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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.

Liu, XJ, Liang QD, Li ZL, Castillo PR, Shi Y, Xu JF, Huang XL, Liao S, Huang WL, Wu WN.  2017.  Origin of Permian extremely high Ti/Y mafic lavas and dykes from Western Guangxi, SW China: Implications for the Emeishan mantle plume magmatism. Journal of Asian Earth Sciences. 141:97-111.   10.1016/j.jseaes.2016.09.005   AbstractWebsite

Late Permian mafic flows and dikes are prominent features in and around the Western Guangxi region in southern China. Based on petrographic, geochemical and Sr-Nd isotopic data, the western Guangxi mafic rocks are geochemically akin to the Emeishan large igneous province (ELIP) high-Ti basalts, except that they possess extremely elevated Ti/Y ratios (750-2000). The Dy/Yb and Ti/Y vs. Dy/Dy* covariations of the mafic rocks indicate a garnet-controlled magmatic differentiation of a mafic melt at relatively great depth. The limited epsilon(Nd)(t) range from +0.41 to +1.81 also suggests minimal crustal contamination of such a melt. Geochemical modeling using TiO2/Yb vs. Nb/Yb and Zr/Y vs. Nb/Y projections indicate that the parental melts of the western Guangxi mafic rocks formed at a low degree (<5%) of partial melting at or over 3.5 GPa, consistent with a deep mantle plume source under a thick continental lithosphere. Thus, the Guangxi extremely high Ti/Y mafic rocks most likely represent a part of outer zone of the ELIP plume magmatism. Results of this study reinforce the previously proposed temporal and spatial distribution of the ELIP. (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.

Hahm, D, Castillo PR, Hilton DR.  2009.  A deep mantle source for high He-3/He-4 ocean island basalts (OIB) inferred from Pacific near-ridge seamount lavas. Geophysical Research Letters. 36   10.1029/2009gl040560   AbstractWebsite

One of the most contentious issues in the debate on the origin of volcanic island and seamount chains is the significance of high He-3/He-4 ratios at such locations. The contemporary hotspot hypothesis calls for the high He-3/He-4 signature to be derived from a distinct source reservoir that lies deep in the mantle. The competing plate stress hypothesis claims that extreme isotopic signals, such as the high He-3/He-4, come from dispersed crustal lithologies in the upper mantle. Here, we show that lavas from the East Pacific Rise-the ridge axis and near-ridge seamounts, which have radiogenic isotope compositions overlapping with other Pacific OIB, do not have high He-3/(4) He ratios. This suggests that high He-3/He-4 is not associated with dispersed, heterogeneous lithologies embedded in the upper mantle. We conclude that the mantle source of high He-3/He-4 OIB is unique to volcanic island and seamount chains and likely resides at depth in the mantle. Citation: Hahm, D., P. R. Castillo, and D. R. Hilton (2009), A deep mantle source for high He-3/(4) He ocean island basalts (OIB) inferred from Pacific near-ridge seamount lavas, Geophys. Res. Lett., 36, L20316, doi: 10.1029/2009GL040560.

Castillo, PR.  2008.  Origin of the adakite-high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico. Geological Society of America Bulletin. 120:451-462.   10.1130/b26166.1   AbstractWebsite

Constraining the origin of the adakite-high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global are magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted Faralon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the malic lower Baja California crust, producing adakite rocks, and the preexisting metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.

Tian, L, Castillo PR, Hawkins JW, Hilton DR, Hannan BB, Pietruszka AJ.  2008.  Major and trace element and Sr-Nd isotope signatures of lavas from the Central Lau Basin: Implications for the nature and influence of subduction components in the back-arc mantle. Journal of Volcanology and Geothermal Research. 178:657-670.   10.1016/j.jvolgeores.2008.06.039   AbstractWebsite

New major and trace element and Sr-Nd isotope data are presented for basaltic glasses from active spreading centers (Central Lau Spreading Center (CLSC), Relay Zone (RZ) and Eastern Lau Spreading Center (ELSC)) in the Central Lau Basin, SW Pacific. Basaltic lavas from the Central Lau Basin are mainly tholeiitic and are broadly similar in composition to mid-ocean ridge basalts (MORB). Their generally high (87)Sr/(86)Sr ratios, combined with relatively low (143)Nd/(144)Nd ratios are more akin to MORB from the Indian rather than Pacific Ocean. In detail, the CLSC, RZ and ELSC lavas are generally more enriched in large ion lithophile elements (Rb, Ba, Sr, and K) than average normal-MORB, which suggests that the mantle beneath the Central Lau Basin was modified by subducted slab-derived components. Fluid mobile/immobile trace element and Sr - Nd isotope ratios suggest that the subduction components were essentially transferred into the mantle via hydrous fluids derived from the subducted oceanic crust; contributions coming from the subducted sediments are minor. Compared to CLSC lavas, ELSC and RZ lavas show greater enrichment in fluid mobile elements and depletion in high field strength elements, especially Nb. Thus, with increasing distance away from the arc, the influence of subduction components in the mantle source of Lau Basin lavas diminishes. The amount of hydrous fluids also influences the degree of partial melting of the mantle beneath the Central Lau Basin, and hence the degree of melting also decreases with increasing distance from the arc. (C) 2008 Elsevier B.V. All rights reserved.

Xu, JF, Castillo PR, Chen FR, Niu HC, Yu XY, Zhen ZP.  2003.  Geochemistry of late Paleozoic mafic igneous rocks from the Kuerti area, Xinjiang, northwest China: implications for backarc mantle evolution. Chemical Geology. 193:137-154. AbstractWebsite

The composition of Kuerti mafic rocks in the Allay Mountains in northwest China ranges from highly geochernically depleted, with very low La, Ta and Nb and high epsilon(Nd(t)) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Allay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochernically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin-island arc system along the paleo-Asian ocean margin was formed in the Allay region during Devonian-Early Carboniferous. New data further indicate that the final orogenic event in the Allay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian. (C) 2003 Elsevier Science B.V. All rights reserved.

Xu, JF, Castillo PR, Li XH, Yu XY, Zhang BR, Han YW.  2002.  MORB-type rocks from the Paleo-Tethyan Mian-Lueyang northern ophiolite in the Qinling Mountains, central China: implications for the source of the low Pb-206/Pb-204 and high Nd-143/Nd-144 mantle component in the Indian Ocean. Earth and Planetary Science Letters. 198:323-337.   10.1016/s0012-821x(02)00536-8   AbstractWebsite

Samples from a basalt and gabbro section of the Paleo-Tethyan (similar to350 Ma) Mian-Lue northern ophiolites (MLNO) in the Qinling Mountains of central China display sub-parallel and relatively smooth incompatible trace element-depleted patterns and have high E-Nd(350 Ma) (8.1-11.3) and low Pb-206/Pb-204((350 Ma)) (16.90-17.25). The MLNO basalts and gabbros are compositionally similar to normal mid-ocean ridge basalts (MORB), particularly to those from the Carlsberg Ridge and Indian Ocean Ridge Triple Junction. The basalts and gabbros also have high Delta7/4 and Delta8/4 isotopic values characteristic of the Dupal isotopic anomaly in the southern hemisphere. Although the MLNO is presently in the northern hemisphere, it was previously located within the southerly location of the Indian Ocean based on palcomagnetic data. Thus, assuming that the low Pb-206/Pb-204 ratio of the MLNO basalts and gabbros is not due to seawater alteration or continental contamination, the unique isotopic signature of both the Paleo-Tethyan oceanic igneous crust and the modern Indian MORB may have come from a very similar, if not identical mantle reservoir. This indicates that a portion of the modern Indian MORB mantle isotopic domain could have been in existence for at least similar to350 Ma. We propose that the low Pb-206/Pb-204 and high Nd-143/Nd-144 isotopic character of the MLNO basalts and gabbros as well as similar Indian MORB originated either from a low mu sub-domain of the depleted asthenospheric mantle in the southern hemisphere or due to contamination of the depleted asthenosphere by deep-rooted plumes carrying a low Pb-206/Pb-204 mantle component, In contrast, the origin of the more common Indian MORB with low Pb-206/Pb-204 and low Nd-143/Nd-144 is most probably associated with the delamination of the Gondwanan continental lithosphere during formation of the Indian Ocean. (C) 2002 Elsevier Science B.V. All rights reserved.

Castillo, PR, Solidum RU, Punongbayan RS.  2002.  Origin of high field strength element enrichment in the Sulu Arc, southern Philippines, revisited. Geology. 30:707-710.   10.1130/0091-7613(2002)030<0707:oohfse>;2   AbstractWebsite

The enrichment of high field strength elements (HFSE) in Sulu Arc lavas has been proposed as a product of metasomatism of the mantle wedge. It is postulated that a dacitic melt, derived from melting of subducted Sulu Sea basaltic crust, stabilizes in the mantle wedge amphibole, which later breaks down and releases HFSEs into the source of basaltic arc lavas. New data for primitive, high-K calc-alkalic basalts that contain the highest HFSEs among Sulu Arc lavas and seafloor basalts subducting along the Sulu Trench have contrasting chemical and isotopic characteristics. This makes it unlikely that the source of HFSE enrichment in Sulu Arc lavas is melt derived from the subducted Sulu Sea basaltic crust or amphibole formed during metasomatism of the mantle wedge by such melt. We propose that HFSE enrichment in Sulu Arc lavas results from melting of a geochemically enriched component in the mantle wedge.

Castillo, PR.  2002.  The origin of some of the adakite-like and Nb-enriched lavas in southern Philippines. Acta Petrologica Sinica. 18:143-151. AbstractWebsite

The term adakite was originally used to designate silica- and sodium-rich, high-Sr/Y and -La/Yb arc lava derived from melting of the basaltic portion of a subducted slab. Adakites generally occur in convergent margins where young and, thus, still hot oceanic slabs are being subducted. Niobium-enriched arc basalts are mildly- to highly-alkalic mafic lavas that contain relatively higher amount of high field-strength elements (HFSE) than normal arc lavas. These basalts commonly occur with adakites and such association has been used to argue that their unusually high HFSE content results from metasomatism of their mantle source by slab-derived melts. Previous regional studies have suggested that the southern Philippines is a type locality for both adakites and Nb-enriched arc basalts. However, more recent detailed investigations show that although some of the southern Philippine arc lavas are adakite-like, these are most likely products of magmatic differentiation of parental lavas derived from a mantle wedge metasomatized mainly by a sediment-derived component. Moreover, HFSE-enrichment in the best examples of Nb-enriched lavas in southern Philippines is most likely due to melting of enriched mantle components that appear to be ubiquitous in the western Pacific margin. These results raise the question whether there are true slab-derived melts and whether Nb-enriched arc lavas are due to metasomatism of the mantle wedge by such melts in southern Philippines.

Hilton, DR, Gronvold K, Macpherson CG, Castillo PR.  1999.  Extreme He-3/He-4 ratios in northwest Iceland: constraining the common component in mantle plumes. Earth and Planetary Science Letters. 173:53-60.   10.1016/s0012-821x(99)00215-0   AbstractWebsite

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. He-3/He-4 ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently similar to 37 R-A (R-A = air He-3/He-4) - significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189-218]). The Sr, Nd and Ph isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517-520]) - in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275-24300]). This observation implies that Selardalur may represent the best estimate available to date of the He-Sr-Nd-Pb isotopic composition of the 5th mantle component common to many mantle plumes. (C) 1999 Elsevier Science B.V. All rights reserved.

Castillo, PR, Natland JH, Niu YL, Lonsdale PF.  1998.  Sr, Nd and Pb isotopic variation along the Pacific-Antarctic risecrest, 53-57 degrees S: Implications for the composition and dynamics of the South Pacific upper mantle. Earth and Planetary Science Letters. 154:109-125.   10.1016/s0012-821x(97)00172-6   AbstractWebsite

Sr, Nd and Pb isotope data for basalts from spreading axes and off-axis volcanoes near the Pacific-Antarctic risecrest, from Vacquier transform to just south of Udintsev transform, reveal an isotopically heterogeneous upper mantle. The isotopic composition of the mantle is represented by three end-members: (1) the 'depleted' source of the bulk of Pacific normal-type mid-ocean ridge basalts (N-MORB); (2) an 'enriched' source that produces basalts of the Hollister Ridge; and (3) a source, restricted to two adjacent sample locales, similar to that of Indian MORE. The distribution of these isotopic heterogeneities along the Pacific-Antarctic risecrest suggests two alternative hypotheses on the nature and dynamics of the south Pacific upper mantle. The whole area could be a single N-MORB mantle domain that shows a weak but continuous increase in Nd-143/Nd-144 from northeast to southwest across more than 2000 km of sea floor. The gradient is unrelated to the Louisville hotspot because Louisville basalts have low Nd-143/Nd-144 and the hotspot's influence along the ridge is spatially limited and near the high Nd-143/Nd-144 southwestern end of the gradient. The gradient appears consistent with a southwestward flow of the Pacific N-MORB-type mantle that has been proposed mainly on the basis of ridge morphology. That the N-MORB mantle domain is continuous across Heezen suggests that large-scale magmatic segmentation is not Sr-87/Sr-86, Delta Nd and Delta 8/4 of samples related to the largest structural offsets of the Pacific ridges. Alternatively, the higher from southwest of the Heezen transform relative to those from the northeast could result from southwestward pumping of both plume and Indian Ocean-type mantle material by the Louisville hotspot. The Heezen transform forms a prominent tectonic and mantle domain boundary that prohibits the Louisville-and Indian Ocean-type mantle from flowing towards and contaminating the depleted Pacific-type source in the northeast. (C) 1998 Elsevier Science B.V.

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.

Baker, PE, Castillo PR, Condliffe E.  1995.  Petrology and geochemistry of igneous rocks from Allison and Resolution guyots, sites 865 and 866. Proceedings of the Ocean Drilling Program, Scientific Results. 143:245-261., College Station, TX, United States (USA): Texas A & M University, Ocean Drilling Program, College Station, TX   10.2973/   AbstractWebsite
Castillo, PR, Floyd PA, France-Lanord C, Alt JC.  1992.  Data report; Summary of geochemical data for Leg 129 igneous rocks. Proceedings of the Ocean Drilling Program, Scientific Results. 129:653-670., College Station, TX, United States (USA): Texas A & M University, Ocean Drilling Program, College Station, TX   10.2973/   AbstractWebsite
Floyd, PA, Castillo PR.  1992.  Geochemistry and petrogenesis of Jurassic ocean crustal basalts, Site 801. Proceedings of the Ocean Drilling Program, Scientific Results. 129:361-388., College Station, TX, United States (USA): Texas A & M University, Ocean Drilling Program, College Station, TX   10.2973/   AbstractWebsite
Floyd, PA, Winchester JA, Castillo PR.  1992.  Geochemistry and petrography of Cretaceous sills and lava flows, sites 800 and 802. Proceedings of the Ocean Drilling Program, Scientific Results. 129:345-359., College Station, TX, United States (USA): Texas A & M University, Ocean Drilling Program, College Station, TX   10.2973/   AbstractWebsite
Castillo, PR, Carlson RW, Batiza R.  1991.  Origin of Nauru Basin Igneous Complex - SR, ND and PB Isotope and REE Constraints. Earth and Planetary Science Letters. 103:200-213.   10.1016/0012-821x(91)90161-a   AbstractWebsite

The tholeiitic basalts and microdolerites that comprise the Cretaceous igneous complex in the Nauru Basin in the western equatorial Pacific have moderate ranges in initial Sr-87/Sr-86 (0.70347-0.70356), initial Nd-147/N-144(0.51278-0.51287), and measured Pb-206/Pb-204 (18.52-19.15), Pb-207/Pb-204 (15.48-15.66) and Pb-208/Pb-204 (38.28-38.81). These isotopic ratios overlap with those of both oceanic island basalts (OIB) and South Atlantic and Indian mid-ocean ridge basalts (MORB). However, the petrography, mineralogy, and bulk rock chemistry of the igneous complex are more similar to MORB than to OIB. Also, the rare earth element contents of Nauru Basin igneous rocks are uniformly depleted in light elements (La/Sm(ch) < 1) indicative of a mantle source compositionally similar to that of MORB. These results suggest that the igneous complex is the top of the original ocean crust in the Nauru Basin, and that the notion that the crust must be 15 to 35 m.y. older based on simple extrapolation and identification of the M-sequence magnetic lineations [1,2] may be invalid because of a more complicated tectonic setting. The igneous complex most probably was extruded from an ocean ridge system located near the anomalously hot, volcanically active, and isotopically distinct region in the south central Pacific which has been in existence since approximately 120 Ma.

Miklius, A, Flower MFJ, Huijsmans JPP, Mukasa SB, Castillo PR.  1991.  Geochemistry of lavas from Taal Volcano, southwestern Luzon, Philippines; evidence for multiple magma supply systems and mantle source heterogeneity. Journal of Petrology. 32:593-627., Oxford, United Kingdom (GBR): Clarendon Press, Oxford AbstractWebsite
Castillo, P, Batiza R.  1989.  Strontium, neodymium and lead isotope constraints on near-ridge seamount production beneath the South Atlantic. Nature. 342:262-265. AbstractWebsite

Studies of seamounts near the East Pacific Rise (EPR) have shown that they are chemically and isotopically more diverse than those erupted on the rise axis. Here we show that the relationship of strontium, neodymium and lead isotope data between seamounts in the South Atlantic and the nearby axis of the slow-spreading Mid-Atlantic Ridge (MAR) is similar to that seen in the Pacific. This indicates that the processes leading to formation of near-ridge seamounts are similar at a wide range of spreading rates. Differences in the specific isotope signatures of lavas from near-ridge seamounts and axes of the EPR and MAR reflect regional differences in the upper-mantle source of mid-ocean-ridge basalts.

Castillo, P, Batiza R, Vanko D, Malavassi E, Barquero J, Fernandez E.  1988.  Anomalously young volcanoes on old hot-spot traces; I, Geology and petrology of Cocos Island; with Suppl. Data 88-18. Geological Society of America Bulletin. 100:1400-1414., Boulder, CO, United States (USA): Geological Society of America (GSA), Boulder, CO   10.1130/0016-7606(1988)100<1400:ayvooh>;2   AbstractWebsite

Cocos Island is the summit of a seamount on the aseismic Cocos Ridge, a proposed trace of the Galapagos hot spot. The island lies on a portion of the ridge that is middle Miocene in age, but K/Ar and paleomagnetic dates indicate that Cocos is only about 2 m.y. old. Cocos thus offers a rare opportunity for an on-land study of seamount volcanism superimposed on an early hot-spot volcanism.Cocos Island was built in three major stages that define three lithostratigraphic units: (1) shield-building, (2) explosive volcanism, and (3) post-explosive volcanism stages. All Cocos rocks belong to the typical oceanic island alkali basalt-to-trachyte series and have fairly homogeneous Sr ( (super 87) Sr/ (super 86) Sr = 0.70299-0.70308), Nd ( (super 143) Nd/ (super 144) Nd = 0.512952-0.513001), and Pb ( (super 206) Pb/ (super 204) Pb = 19.214-19.251; (super 207) Pb/ (super 204) Pb =15.553-15.596; (super 206) Pb/ (super 204) Pb = 38.899-39.036) isotopic ratios. The Cocos rock series was generated by fractional crystallization of olivine, clinopyroxene, plagioclase, ilmenite, and apatite from similar alkali basalt parental magmas. Sr, Nd, and Pb isotopic ratios indicate that Cocos and Galapagos volcanic rocks may have come from a common, although heterogeneous, mantle reservoir, and this implies that the young Cocos volcano is still a part of the Galapagos hot-spot signal.

Castillo, P.  1988.  The Dupal anomaly as a trace of the upwelling lower mantle. Nature (London). 336:667-670., London, United Kingdom (GBR): Macmillan Journals, London   10.1038/336667a0   AbstractWebsite

The Dupal anomaly records Sr and Pb isotopic anomalies in lavas from oceanic islands in the S Atlantic and Indian oceans. It is shown that the two Dupal anomaly maxima appear to be directly associated with two large-scale regions of low seismic velocity in the lower mantle, and are correlated with the loci of active hotspots. This suggests that large-scale structural features in the lower mantle produce geochemical characteristics on the surface, and may place constraints on the chemical structure of the mantle and on mantle convection.