainly produced during the Eocene magmatic flare-up associated with the Arabia-Eurasia convergence. The rock suite includes basalt, trachyandesite/andesite and trachydacite/rhyolite lavas and pyroclastic deposits that evolved compositionally from calc-alkalic to shoshonitic. Their normalized trace element concentration patterns are moderately enriched in light rare earth element and depleted in high field-strength elements (HFSE; e.g., Nb, Ta, Ti). They have narrow ranges of initial Pb isotopic ratios and Nd-143/Nd-144(j), but highly variable Sr-87/Sr-86(j). The new analyses indicate that the parental magmas of the volcanic rocks were derived from a mantle source that had been enriched by fluids released from a subducted oceanic slab. The fluids introduced significant amounts of large ion lithophile elements, but negligible HFSE to the source. The parental magmas underwent fractional crystallization and assimilation of upper crustal materials to produce the range of volcanic rocks. Integration of new analyses with regional data suggests that the Eocene volcanic rocks from north-central Iran, together with-coeval volcanic rocks in eastern Pontides and northern Anatolia, were most probably derived from a lithospheric mantle source that had been previously metasomatized by fluids derived from a subducted slab before and during the Arabia-Eurasia collision. (C) 2018 Elsevier B.V. All rights reserved.

Castillo, PR, Macisaac C, Perry S, Veizer J.  2018.  Marine carbonates in the mantle source of oceanic basalts: Pb isotopic constraints. Scientific Reports. 8   10.1038/s41598-018-33178-4   AbstractWebsite

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high Pb-206/Pb-204 coming from one of the end-components with a long time-integrated high U-238/Pb-204 or mu (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high Pb-206/Pb-204, indicating a HIMU-like source. Moreover, measured Th-232/U-238 (kappa) for most MORB are lower than those deduced from their Pb-208/Pb-204 and Pb-206/Pb-204. Such high mu and low kappa features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1st and 2nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

Panter, KS, Castillo P, Krans S, Deering C, McIntosh W, Valley JW, Kitajima K, Kyle P, Hart S, Blusztajn J.  2018.  Melt origin across a rifted continental margin: A case for subduction-related metasomatic agents in the lithospheric source of alkaline basalt, NW Ross Sea, Antarctica. Journal of Petrology. 59:517-557.   10.1093/petrology/egy036   AbstractWebsite

Alkaline magmatism associated with the West Antarctic rift system in the NW Ross Sea (NWRS) includes a north-south chain of shield volcano complexes extending 260km along the coast of Northern Victoria Land (NVL), numerous small volcanic seamounts located on the continental shelf and hundreds more within an 35 000km 2 area of the oceanic Adare Basin. New 40 Ar/39 Ar age dating and geochemistry confirm that the seamounts are of Pliocene-Pleistocene age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent, as well as to a much broader region of diffuse alkaline volcanism that encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late-stage breakup of Gondwana at 100 Ma, suggesting that the magmatism is interrelated, yet the mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and the roles of lithospheric and sublithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with > 6wt % MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica-undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, as well as LREE/HREE (heavy REE) and Nb/Y ratios. Radiogenic isotopes also vary, with Nd and Pb isotopic compositions increasing and Sr isotopic compositions decreasing oceanward. These variations cannot be explained by shallow-level crustal contamination or by changes in the degree of mantle partial melting, but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica-undersaturated and incompatible element enriched basalts best represent the composition of the sub-lithospheric magma source with low 87 Sr/86 Sr ( 0 7030) and d 18 Oolivine ( 5 0&), and high 143 Nd/144 Nd ( 0 5130) and 206 Pb/204 Pb ( 20). The isotopic ` endmember' signature of the sub-lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small-degree melts (carbonate-rich silicate liquids) to form amphibole-rich metasomes. Later melting of the metasomes produced silica-undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continentward. Ancient and/ or more recent ( 550-100 Ma) subduction along the Pan-Pacific margin of Gondwana supplied the recycled subduction-related material to the asthenosphere. Melting and carbonate metasomatism were triggered during major episodes of extension beginning in the Late Cretaceous, but alkaline magmatism was very limited in its extent. A significant delay of 30 to 20 Myr between extension and magmatism was probably controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle and/ or by edge-driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton.

Shi, Y, Pei XL, Castillo PR, Liu XJ, Ding HH, Guo ZC.  2017.  Petrogenesis of the similar to 500 Ma Fushui mafic intrusion and Early Paleozoic tectonic evolution of the Northern Qinling Belt, Central China. Journal of Asian Earth Sciences. 141:74-96.   10.1016/j.jseaes.2016.09.003   AbstractWebsite

The Fushui mafic intrusion in the Qinling orogenic belt (QOB) is composed of meta-gabbro, meta-gabbrodiorite, diorite, and syenite. Most of these rocks are metamorphosed under the upper greenschist facies to lower amphibolite facies metamorphism. Zircon separates from eight samples have LA-ICP-MS U-Pb ages of 497-501 Ma which are taken to be the emplacement age of magmas that formed the Fushui intrusion. Most of the zircon grains exhibit negative epsilon(HF) values, correspond to T-DMS model ages of late Paleoproterozoic-early Mesoproterozoic or Neoproterozoic and suggest that the mafic rocks were most probably derived from mafic melts produced by partial melting of a previously metasomatized litho spheric mantle. The intrusion is not extensively contaminated by crustal materials and most chemical compositions of rocks are not modified during the greenschist to amphibolite-facies metamorhism. Rocks from the intrusion have primitive mantle-normalized trace element patterns with significant enrichment in light-REE and large ion lithophile elements (LILE) and depletion in high field-strength elements (HFSE). On the basis of the trace element contents, the Fushui intrusion was derived from parental magmas generated by <10% partial melting of both phlogopite-lherzolite and garnet-lherzolite mantle sources. These sources are best interpreted to be in a subduction-related arc environment and have been modified by fluids released from a subducting slab. The formation of the Fushui intrusion was related to the subduction of the Paleotethyan Shangdan oceanic lithosphere at similar to 500 Ma. (C) 2016 Elsevier Ltd. All rights reserved.

Yan, QS, Castillo P, Shi XF, Wang LL, Liao L, Ren JB.  2015.  Geochemistry and petrogenesis of volcanic rocks from Daimao Seamount (South China Sea) and their tectonic implications. Lithos. 218:117-126.   10.1016/j.lithos.2014.12.023   AbstractWebsite

The South China Sea (SCS) experienced three episodes of seafloor spreading and left three fossil spreading centers presently located at 18 degrees N, 17 degrees N and 15.5 degrees N. Spreading ceased at these three locations during magnetic anomaly 10, 8, and 5c, respectively. Daimao Seamount (16.6 Ma) was formed 10 my after the cessation of the 17 degrees N spreading center. Volcaniclastic rocks and shallow-water carbonate facies near the summit of Daimao Seamount provide key information on the seamount's geologic history. New major and trace element and Sr-Nd-Pb isotopic compositions of basaltic breccia clasts in the volcaniclastics suggest that Daimao and other SCS seamounts have typical ocean island basalt-like composition and possess a 'Dupal' isotopic signature. Our new analyses, combined with available data, indicate that the basaltic foundation of Daimao Seamount was formed through subaqueous explosive volcanic eruptions at 16.6 Ma. The seamount subsided rapidly (>0.12 mm/y) at first, allowing the deposition of shallow-water, coral-bearing carbonates around its summit and, then, at a slower rate (<0.12 mm/y). We propose that the parental magmas of SCS seamount lavas originated from the Hainan mantle plume. In contrast, lavas from contemporaneous seamounts in other marginal basins in the western Pacific are subduction-related. (C) 2015 Elsevier B.V. All rights reserved.

Yan, QS, Shi XF, Castillo PR.  2014.  The late Mesozoic-Cenozoic tectonic evolution of the South China Sea: A petrologic perspective. Journal of Asian Earth Sciences. 85:178-201.   10.1016/j.jseaes.2014.02.005   AbstractWebsite

This paper presents a review of available petrological, geochonological and geochemical data for late Mesozoic to Recent igneous rocks in the South China Sea (SCS) and adjacent regions and a discussion of their petrogeneses and tectonic implications. The integration of these data with available geophysical and other geologic information led to the following tectono-magmatic model for the evolution of the SCS region. The geochemical characteristics of late Mesozoic granitic rocks in the Pearl River Mouth Basin (PRMB), micro-blocks in the SCS, the offshore continental shelf and Dalat zone in southern Vietnam, and the Schwaner Mountains in West Kalimantan, Borneo indicate that these are mainly I-type granites plus a small amount of S-type granites in the PRMB. These granitoids were formed in a continental arc tectonic setting, consistent with the ideas proposed by Holloway (1982) and Taylor and Hayes (1980, 1983), that there existed an Andean-type volcanic arc during later Mesozoic era in the SCS region. The geochonological and geochemical characteristics of the volcanics indicate an early period of bimodal volcanism (60-43 Ma or 32 Ma) at the northern margin of the SCS, followed by a period of relatively passive style volcanism during Cenozoic seafloor spreading (37 or 30-16 Ma) within the SCS, and post-spreading volcanism (tholeiitic series at 17-8 Ma, followed by alkali series from 8 Ma to present) in the entire SCS region. The geodynamic setting of the earlier volcanics was an extensional regime, which resulted from the collision between India and Eurasian plates since the earliest Cenozoic, and that of the post-spreading volcanics may be related to mantle plume magmatism in Hainan Island. In addition, the nascent Hainan plume may have played a significant role in the extension along the northern margin and seafloor spreading in the SCS. (C) 2014 Elsevier Ltd. All rights reserved.

Liu, XJ, Xu JF, Castillo PR, Xiao WJ, Shi Y, Feng ZH, Guo L.  2014.  The Dupal isotopic anomaly in the southern Paleo-Asian Ocean: Nd-Pb isotope evidence from ophiolites in Northwest China. Lithos. 189:185-200.   10.1016/j.lithos.2013.08.020   AbstractWebsite

It has been suggested that the Dupal isotopic anomaly in the mantle can be traced in the Paleozoic ophiolites from the Neo- and Paleo-Tethyan Ocean (275-350 Ma). The Karamaili ophiolite (KO) and Dalabute ophiolite (DO) in the eastern and western corners, respectively, of the Junggar basin in NW China represent remnants of the relatively older (>350 Ma) Paleo-Asian Ocean (PAO) crust. Thus, these ophiolites can provide additional constraints on the long-term composition and evolution of the Paleozoic suboceanic mantle. We present new major-trace element and Sr, Nd and high-precision Pb isotope data for the basalts, gabbros and a plagioclase separate from the KO and DO. Our results indicate that the PAO crust indeed has a Dupal-like isotopic signature. In detail, all samples have relatively low epsilon(Nd(t)) and high Pb-208/Pb-204((t)) for given Pb-206/(204) Pb-(t) ratios (i.e., positive Delta 8/4 values), similar to the Dupal isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). The trace element signature of DO mafic rocks is similar to that of normal- and enriched-MORB whereas that of the KO is transitional between MORB and arc basalt. Therefore, the DO mantle domain reflects the PAO asthenosphere and the KO domain additionally shows the influence of the subduction process. Geochemical modeling using Th/Nd as well as Nd and Pb isotopic ratios indicates that up to 2% subduction component had been added to a depleted Indian MORB-type mantle to produce the bulk of KO rocks. The subduction component in the KO rocks consisted of variable proportions of <= 1% partial melt of unradiogenic sediment similar to modern lzu-Bonin trench sediment and hydrous fluid dehydrated from the subducted altered oceanic crust. The Devonian asthenospheric mantle beneath the southern PAO is isotopically heterogeneous, but lends support to the idea that the Dupal isotopic anomaly existed prior to the opening of the Indian Ocean. Finally, plate tectonic reconstruction indicates that the anomaly was present in the Neo- and Paleo-Tethyan oceans in the southern hemisphere and in the southern part of PAO in the northern hemisphere during the late Paleozoic. C) 2013 Elsevier B.V. All rights reserved.

Yan, QS, Castillo PR, Shi XF.  2012.  Geochemistry of basaltic lavas from the southern Lau Basin: input of compositionally variable subduction components. International Geology Review. 54:1456-1474.   10.1080/00206814.2012.664031   AbstractWebsite

We present new major element, trace element, and Sr-Nd-Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e. g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr-Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher Sr-87/Sr-86 for a given Nd-143/Nd-144 ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.

Macpherson, CG, Chiang KK, Hall R, Nowell GM, Castillo PR, Thirlwall MF.  2010.  Plio-Pleistocene intra-plate magmatism from the southern Sulu Arc, Semporna peninsula, Sabah, Borneo: Implications for high-Nb basalt in subduction zones. Journal of Volcanology and Geothermal Research. 190:25-38.   10.1016/j.jvolgeores.2009.11.004   AbstractWebsite

New analyses of major and trace element concentrations and Sr, Nd and Pb isotopic ratios are presented for Plio-Pleistocene basalts and basaltic andesites from the Semporna peninsula in Sabah, Borneo, at the southern end of the Sulu Arc. Depletion of high field strength elements (HFSE), which is characteristic of many subduction-related magmatic suites, is present in more evolved Semporna rocks but is associated with radiogenic Sr and Pb, and less radiogenic Nd isotopic ratios and results from contamination of mafic melt by, possibly ancient, crustal basement. The most mafic lavas from Semporna, and elsewhere in the Sulu Arc, display no HFSE depletion relative to other elements with similar compatibility. High-Nb basalt from Semporna formed when mantle resembling the source of Ocean Island Basalt (OIB) upwelled into lithospheric thin spots created during earlier subduction. This mantle did not experience enrichment by fluids or melt derived from subducted crust. The presence of similar lavas throughout the Sulu Arc and around the South China Sea suggests that the OIB-like component resides in the convecting upper mantle. Depletion of light rare earth elements, with respect to other incompatible elements, throughout the Sulu Arc could result from melt-mantle interaction during magma transport through the lithosphere. Such depletion is absent in suites from the South China Sea, where magma probably migrated along large, lithosphere-penetrating structures. Semporna high-Nb basalts are not associated with adakitic magmatism which is a frequent, but not ubiquitous, association in some active subduction zones. Both geochemical signatures are developed early in the history of a melt pulse, either in the source (high-Nb basalt) or during deep differentiation (adakite). Preservation of these distinctive geochemical signatures is favoured in settings that minimise (1) interaction with other, more copious melt types, and/or (2) subsequent differentiation in the shallow crust. Where found, the high-Nb basalt-adakite association is a result of transport through favourable lithospheric conditions and not due to any link between their mantle Sources. (C) 2009 Elsevier B.V. All rights reserved.

Castillo, PR, Lonsdale PF, Moran CL, Hawkins JW.  2009.  Geochemistry of mid-Cretaceous Pacific crust being subducted along the Tonga-Kermadec Trench: Implications for the generation of arc lavas. Lithos. 112:87-102.   10.1016/j.lithos.2009.03.041   AbstractWebsite

The Pacific Plate is currently being subducted at a rapid rate beneath the Indo-Australian Plate along the Tonga-Kermadec Trench in the southwestern Pacific. It has long been assumed that the lithosphere being subducted is relatively old and homogeneous in composition. Basaltic lavas dredged from the upper crust of the incoming lithosphere along the length of the trench are mid- to late-Cretaceous in age. Although the samples are mainly N-MORB, they range from tholeiitic to alkalic basalts. Concentrations of incompatible trace elements show a high degree of variability (e.g., Ba = 8 to 270 ppm, Rb = 0.4 to 39 ppm, Sr = 60 to 625 ppm, and Nd = 2.3 to 25 ppm). Neodymium, Sr and Pb isotopic data also show wide ranges (epsilon(Nd)(T) = 5.7-11.2; (87)Sr/(86)Sr(i) = 0.70224-0.70311; (206)Pb/(204)Pb(i) = 17.84-20.22). More importantly, the basaltic crust being subducted displays a latitudinal compositional variation that is similar to that shown by the Tonga-Kermadec arc lavas. Previous studies have proposed that the variably depleted sub-arc mantle, which was preconditioned through a backarc melt extraction or displacement process, is mainly responsible for the latitudinal variation in the Tonga-Kermadec arc lavas. However, our new results suggest a greater role of the lithospheric input into the source of arc lavas. The three end-member possibilities linking the latitudinal variation of the lithospheric input to the source of arc lava output are: (I) the mantle wedge beneath the volcanic arc on the west side of the trench, the main source of Tonga-Kermadec arc lavas, is a western extension of the Cretaceous Pacific upper mantle east of the trench; (2) the altered oceanic crust melts and the resultant slab melt modifies the mantle source of arc lavas; and (3) fluids dehydrated from the altered oceanic crust effectively transfer the compositional signature of the subducted slab into the mantle source of arc lavas. (C) 2009 Elsevier B.V. All rights reserved.

Castillo, PR, Janney PE, Solidum RU.  1999.  Petrology and geochemistry of Camiguin Island, southern Philippines: insights to the source of adakites and other lavas in a complex arc setting. Contributions to Mineralogy and Petrology. 134:33-51.   10.1007/s004100050467   AbstractWebsite

Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines. The island consists of four volcanic centers which have erupted basaltic to rhyolitic calcalkaline lavas during the last similar to 400 ka. Major element, trace element and Sr, Nd and Pb isotopic data indicate that the volcanic centers have produced a single lava series from a common mantle source. Modeling results indicate that Camiguin lavas were produced by periodic injection of a parental magma into shallow magma chambers allowing assimilation and fractional crystallization (AFC) processes to take place. The chemical and isotopic composition of Camiguin lavas bears strong resemblance to the majority of lavas from the central Mindanao volcanic field confirming that Camiguin is an extension of the tectonically complex Central Mindanao Are (CMA). The most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab. Some Camiguin high-silica lavas are similar to high-silica lavas from Mindanao, which have been identified as "adakites" derived from direct melting of a subducted basaltic crust. More detailed comparison of Camiguin and Mindanao adakites with silicic slab-derived melts and magnesian andesites from the western Aleutians, southernmost Chile and Batan Island in northern Philippines indicates that the Mindanao adakites are not pure slab melts. Rather, the CMA adakites are similar to Camiguin high-silica lavas which are products of an AFC process and have negligible connection to melting of subducted basaltic crust.

Janney, PE, Castillo PR.  1997.  Geochemistry of Mesozoic Pacific mid-ocean ridge basalt: Constraints on melt generation and the evolution of the Pacific upper mantle. Journal of Geophysical Research-Solid Earth. 102:5207-5229.   10.1029/96jb03810   AbstractWebsite

We present major and trace element and Sr-Nd-Pb isotope results on Mesozoic (130-151 Ma) mid-ocean ridge basalt (MORE) recovered from five Deep Sea Drilling Project sites in the central and northwestern Pacific Ocean. Seawater alteration is responsible for much of the major element variability in these basalts, but magmatic variations are still discernible. Major element modeling of the least altered samples indicates that the basalts were generated by degrees and pressures of melting identical to those of modern Pacific MORE, and this, in addition to the similarity in spreading rates between the East Pacific Rise and Mesozoic Pacific ridges, suggests that the style of mantle upwelling and melting at spreading centers is spreading rate dependent. In general, the five Mesozoic MORE units, like Jurassic Pacific MORE from Ocean Drilling Program Site 801, are depleted in highly incompatible elements relative to average N-MORB and display a wide range in Nd and Pb isotopic ratios (epsilon(Nd)(T) = 8.4-11.6; Pb-206/Pb-204(i) = 17.9-18.6) but have a low and uniform Sr isotopic composition (Sr-87/Sr-86(i) =0.7023-0.7026). This isotopic variation can be explained by mixing a depleted mantle source with small amounts of recycled oceanic crust (HIMU). In contrast to the older MORE, mid-Cretaceous Pacific MORE (approximate to 115-100 Ma) are moderately to strongly enriched in highly incompatible elements with an ''enriched mantle'' isotopic affinity. The shift in MORE composition coincides with the onset of effusive mid-Cretaceous intraplate volcanism in the Pacific and reflects widespread contamination of the Pacific upper mantle with materials derived from the plumes or plume heads responsible for mid-Cretaceous oceanic plateaus and seamount chains.

Graham, DW, Castillo PR, Lupton JE, Batiza R.  1996.  Correlated He and Sr isotope ratios in South Atlantic near-ridge seamounts and implications for mantle dynamics. Earth and Planetary Science Letters. 144:491-503.   10.1016/s0012-821x(96)00172-0   AbstractWebsite

He-4/He-3 and Sr-87/Sr-86 ratios are highly anti-correlated for a suite of seamount glasses from both sides of the Mid-Atlantic Ridge at 26 degrees S; the linear correlation coefficient (r(2)) is 0.99 for 5 localities at 3 different seamounts. The seamounts are located on crust up to 2.5 myr old, and have He-4/He-3 as low as 65,400 (He-3/He-4 = 11 R(A)) and Sr-87/Sr-86 as high as 0.70350. These isotopic values are significantly lower and higher, respectively, than those for basaltic glasses recovered from 13 localities along the adjacent ridge axis, where the lowest He-4/He-3 ratio is 92,000 (He-3/He-4 = 7.8 R(A)) and the highest (87)/(86) Sr is 0.70258. Geophysical studies and the small (1-2%) degree of helium isotope disequilibrium between vesicles and glass for three seamount lavas suggest that the seamounts formed on or near the ridge axis. Because no off-ridge hotspots are present in this area, formation of the seamounts probably involved capture by the ridge of a passive mantle heterogeneity of 'blob' during rift propagation and tectonic evolution of the Moore fracture zone. The He-Sr-Nd-Pb isotopic results for the seamounts show a general trend toward compositions observed for the Reunion hotspot in the Indian Ocean. Collectively, the seamount and ridge axis results are somewhat enigmatic. In addition to the highly correlated He and Sr isotopes at the seamounts, a fair correlation exists between He and Nd isotopes (r(2) = 0.70). in contrast, a correlation between He and Pb isotopes is absent for the seamount glasses, while an independent, positive correlation exists between He-4/He-3 and Pb-206/Pb-204 for axial lavas. Apparently, different processes are responsible for the seamount He-Sr-Nd isotope relationships and for the nearby ridge He-Pb isotope relationship. If these relations are only of local significance and result from complications inherent in multi-stage mixing of more than two mantle components, then they imply that the upper mantle may contain domains with variable 4He/ 3He ratios, in some cases significantly lower than 80,000 (He-3/He-4 > 9 R(A)), On the other hand, binary mixing adequately explains the linear He-Sr isotope trend in the seamount lavas. This linear trend suggests similar He-3/Sr-86 ratios in the local MORE mantle source and in the source region of the low He-4/He-3 blob, which is most likely the lower mantle or the transition zone region. This similarity in He-3/Sr-86 is inconsistent with a lower mantle 3 He/Sr-86 ratio that exceeds the upper mantle ratio by at least a factor of 501 deduced from geochemical models of mantle evolution. Consequently, rare gas models involving a steady-state upper mantle and quasi-closed lower mantle may be inappropriate if applied at length scales on the order of similar to 100 km, characteristic of mid-ocean ridge segments.