Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, Korea

Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)_N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.     

The Harrat Al-Birk basalts in southwest Saudi Arabia:characteristic alkali mafic magmatism related to Red Sea rifting

Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~ 5 Ma.This volcanic field is almost monotonous and is dominated by basalts that include mafic-ultramafic mantle xenoliths(gabbro,websterite,and garnet-clinopyroxenite).The present work presents the first detailed petrographic and geochemical notes about the basalts.They comprise vesicular basalt,porphyritic basalt,and flow-textured basalt,in addition to red and black scoria.Geochemically,the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti,sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment.There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe-Ti oxides.The latter have abundant titanomagnetite and lesser ilmenite.There is a remarkable enrichment of light rare earth elements and depletion in Ba,Th and K,Ta,and Ti.The geochemical data in this work suggest Harrat Al-Birk basalts represent products of watersaturated melt that was silica undersaturated.This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts.Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites.The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe-Ti oxides,particularly ilmenite.Accordingly,the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths(e.g.Nemeth et al.in The Pleistocene Jabal Akwa A1 Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region,SW Saudi Arabia 2014).     

Chemical heterogeneity of the Archaean mantle,composition of the earth and mantle evolution

New rare earth element (REE) data for Archaean basalts and spinifex-textured peridotites (STP) show a range of La/Sm ratios (chondrite-normalized) from 0.36 to 3.5, with the bulk of the data in the range 0.7–1.3. This supports the hypothesis, based on Sr isotope initial ratios, that the Archaean mantle was chemically heterogeneous. We suggest that the bulk mantle source for Archaean basaltic magmas was close to an undepleted earth material. An average chemical composition of the Archaean mantle is estimated using chemical regularities observed in Archaean STP and high-magnesian basalts. TiO₂ and MgO data show an inverse correlation which intersects the MgO axis at about 50% MgO (Fo₉₂). TiO₂ abundance in the mantle source is measured on this plot by assigning anMgO= 38% for the mantle. Concentrations of other elements are also estimated and these data are then used to obtain a composition for the bulk earth. We suggest an earth model with about 1.35 times ordinary chondrite abundances of refractory lithophile elements and about 0.2 times carbonaceous type 1 chondrite abundances of moderately volatile elements (such as Na, Rb, K, Mn). P shows severe depletion in the model earth relative to carbonaceous chondrites, a feature either due to volatilization or core formation (preferred). Our data support the hypothesis of Ringwood that the source material for the earth is a carbonaceous chondrite-like material.The generation of mid-ocean ridge basalts (MORB) is examined in the light of the model earth composition and Al₂O₃/TiO₂, CaO/TiO₂ ratios. It is suggested that for primitive basalts, these values can be used to predict the residual phases in their source. Comparison of chemical characteristics of inferred sources for 2.7-b.y. Archaean basalts and modern “normal” MORB indicates that the MORB source is severely depleted in highly incompatible elements such as Cs, Ba, Rb, U, Th, K, La and Nb, but has comparable abundances of less incompatible elements such as Ti, Zr, Y, Yb. The cause of the depletion in the MORB source is examined in terms of crust formation and extraction of silica-undersaturated melts. The latter seems to be a more likely explanation, since the degree of enrichment of highly incompatible elements in the crust only accounts for up to 40% of their abundances in the bulk earth and cannot match the depletion pattern in normal MORB. A large volume of material, less depleted than the source for normal MORB must therefore exist in the mantle and can serve as the source for the ocean island basalts and “normal” MORB.Three different mantle evolution models are examined and each suggests that the mantle is stratified with respect to abundances of incompatible trace elements. We suggest that no satisfactory model is available to fully explain the spectrum of geochemical and geophysical data. In particular the Pb and Sr isotope data on oceanic basalts, the depletion patterns of MORB and the balance between lithophile abundances in the crust and mantle, are important geochemical constraints to mantle models. Further modelling of the mantle evolution will be dependent on firmer information on the role of subduction, mantle convection pattern, and basalt production through geologic time together with a better understanding of the nature of Archaean crustal genesis.     

Flux versus decompression melting at stratovolcanoes in southeastern Guatemala

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental arc setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua–Pacaya and Tecuamburro–Moyuta.The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral–aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)_N ratios of ∼1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting.     

Geochemistry of basalts drilled in the North Atlantic by IPOD Leg 49: Implications for mantle heterogeneity

IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions.It is apparent that few of the recovered basalts have the geochemical characteristics of typical “depleted” midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge.Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb,⁸⁷Sr/⁸⁶Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in “MORB”.The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an “enriched” and a “depleted” source, but rather a range of sources heterogeneous on different scales for different elements — to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.     

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth''s core

We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earth's core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earth's primitive mantle.

Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle ( 0.036) is too low to represent the primitive value ( 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.     

Petrogenesis of diabase from accretionary prism in the southern Qiangtang terrane,central Tibet: Evidence from U–Pb geochronology,petrochemistry and Sr–Nd–Hf–O isotope characteristics

The Bangong–Nujiang suture (BNS) between the Lhasa and Qiangtang terranes is an important boundary and its petrogenesis is controversial. Diabase from the accretionary prism in the southern Qiangtang terrane yields a zircon U–Pb age of 181.3 ± 1.4 Ma. All the diabases show tholeiitic basalt compositions, gentle enrichment patters of light rare earth elements (REE), variable enrichment in incompatible element concentrations (e.g. Th and Rb), and no anomaly in high field strength elements (e.g. Nb and Ta), similar to that of enriched mid‐ocean ridge basalt (E‐MORB). They have relatively homogeneous whole rock Nd (εNd(t) = 7.3–9.1) and zircon Hf–O isotopic compositions (εHf(t) = 14.8–16.1, and δ¹⁸O = 4.57–6.12‰), possibly indicating melting of the depleted mantle and no significant crustal contamination during the petrogenesis. The element variations suggest that the diabases were formed by plume–ridge interaction at a mid‐ocean ridge within the Bangong–Nujiang ocean.     

Petrogenesis of basalts from the FAMOUS area: Mid-Atlantic Ridge

Fresh basalt glasses most of which have Mg/(Mg + Fe²⁺) of 0.66–0.72 from outcrops within 3 km of one other in the rift valley at the Project FAMOUS locality have been analyzed for major, minor and trace elements in order to determine their petrogenesis.Transition metal abundances of the FAMOUS samples are similar to a wide variety of continental and oceanic basalts with high MgO and Ni, all of which show remarkably little variation, with the exception of Cu, Zn and Ti, on a chondrite-normalized plot. Modelling of these data suggests that the mantle beneath both continents and oceans is systematically fractionated relative to chondrites. This fractionation provides a constraint for models of earth formation and subsequent evolution.The abundances of the rare earth and the incompatible elements, Ba, La, Th, U, and Nb, vary by more than a factor of three and the La/Yb and La/Sm ratios vary by factors of 3.1 and 1.6, respectively, in samples with similar, high Mg/(Mg + Fe²⁺). There is no correlation between the degree of light-REE enrichment and the heavy-REE abundance. Furthermore, the trace element variations do not appear correlated with respect to location in the rift valley or to time of eruption. These trace element features demonstrate that successive eruptions in one small area of the rift valley can show wide variations in trace element chemistry over a short span of time; they preclude the derivation of these basalt glasses from a single magma chamber.Despite the heterogeneities in REE and the variable trace element abundances, a homogeneous mantle source is suggested by the similarities among the samples in the incompatible element ratios of La/Ce, Ba/Th, Zr/Nb and K/Ba and the small range in⁸⁷Sr/⁸⁶Sr isotope ratios observed in other samples from the FAMOUS region (White and Bryan, 1977). Thus, trace element heterogeneities appear to be generated by processes in the mantle during melting. However, processes such as batch partial melting, fractional fusion, fractional crystallization, zone refining, or mixing of magmas or sources acting alone are incapable of explaining the lack of correlation between the light and heavy REE.It is suggested that the observed variations are a consequence of dynamic partial melting of a homogeneous mantle source region. This process includes varying degrees of partial melting of an uprising mantle source with continuous but incomplete removal of melt as melting proceeds, varying extents of batch partial melting, and zone refining. Dynamic melting can produce different melts from a homogeneous source which have different degrees of light-REE enrichment and crossing REE patterns. The variable trace element abundances which may be produced through dynamic melting may be the cause of the apparent decoupling of major and trace elements (Bryan et al., 1976) which previously has been suggested for the FAMOUS region (Bryan and Moore, 1977).     

Magma migration beneath an ocean ridge

Basaltic volcanism which forms the oceanic crust at mid-ocean ridges is the result of pressure release melting associated with ascending mantle convection. We present a model that gives the distribution of melting beneath the ridge and the subsequent migration of magma through the asthenosphere. In order to produce the degree of partial melting associated with the basaltic rocks making up the ocean crust, melting must extend to a depth of at least 70 km. Small degrees of partial melting are expected to result in an interconnected permeability along grain intersections. Due to the differential buoyancy of the magma relative to the residual solid the magma will be rapidly driven upwards. Solid-state creep allows the solid matrix to collapse as the magma migrates upwards and the lithostatic pressure in the matrix is nearly equal to the fluid pressure in the magma. The percentage partial melt present is only slightly greater than that necessary for the development of interconnected permeability and is much less than the degree of partial melting. The first partial melt fraction produced at the greatest depths migrates upwards and mixes with the later partial melt fractions produced at shallower depths. The uniformity of this mixing will have a profound effect on the chemistry of the basalts of the oceanic crust.     

Across-arc geochemical variation of Quaternary lavas in West Java, Indonesia: Mass-balance elucidation using arc basalt simulator model

The Sunda Arc of Indonesia developed along the convergent margin between the Eurasian and the Australian Plates. More than 100 Quaternary volcanic centers occur along the arc. The West Java Arc is a segment of the Sunda Arc in which more than 10 volcanic centers are located, corresponding to the 120 to 200 km depth contours of the Wadati–Benioff zone. The geochemistry of 207 Quaternary lavas from six centers across the arc was investigated. The lavas range from basalt to dacite. Incompatible element abundances increase from the volcanic front to the rear‐arc in response to a change from low‐K to high‐K suites. Nd–Sr isotope compositions of the basalts scatter between mid‐ocean ridge basalt (MORB) source mantle and Indian Ocean sediment (SED) compositions, with volcanic front low‐K basalts having more radiogenic Nd than the rear‐arc basalts. It is suggested that mixing between slab‐derived fluids mainly from the SED and melt from MORB source mantle played a significant role in determining the geochemistry of the West Java basalts. Incompatible element patterns in primitive mantle normalized multi‐element plots are almost identical across the arc, except for greater inclination and weaker positive Sr spikes in the rear‐arc basalts. This suggests a lower degree of partial melting in the rear‐arc mantle, accompanied by change in SED fluid composition between the volcanic front and the rear‐arc. The latter is confirmed by fluid‐fluxed melting model calculations using multiple trace elements and Nd and Sr isotopes. All the West Java Arc lavas require deficit of Sr from the slab SED. This may occur due to selective breakdown of Sr‐rich hydrous silicate minerals, such as zoisite, at shallower depths before the SED component reaches the depth of dehydration effective for magma genesis. The rear‐arc basalts need further Sr deficits along with lesser fluid. These features are commonly observed in many arc basalts, and are likely attributable to the same mechanism.     

The geochemistry of oceanic basalts in the vicinity of transform faults: Observations and implications

Consideration of geochemical data from basalts formed near major Atlantic and Pacific transform faults reveals two significant sets of observations. First, compared to basalts formed far from the transform, basalts near the ridge/transform intersection have, for the same MgO contents, higher abundances of TiO₂ and other incompatible elements, higher La/Sm and La/Yb ratios, and often higher FeO. These enrichments are distinct from and occur in addition to the more variable and fractionated compositions which have been frequently noted [10–13]. Modeling of this “transform fault effect” using data from the Tamayo/EPR intersection suggests the chemical systematics are caused by decreasing extents of melting as the transform is approached.Second, there are chemical discontinuities in the major element, trace element and isotopic chemistry of basalts across many transforms. These “transform discontinuities” occur in normal ocean crust as well as around hot spots.Consideration of the melting zone in the mantle suggests that the transform fault effect is a natural consequence of the ridge/transform plate boundary. The melting zone beneath a ridge segment must terminate across the transform, leading to lower extents of melting at the transform edge. The surface manifestation of the change in the melting zone may be affected by the age of the transform offset, the spreading rate, the transform spacing and the interaction of mantle flow with the local thermal structure; it may be obscured by episodic magma chamber processes and mantle heterogeneity.The significance of transform discontinuities depends on whether they persist with age. If they do not, then temporally variable crust-forming processes may produce changes along a flow line similar to those at zero age across a transform. If, on the other hand, a discontinuity persists with age, then the transform may be related to a fundamental discontinuity in the underlying mantle. Long-lived transform discontinuities would have profound implications for the nature of plate motions, mantle convection and mantle heterogeneity.     

Rare earth and trace element characteristics of ophiolitic metabasalts from the Alpine-Apennine belt

The Western Mediterranean Jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts. In this paper we report analyses of rare earth elements (REE), transition elements (Ni, Co, Cr, V, Sc) and high field strength elements (Ti, P, Y, Zr, Ta, Hf) for metabasalts from representative ophiolite outcrops in the Eastern Alps, Corsica and the Northern Apennines (Liguria).The chemical characteristics of the metabasalts range from “normal” to “transitional” mid-ocean ridge basalt (MORB). Most chemical variation in the metabasalts from the different areas can be explained by low-pressure fractional crystallization, by differences in degree of partial melting, and by minor chemical heterogeneities of the source, but the “transitional” MORB characteristics of some metabasalts from Corsica (Balagne) might reflect formation from a source with different mineral and chemical composition. The estimated REE pattern of the source of the Liguria-type metabasalts corresponds to the pattern for certain Ligurian ultramafic rocks, which might therefore represent the residue from extraction of some ophiolitic lavas.While the Liguria-type ophiolitic metabasalts might represent products of a “normal”, for instance, oceanic ridge, the “transitional” metabasalts might be lavas erupted, during the early stages of opening of a small ocean basin, or along the continental margin of a larger ocean basin.     

Oceanic mantle beneath the southern Rio Grande rift

Major element and modal trends in spinel lherzolites from Salt Lake Crater, Hawaii, and Kilbourne Hole, New Mexico, are essentially identical. These trends have developed through 5–30% partial melting of the same pyrolite-like parent composition. The melting events probably took place at mid-ocean ridges.Using Cr/(Cr + Al + Fe³⁺) in spinel as an index of the degree of depletion in fusible components in each of a large number of lherzolite inclusions, we have subdivided both suites into relatively parental, intermediate, and refractory groups, and computed an average bulk composition for each group. Compositions of hypothetical melts generated from these rocks are characterized by high MgO (about 18%), low Ti and K, and CaO/Al₂O₃ around 1.0.The strongly similarity between the two xenolith suites suggests that south central New Mexico is underlain by oceanic mantle. We propose that the Kilbourne Hole lherzolites represent oceanic lithosphere which was buried beneath offshore sediments during a Precambrian ocean basin formation, later to be preserved when subduction further offshore created new additions to the continent above that near-shore oceanic lithosphere. In addition, we propose that the Mg-rich, Ti- and K-poor calculated melts reported here are the primitive precursors to mid-ocean ridge basalts. Early dunite and wehrlite cumulates from some ophiolite sequences may record the fractionation histories these liquids follow prior to eruption at the surface.     

Geochemistry of rare earth elements in basalts from the Walvis Ridge: implications for its origin and evolution

Selected basalts from a suite of dredged and drilled samples (IPOD sites 525, 527, 528 and 530) from the Walvis Ridge have been analysed to determine their rare earth element (REE) contents in order to investigate the origin and evolution of this major structural feature in the South Atlantic Ocean. All of the samples show a high degree of light rare earth element (LREE) enrichment, quite unlike the flat or depleted patterns normally observed for normal mid-ocean ridge basalts (MORBs). Basalts from Sites 527, 528 and 530 show REE patterns characterised by an arcuate shape and relatively low (Ce/Yb)_N ratios (1.46–5.22), and the ratios show a positive linear relationship to Nb content. A different trend is exhibited by the dredged basalts and the basalts from Site 525, and their REE patterns have a fairly constant slope, and higher (Ce/Yb)_N ratios (4.31–8.50).These differences are further reflected in the ratios of incompatible trace elements, which also indicate considerable variations within the groups. Mixing hyperbolae for these ratios suggest that simple magma mixing between a “hot spot” type of magma, similar to present-day volcanics of Tristan da Cunha, and a depleted source, possibly similar to that for magmas being erupted at the Mid-Atlantic Ridge, was an important process in the origin of parts of the Walvis Ridge, as exemplified by Sites 527, 528 and 530. Site 525 and dredged basalts cannot be explained by this mixing process, and their incompatible element ratios suggest either a mantle source of a different composition or some complexity to the mixing process. In addition, the occurrence of different types of basalt at the same location suggests there is vertical zonation within the volcanic pile, with the later erupted basalts becoming more alkaline and more enriched in incompatible elements.The model proposed for the origin and evolution of the Walvis Ridge involves an initial stage of eruption in which the magma was essentially a mixture of enriched and depleted end-member sources, with the N-MORB component being small. The dredged basalts and Site 525, which represent either later-stage eruptives or those close to the hot spot plume, probably result from mixing of the enriched mantle source with variable amounts and variable low degrees of partial melting of the depleted mantle source. As the volcano leaves the hot spot, these late-stage eruptives continue for some time. The change from tholeiitic to alkalic volcanism is probably related either to evolution in the plumbing system and magma chamber of the individual volcano, or to changes in the depth of origin of the enriched mantle source melt, similar to processes in Hawaiian volcanoes.     

Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts evidence for mantle heterogeneity beneath Iceland

Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in ¹⁴³Nd/¹⁴⁴Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in ¹⁴³Nd/¹⁴⁴Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in ⁸⁷Sr/⁸⁶Sr (0.7031–0.7032). These ⁸⁷Sr/⁸⁶Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB.We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and ¹⁴³Nd/¹⁴⁴Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, ⁸⁷Sr/⁸⁶Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.     

A hafnium isotope and trace element perspective on melting of the depleted mantle

New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform (¹⁷⁶Hf/¹⁷⁷Hf≈0.28313–0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109–123; Patchett, Lithos 16 (1983) 47–51]. Atlantic MORB have ¹⁷⁶Hf/¹⁷⁷Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077–1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low ¹⁷⁶Hf/¹⁷⁷Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf–Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB): ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions – the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] – can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges.     

Causes of spatial compositional variations in Mariana arc lavas: Trace element evidence

New inductively coupled plasma mass spectrometry (ICP-MS) trace element data are presented on a suite of arc lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged from seamounts in the Central Island Province (CIP), the Northern Seamount Province (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples concluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source and an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along the Mariana arc can be better explained by the addition of spatially varying subduction components to a spatially varying mantle source. The data suggest that the subduction component in the CIP and VA is dominated by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is dominated by more silicic fluids derived from volcanogenic sediments as well as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge basalt source by loss of a small melt fraction, while the mantle wedge in the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of seamounts controls the arc and back-arc geometries, so the concomitant variation between subducted material and mantle composition may be no coincidence. The high field strength element (HFSE) data indicate a high degree of melting (∼ 25–30%) throughout the arc, ∼ 10% of which may be attributed to decompression and ∼ 20% to fluid addition.     

Genesis of the Madang Cenozoic sodic alkaline basalt in the eastern margin of the Tibetan Plateau and its continental dynamic implications

The Madang Cenozoic sodic alkaline basalt occurred in the eastern margin of the Tibetan Plateau, where is a key tectonic transform region of Tibet, North China, and Yangtze blocks. The basalts are characterized by the variation in SiO₂=42%–51%, Na₂O/K₂O>4, belonging to the sodic alkaline basalt series. The rocks are enriched in Ba, Th, Nb, Ta, relative to a slight depletion in K, Rb in the trace and rare earth element (REE) spider diagrams that are similar to the typical oceanic island alkaline basalt. The Sr-Nd-Pb isotopic compositions suggest that they are derived from a mixed mantle reservoir. The western Qinling-Songpan tectonic region was controlled by Tibet, North China and Yangtze blocks since Cenozoic, therefore, the region was in the stage of the substance converge from the mantle to upper crust, producing a mixed mantle reservoir in the studied area. The Madang basalts occurred in the specific tectonic background, they result from partial melting of a mixed asthenospheric mantle reservoir in the western Qinling-Songpan tectonic node.

     

Bimodal tholeiitic and mildly alkalic basalts from Bhir area,central Deccan Volcanic Province,India: Geochemistry and petrogenesis

Bimodal tholeiitic and mildly alkalic basalts occur near Bhir, in the central part of Deccan Volcanic Province (DVP). Major and trace element concentrations show that, of the ten flows, nine are tholeiitic and one is an alkalic basalt. The Bhir basalts have a wide range of chemical composition. Geochemical variations in the stratigraphic section define three distinct phases of evolution (zones 1 to 3). Crystal fractionation of plagioclase, clinopyroxene, olivine and Fe–Ti oxide expanded the compositional range. Low Mg#s (39–55), low concentrations of Ni and Cr and high Zr suggest the evolved nature of the Bhir basalts. Fractionation modeling suggests about 42% fractional crystallization.In spite of the dominant role of fractional crystallization in the evolution of Bhir basalts, some other processes must be sought to explain the chemical variations. Crustal contamination, magma mixing and degree of partial melting are suggested to explain the observed chemical variations. Resorption, reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing. Samples of flows one and four suspected of being contaminated all have enriched SiO₂ and LILE (K, Rb, and Ba) contents and depletion in Ti and P, believed to be due to ‘granitic’ crustal contamination.As compared to tholeiitic basalts, the alkalic basalts are characterized by low SiO₂ and high TiO₂, Na₂O, K₂O and P₂O₅. Alkalic basalts are richer in LILE (Rb and Ba), HFSE (Nb, Zr, and Y) and REE than the tholeiitic basalts. The alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the DVP. Based on major, trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of OIB was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and/or inhomogeneities in the mantle source.     

Fine-scale isotopic variation in Mariana Trough basalts: evidence for heterogeneity and a recycled component in backarc basin mantle

Fine-scale sampling with alvin and by dredging of the axial ridge in the Mariana Trough between 17°40′N and 18°30°N recovered basalts with isotopic compositions that span the range between N-type MORB and Mariana island arc basalts. There is a local tectonic-morphological control on basalt compositions; MORB-like basalts are found on the deeper ridge segment bounded by the Pagan transform and the ridge offset at 17°56′N, while basalts from the shallower ridge to the north are typical Mariana Trough basalts (MTB) having compositions intermediate between the two endmember rock types. Arc-like basalts were recovered from one site on the axial ridge.The discovery of basalts with such diverse isotopic characteristics from a short (100 km) section of this backarc spreading center constrains the chemical characteristics and distribution of mantle source variability in the Mariana Trough. SrNdPb isotopic variability suggests that the MTB source is heterogeneous on the scale of individual melt batches. The principal component in the MTB mantle source region is depleted peridotite similar to the source of MORB. The enriched component, most evident in the arc-like basalts and intimately mixed in MTB, has isotopic characteristics similar to those observed in the Mariana arc basalts. The isotopic data suggest that source variability for Mariana axial ridge basalts can be explained by mixed arc-like and MORB-like mantle. We hypothesize that there are fragments of old oceanic lithosphere in the backarc source region. This lithospheric component may reflect remnants of subducted seafloor or forearc-volcanic arc mantle that predate rifting in the backarc basin.