Showing papers on "Incompatible element published in 1991"

Element Fluxes Associated with Subduction Related Magmatism

Abstract: Destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element (REE) ratios. Most island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87 Sr/ 86 Sr = 0.7033, 143 Nd/ 144 Nd = 0.51302, 206 Pb/ 204 Pb = 18.76 , 207 Pb/ 204 Pb = 15.57, and 208 Pb/ 204 Pb = 38.4. However, they also have Rb/Sr (0.03), Th/U (2.2) and Ce/Yb (8.5) ratios which are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted heavy REE, and much more variable isotope ratios. Such rocks are found in the Aeolian Islands, Grenada, Indonesia and Philippines, and their isotope and trace element features have been attributed both to contributions from subducted sediment, and/or old trace element enriched material in the mantle wedge. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid-ocean ridge basalts, since these may contain ca. 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that not more than 15% of the Sr and Th in an average arc magma were derived from subducted material, and that the rest were derived from the mantle wedge. The fluxes of elements which cannot be characterized isotopically are more difficult to constrain, but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.

Degassing history of water, sulfur, and carbon in submarine lavas from Kilauea Volcano, Hawaii

Abstract: Major, minor, and dissolved volatile element concentrations were measured in tholeiitic glasses from the submarine portion (Puna Ridge) of the east rift zone of Kilauea Volcano, Hawaii. Dissolved H_(2)O and S concentrations display a wide range relative to nonvolatile incompatible elements at all depths. This range cannot be readily explained by fractional crystallization, degassing of H20 and S during eruption on the seafloor, or source region heterogeneities. Dissolved C0_2 concentrations, in contrast, show a positive correlation with eruption depth and typically agree within error with the solubility at that depth. We propose that most magmas along the Puna Ridge result from (I) mixing of a relatively volatile-rich, undegassed component with magmas that experienced low pressure (perhaps subaerial) degassing during which substantial H_(2)O, S, and C0_2 were lost, followed by (2) fractional crystallization of olivine, clinopyroxene, and plagioclase from this mixture to generate a residual liquid; and (3) further degassing, principally of C0_2 for samples erupted deeper than 1000 m, during eruption on the seafloor. The degassed end member may form at upper levels of the summit magma chamber (assuming less than lithostatic pressure gradients), during residence at shallow levels in the crust, or during sustained summit eruptions. The final phase of degassing during eruption on the seafloor occurs slowly enough to achieve melt/vapor equilibrium during exsolution of the typically CO_(2)-rich vapor phase. We predict that average Kilauean primary magmas with 16% MgO contain ~0.47 wt% H_(2)O, ~900 ppm S, and have δD values of ~-30 to -40‰. Our model predicts that submarine lavas from wholly submarine volcanoes (i.e., Loihi), for which there is no opportunity to generate the degassed end member by low pressure degassing, will be enriched in volatiles relative to those from volcanoes whose summits have breached the sea surface (i.e., Kilauea and Mauna Loa).

Dynamic melting of the Iceland plume

Abstract: Icelandic high-magnesia basalts show striking correlations between major element abundances and incompatible element and radiogenic isotope ratios. The most MgO-rich lavas have the most depleted incompatible element ratios and among the least radiogenic lead isotopes recorded in Atlantic mid-ocean-ridge basalts, highlighting a decoupling of the major and trace element characteristics expected of plume melts. This paradox can be explained by the process that mixes melts segregated from different depths of the melting column. The resulting model provides insight into the processes governing melt compositions at spreading ridges.

The evolution of Mauna Kea Volcano, Hawaii: Petrogenesis of tholeiitic and alkalic basalts

Abstract: Mauna Kea Volcano has three exposed rock units. Submarine shield-building tholeiites form the oldest unit. Subaerial, interbedded tholeiitic and alkalic basalts form the intermediate age unit (70–240 Ka), and they are partially covered by evolved alkalic lavas, hawaiites and mugearites (4–66 Ka). In contrast to other Hawaiian volcanoes, such as Haleakala and Kauai, lavas from Mauna Kea do not define systematic temporal variations in Pb, Sr or Nd isotopic ratios. However with decreasing age the tholeiitic basalts are increasingly enriched in incompatible elements; therefore the shield and postshield tholeiites were derived from compositionally distinct parental magmas. Submarine shield lavas from the east rift contain forsterite-rich olivine (up to Fo90.5) providing evidence for MgO-rich (14.4 to 17%) magmas. Postshield tholeiitic and alkalic basalts with similar isotopic ratios may have been derived from the same source composition by different degrees of partial melting. If a compositionally and isotopically homogeneous source and a batch melting model are assumed, inversion of incompatible element abundance data for the postshield basalts requires low degrees (<2%) of melting of a garnet Iherzolite source which had near-chondritic abundances of heavy rare-earth elements (REE) but less than chondritic abundances of highly incompatible elements such as Ba, Nb and light REE. As the volcano migrated away from the hotspot, eruption rates decreased enabling high Fe-Ti basalts to form by fractional crystallization in shallow crustal magma chambers. The associated phenocryst-rich, high-MgO postshield lavas (picrites and ankaramites) are products of phenocryst accumulation. Eventually basaltic eruptions ceased, and the youngest Mauna Kea lavas are exclusively hawaiites and mugearites which formed from alkalic basalt parental magmas by clinopyroxene-dominated fractionation at lower crustal pressures.

The tholeiite to alkalic basalt transition at Haleakala Volcano, Maui, Hawaii

Abstract: Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (∼1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of ∼10% to 15%, while the intercalated alkalic lavas reflect a range of ∼6.5% to 8% melting. However, within the uppermost 50 m of section. 87Sr/86Sr decreases from 0.70371 to 0.70328 as eruption age decreased from ∼0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at ∼0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.

New data on magmatic H 2 O contents of pantellerites, with implications for petrogenesis and eruptive dynamics at Pantelleria

Abstract: Infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at Pantelleria, Italy, show that the magmas contained moderate pre-eruptive H2O contents, ranging from 1.4 to 2.1 wt.%. Melt H2O concentrations increase linearly with incompatible elements, demonstrating that H2O contents were not buffered significantly during fractionation by any crystalline or vapor phase. The relatively low H2O contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt. Preeruptive H2O concentrations do not correlate with the volume or explosivity of pantellerite eruptions; decompression history is critical in determining the style of pantellerite (and other) eruptions.

Melting in the lithospheric mantle: Inverse modelling of alkali-olivine basalts from the Big Pine Volcanic Field, California

Abstract: The variations in trace element abundances of a suite of alkali-olivine basalts from the Big Pine volcanic field, California, have been ‘inverted’ following the method of Hofmann and co-workers to obtain source concentration and distribution coefficient data. The high Mg-numbers and ne-normative mineralogy of these lavas allow a simple correction to be made for fractional crystallisation, and together with a limited range in 87Sr/86Sr (0.7056–0.7064), suggest derivation from a relatively homogeneous source region. Negative correlations between SiO2 and P2O5, and SiO2 and Rb in the calculated primary magmas imply that both major and trace elements vary in a coherent fashion as a function of the degree of partial melting. The Big Pine lavas are characterised by high ratios of large-ion lithophile to high-field strength elements (Ba/Nb>60), and the inverse procedure demonstrates that this reflects source concentrations, as opposed to a mineralogical control. The calculated mantle source is further characterised by generally high abundances of Sr, Ba, K, and Th relative to Nb and Ta which imply that incompatible element enrichment of the source occurred above a subduction zone. A model Sm/Nd age of 1.8 Ga for this enrichment coincides with the regional crustal formation age. Such features imply that both the major and trace element components of the Big Pine lavas are derived from within lithospheric mantle, perhaps mobilised by the high geothermal gradients which characterise the extensional environment of the Basin and Range Province. A comparison with other Cenozoic mafic lavas throughout the western United States suggests that a substantial proportion of the mantle lithosphere in this area has similar chemical characteristics to the source of the Big Pine lavas. If this is the case, then it implies that convergent margins represent an important tectonic environment for the formation of lithospheric mantle.

Development of the Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain

Abstract: Glass Mountain, California, consists of >50 km3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization. The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2 Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus between extrusion of the last Glass Mountain lava and the climactic eruption.

Geochemistry of peridotites and mafic igneous rocks from the Central Dinaric Ophiolite Belt, Yugoslavia

Abstract: In the Central Dinaric Ophiolite Belt (CDOB) peridotites and associated metamorphic rocks of various grades tectonically overlie an olistostrome melange of middle to late Jurassic age. Peridotites and underlying slices of mafic granulites (partially transformed to gamet amphibolites) are intruded by doleritic dikes which do not occur in the melange. The melange contains blocks of subgreywackes and cherts as well as those of pillow lavas and massive diabase (spilites). CDOB peridotites are in the spinel peridotite facies, but locally spinel-plagioclase peridotites occur as well. All peridotites have lherzolitic compositions showing several significant element correlations: Al2O3, CaO, TiO2, Na2O and Cu are negatively correlated and Ni is positively correlated with MgO. Recent estimates of primitive mantle compositions lie near the low-MgO end point of each correlation trend. Al/Ti and Ca/Al ratios of CDOB lherzolites are for the most part higher than the range observed in chondrites. However, when a few samples with extreme compositions are excluded, Al/Ti and Ca/Al are positively correlated with MgO, and the samples at the low-MgO end have near-chondritic Ca/Al but slightly higher than chondritic Al/Ti ratios. Chondrite-normalized REE patterns of CDOB lherzolites show extreme depletions in LREE providing strong evidence for the absence of any metasomatic renrichment. The lack of correlation between highly incompatible elements (LREE) and moderately incompatible elements (HREE, Ti, Na, Al, Ca) together with the extremely low La/Sm ratios suggest that fractional or very small increment melt removal played a role in the genesis of these lherzolites. Four out of five lherzolites yield and apparent Sm-Nd isochron age of 136±15 Ma with an ɛNb of 6.0±1.1 (bulk rocks and clinopyroxene separates). One sample has an exceptionally high ɛNd of about 23. The mafic igneous rocks scatter around the lower end of the 136 Ma reference isochron allowing, but not proving, a genetic relationship with a mantle having a Nd isotopic composition which is similar to that of CDOB lherzolites. LIL element abundances of spilites and doleritic dike rocks suggest some hydrothermal alteration. In primitive mantle-normalized concentration diagrams none of these mafic igneous rocks shows a significant negative Nb-Ta anomaly. Chondrite-normalized REE patterns of both rock types are essentially flat. Whereas the inferred primary compositions of the spilites compare well with those of E-type MORBs, the doleritic dike rocks show elemental ratios similar to those normally found in back-arc basin tholeiites.

Destructive margin magmatism and the contributions from the mantle wedge and subducted crust

Abstract: Destructive plate margin magmas may be considered in two groups on the basis of their REE ratios. Many island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87Sr/86Sr = 0.7033 (±0.0002), 143Nd/ 144Nd = 0.51302 (±0.December 20014), 206Pb/204Pb = 18.76 (±0.13), 207Pb/204Pb = 15.57 (±0.02) and 208Pb/204Pb = 38.4 (±0.18). However, they also have Rb/Sr (0.027 ± 0.025), Th/U (2.18 ±0.5) and Ce/Yb (8.4 ±2.7) ratios that are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted HREE and much more variable isotope ratios, and these have been attributed to both contributions from subducted sediment, and old, trace element‐enriched material in the mantle wedge. The Th isotope ratios of destructive margin rocks are similar to those in mid ocean ridge basalt (MORB) and ocean island basalt (OIB). However, while many MORB and OIB are displaced to low (238U/230Th) on the (230Th/232Th)‐(238U/232Th)...

Geochemistry of Quaternary lavas from NE Sulawesi: transfer of subduction components into the mantle wedge

Abstract: Major and trace element, and Sr-Nd isotope compositions were determined for Quaternary volcanic rocks from NE Sulawesi (the Sangihe are), Indonesia, in order to examine the origin of across-arc variation in lava and magma source chemistry. The arc is formed in an intraoceanic tectonic setting and is not associated with a backarc basin, thereby minimizing possible contributions from non-arc geochemical reservoirs. The geochemistry of these arc lavas is likely to provide essential information about the chemical characteristics of subduction components. All incompatible elements, except Pb, increase away from the volcancic front. Major element data for Mg-rich lavas together with available experimental data, suggest that primary magmas are produced at higher pressured by smaller degrees of partial melting beneath the backarc-side volcanoes. Rb/K and Ba/Pb are higher, and 87Sr/86Sr and 143Nd/144Nd are lower in backarc-side lavas. These variations may be attributed to generation of hydrous fluids in the downdragged hydrous peridotite layer at the base of the mantle wedge through the following reactions: decompositions of pargasitic amphibole to form phlogopite and breakdown of phlogopite to crystallize K-richterite, beneath the volcanic front and the backarc-side volcanoes, respectively.

Lunar ferroan anorthosites and mare basalt sources: The mixed connection

Abstract: Global overturn of a hot, gravitationally unstable lunar mantle immediately following the solidification of a magma ocean explains several characteristics of lunar petrology. Lunar mare basalt sources are inferred to be depleted in europium and alumina. These depletions are consensually attributed to complementary plagioclase floating from a magma ocean. However, in contrast to the mare basalt source parent magma, the ferroan anorthosite parent magma was more evolved by virtue of its lower Mg/Fe ratio and Ni abundances, although less evolved in its poverty of clinopyroxene constituents, flat rare earth pattern, and lower incompatible element abundances. The europium anomaly in mare sources is inferred to be present at 400 km depth, too deep to have been directly influenced by plagioclase crystallization. Massive overturning of the post-magma ocean mantle would have carried down clinopyroxene, ilmenite, and phases containing fractionated rare earths, europium anomalies, and some heat-producing radionuclides.

Contamination of basaltic magma by mafic crust at Amboy and Pisgah Craters, Mojave Desert, California

Abstract: Quaternary alkali basalts from Pisgah Crater and Amboy Crater in southern California exhibit unusual chemical and isotopic variations which probably result from assimilation of mafic crust. Although lavas from both volcanoes are alkali basalts and hawaiites with isotopic and chemical characteristics that are similar to ocean island basalts (OIB) (e.g., eNd=2.6 – 5.9, 87Sr/86Sr = 0.7038 – 0.7049, Hf/Ba=0.014 – 0.017), they display highly correlated and unusual variations in their chemical and isotopic compositions. At each volcano, MgO decreased during the eruptive sequence from ≈8.5 wt % to ≈4.5 wt %. Incompatible elements are positively correlated with MgO and therefore also decreased during the eruptive sequence. Nd, Sr, and Pb isotope ratios correlate strongly with MgO. Compositional and isotopic data cannot be explained by any combination of closed-system fractionation, partial melting of the mantle, or silicic contamination. These data indicate that the basalts represent mixing between a high-MgO, high-eNd component, common to both Pisgah and Amboy craters, with lower-MgO and -eNd components that are unique to each center. The high-MgO component most likely is a primitive mantle-derived magma, based in part on its similarity to nearby xenolith-bearing Quaternary basalts. The low-MgO components are interpreted to be partial melts of mafic crust. If our model is correct, then both volcanoes evolved from eruption of nearly pure mantle melts early in their history to eruption of nearly pure remelted mafic crust late in their history. The crustal source could not have been underplated Mesozoic or younger oceanic crust, but its age is otherwise unconstrained; nearby Mesozoic gabbros and Proterozoic diabases have appropriate isotopic compositions. The basalt data provide no evidence that ancient enriched lithospheric mantle currently underlies the Mojave Desert. If such mantle was present at any time beneath this region, it must have been removed during one or more of the many tectonic events that affected the Mojave Desert during the Phanerozoic. Regional variability of isotope ratios in basalts is commonly interpreted to reflect variability of the underlying mantle. Data from this study raise the possibility that some of this variability may result from cryptic contamination of OIB-like basalts by mafic crust.

Chemical and isotopic constraints on the origin of basalts from Ninetyeast Ridge, Indian Ocean: results from DSDP Legs 22 and 26 and ODP Leg 121

Abstract: There are significant variations in isotope ratios and highly incompatible element ratios between sites, which suggest that the mantle source for the ridge basalts was compositionally variable. The authors agree with previous models that relate Ninetyeast Ridge to a mantle plume in the southern Indian Ocean. The tholeiitic, iron-enriched, and voluminous character of the ridge basalts is typical of oceanic islands associated with plumes on or near a mid-ocean ridge. The major element data, like the gravity data, strongly suggest that the ridge was erupted on or very close to an active spreading center. Isotopically, the most likely plume that created the excess magmatism on the Ridge is the Kerguelen-Heard plume system, but the Ninetyeast Ridge basalts do not represent a simple mixing of the Kerguelan plume and mid-ocean Ninetyeast Ridge basalt mantle. -from Authors

Precaldera lavas of the southeast San Juan Volcanic Field: Parent magmas and crustal interactions

Abstract: Early intermediate composition volcanic rocks of the Oligocene (circa 34–29 Ma) southeast San Juan volcanic field, southern Colorado, comprise the Conejos Formation. Conejos lavas include both high-K calc-alkaline and alkaline magma series (54–69% SiO2) ranging in composition from basaltic andesite (basaltic trachyandesite) to dacite (trachydacite). The subsequent Platoro caldera complex (29–27 Ma) was superimposed on a cluster of broadly precursory Conejos stratocones. Precaldera volcanism occurred in three pulses corresponding to three time-stratigraphic members: (1) the Horseshoe Mountain member, (2) the Rock Creek member, and (3) the Willow Mountain member. Each member exhibits distinctive phenocryst modes and incompatible trace element contents. Horseshoe Mountain lavas (hornblende-phyric) have relatively low alkali and incompatible element abundances, Rock Creek lavas (anhydrous phenocrysts) and ash-flow tuffs have the highest abundances, and Willow Mountain lavas (diverse mineralogy) are intermediate. All Conejos lavas exhibit low ratios of lead (206Pb/204Pb = 17.5 to 18.2) and neodymium (eNd = −8 to −4) isotopes and high 87Sr/86Sr (0.7045 to 0.7056) compared to depleted asthenospheric mantle. These values lie between those of likely mantle compositions and the isotopic composition of Proterozoic crust of the southern Rocky Mountains. Mafic lavas of the Horseshoe Mountain member have the lowest Pb and Nd isotope ratios among Conejos members but trend toward higher isotopic values with increasing degrees of differentiation. Compositions within the Rock Creek series trend toward higher Pb and lower Nd isotope ratios with increasing SiO2. Willow mountain volcanic sequences define diverse chemical-isotopic correlations. We interpret the chemical and isotopic differences observed between mafic lavas of each member to reflect derivation from compositionally distinct mantle derived parent magmas that have experienced extensive deep level crustal contamination. Chemical and isotope correlations within each member indicate Conejos magmas differentiated via staged polybaric, multiprocess evolutionary paths. Stage I occurred near the base of the crust as mantle-derived basalt evolved to basaltic andesite by the MASH mechanism. Stage II followed ascent to shallower crustal levels where basaltic andesite differentiated to more evolved compositions by combined fractionation and assimilation of heterogeneous crust.

Petrological and geochemical constraints on the genesis of Mesozoic-Cenozoic magmatism of King George Island, South Shetland Islands, Antarctica

Abstract: Petrological and geochemical data are reported for a series of Late Cretaceous-Middle Miocene volcanic, hypabyssal and intrusive rocks from King George Island (KGI) and from nearby Ridley Island, South Shetland Islands. Major element data indicate a calc-alkaline, basic to intermediate composition for the analysed samples. Although emplaced on a continental margin, the KGI rocks generally display low abundances of incompatible trace elements, close to those typically observed in calc-alkaline suites erupted in intraoceanic island arcs. A few samples have a significant negative Ce anomaly. Many incompatible elements define smooth positive trends on interelemental variation diagrams which suggests that magmas erupted at different times on KGI maintained a rather constant composition in terms of incompatible element ratios. Geochemical modelling, based on Sr isotope ratios and incompatible element ratios, suggests that the primary calc-alkaline magmas of KGI were all generated in an upper mantle modified by addition of small amounts of pelagic sediments dragged down by subduction processes.

Geochemistry of the Mesozoic regional basic dykes of western Dronning Maud Land, Antarctica

Abstract: Regional dolerite dykes of Mesozoic age in western Dronning Maud Land are variable in both major and trace element composition and include picritic types (MgO>18 wt%). The range in incompatible element concentrations is considerable (e.g. Zr 40–478 ppm) and shows little correlation with MgO content. Both high-and low-Ti, Zr (HTZ and LTZ) magma types are present and there is a spread of compositions between these types. Major element oxide variations in dykes having MgO>10 wt% indicate that olivine and orthoyproxene fractionation occurred, presumably at an early high-pressure stage of magma evolution. Major element oxide variations in dykes having MgO<10 wt% indicate control by olivine and clinopyroxene. A minority of the more evolved dykes are compositionally similar to the nearby Kirwan basalts, but the majority cannot be related to the Kirwan basalts by any simple petrogenetic process as they contain higher concentrations of incompatible elements and have higher Mg-numbers. The HTZ Dronning Maud Land dolerites have incompatible trace element concentrations which are very similar to the HTZ basalt magma types of the Karoo of southern Africa with the exception of lower K and Rb in DML dolerites. The HTZ dolerites occur in the part of Dronning Maud Land which appears to have been tectonically stable since the Archaean and are not found to intrude the surrounding high-grade (about 1000 Ma) metamorphic rocks of the Sverdrup Group. These data provide qualified support for models which seek to relate spatially the HTZ Mesozoic basalt types of Gondwana to sources beneath stable Archaean cratons.

Compositional changes in Trans‐Pecos Texas Magmatism Coincident with Cenozoic stress realignment

Abstract: The composition and inferred sources of Cenozoic magmas in Trans-Pecos Texas changed abruptly between 32 and 28 Ma, coincident with a fundamental change in stress regime. Magmas emplaced between 48 and 32 Ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas. These rocks have low Nb and Ta compared to Zr, Hf, La, Ba, and K, characteristics typical of continental volcanic arcs. However, the Texas rocks are more alkalic, have higher concentrations of incompatible elements, and have higher ratios of Nb and Ta to Zr, Hf, La, Ba, and K than is typical of rocks from arcs near trenches. These differences reflect relative position within the Cordilleran arc. Trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios. Trans-Pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone. Magmas emplaced after 31 Ma were either bimodal, alkali basalt-rhyolite, or exclusively basaltic. They have high absolute abundances of incompatible trace elements and high ratios of Nb and Ta to Zr, Hf, La, Ba, and K. The mafic rocks are typical of continental rift or ocean island basalts. Trace element models using Nb, Ta, Y, Yb, Hf, and Zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting, variable amounts of fractional crystallization, but little or no assimilation. The change in magma compositions and sources is best illustrated by a drop in Zr/Nb, from between 18 and 6.25 in the earlier, arc rocks to between 6.25 and 2 in the later, rift-related rocks. Stress regime appears to play an important role in the process of magma generation and evolution. The change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 Ma occurred in a continental volcanic arc, whereas later activity occurred in a setting of intraplate extension. The magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3–4 m.y.

Sr, Nd and Pb Isotopic Constraints in the Genesis of a Calc-Alkaline Plutonic Suite in Oman Ophiolite Related to the Obduction Process

Abstract: K-mineral bearing felsic intrusions in the Oman ophiolite have been studied for incompatible elements, and Pb, Sr, Nd isotopes. The magmatic bodies (mainly from the Khawr Fakkan massif) represent differentiated tens of meter sized intrusions or metric dikes cross-cutting the ophiolite at mantle level. Metamorphic schists and quartzites having reached anatexis, belonging to the sole of the same massif are also studied for comparison.

9. geochemistry and petrology of basalts from leg 136, central pacific ocean1

Abstract: About 13 m of Cretaceous, tholeiitic basalt, ranging from normal (N-MORB) to transitional (T-MORB) mid-ocean-ridge basalts, was recovered at Ocean Drilling Program Site 843 west of the island of Hawaii. These moderately fractionated, aphyric lavas are probably representative of the oceanic basement on which the Hawaiian Islands were built. Whole-rock samples from parts of the cores exhibiting only slight, low-temperature, seawater alteration were analyzed for major element, trace element, and isotopic composition. The basalts are characterized by enrichment in the high field strength elements relative to N-MORB, by a distinct positive Eu anomaly, and by Ba/Nb and La/Nb ratios that are much lower than those of other crustal or mantle-derived rocks, but their isotope ratios are similar to those of present-day N-MORB from the East Pacific Rise. Hole 843A lavas are isotopically indistinguishable from Hole 843B lavas and are probably derived from the same source at a lower degree of partial melting, as indicated by lower Y/Nb and Zr/Nb ratios and by higher concentrations of light and middle rare earth elements and other incompatible elements relative to Hole 843B lavas. Petrographic and trace-element evidence indicates that the Eu anomaly was the result of neither Plagioclase assimilation nor seawater alteration. The Eu anomaly and the enrichments in Ta, Nb, and possibly U and K relative to N-MORB apparently are characteristic of the mantle source. Age-corrected Nd and Sr isotopic ratios indicate that the source for the lavas recovered at ODP Site 843 was similar to the source for Southeast Pacific MORB. An enriched component within the Cretaceous mantle source of these basalts is suggested by their initial 208 Pb/ 204 Pb- 206 P b/ 204 Pb and eNd- Pb/ Pb ratios. The Sr-Pb isotopic trend of Hawaiian post-shield and post-erosional lavas cannot be explained by assimilation of oceanic crust with the isotopic composition of the Site 843 basalts.

Neodymium isotope evidence for ultra-depleted mantle in the early Proterozoic

Abstract: THE episodic extraction of juvenile continental crust from the Earth's mantle over the past 4 Gyr has led to a progressive depletion of incompatible elements in the upper mantle1,2. A knowledge of the degree and uniformity of this mantle depletion throughout Earth history is important for understanding the growth of continents, the evolution of the crust-mantle system and the nature of mantle convection through time. Here we report initial 143Nd/144Nd ratios, for 1.8-Gyr-old mafic volcanics from the Harts Range meta-igneous complex of central Australia which are the highest yet reported for Proterozoic igneous rocks (ɛNd = +6.9 to +8.2 for the least contaminated samples). These ratios far exceed those proposed in models3–5 for the isotopic evolution of the depleted mantle at this time, and imply the existence of a mantle reservoir that had been highly depleted for at least 1 Gyr. This provides strong evidence that major periods of continental growth, such as that in the late Archaean, produced long-lived heterogeneities in the upper mantle.

Amphibolites of the Ashe and Alligator Back Formations, North Carolina: Samples of Late Proterozoic-early Paleozoic oceanic crust

Abstract: The amphibolites of the Late Proterozoic-early Paleozoic Ashe and Alligator Back Formations in western North Carolina represent metamorphosed tholeiitic basalts, unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the Grenville-age basement rocks. On the basis of their geochemistry, the amphibolites are divisible into three compositional groups (I, II, and III), the protoliths of which appear to have evolved from different parental magmas at a spreading center. Group II (intermediate-Ti) amphibolite, the predominant variety in the study area, constitutes a highly fractionated suite (Zr ≃ 40-200 ppm, TiO2 ≃ 0.7-2.3 wt.%) with geochemical characteristics similar to those of N-type MORB formed from a heterogeneous mantle source. Group III (high-Ti) amphibolite is enriched in high-field-strength incompatible elements (Zr, Nb, Ti, Y, REE's), especially Ti (TiO2 = 2.8-3.4 wt.%), and is interpreted as T-type MORB on the basis of its Zr/Nb and Y/Nb ratios. Group I (low-Ti) amphibolite, restricted to the Ashe Formation, is characterized by extremely low abundances of incompatible elements, including Ti (TiO2 < 0.45 wt.%), and U-shaped REE profiles, requiring the mixing of a depleted MORB-type mantle source with a fluid phase enriched in LREE's. The juxtaposition of depleted, low-Ti basalt (group I amphibolite) and MORB-like basalts (group II and group III amphibolites) is suggestive of a back-arc basin setting, as has been postulated for many ophiolites, but it can also be accounted for by multi-stage melting of an upper-mantle source adjacent to a mantle plume in a mid-oceanic-ridge environment. In either case, the Ashe and Alligator Back amphibolites represent metamorphosed oceanic crust material generated at a spreading center.