Paterno R. Castillo

Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones

Chemical evaluation of fluids affected during progressive water-sediment interactions provides critical information regarding the role of slab dehydration and/or crustal recycling in subduction zones. To place some constraints on geochemical processes during sediment subduction, reactions between decollement sediments and synthetic NaCl-CaCl2 solutions at 25–350°C and 800 bar were monitored in laboratory hydrothermal experiments using an autoclave apparatus. This is the first attempt in a single set of experiments to investigate the relative mobilities of many subduction zone volatiles and trace elements but, because of difficulties in conducting hydrothermal experiments on sediments at high P-T conditions, the experiments could only be designed for a shallow (∼ 10 km) depth. The experimental results demonstrate mobilization of volatiles (B and NH4) and incompatible elements (As, Be, Cs, Li, Pb, Rb) in hydrothermal fluids at relatively low temperatures (∼ 300°C). In addition, a limited fractionation of light from heavy rare earth elements (REEs) occurs under hydrothermal conditions. On the other hand, the high field strength elements (HFSEs) Cr, Hf, Nb, Ta, Ti, and Zr are not mobile in the reacted fluids. The observed behavior of volatiles and trace elements in hydrothermal fluids is similar to the observed enrichment in As, B, Cs, Li, Pb, Rb, and light REEs and depletion in HFSEs in arc magmas relative to magmas derived directly from the upper mantle. Thus, our work suggests a link between relative mobilities of trace elements in hydrothermal fluids and deep arc magma generation in subduction zones. The experimental results are highly consistent with the proposal that the addition of subduction zone hydrous fluids to the subarc mantle, which has been depleted by previous melting events, can produce the unique characteristics of arc magmas. Moreover, the results suggest that deeply subducted sediments may no longer have the composition necessary to generate the other distinct characteristics, such as the B-δ11 B and B-10Be systematics, of arc lavas. Finally, the mobilization of B, Cs, Pb, and light REEs relative to heavy REEs in the hydrothermal fluids fractionate the ratios of B/Be, B/Nb, Cs/Rb, Pb/Ce, La/Ba and LREE/HREE, which behave conservatively during normal magmatic processes. These results demonstrate that the composition of slab-derived fluids has great implications for the recycling of elements; not only in arc magmas but also in mantle plumes.

Extreme 3He/4He ratios in northwest Iceland: constraining the common component in mantle plumes

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. 3He/4He ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently ˜37 RA (RA = air 3He/4He) — 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 Pb 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.

Geochemistry of late Paleozoic mafic igneous rocks from the Kuerti area, Xinjiang, northwest China: implications for backarc mantle evolution

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high eNd(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 Altay 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 geochemically 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 Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian.

MORB-type rocks from the Paleo-Tethyan Mian-Lueyang northern ophiolite in the Qinling Mountains, central China: implications for the source of the low 206Pb/204Pb and high 143Nd/144Nd mantle component in the Indian Ocean

Abstract Samples from a basalt and gabbro section of the Paleo-Tethyan (∼350 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 ϵ Nd (350 Ma) (8.1–11.3) and low 206 Pb/ 204 Pb (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 Δ7/4 and Δ8/4 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 paleomagnetic data. Thus, assuming that the low 206 Pb/ 204 Pb 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 ∼350 Ma. We propose that the low 206 Pb/ 204 Pb and high 143 Nd/ 144 Nd isotopic character of the MLNO basalts and gabbros as well as similar Indian MORB originated either from a low μ 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 206 Pb/ 204 Pb mantle component. In contrast, the origin of the more common Indian MORB with low 206 Pb/ 204 Pb and low 143 Nd/ 144 Nd is most probably associated with the delamination of the Gondwanan continental lithosphere during formation of the Indian Ocean.

Origin of the adakite–high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico

Constraining the origin of the adakite–high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc 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 Farallon-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 mafic lower Baja California crust, producing adakite rocks, and the preexist-ing metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.

Pin-pricking the elephant: Evidence on the origin of the Ontong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg 192 basalts

Abstract Age-corrected Pb, Sr and Nd isotope ratios for early Aptian basalt from four widely separated sites on the Ontong Java Plateau that were sampled during Ocean Drilling Program Leg 192 cluster within the small range reported for three earlier drill sites, for outcrops in the Solomon Islands, and for the Nauru and East Mariana basins. Hf isotope ratios also display only a small spread of values. A vitric tuff with εNd(t) = +4.5 that lies immediately above basement at Site 1183 represents the only probable example from Leg 192 of the Singgalo magma type, flows of which comprise the upper 46–750 m of sections in the Solomon Islands and at Leg 130 Site 807 on the northern flank of the plateau. All of the Leg 192 lavas, including the high-MgO (8–10 wt%) Kroenke-type basalts found at Sites 1185 and 1187, have εNd(t) between +5.8 and +6.5. They are isotopically indistinguishable from the abundant Kwaimbaita basalt type in the Solomon Islands, and at previous plateau, Nauru Basin and East Mariana Basin drill sites. The little-fractionated Kroenke-type flows thus indicate that the uniform isotopic signature of the more evolved Kwaimbaita-type basalt (with 5–8 wt% MgO) is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers, but instead must reflect the signature of an inherently rather homogeneous (relative to the scale of melting) mantle source. In the context of a plume-head model, the Kwaimbaita-type magmas previously have been inferred to represent mantle derived largely from the plume source region. Our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event (e.g. a small amount of partial melting) approximately 3 Ga or earlier, and subsequently evolved in nearly closed-system fashion until being tapped by plateau magmatism in the early Aptian. These results are consistent with current models of a compositionally distinct lower mantle and a plume-head origin for the plateau. However, several other key aspects of the plateau are not easily explained by the plume-head model. The plateau also poses significant challenges for asteroid impact, Icelandic-type and plate separation (perisphere) models. At present, no simple model appears to account satisfactorily for all of the observed first-order features of the Ontong Java Plateau.

Geochemistry of Mesozoic Pacific mid‐ocean ridge basalt: Constraints on melt generation and the evolution of the Pacific upper mantle

We present major and trace element and Sr-Nd-Pb isotope results on Mesozoic (130-151 Ma) mid-ocean ridge basalt (MORB) 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 MORB, 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 MORB units, like Jurassic Pacific MORB 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 (e Nd (T) = 8.4-11.6; 206 Pb/ 204 Pb i = 17.9-18.6) but have a low and uniform Sr isotopic composition ( 87 Sr/ 86 Sr 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 MORB, mid-Cretaceous Pacific MORB ( 115-100 Ma) are moderately to strongly enriched in highly incompatible elements with an enriched mantle isotopic affinity. The shift in MORB 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.

Anomalously young volcanoes on old hot-spot traces: I. Geology and petrology of Cocos Island

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 ( 87 Sr/ 86 Sr = 0.70299-0.70308), Nd ( 143 Nd/ 144 Nd = 0.512952-0.513001), and Pb ( 206 Pb/ 204 Pb = 19.214-19.251; 207 Pb/ 204 Pb =15.553-15.596; 206 Pb/ 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.

Basalts from the Central Pacific Basin: Evidence for the origin of Cretaceous igneous complexes in the Jurassic western Pacific

Studies of marine magnetic anomalies suggest that oceanic crust of Jurassic age underlies the Nauru, East Mariana and northwestern Central Pacific basins of the west central Pacific Ocean. However, the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program have only recovered basalts of Cretaceous age from these basins, indicating either that large areas of the Jurassic western Pacific are covered by Cretaceous intraplate igneous complexes or that Cretaceous ocean crust is present in these areas. We present chemical and isotopic data on basalts and dolerites recovered by DSDP Leg 17 from the Central Pacific Basin (CPB). Drilling in the predicted Jurassic-age portion of the CPB recovered Late Albian (100–105 Ma) tholeiitic pillow basalts at Site 169 and Late Cretaceous alkalic dolerite sills at Sites 170 and 169 above the extrusives. Early Cretaceous crust was recovered from Site 166. The Site 169 tholeiites are LREE depleted but slightly enriched in highly incompatible elements relative to normal mid-ocean ridge basalt (MORB), giving them trace element ratios similar to MORB erupted near hot spots. The Sr, Nd, and Pb isotopic compositions of the tholeiites (87Sr/86Sri = 0.70341–0.70348; eNd(t) = +6.2–6.4; 206Pb/204Pbmeas = 18.63–18.68) overlap with MORB from the Indian Ocean, but fall outside of the Sr and Nd isotopic ranges for Pacific MORB. The Site 169 tholeiites are compositionally almost identical to basalts from the Nauru and East Mariana basins and are isotopically similar to some Ontong Java Plateau basalts. Site 166 crustal lavas are similar to normal-MORB from the East Pacific Rise. Chemical and isotopic data for the Site 169 tholeiites are consistent with an origin at a spreading center contaminated with EM I-type plume materials, probably from the Ontong Java plume head. Based on geochemical and geophysical data from the region, we propose that the Site 169 tholeiites, as well as basalts from the Nauru and East Mariana basins, were created at a system of short-lived mid-Cretaceous spreading centers extending from the East Mariana Basin into the northwestern Central Pacific Basin, and that rifting of Jurassic crust was initiated as a result of the rapid formation of Ontong Java Plateau. The Sites 169 and 170 sills appear to have been intruded as a result of near-ridge, non-hot spot volcanism similar to that producing young seamounts in the eastern Pacific today. However, the intrusion of these sills, as well as their HIMU (high μ) isotopic affinity, may have been influenced by nearby mantle plumes.

Origin of high field strength element enrichment in the Sulu Arc, southern Philippines, revisited

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.