Kahoolawe Island,Hawaii: The role of an ‘inaccessible’ shield volcano in the petrology of the Hawaiian islands and plume

Kahoolawe volcano (～10×17 km) forms one of the eight major Hawaiian islands. Access for geologic sampling has long been restricted due to military and preservation policies. However, limited visits to Kahoolawe in the 1980s yielded >200 samples, many of which have since been used to study the volcano within the framework of Hawaiian shield and mantle source geochemistry, petrology, mineralogy, and igneous processes.Kahoolawe is a tholeiitic shield with tholeiitic caldera-filling lavas, and at least seven postshield vents that erupted tholeiitic and (sparse) alkalic lavas. On smaller scales are a gabbro intrusion and ultramafic and gabbroic xenoliths in some postshield lavas. There is no evidence for rejuvenated volcanism. In its structural setting, Kahoolawe lies along the “Loa” trend of Hawaiian shields.Major element compositions of shield and caldera-filling lavas are similar and cluster at ～6–7 wt% MgO, range from ～5.5 to 16 wt% MgO, and include ～9 wt% MgO examples that can be modeled as parental to the evolved lavas. For example, least squares mass balancing demonstrates that from ～15% to 30% crystallization of olivine (±orthopyroxene), clinopyroxene, and plagioclase accounts for the ～5.5–6 wt% MgO range of tholeiitic lavas. Greater differentiation occurred in the gabbro (diabasic) intrusive body as a segregation vein with ～2.9 wt% MgO, and extreme differentiation produced local, small-volume rhyolitic melts that segregated into lava vesicles. Postshield lavas are mainly tholeiitic, have ～5–7 wt% MgO, and overlap shield compositions. Some are alkalic, as low as ～3.9 wt% MgO (hawaiite), and can be modeled as liquids after a ～9 wt% MgO alkalic magma crystallized ～30% olivine, clinopyroxene, plagioclase, and magnetite.Important aspects of Sr, Nd, Hf, and Pb isotopic ratios in Kahoolawe shield and caldera-filling lavas are slightly higher ⁸⁷Sr/⁸⁶Sr than in Koolau shield lavas (Oahu island; Makapuu-stage; e.g., Koolau mantle ‘endmember’) when compared at a given ¹⁴³Nd/¹⁴⁴Nd (e.g., ～0.7042 at 0.5128), ²⁰⁶Pb/²⁰⁴Pb largely at the low end of the range for Hawaiian shields (e.g., ～18), and ε_Hf generally comparable to the values of other Hawaiian shields and ocean islands (e.g., ε_Hf 8 at ε_Nd 4). The isotopic ratios overall suggest small-scale source heterogeneity, considering the island size, and that Kahoolawe shield and caldera lavas were derived from a Hawaiian plume source containing recycled oceanic crust of gabbro and sediments. Based on certain geochemical indicators, however, such as Ce/Sr and La/Nb vs. ⁸⁷Sr/⁸⁶Sr, the source contained slightly less gabbro component than other shield sources (e.g., Koolau). Isotopic data for Kahoolawe postshield lavas are scarce, but those available generally overlap the shield data. However, ratios among certain alteration-resistant incompatible trace elements (e.g., Zr/Nb) discriminate some postshield alkalic from shield lavas. But because the different ratios for those postshield lavas can be explained by smaller partial-melting percentages of the shield source and by differentiation, neither isotopes nor trace elements identify postshield magmas as originating in a source unlike that for the shield lavas.     

Sr–Nd–Pb isotopic constraints on the nature of the mantle sources involved in the genesis of the high-Ti tholeiites from northern Paraná Continental Flood Basalts (Brazil)

There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133 Ma) ⁸⁷Sr/⁸⁶Sr ratios of 0.70538–0.70642, ¹⁴³Nd/¹⁴⁴Nd of 0.51233–0.51218, ²⁰⁶Pb/²⁰⁴Pb of 17.74–18.25, ²⁰⁷Pb/²⁰⁴Pb of 15.51–15.57, and ²⁰⁸Pb/²⁰⁴Pb of 38.18–38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P₂O₅/K₂O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr–Nd–Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γ_Os values range from +1.0 to +2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred.     

Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

Petrology and geochemistry of basalts from the American-Antarctic Ridge,Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from 4° W to 18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have enriched geochemical characteristics (T- and P-type MORB).These different types of MORB are readily distinguished in terms of their incompatible trace element and isotopic characteristics: N-type MORB have high Zr/Nb (17–78), Y/Nb (4.6–23) and ¹⁴³Nd/¹⁴⁴Nd (0.51303–0.51308) ratios, low Zr/Y (2.2–4.2) and ⁸⁷Sr/⁸⁶Sr (0.70263–0.70295) ratios and have (La/Sm)_N<1.0; T-type MORB have lower than chondritic Zr/Nb ratios (8.8–15.5), relatively low Y/Nb (1.9–4.3) and ¹⁴³Nd/¹⁴⁴Nd (0.51296–0.51288) ratios and relatively high Zr/Y (3.1–4.7), ⁸⁷Sr/⁸⁶Sr (0.70307–0.70334) and (La/Sm)_N (1.1–1.5) ratios; the single sample of P-type MORB has low Zr/Nb (6.3), Y/Nb (0.9) and ¹⁴³Nd/¹⁴⁴Nd (0.51287) ratios and high Zr/Y (7.1), ⁸⁷Sr/⁸⁶Sr (0.70351) and (La/Sm)_N (2.4) ratios. The geochemical characteristics of this sample are essentially identical to those of the Bouvet Island lavas.Geochemically enriched MORB are less abundant on the American-Antarctic Ridge than on the Southwest Indian Ridge but their geochemical characteristics are identical. The compositions of T- and P-type MORB are consistent with a regional mixing model involving normal depleted mantle and Bouvet plume type magma. On a local scale the composition of T-type MORB is consistent with derivation from depleted mantle which contains 4% veins of P-type melt.We propose a model for the evolution of the American-Antarctic Ridge lavas in which N-type MORB is derived from mantle with negligible to low vein/mantle ratios, T-type MORB is derived from domains with moderate and variable vein/mantle ratios and P-type MORB from regions with very high vein/mantle ratios where vein material comprises the major portion of the melt. The sparse occurrence of enriched lavas and by implication enriched mantle beneath the American-Antarctic Ridge, some distance (500–1,200 km) from the Bouvet plume location, is interpreted to be the result of lateral dispersion of enriched mantle domains by asthenospheric flow away from the Bouvet mantle plume towards the American-Antarctic Ridge.     

Origin of Sr,Nd and Pb isotopic systematics in high-Sr basalts from central Arizona

Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (⁸⁷Sr/⁸⁶Sr = 0.7052 to 0.7055, _Nd= –9.2 to –10.7); alkali olivine basalts (⁸⁷Sr/ ⁸⁶Sr = 0.7049 to 0.7054, _Nd= –2 to 0.2); basanite and hawaiites (⁸⁷Sr/⁸⁶Sr = 0.7049 to 0.7053, _Nd= –3.5 to –7.8); and quartz tholeiites (⁸⁷Sr/⁸⁶Sr = 0.7047, _Nd= –1.4 to –2.6). Post-faulting lavas have lower ⁸⁷Sr/⁸⁶Sr (<0.7045) and _Nd from –3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.     

Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California

This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.70597–0.70653; ²⁰⁶Pb/ ²⁰⁴Pb=18.862–19.018; ²⁰⁷Pb/²⁰⁴Pb=15.640–15.686; ²⁰⁸Pb/ ²⁰⁴Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (⁸⁷Sr/⁸⁶=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high ⁸⁷Sr/⁸⁶Sr (71141–0.71240) and low ²⁰⁶Pb/²⁰⁴Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H₂O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.     

Geochemistry of basaltic lavas from the southern Lau Basin: input of compositionally variable subduction components

We present new major element, trace element, and Sr–Nd–Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e.g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr–Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher ⁸⁷Sr/⁸⁶Sr for a given ¹⁴³Nd/¹⁴⁴Nd ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.     

Geochemical and Sr-Nd-Pb isotopic characteristics of Late Cenozoic leucite lamproites from the East European Alpine belt (Macedonia and Yugoslavia)

In the East European Alpine belt, leucite-sanidine-phlogopite-olivine-bearing volcanic rocks of Late Cenozoic age occur at eight localities within the Vardar suture zone and at one locality in the Southern Carpathian fold-and-thrust belt. Most of these volcanics are characterized by high Mg# (66.6–78.6), high abundances of Ni (117–373 ppm) and Cr (144–445 ppm) as well as high primary K₂O contents (5.63–7.01 %) and K₂O/Na₂O values (1.93–4.91). Rocks with more differentiated compositions are rare. A lamproite affinity of these rocks is apparent from their relatively low contents of Al₂O₃ (9.9–14.3 wt%) and CaO (6.2–8.3 wt%) in combination with high abundances of Rb (85–967 ppm), Ba (1,027–4,189 ppm), Th (18.9–76.5 ppm), Pb (19–54 ppm), Sr (774–1,712 ppm) and F (0.16–0.52 wt%), and the general lack of plagioclase. Although eruption of the magmas took place in post-collisional extensional settings, significant depletions of Nb and Ta relative to Th and La, low TiO₂ contents (0.92–2.17 %), low ratios of Rb/Cs, K/Rb and Ce/Pb as well as high ratios of Ba/La and Ba/Th suggest close genetic relationships to subduction zone processes. Whereas Sr and Nd isotope ratios show relatively large variations (⁸⁷Sr/⁸⁶Sr = 0.7078–0.7105, ¹⁴³Nd/¹⁴⁴Nd = 0.51242–0.51215), Pb isotope ratios display a very restricted range with ²⁰⁶Pb/²⁰⁴Pb = 18.68–18.88 and variable but generally high 7/4 (11–18) and 8/4 (65–95) values. The observed petrographic, geochemical and isotopic characteristics are best explained by a genetic model involving preferential melting of phlogopite-rich veins in an originally depleted lithospheric mantle source, whereby the metasomatic enrichment of the mantle source is tentatively related to the addition of components from subducted sediments during consumption of Tethyan oceanic lithosphere.Editorial responsibility: J. Hoefs     

Geochemistry of tholeiites from Lanai,Hawaii

Lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the Hawaiian hotspot. This volcano apparently became extinct during the shield-building stage of volcanism, as shown by the absence of both alkalic cap and post-erosional lavas. Major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that Lanai tholeiites span a large range in composition. Some Lanai lavas are unique geochemically among Hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any Hawaiian lavas and well-developed positive Eu anomalies. The geochemical characteristics of these low-abundance Lanai tholeiites are not the result of alteration, differences in mantle source modal mineralogy, the presence of residual accessory mantle phases or fractional crystallization of such phases, assimilation of depleted [MORB] wall-rock, or accumulation/resorption of phenocrysts or xenocrysts. Incompatible trace element ratios (e.g., Nb/La, Nb/Th, La/Th, La/Hf, Ce/Pb) in Lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements. Large variations in La/Sm, La/Yb, and absolute REE abundances at constant MgO suggest that Lanai tholeiites formed by variable amounts of partial melting. However, large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source, but must reflect geochemical heterogeneities in the Lanai source. Partial melting modeling indicates that the mixed Lanai source is probably LREE-enriched [i.e., (La/Yb)_CN>1]. One component in the Lanai source, exemplified by the low-abundance tholeiites, has markedly lower REE/HFSE, Th/HFSE, alkali/HFSE, and Ce/Pb ratios than other Lanai or Hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the Hawaiian plume source. The positive Eu anomalies that characterize the low-abundance Lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component. Progressive temporal geochemical variations in Lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time. This change could have resulted from a progressive decrease in the extent of partial melting of the Lanai source. The compositional variability of Lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting. Lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger-volume Hawaiian shields. Therefore, volume differences between Hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting. The premature cessation of eruptive activity at Lanai may be attributed to relatively large degrees of partial melting of a small plume.     

Isotope and trace element evidence for three component mixing in the genesis of the North Luzon arc lavas (Philippines)

Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in ⁸⁷Sr/⁸⁶Sr (0.70327–0.70610), ¹⁴³Nd/¹⁴⁴Nd (0.51302–0.51229) and ²⁰⁸Pb^*/²⁰⁶Pb^* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high 8/4, and to a lesser extent high 7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS subduction component interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (<5%) of subducted S China Sea terrigenous sediment. The ⁸⁷Sr/⁸⁶Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁸Pb^*/²⁰⁶Pb^*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.     

Chemical and isotopic variations along the superfast spreading East Pacific Rise from 6 to 30°S

Chemical data of 39 fresh basaltic glasses from the East Pacific Rise (EPR) between 6 and 30°S and Pb, Sr, and Nd isotopic compositions of 12 basalt glasses are presented. Major and trace element data indicate a wide compositional range, including primitive basalts (Mg#=0.67) and highly evolved FeTi-basalts (Mg#=0.34) [molMg/(Mg+Fe²⁺)]. The compositional range can be attributed to low-pressure fractional crystallization. Fractionation-corrected major element concentrations provide evidence for varying mantle melting conditions. Calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar, at temperatures between 1430 and 1280°C, and total degrees of partial melting between 17 and 20% by weight. Leached and hand-picked basalt glasses display large variations in ⁸⁷Sr/⁸⁶Sr (0.70235–0.70270), ¹⁴³Nd/¹⁴⁴Nd (0.51312–0.51323), and ²⁰⁶Pb/²⁰⁴Pb (18.064–18.665), but are similar to other N-type MORB from the EPR. The isotopic ratios of basalts from 13 to 23°S show strong correlations and delineate two systematic trends. From 23 to 17°S, ⁸⁷Sr/⁸⁶Sr and Pb isotope ratios increase and ¹⁴³Nd/¹⁴⁴Nd decrease in agreement with previous results (Mahoney et al. 1989). A reverse trend is indicated by basalts from 17 to 13°S. However, K/Ti and (La/Sm)_N continuously increase from 23 to 13°S. This opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17°S. North of 13.5°S (Garrett Fracture Zone), isotopic data show no systematic variation with ridge location and display an overall weaker covariation. The results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions (P, T, F) (F, degrees of melting). There is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation. Our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30°S. We interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components. In contrast, variations in trace elements are attributed to young mantle differentiation processes.     

Geochemical and isotopic variations in the calc-alkaline rocks of Aeolian arc,southern Tyrrhenian Sea,Italy: constraints on magma genesis

New geochemical and isotopic data are reported for calc-alkaline (CA) volcanics of the Aeolian arc. Three main groups are recognized: the Alicudi and Filicudi volcanics in the western part of the arc; the Panarea, Salina and Lipari (henceforth termed PSL) volcanics in the central part of the arc and the Stromboli suite which makes up the eastern part of the arc. Each group is characterized by distinctive isotopic ratios and incompatible element contents and ratios. ⁸⁷Sr/⁸⁶Sr values (0.70352–0.70538) increase from west to northeast, and are well correlated with ¹⁴³Nd/¹⁴⁴Nd (Nd from +4.8 to -1.5). Pb isotope ratios are fairly high (6/4=19.15–19.54; 7/4=15.61–15.71; 8/4=38.97–39.36), with a general increase of 7/4 and 8/4 values from Alicudi to PSL islands and Stromboli. LILE contents and some incompatible element ratios (e.g. Ba/La, La/Nb, Zr/Nb, Rb/Sr) increase from the western to the central part of the arc, whereas HFSE and REE abundances decrease. Opposite variations are often observed in the volcanics toward the north-east from PSL islands. To account for these features and the decoupling observed between isotopic compositions and incompatible element abundances and ratios, it is suggested that a mantle source with affinities to the MORB source is metasomatized by slab-derived, crustal components. The proportion of crustal material entrained in the mantle source increases from Alicudi to Stromboli, according to the Sr and Nd isotope variations. It is also proposed that slab derived hydrous fluids play an important role, but which is variable in different sectors of the arc. This is attributed to the metasomatizing agent having variable fluid/melt ratios, reflecting different types of mass transfer from the subducted contaminant (probably pelagic sediments) to the mantle wedge. Thus, it is suggested that the slab derived end-member has a high hydrous fluid/melt ratio in the PSL mantle source and a correspondingly lower ratio in the Alicudi and Stromboli sources.     

Pb-Sr-Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany

 The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic megacrysts have very radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an old crustal component as reflected in the high ²⁰⁷Pb/²⁰⁴Pb ratios of many basalts from the EVP. This inference is supported by ²⁰⁶Pb/²⁰⁴Pb ratios of ∼17.6 to 18.3 and ɛ_Nd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement. The ⁸⁷Sr/⁸⁶Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions (ɛ_Nd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements (LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts (OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle. Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K₂O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western Europe. Received: 9 March 1995/Accepted: 24 July 1995     

Geochemistry of lamprophyres from the Western Alps,Italy: implications for the origin of an enriched isotopic component in the Italian mantle

Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected ⁸⁷Sr/⁸⁶Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: ²⁰⁶Pb/²⁰⁴Pb = 18.669–18.895, ²⁰⁷Pb/²⁰⁴Pb = 15.605–15.689 and ²⁰⁸Pb/²⁰⁴Pb = 38.224–39.134. ⁸⁷Sr/⁸⁶Sr ratios show a negative correlation with ¹⁴³Nd/¹⁴⁴Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.     

The geochemistry of Dinantian volcanism in south Kintyre and the evidence for provincialism in the southern Scottish mantle

Scottish Dinantian transitional to mildly alkaline volcanism is represented by abundant outcrops in the Midland Valley, Southern Uplands and Highlands provinces. Dinantian volcanic rocks from Kintyre in the Scottish Highlands range in composition from basalt through basaltic hawaiite, hawaiite, mugearite and benmoreite to trachyte, the compositions of the evolved types being largely due to differentiation from the basaltic parents.Recent geochemical investigations of Scottish Caledonian granitoids, Siluro-Devonian Old Red Sandstone (ORS) lavas and xenolith suites from numerous vents and dykes of Permo-Carboniferous to Tertiary age have revealed that the Scottish crust and upper mantle both increase in age and are increasingly enriched in incompatible elements towards the north and northwest. The upper mantle and lower crust below the Highlands province are therefore largely considered to be more enriched and in parts older than those below the Midland Valley and Southern Uplands. Dinantian alkali basalts from these latter two provinces have Nd values predominantly in the range +3 to +6, initial ⁸⁷Sr/⁸⁶Sr values of 0.7029–0.7041 and ²⁰⁷Pb/ ²⁰⁴Pb values of 15.48–15.60. However, similar basalts from Kintyre and Arran in the Highlands have lower Nd (+0.1 to +3.4) and ²⁰⁷Pb/²⁰⁴Pb (for given ²⁰⁶Pb/²⁰⁴Pb ratios; 15.49–15.51) and slightly higher ⁸⁷Sr/⁸⁶Sr (0.7033–0.7046). This regional variation correlates well with the differences seen between Midland Valley and Highland magmas in the ORS calc-alkaline suite (Thirlwall 1986) and it is suggested that both the ORS and Dinantian basic rocks are derived from similar types of mantle, although no lithospheric slab component is present in the later Dinantian suites. Isotopically-distinct portions of mantle therefore appear to have been present below the Highland and Midland Valley-Southern Upland provinces from at least Caledonian to Carboniferous times. The combined incompatible element and Sr-Nd-Pd isotopic evidence from Kintyre and Arran basaltic rocks does not allow unequivocal distinction between a lithospheric mantle and a sublithospheric mantle source. The observed correlation between isotopic composition of Dinantian basalts and the chemical composition of the lithosphere, together with the apparent involvement of long-term separated source reservoirs suggests that Kintyre and Arran lavas were derived largely from a lithospheric mantle source. On the other hand, the isotopic enrichment of Kintyre basaltic rocks is not extreme; trace element and isotopic compositions are still comparable to modem OIB. Sublithospheric mantle could therefore also be a viable source for Kintyre and Arran Dinantian volcanism.     

Magma source components in an arc-continent collision zone: the Flores-Lembata sector,Sunda arc,Indonesia

Major, trace-element, and Sr-, Nd-and Pbisotope data are presented for volcanics from 12 active or recently active volcanoes from the islands of Flores, Adonara, Lembata and Batu Tara in the eastern Sunda are. The volcanics vary in composition from low-K tholeiite, through medium-and high-K calcalkaline types to the K-rich leucite basanites of Batu Tara. From the tholeiites to the leucite basanites there are marked increases in the concentrations of LILE (K, Rb, Ba, Sr), LREE and La/Yb, and all the volcanics have high Ba/ Nb, La/Nb and Ba/La compared with mid-ocean ridge and intraplate eruptives. K/Cs values are generally lower than OIB values, and overlap those of other arc volcanics and northeast Indian Ocean sediments. The volcanics exhibit a broad range of ⁸⁷Sr/⁸⁶Sr (0.70468–0.70706), ¹⁴³Nd/¹⁴⁴Nd (0.512946–0.512447), and a moderate range in ²⁰⁶Pb/²⁰⁴Pb (18.825–19.143), ²⁰⁷Pb/ ²⁰⁴Pb (15.643–15.760) and ²⁰⁸Pb/²⁰⁴Pb (38.97–39.51). Trace-element and isotopic data suggest that the mantle beneath the eastern Sunda arc is a complex heterogeneous mixture of 3 or 4 major source components: MORB-source or depleted MORB-source, OIB-source and subducted Indian Ocean sediment. The low-K tholeiites were probably formed by relatively large degrees of melting of depleted MORB-source mantle, modified by subduction-related fluids, whereas the trace-element and isotopic characteristics of the K-rich volcanics suggest that they were derived from an OIB source which and been modified by a subduction-related melt component. The source components of the medium-to high-K calcalkaline rocks are more difficult to determine, and probably include mixtures of MORB-source or OIB-source, and melt/fluid derived from subducted oceanic sediment. Minor-and trace-element modelling calculations indicate substantial difficulties in producing the relatively low Ti-contents of arc volcanics by melting OIB-source mantle. Where OIB mantle is considered to be an important component of arc magmas it is suggested that the HFSE are buffered to relatively low concentration by a residual Ti-rich accessory phase.     

Paleoproterozoic subduction-related metasomatic signatures in the lithospheric mantle beneath NE Brazil: inferences from trace element and Sr–Nd–Pb isotopic compositions of Neoproterozoic high-K igneous rocks

Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/Yb_CN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (_Nd from −9.3–−20.1) isotopic compositions. T_DM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb=16–17.3, ²⁰⁷Pb/²⁰⁴Pb=15.1–15.6, ²⁰⁸Pb/²⁰⁴Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust.     

Permian basaltic rocks in the Tarim basin, NW China: Implications for plume–lithosphere interaction

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO₂ (2.43%–4.59%, weight percent), high Fe₂O₃ + FeO (12.63%–17.83%) and P₂O₅ (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t = 280 Ma)⁸⁷Sr/⁸⁶Sr ratio ( 0.7048) and positive εNd(t) (3.42–4.66) values. Group 2 has relatively large initial ⁸⁷Sr/⁸⁶Sr ratio (0.7060–0.7083) and negative εNd(t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of (²⁰⁶Pb/²⁰⁴Pb)t, (²⁰⁷Pb/²⁰⁴Pb)t and (²⁰⁸Pb/²⁰⁴Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.     

Evolution of alkalic lavas at Haleakala Volcano,east Maui,Hawaii

The postshield and posterosional stages of Haleakala Volcano contain intercalated alkalic basalt and evolved alkalic lavas. Isotopic and incompatible element abundance ratios in the Haleakala postshield basalts changed systematically with time, providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources. Specifically, a depleted, i.e. low⁸⁷Sr/⁸⁶Sr and high¹⁴³Nd/¹⁴⁴Nd, mantle component is more abundant in younger lavas. However, as magma-production rates decreased during the postshield and posterosional stages, basaltic melts in magma reservoirs cooled and fractionated, leading to evolved residual melts such as hawaiite. Because primary basalt compositions changed with time, the evolved Haleakala lavas formed from a range of parental compositions. However, basalts and evolved lavas of similar age and isotopic ratios (Sr and Nd) have major and trace element contents that are consistent with a crystal-fractionation model. Although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both Haleakala and Mauna Kea volcanoes, there are important differences between their lavas. For example, compositional differences between the hawaiite suites at Haleakala and Mauna Kea indicate that, on average, the evolved lavas at Haleakala formed at lower pressures. Also, at Haleakala basalts are intercalated with hawaiites, whereas at Mauna Kea basalts and hawaiites are separated by a sharp boundary. These differences probably reflect a higher magma supply rate to the Haleakala volcano.     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove