Showing papers on "Basalt published in 2011"

Identification of Source Lithology in the Hawaiian and Canary Islands: Implications for Origins

Abstract: Results are reported of an exploration of mantle source lithology for intraplate magmas using whole-rock and olivine phenocryst compositions.This analysis includes modern mid-ocean ridge basalts and Archean komatiites as low- and high-temperature reference frames. It is shown that the Ni, Ca, Mn, and Fe/Mn contents of olivine phenocrysts in modern mid-ocean ridge basalts and Archean komatiites are consistent with a normal peridotite source. In contrast, olivine phenocrysts in shield-building lavas on Hawaii are higher in Ni and Fe/ Mn, and lower in Mn and Ca than those expected to crystallize from meltsofa normal peridotite source, and point tothe importance ofpyroxenite as proposed by Sobolev and co-workers. Hawaiian shield stage lavas and their olivine phenocrysts are similar to those expected from partial melts of a 100% stage 2 pyroxenite source. Such a source might form from a variety of melt^rock, melt^melt, and rock^rock reactions. Primary pyroxenite-derived magmas have a range of SiO2 contents that are positively correlated with 187 Os/ 188 Os and negatively correlated with 3 He/ 4 He.These results are consistent with a Hawaiian plume containing recycled crust within a peridotite matrix.Variable amounts of free silica are inferred in Hawaiian pyroxenite sources, which contribute to the production of SiO2-rich magmas. In contrast, peridotite and olivine pyroxenite melting are inferred to produce SiO2-poor pre-shield magmas at Loihi.The interaction of SiO2-rich and -poor magmas in the Hawaiian plume will trigger crystallization, not mixing. Mixing is permitted at low pressures in melt conduits and magma chambers, and work on olivine-hosted melt inclusions will be useful to evaluate its importance. In contrast to Hawaii, many ocean island basalts in localities such as the Canary Islands are deficient in SiO2, and may have been generated by partial melting of olivine pyroxenites that formed by solid-state reaction between recycled crust þ peridotite in the lower mantle. There is likely to be a wide range of whole-rock pyroxenite compositions in the mantle, as well assignificant variability in Mn and Fe/Mn in both peridotite partial melts and their olivine phenocrysts. In general, there are not likely to be welldefined end-member peridotite and pyroxenite sources in the mantle. Nevertheless, taxonomical difficulties encountered in source lithology identification may yield rich rewards, such as a better understanding of the relationship between lithological diversity in the lower mantle and its petrological expression in intraplate magmatism.

SIMS zircon U-Pb geochronology and Sr-Nd isotopes of Ni-Cu-Bearing Mafic-Ultramafic Intrusions in Eastern Tianshan and Beishan in correlation with flood basalts in Tarim Basin (NW China): Constraints on a ca. 280 Ma mantle plume

Abstract: Zircon SIMS U-Pb dating of the Poshi, Hongshishan, Bijiashan, and Huangshan Ni-Cu-bearing and Xiangshan Ni-Cu-Ti-Fe-bearing mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift yields a relatively restricted range of 278.6 Ma to 284.0 Ma. The histogram of compiled age data of basalts in the Tarim Basin and mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift has a peak of 280 Ma, which probably represents the time of mantle plume activity. The basalts have lower eNd(t) values in the range of −9.2 ∼ −1.7 and Mg# of 2 wt.%), indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting. The mafic-ultramafic intrusions have higher eNd(t) of −1.3 ∼ 11.2 and Mg# of 33 ∼ 90, and lower TiO2

High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions

Abstract: The Moon has long been thought to be highly depleted in volatiles such as water, and indeed published direct measurements of water in lunar volcanic glasses have never exceeded 50 parts per million (ppm). Here, we report in situ measurements of water in lunar melt inclusions; these samples of primitive lunar magma, by virtue of being trapped within olivine crystals before volcanic eruption, did not experience posteruptive degassing. The lunar melt inclusions contain 615 to 1410 ppm water and high correlated amounts of fluorine (50 to 78 ppm), sulfur (612 to 877 ppm), and chlorine (1.5 to 3.0 ppm). These volatile contents are very similar to primitive terrestrial mid-ocean ridge basalts and indicate that some parts of the lunar interior contain as much water as Earth's upper mantle.

Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

Abstract: Water has been found in many lunar rock samples, but its sources are unknown. Isotopic analyses of Apollo samples of lunar mare basalts and highlands rocks suggest that a significant volume of water was delivered to the Moon by comets shortly after its formation by giant impact.

Ages and stratigraphy of lunar mare basalts: A synthesis

Abstract: The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Accurate estimates of mare basalt ages are necessary to place constraints on the duration and the flux of lunar volcanism, as well as on the petrogenesis of lunar mare basalts and their relationship to the thermal evolution of the Moon. Here, we report on ages derived from crater size-frequency distribution measurements for exposed mare basalt units on the lunar nearside hemisphere. Crater size-frequency distribution measurements provide a statistically robust and accurate method to derive absolute model ages of unsampled regions on the Moon and other planetary surfaces. This paper summarizes and synthesizes results from our crater-counting efforts over more than 10 yr. We have dated basalts in Oceanus Procellarum, Imbrium, Serenitatis, Tranquillitatis, Humboldtianum, Australe, Humorum, Nubium, Cognitum, Nectaris, Frigoris, and numerous smaller occurrences like impact craters and sinus and lacus areas. Our investigations show that (1) in the investigated basins, lunar volcanism was active for almost 3 b.y., starting at ~3.9–4.0 b.y. ago and ceasing at ~1.2 b.y. ago, (2) most basalts erupted during the late Imbrian Period at ~3.6–3.8 b.y. ago, (3) significantly fewer basalts were emplaced during the Eratosthenian Period, and (4) basalts of possible Copernican age have been found only in limited areas in Oceanus Procellarum. Our results confirm and extend the general distribution of ages of mare basalt volcanism and further underscore the predominance of older mare basalt ages in the eastern and southern nearside and in patches of maria peripheral to the larger maria, in contrast to the younger basalt ages on the western nearside, i.e., in Oceanus Procellarum. New data from the recent international armada of lunar spacecraft will provide mineralogical, geochemical, morphological, topographic, and age data that will further refine our understanding of the flux of lunar mare basalts and their relation to petrogenetic trends and lunar thermal evolution.

Formation of high δ18O fayalite-bearing A-type granite by high-temperature melting of granulitic metasedimentary rocks, southern China

Abstract: The genesis of A-type granites has been controversial. Fayalite granite is a member of the most reduced A-type granites, commonly thought to have been primarily sourced from tholeiitic rocks. In this paper we report petrography, whole-rock geochemistry, Sr-Nd isotope, and in situ zircon Hf-O isotope results for a fayalite-bearing A-type granite suite at Jiuyishan in southern China. High zircon δ 18 O (8.0‰–9.8‰), negative zircon eHf(t) (–6.2 to –2.3), and evolved whole-rock Sr-Nd isotopes ( I Sr = 0.7151–0.7181; eNd(t) = –7.4 to –6.6) indicate the reworking of old supracrustal rocks. Isotopic and geochemical results, particularly downward inflections of Zr and Ba at ∼70 wt% SiO 2 , point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite. Integrated petrological, geochemical, and isotopic studies present the first convincing case that reduced A-type granites can also be generated by high-temperature (>960 °C) melting of granulitic metasedimentary rocks, likely related to the upwelling of the asthenosphere and/or underplating and intrusion of basaltic melts. We emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity ( f O 2 ), low f H 2 O , and high temperature.

Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume

Abstract: Volcanoes formed above the Hawaiian mantle plume exhibit a striking contrast in the geochemical characteristics of the lavas erupted at the northern Kea compared with the southern Loa volcanoes. Isotopic data show that these trends have persisted for more than 5 million years and may mirror compositional heterogeneities in the deep mantle. Linear chains of volcanic ocean islands are one of the most distinctive features on our planet. The longest, the Hawaiian–Emperor Chain, has been active for more than 80 million years, and is thought to have formed as the Pacific Plate moved across the Hawaiian mantle plume, the hottest and most productive of Earth's plumes. Volcanoes fed by the plume today form two adjacent trends, including Mauna Kea and Mauna Loa, that exhibit strikingly different geochemical characteristics. An extensive data set of isotopic analyses shows that lavas with these distinct characteristics have erupted in parallel along the Kea and Loa trends for at least 5 million years. Seismological data suggest that the Hawaiian mantle plume, when projected into the deep mantle, overlies the boundary between typical Pacific lower mantle and a sharply defined layer of apparently different material. This layer exhibits low seismic shear velocities and occurs on the Loa side of the plume. We conclude that the geochemical differences between the Kea and Loa trends reflect preferential sampling of these two distinct sources of deep mantle material. Similar indications of preferential sampling at the limit of a large anomalous low-velocity zone are found in Kerguelen and Tristan da Cunha basalts in the Indian and Atlantic oceans, respectively. We infer that the anomalous low-velocity zones at the core–mantle boundary are storing geochemical anomalies that are enriched in recycled material and sampled by strong mantle plumes.

Monte Carlo Simulations of Metasomatic Enrichment in the Lithosphere and Implications for the Source of Alkaline Basalts

Abstract: One hypothesis for the origin of alkaline lavas erupted on oceanic islands and in intracontinental settings is that they represent the melts of amphibole-rich veins in the lithosphere (or melts of their dehydrated equivalents if metasomatized lithosphere is recycled into the convecting mantle). Amphibole-rich veins are interpreted as cumulates produced by crystallization of low-degree melts of the underlying asthenosphere as they ascend through the lithosphere. We present the results of trace-element modelling of the formation and melting of veins formed in this way with the goal of testing this hypothesis and for predicting how variability in the formation and subsequent melting of such cumulates (and adjacent cryptically and modally metasomatized lithospheric peridotite) would be manifested in magmas generated by such a process. Because the high-pressure phase equilibria of hydrous near-solidus melts of garnet lherzolite are poorly constrained and given the likely high variability of the hypothesized accumulation and remelting processes, we used Monte Carlo techniques to estimate how uncertainties in the model parameters (e.g. the compositions of the asthenospheric sources, their trace-element contents, and their degree of melting; the modal proportions of crystallizing phases, including accessory phases, as the asthenospheric partial melts ascend and crystallize in the lithosphere; the amount of metasomatism of the peridotitic country rock; the degree of melting of the cumulates and the amount of melt derived from the metasomatized country rock) propagate through the process and manifest themselves as variability in the trace-element contents and radiogenic isotopic ratios of model vein compositions and erupted alkaline magma compositions. We then compare the results of the models with amphibole observed in lithospheric veins and with oceanic and continental alkaline magmas. While the trace-element patterns of the near-solidus peridotite melts, the initial anhydrous cumulate assemblage (clinopyroxene ± garnet ± olivine ± orthopyroxene), and the modelled coexisting liquids do not match the patterns observed in alkaline lavas, our calculations show that with further crystallization and the appearance of amphibole (and accessory minerals such as rutile, ilmenite, apatite, etc.) the calculated cumulate assemblages have trace-element patterns that closely match those observed in the veins and lavas. These calculated hydrous cumulate assemblages are highly enriched in incompatible trace elements and share many similarities with the trace-element patterns of alkaline basalts observed in oceanic or continental setting such as positive Nb/La, negative Ce/Pb, and similiar slopes of the rare earth elements. By varying the proportions of trapped liquid and thus simulating the cryptic and modal metasomatism observed in peridotite that surrounds these veins, we can model the variations in Ba/Nb, Ce/Pb, and Nb/U ratios that are observed in alkaline basalts. If the isotopic compositions of the initial low-degree peridotite melts are similar to the range observed in mid-ocean ridge basalt, our model calculations produce cumulates that would have isotopic compositions similar to those observed in most alkaline ocean island basalt (OIB) and continental magmas after ~0·15 Gyr. However, to produce alkaline basalts with HIMU isotopic compositions requires much longer residence times (i.e. 1–2 Gyr), consistent with subduction and recycling of metasomatized lithosphere through the mantle. EM magmas cannot readily be explained without appealing to other factors such as a heterogeneous asthenosphere. These modelling results support the interpretation proposed by various researchers that amphibole-bearing veins represent cumulates formed during the differentiation of a volatile-bearing low-degree peridotite melt and that these cumulates are significant components of the sources of alkaline OIB and continental magmas. The results of the forward models provide the potential for detailed tests of this class of hypotheses for the origin of alkaline magmas worldwide and for interpreting major and minor aspects of the geochemical variability of these magmas.

Domains of depleted mantle: New evidence from hafnium and neodymium isotopes

Abstract: Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are influenced by a heretofore unrecognized depleted component.

Timing of initiation of extension in the Tianshan, based on structural, geochemical and geochronological analyses of bimodal volcanism and olistostrome in the Bogda Shan (NW China)

Abstract: This paper describes an olistostrome formation and accompanied bimodal volcanic rocks occurring in the Baiyanggou area, south of Bogda Shan The main lithotectonic units consist of olistostrome, volcanic rocks and turbidite The olistostrome is tectonically underlain by Upper Carboniferous limestone and sandstone along a NEE-trending detachment fault Paleo-growth fault is locally observed The olistostrome unit includes plenty of blocks of limestone, sandstone, rhyolite and volcaniclastic rocks, and a matrix of graywacke Limestone blocks are dated as Pennsylvanian-Bashkirian in age by the coral and brachiopod fossils that are extensively recognized in the Upper Carboniferous strata The volcanic unit consists of pillowed and massive basalt and rhyolite, the latter occur as an 8- to 10-meter-thick layer above the olistostrome unit The turbidite unit is mainly composed of chert, siliceous mudstone and sandstone, within which the Bouma sequence can be locally recognized Meter-wide gabbro and diabase dykes intrude these three units Geochemically, rhyolites are characterized by high ACNK value of >11, depletion of Ba, Nb and Sm, and enrichment in Rb, Th and Zr Basaltic rocks are rich in K2O, they show a LREE-enriched pattern and depletion in Ba, Nb and Zr, and enrichment in Ti, Ce and Hf, similar to continental rift-type tholeiite series A gabbro porphyrite intruding the olistostrome was dated at 288 ± 3 Ma by a sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb method, and a rhyolite at 297 ± 2 Ma by a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) zircon U–Pb method The Baiyanggou olistostrome and accompanying bimodal volcanic series are linked to an extensional setting that developed in the south of the Bogda Shan Several lines of evidence, eg occurrence of large-scale strike-slip shear zones, large number of mantle-derived magmatic rocks and available geochronological data, demonstrate a significant geodynamic change from convergence to extension in the Chinese Tianshan belt, even in the whole Central Asian Orogenic Belt The extension in the Chinese Tianshan belt is initiated at ca 300 Ma, ie around Carboniferous–Permian boundary times, and the peak period of intra-plate magmatism occurred in the interval of 300–250 Ma

Compositional trends of Icelandic basalts: Implications for short-length scale lithological heterogeneity in mantle plumes

Abstract: Lithological variations in the mantle source regions under mid-ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we combine compositional observations from Icelandic basalts and modelling of melting of a bi-lithologic peridotite-pyroxenite mantle to demonstrate that, while short-lengthscale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of endmember Icelandic melts, we find enriched melts to be characterised by low SiO2 and CaO, but high FeO. We quantitatively compare endmember compositions to experimental partial melts generated from a range of lithologies, pressures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilised by the addition of up to 40% mid-ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be over-represented in accumulated melts compared with its abundance in the source. Modelling of peridotite-pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations.

Very recent and wide-spread basaltic volcanism on Mars

Abstract: New spacecraft data provide increasing evidence for a dynamic environment on presentA¢Â�Â�day Mars. Exogenic processes such as impact cratering, mass wasting processes, and active dune migration have all been observed to modify the surface. No traces of current endogenic activity have been found yet, but some studies point to very localized volcanism in the last few millions of years. However, no systematic study of young volcanic surfaces had been performed so far. We present absolute model age determinations of plains volcanism on Mars as derived from impact crater sizefrequency distributions. Extended areas in Tharsis, the largest volcanoA¢Â�Â�tectonic region on Mars, have been resurfaced by lava flows in the last few tens of millions of years. We also present results on the rheologic properties of these lava flows, inferred from morphometric measurements. Yield strengths are in the range of 100A¢Â�Â�300 Pa, and viscosities reach values of 10^2 to 10^3 Pa s, indicating basaltic compositions. The results imply that Mars retained until recently, and probably still retains, enough internal heat to produce wideA¢Â�Â�spread plainA¢Â�Â�style volcanism, producing lowA¢Â�Â�viscosity lava flows throughout large parts of Tharsis

Large-scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the Daly gap

Abstract: The dearth of intermediate magmatic compositions at the Earth's surface, referred to as the Daly gap, remains a major issue in igneous petrology. The initially favored explanation invoking silicate liquid immiscibility during evolution of basalt to rhyolite has lost support because of the absence of any firm geological evidence for separation of Fe- and Si-rich liquids in igneous rocks. This work presents a record of large-scale magmatic differentiation due to immiscibility in the tholeiitic Sept Iles intrusion (Canada), one of the largest layered plutonic bodies on Earth. Gabbroic cumulate rocks from the Critical Zone of this intrusion show a bimodal distribution in density and P2O5 content, despite identical major element chemistry of the principal magmatic phases. Immiscibility is supported by the presence of contrasting Fe-rich and Si-rich melt inclusions trapped in cumulus apatite. Phase diagrams and well-documented occurrences of small-scale immiscibility confirm that liquid-liquid unmixing and the separation of Fe-rich and Si-rich liquids may contribute significantly to the Daly gap along the tholeiitic liquid line of descent.

Implications of the Nuvvuagittuq Greenstone Belt for the Formation of Earth's Early Crust

Abstract: of the belt formed at � 4·28 Ga, which would make it the only known remnant of Hadean crust preserved on Earth. The dominant lithology of the belt has a mafic composition that consists of gneisses ranging from cummingtonite amphibolite to garnet^biotite schist composed of variable proportions of cummingtonite þ biotite þ quartz, � plagioclasegarnetanthophyllitecordierite. The composition of this unit ranges from basalt to andesite and it is divided into two distinct geochemical groups that are stratigraphical- ly separated by a banded iron formation (BIF). At the base of the se- quence, the mafic unit is mainly basaltic in composition and generally has relatively low Al2O3 and high TiO2 contents, whereas above the BIF, the unit is characterized by high Al2O3 and low TiO2 contents and exhibits a wider range of compositions from bas- altic to andesitic. The low-Ti unit can be further subdivided into a trace element depleted and a trace element enriched subgroup. The high-Ti unit is characterized by relatively flat REE patterns as opposed to the low-Ti gneisses, which display light REE-enriched profiles with flat heavy REE slopes. The incompatible element depleted low-Ti rocks have U-shaped REE profiles.The geochemical groups have compositional analogues in three types of ultramafic sills that exhibit the same stratigraphic succession. Generally, the mafic gneisses have low Ca, Na and Sr contents, with many samples having CaO contents51wt %. Such low Ca contents are unlikely to represent the original composition of their igneous precursors and are interpreted to reflect intensive alteration of plagioclase. These compositional characteristics along with the presence of cordier- ite þ anthophyllite suggest that the protoliths of the mafic gneisses were mafic volcanic rocks exhibiting variable degrees of hydrothermal alteration. The high-Ti compositional type shares geochemical char- acteristics with tholeiitic volcanic suites with low Al2O3 and high TiO2 contents and is consistent with crystal fractionation at low pressures under dry conditions. In contrast, the low-Ti compositional group is geochemically similar to boninitic and calc-alkaline volcanic suites.The high Al2O3 and lowTiO2 contents in the andesitic com- positions suggest the early crystallization of Fe^Ti oxides and late appearance of plagioclase, and are more consistent with fractionation at elevated water pressures. The succession from 'tholeitic' to 'calc-alkaline' magmatism seen in the Nuvvuagittuq greenstone belt is typical of the volcanic successions of many younger Archean green- stone belts. Regardless of the exact tectonic setting, this volcanic suc- cession suggests that the geological processes responsible for the formation and evolution of Archean greenstone belts were active at 3·8 Ga and perhaps as early as 4· 3G a.

The Petrology and Geochemistry of St. Helena Alkali Basalts: Evaluation of the Oceanic Crust-recycling Model for HIMU OIB

Abstract: We present major element, trace element, and petrographic data on alkali basalts from St. Helena, and examine the geochemical characteristics of a recycled component involved in the source of HIMU (Pb/ Pb420·5) ocean island basalts. Petrographic and compositional variations in the St. Helena basalts are best explained by the combined effect of fractional crystallization and accumulation of phenocrysts. Primary melt compositions are estimated by correcting for the effects of crystal^liquid differentiation by reconstructing the order of crystallization and the relative amount of fractionated phases.This calculation indicates that the St. Helena alkali basalts are derived from a common primary magma with 14^20 wt % MgO. Simple partial melting of fertile mantle peridotite, depleted mid-ocean ridge basalt (MORB)-source mantle, or garnet pyroxenite fails to produce the St. Helena primary melt. Instead, this primary melt can be reproduced if there are contributions from ancient recycled oceanic crust and depleted peridotite [(Rb/ Nb)PM1⁄4 0·38^0·80]. Subducted sediment can be excluded to explain the low (Rb, Ba, U)/Nb and Ce/Pb of St. Helena basalts. Geochemical modeling using major and trace element abundances, together with Sr, Nd, Pb, and Hf isotope ratios, indicates that the St. Helena primary melt can be formed by 1^2% melting of a peridotitic source that was refertilized by a small amount (8^18%) of melt derived from recycled oceanic crust. This source has a similar trace element pattern to modern normal (N)-MORB, but element abundances are 0·1^0·2 times N-MORB values. The calculated recycled crust has a wide range of present-day Pb isotopic ratios (Pb/ Pb of 21·7^79·3 and Pb/ Pb of 40·8^89·3), Sr/ Sr of 0·7018^0·7028, Nd/Nd of 0·51274^0·51285, and Hf/Hf of 0·28262^0·28293 after a residence time of 1·2^2·8 Gyr. Rb, Ba, Pb, Sr, and light rare earth element abundances in the recycled crust are depleted compared with modern N-MORB, whereasTh, U, Sm, and Nd abundances fall within the range of compositional variations in modern N-MORB. The trace element compositions of the recycled oceanic crust can be explained by element behavior during seafloor alteration and subduction zone dehydration of oceanic crust. Therefore, recycling of ancient subducted oceanic crust is a potential process for producing the St. Helena HIMU basalts.