Isotopic Compositions Of Basalts Research Articles

The indium isotopic composition of the bulk silicate Earth

Indium (In) behaves as a moderately volatile metal during nebular and planetary processes, and its volatility depends strongly on oxygen fugacity. The In isotopic composition of the bulk silicate Earth (BSE) could provide a critical constraint on the nature of Earth’s building blocks and mechanisms that lead to its volatile depletion. However, accurately and precisely determining the isotopic composition of In of the silicate Earth is challenging due to its low abundance in igneous rocks and the presence of significant isobaric interferences on its isotopes (e.g., 113Cd+ on 113In+ and 115Sn+ on 115In+). Here, we present a purification procedure for In from rock matrices and report the first dataset of In isotopic compositions of 30 terrestrial igneous rocks, one biotite geostandard, and one carbonaceous chondrite (Allende) measured on a Nu Sapphire collision-cell equipped multi-collector inductively-coupled-plasma mass-spectrometer (CC-MC-ICP-MS) with an external reproducibility of 0.11‰ (2SD). At this level of precision, we find no statistically significant difference in the In isotopic compositions of mid-ocean-ridge basalts (MORB), oceanic island basalts (OIB), and continental flood basalts (CFB). Furthermore, Canary Islands, Iceland and Afar lavas display no analytically resolvable In isotopic variations from basalts to rhyolites. Therefore, In isotope fractionation during igneous processes is smaller than our analytical uncertainty and the In isotopic compositions of basalts are likely to be representative samples of their mantle sources. The twenty-one terrestrial basalts from diverse geological settings have an average δ115In of 0.35 ± 0.07 ‰ (2SD). This value represents the current best estimate of the In isotopic composition of the mantle as well as of the bulk silicate Earth, assuming limited In isotope fractionation during mantle partial melting, and due to the small contribution of the continental crust to the In budget (<5%). This isotopic composition provides a baseline with which to compare with chondrites and differentiated planetary bodies in future studies.

Basalt from the Extinct Spreading Center in the West Philippine Basin: New Geochemical Results and Their Petrologic and Tectonic Implications

We present geological, bulk-rock geochemical and Sr–Nd–Hf isotopic data for mafic rocks from the West Philippine Basin (WPB). These mafic rocks comprise pillow basalts characterized by a vesicular structure. The mid-ocean ridge basalt (MORB)-normalized trace element patterns of basalts from the study area display depletions in Nb. In addition, the chondrite-normalized lanthanide patterns of basalts from the WPB are characterized by significant depletions in the light lanthanides and nearly flat Eu to Lu segments. The investigated rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703339–0.703455 and high εNd(t) values (8.0 to 8.7). Furthermore, basalts from the WPB have 176Hf/177Hf ratios that range from 0.28318 to 0.28321 and high εHf(t) from 15.2 to 16.3. Semi-quantitative modeling demonstrates that the parental melts of basalts from the study area were derived by ~20% adiabatic decompression melting of a rising spinel-bearing peridotite source. The Sr–Nd–Hf isotopic compositions of basalts from the WPB indicate that their parental magmas were derived from an upper mantle reservoir possessing the so-called Indian-type isotopic anomaly. Interpretation of the isotopic data suggests that the inferred mantle source was most likely influenced by minor inputs of a sediment melt derived from a downgoing lithospheric slab. Collectively, the petrographic and geochemical characteristics of basalts from the study area are analogous to those of mafic rocks with a back-arc basin (BAB)-like affinity. As such, the petrogenesis of basalts from the WPB can be linked to upwelling of an Indian-type mantle source due to lithospheric slab subduction that was followed by back-arc spreading.

Geochemistry and Geochronology of the Accreted Mafic Rocks From the Hengchun Peninsula, Southern Taiwan: Origin and Tectonic Implications

AbstractThe accretionary prism of the Hengchun Peninsula at the southernmost tip of the active Taiwan orogen has newly emerged due to the transition from a subduction zone to an arc‐continent collision zone. The mafic rocks within the Hengchun accretionary prism, whose provenance has long been controversial, are highly indicative of the mantle nature and tectonic evolution around the northern Manila Trench. Based on geochemical analyses, the accreted basalts are classified as normal and enriched mid‐ocean ridge basalts (N‐MORB and E‐MORB) and alkaline ocean island basalts. The N‐MORBs, which exhibit geochemical signatures indicative of virtually unmetasomatized depleted MORB mantle and yield zircon U‐Pb ages of 22.8 ± 0.3 and 24.2 ± 1.1 Ma, were off‐scraped from the subducted oceanic crust of the South China Sea (SCS). The alkaline ocean island basalts are considered fragments of subducted seamount due to their geochemical similarity with seamount basalts from the SCS. The Hf‐Nd isotopic compositions of basalts from the north SCS may represent the contributions of recycled oceanic crust and the upwelling Hainan Plume. The E‐MORBs, which exhibit similar geochemical compositions as rocks from the Huatung Basin and yield an 40Ar/39Ar age of 135.0 ± 5.6 Ma, were removed from the overriding plate due to the excessive shear stress between the converging plates and were laterally transported by strike‐slip movement; alternatively, they were off‐scraped from the subducted Luzon forearc slices. The temporary uplift and collapse of the accretionary prism caused by seamount subduction may have been responsible for the genesis of the lenticular Shihmen Conglomerate.

Could sedimentary carbonates be recycled into the lower mantle? Constraints from Mg isotopic composition of Emeishan basalts

Whether or not sedimentary carbonates can be recycled into the lower mantle through subduction remains unclear. To further elucidate this issue, we investigate the Mg isotopic composition of Permian basalts from the Emeishan large igneous province (ELIP). Emeishan basalts can be divided into two major groups: the low-Ti and high-Ti basalts, which exhibit distinct major, trace element, and Sr–Nd–Pb isotopic compositions. However, they both possess mantle-like Mg isotopic compositions with δ26Mg values of −0.33 to −0.19‰ and −0.35 to −0.19‰, respectively. Both the low-Ti and high-Ti basalts have experienced compositional evolution, e.g., fractional crystallization or crustal contamination, because their Mg# (39.8–61.1) values are significantly lower than that of primary melt (~72.0). However, their δ26Mg values do not vary with Mg#, indicating that the Mg isotopic compositions of the basalts were inherited from their mantle source. The low-Ti basalts are enriched in light rare earth elements with low εNd(t) values (mostly ranging from −10.5 to −8.8) and low Ce/Pb, high Ba/Th ratios, suggesting that they were derived from the lithospheric mantle that was metasomatized by slab-derived fluids. Their mantle-like Mg isotopic composition suggests that the metasomatism of slab-derived fluids did not affect their Mg isotopic composition. This is consistent with most of the reported Arc basalts with mantle-like Mg isotopic compositions. On the other hand, the high-Ti basalts display an OIB-like trace element pattern and positive εNd(t) values (+0.2~+2.1), which were possibly derived from a mantle plume that originated from the lower mantle. Their mantle-like Mg isotopic composition indicates that sedimentary carbonates recycled into the lower mantle beneath the ELIP were limited (<2%).

Characteristics of silicon and oxygen isotopic compositions of basalts near East Pacific Rise 13°N

In this study, 13 groups of silicon and oxygen isotopes and major elements of the basalts near the East Pacific Rise 13°N are used to study the fractionation of silicon and oxygen isotopes. Among these data, Δ30Si values of basalts vary from −0.4‰ to 0.2‰ with a mean value of Δ30Si of (−0.18±0.22)‰. The δ18O values range from 4.1‰ to 6.4‰ with a mean Δ18O value of (+5.35±0.73)‰. Since the δ30Si values increase in the series of basalt-basaltic andesite-andesite, and δ18O values display a positive correlation with the SiO2 content, we propose that the fractionation of silicon and oxygen isotopes is influenced by the SiO2 content in igneous rocks. Compared with the igneous rocks from Manus Basin with clinopyroxene as their dominant mineral phase, MORBs in this study containing olivine and plagioclase as primary minerals have lower Δ18O and Δ30Si values, indicating that the fractionation of silicon and oxygen isotopes is also affected by different Si-O bridges in silicate minerals. Furthermore, our samples from the EPR are defined as E-MORB based on K/Ti ratios. Probably, the difference in Δ30Si and Δ18O between our samples and a normal MORB are cause by the enriched components in E-MORBs.

Geochemical provincialism in the Iceland plume

We present new Pb–Sr–Nd isotope, major and trace element data for a suite of basalts from central Iceland. We combine this new data with existing sample sets and interrogate it using spatial statistical methods. On the ∼100km scale of a volcanic zone we find strong correlation between the three isotope systems. However, on greater length scales we identify two types of spatial structure, both of which are most strongly observed in the Pb isotopes. Firstly, the mean Pb-isotopic composition of basalts becomes progressively less radiogenic from south to north Iceland, with our central Iceland dataset falling at intermediate compositions. Secondly, there is a shift in the pseudo-binary mixing array that samples fall along as the neovolcanic zones are stepped through south to north, both in Pb–Pb, and Pb–Sr/Nd isotope space. The Pb isotope systematics of Icelandic basalts therefore appear to be decoupled from those of Sr and Nd isotopes on length scales >140km. Only within individual neovolcanic zones are there coherent relationships between the Pb, Sr and Nd isotopic compositions of basalts. The spatial structure uniquely recorded by Pb isotopes complements observations from previous authors that Pb isotope dynamics are fundamentally distinct from other isotope systems.Iceland lies at the centre of a geochemical transition that occurs in the North Atlantic mantle. Our analysis shows that this shift occurs progressively from the southwest to north east of the island. However, geochemical spatial structure on Iceland does not map simply into adjacent ridges. We find that at length scales >200km partial melting of the plume head during outflow can explain some of the transition to unradiogenic Pb isotopic compositions north of Iceland. However, the shift in binary mixing arrays to sources with higher time integrated Th/U, requires the North Atlantic mantle to be highly provincial in its Pb isotopic composition.The spatial trends that we observe on Iceland are similar to those seen on Hawaii between the Loa and Kea volcanic chains, which also show shifts in mean Pb isotopic composition and binary mixing array. However, on Iceland we are able to see that, rather than representing dichotomous compositional domains on either side of the island, the change in mean composition and mixing array occurs continuously. Geochemical structure only shows segmentation on the scale of a neovolcanic zone. Discrete arrays in isotopic space on the scale of volcanic systems may develop in response to mixing in the melt transport and storage system and require neither the shallow nor the deep mantle to exhibit sharp (<100km) lateral contrasts in composition.

MORB-like mantle beneath Lanzerote Island, Canary Islands

Our newly obtained data on the He-Ar and Sr-Nd isotopic systematics of mantle xenoliths and their host basalts suggest the absence of a mantle plume beneath Lanzerote Island. The R/Ra ratio of the xenoliths lies within the range typical of MORB. The He isotopic composition of basalts from Lanzerote Island provides evidence of the mixing of two sources: MORB and atmospheric. The He isotopic ratios of both the xenoliths and the basalts do not show any correlations with the Sr and Nd isotopic characteristics. This is the e-book version of the article, published in the Russian Journal of Earth Sciences (doi:10.2205/2012ES000515). It is generated from the original source file using LaTeX’s epub.cls class.

Role of recycled oceanic basalt and sediment in generating the Hf–Nd mantle array

The isotopic composition of oceanic basalts suggests that they are composed of true recycled oceanic crust and sediments, which are mixed with the depleted mantle. Following its subduction, oceanic crust either contributes to the source of island-arc volcanic rocks or it is recycled into the mantle1. Most2,3, but not all authors4 believe that recycled crust is incorporated into the plume source of oceanic basalts. The hafnium (Hf) and neodymium (Nd) isotopic compositions of basalts from oceanic islands and mid-ocean ridges exhibit a linear relationship—the mantle array—which is thought to result from mixing between material from the depleted mantle and an enriched recycled component. Here, we model the Hf–Nd isotopic composition of oceanic basalts as a mixture of recycled oceanic crust and depleted mantle and find that recycling of basalt alone is not sufficient to reproduce the mantle array. We conclude that oceanic sediments, which have a relatively high 176Hf/177Hf ratio, must also be recycled. Combining oceanic sediments with recycled oceanic basalts and subsequent mixing with depleted mantle peridotite produces Hf and Nd isotopic compositions that coincide with the mantle array. The composition of bulk continental crust requires the existence of a complementary low 176Hf/177Hf reservoir, which we suggest is zircon-rich sediment.

Correlated trace element-Pb isotope enrichments in Indian MORB along 18–20°S, Central Indian Ridge

The Central Indian Ridge (CIR), between 18° and 20°S, shows topographic and chemical characteristics, which suggest interaction of the ridge with a mantle plume. In order to investigate the previously postulated input from the Réunion plume (presently located ∼ 1000 km off-axis to the west) on the CIR, we present chemical and isotopic compositions of basalts, collected on and off the CIR axis between 18° and 20°S. We distinguish two geographical groups of samples, called On-Axis and Gasitao, respectively. The On-Axis group is characterized by unradiogenic Sr and Pb isotope ratios and high ε Nd values. Gasitao group basalts have lower SiO 2, are more depleted in incompatible elements and have more radiogenic Sr–Pb isotope ratios and lower ε Nd values than the On-Axis group. The two groups form two distinct, subparallel linear arrays in 207Pb/ 204Pb– 206Pb/ 204Pb space. While the Gasitao array trends towards Réunion plume compositions, and therefore appears to contain some Réunion-type plume material, this is not the case for the On-Axis array. Along the ridge axis, Pb isotopes become more radiogenic from south to north, and incompatible trace elements become more enriched, but the compositional field of Réunion lavas is not a suitable end member for the Pb isotope and highly incompatible element trends (e.g. Ba/Nb). This indicates that the geochemical enrichment seen in the On-Axis region is not related to Réunion-type plume material. Basalts from both groups show both positive and negative Eu anomalies, which are strongly correlated with Sr/Nd ratios, thus indicating both gains and losses of feldspar phenocrysts. However, this has little effect on ratios of other trace elements. The trace element enrichment patterns are strongly correlated with Pb isotope ratios, with the most E-MORB-like samples having the most radiogenic Pb isotopic compositions. Using the trace element (TE)/Pb ratios versus 206Pb/ 204Pb correlations, and by extrapolating these linear correlations to TE/Pb = 0, we constrain possible 206Pb/ 204Pb ratios of the enriched and depleted endmembers. These lie at 18.3 ≤ 206Pb/ 204Pb ≤ 18.8 for the depleted and enriched components, respectively, and not very far outside the range of the actual data. We infer that the CIR MORB, between 18° and 20°S are generated by partial melting of a heterogeneous source consisting of an enriched component and a normal, depleted upper-mantle peridotite. The nature of the enriched component is a matter of speculation. As noted, its composition is different from known Réunion plume compositions. Instead, it may represent recycled (oceanic) crustal material, perhaps derived from a subducted oceanic island. It could also be formed by a “metasomatic” enrichment process similar to that modeled by Donnelly et al. [K.E. Donnelly, S.L. Goldstein, C.H. Langmuir, M. Spiegelman. Origin of enriched ocean ridge basalts and implications for mantle dynamics. Earth Planet. Sci. Lett. 226 (2004) 347–366] to explain “E-type” MORB compositions. In either case, the location of the enriched anomaly on the CIR near the intersection with the Gasitao Ridge appears to be coincidental, because the Gasitao enrichment can be traced to the Réunion plume, whereas the On-Axis group enrichment cannot. We speculate that the Réunion plume flow might be deflected towards the South by the hot upwelling E-MORB mantle, because the southernmost On-Ridge sample does fall on the Gasitao-Réunion trend.

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

An oceanic flood basalt province within the Caribbean plate

The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar– 39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ∼90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau (i.e., large volume, rapidly erupted, regionally extensive volcanism). The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galápagos hotspot at its inception. The trace-element and isotopic compositions of the ∼90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curaçao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, and/or tectonic emplacement of the plateau between North and South America. Still younger volcanics are found in the Dominican Republic (69 Ma) and the Quepos Peninsula of Costa Rica (63 Ma). The latter occurrence conceivably formed over the Galápagos hotspot and subsequently accreted to the western edge of the plateau during subduction of the Farallon plate.

Nd and Sr isotopic compositions of basalts from the Alatao Pass, Xinjiang

THE studied region is near the Alatao Pass in the northwestern Xinjiang Uygur AutonomousRegion.It is near the northeastern margin of the Gongneis Plate tectonically.The Aibihufault,situated in the northeastern border of the plate,striking NW,is a component of theBoloholo suture zone which separates the Gongneis Plate from the Junggar Plate.The Pre-cambrian basement crops out in the Wenquan County,about 100 km west of the studied re-

Crustal interaction during construction of ocean islands: PbSrNdO isotope geochemistry of the shield basalts of Gran Canaria, Canary Islands

The isotopic compositions of ocean island basalts are usually taken to reflect the compositions of their mantle sources. We report PbSrNdO isotopic data for 52 samples, and chemical data for 70 samples from the major early Miocene subaerial shield construction phase of Gran Canaria, an ocean island located on the passive margin of northwest Africa. Small systematic differences in isotopic composition exist between the five vertical sequences of basalts sampled, with tight collinear PbSrNdO isotopic correlations in the lower parts of three sections, and very restricted isotopic compositions elsewhere. Negative 206 Pb 204 Pb − 207 Pb 204 Pb correlations are observed, with increases in Δ7 4 Pb and 87 Sr 86 Sr , and decrease in ϵ Nd, accompanied by increasing δ 18O cpx values. These require that isotopic compositions of the basalts were primarily controlled by mixing between high- 206 Pb 204 Pb mantle and a crustal component. This crustal component must have been introduced into the magmas within the ocean island crust, since relatively large variations in δ 18O cpx values (+5.2 to +6.8‰) are observed over a narrow range in 87 Sr 86 Sr (0.70320 to 0.70390). The isotopic data of the lower parts of the sections can be precisely modelled by up to 8% bulk assimilation of NW African passive margin sediments by the least contaminated lavas: this is sufficient for 50% of the Pb in the most contaminated basalt to be sediment-derived, and to change substantially Ce Pb , Ba Nb and La Nb ratios. No correlations exist between isotopic composition and fractionation indices, even in lavas that are closely spatially related, suggesting that contamination processes were not simple AFC-type processes. It follows that the absence of isotope composition-fractionation index relationships cannot be used as evidence that crustal contamination did not take place. It is very difficult to identify unequivocally an uncontaminated magma composition, since the least sediment-contaminated lavas, and most samples with 206 Pb 204 Pb > 19.55 , have δ 18O cpx values significantly below MORB and lunar values, and below mantle δ 18O cpx values determined by laser fluorination. Uncontaminated magmas could be derived from a mantle source with high 206 Pb 204 Pb (> 19.8), high Ce Pb and negative Δ7 4 Pb that resembles resembles the compositions of the late Miocene undersaturated volcanics of Gran Canaria. If so, this source must have low δ 18O values (≤ +5‰), requiring an origin by recycling of hydrothermally-altered oceanic crust. Alternatively, the uncontaminated magmas could be derived from a mantle source on the Northern Hemisphere Reference Line with 206 Pb 204 Pb ≈ 19.6 , which would imply that magmas with higher 206 Pb 204 Pb and low δ 18O values, which are mostly quite evolved, acquired their characteristics by assimilation of hydrothermally-altered igneous crust with HIMU characteristics.

The strontium isotopic composition of basalts from the East Pacific and Chile Rises and abyssal hills in the Eastern Pacific Ocean

Strontium isotopic differences between the basalts from the East Pacific and the Chile Rises (0.7026–0.7035, average 0.7028) and abyssal hills (0.7025–0.7037, average 0.7033) suggest that these are produced from different source materials. A progressive increase of K/Rb with decrease in Sr87/Sr86 ratio is interpreted in terms of relative depletion of dispersed elements in the source materials.

The strontium isotopic composition of basalts from the Gordo and Juan de Fuca Rises, northeastern Pacific Ocean

The strontium isotopic compositions have been determined for twelve tholeiitic basalts dredged from the Gordo and Juan de Fuca Rises. Sr87/Sr86 ratios range from 0.7012 to 0.7031 and average 0.7026. These data, combined with other data from the East Pacific Rise indicate that tholeiite basalts being erupted along the active rises, in the Pacific Ocean, contain less radiogenic Sr87 than basalts erupted on the islands. These isotopic differences between the ocean-ridge tholeiite and the more alkali island basalts indicate that variations in Rb/Sr have persisted in the mantle for billions of years. The possible origins and distribution of these heterogeneties are discussed.