Dodie James

INCOMPATIBLE TRACE-ELEMENTS IN OIB AND MORB AND SOURCE ENRICHMENT IN THE SUB-OCEANIC MANTLE

The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched and fractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as CePb, are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as CeU, are relatively uniform. The CePb ratios in enriched OIB are correlated with unusually high UPb and low KU. These UPb ratios would have generated excessively radiogenic Pb if they were long-term (109 yr) features of the source such as might result from core formation or recycling of hydrothermally altered ocean floor basalts. However, volcanic centers with high UPb do have high 206Pb204Pb for their 207Pb204Pb, a feature that is most easily modelled by enrichment in U relative to Pb about 108 yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole (≤ 2%), sulfide (≤ 0.2%) and phlogopite (≤ 0.2%) at about the time of formation of the lithosphere, would generate a ‘near-surface fractionated’ (NSF) source with low KU and high UPb, Δ206Pb204Pb and CePb, while maintaining CeU, NbU, BaCe and BaNb that are only slightly fractionated relative to other OIB. An important feature of the modelling of NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from UThRa disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes place during the production of the erupted magmas. The CeU, BaCe and UPb ratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistent with the degree of isotopic variability in OIB relative to MORB and is difficult to explain unless the variations in incompatible trace element ratios in MORB are dominated by effects other than melting. Ratio-element plots provide evidence that the incompatible element ratios of MORB are affected by OIB-component contamination in the source or in transit to the surface and this is consistent with covariation between trace element ratios and some isotopic compositions in MORB. The ratios and concentrations of highly incompatible trace elements in MORB vary as a consequence of this contamination, as well as degree of partial melting. The relative uniformity and near chondritic proportions of CeU and, to a lesser extent, BaCe in OIB compared with MORB are difficult to reconcile with recycling models that advocate material resembling present-day MORB or hydrothermally altered MORB as the dominant component of OIB sources but are consistent with NSF mantle recycling. Similarly, the Ba/U/Ce ratios are inconsistent with models in which the OIB source was affected by Ca perovskite fractionation in a magma ocean on the early Earth.

Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time

Widespread basic magmatism across much of the western United States in the late Cenozoic followed the cessation of subduction along the Pacific coast. This volcanism accompanied lithospheric extension, block faulting and regional uplift In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO>4%) lava samples from all the major volcanic fields in the region. The data were divided into seven sets representing the main tectonomagmatic provinces: Basin and Range (BR), Western Great Basin, Transition Zone (TZ), Colorado Plateau, Snake River Plain, Southern Rocky Mountains, and Great Plains. It was further divided into relatively recent ( 5 Ma) subsets on the basis of field relations and K-Ar data. The 5 Ma) subset shows no great differences between the BR and the other tectonomagmatic provinces; all have high La/Nb and Ba/Nb. Crustal contamination alone cannot be responsible for these variations. We conclude that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. Pelagic sediment, returned to the mantle by subduction, is a possible agent for fluids rich in Ba, radiogenic Sr and unradiogenic Nd, but very poor in Nb. At least some of this enrichment must have accompanied the formation of the Proterozoic crust. It appears that subduction-enriched lithospheric mantle was involved in the generation of all extension-related basic magmas across the western United States until relatively recently. Only in the younger BR and parts of the TZ have asthenosphere-derived magmas, uncontaminated by lithosphere, reached the surface. These observations conflict with models in which uplift and extension are caused by the replacement of mantle lithosphere by asthenosphere. Triey are best explained by the progressive erosion of the lithospheric manue over a plume currently located beneath the Southern Rocky Mountains. Supplemental data are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B91-001;

Enrichment of the continental lithosphere by OIB melts: Isotopic evidence from the volcanic province of northern Tanzania

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Population density and zoning of olivine phenocrysts in tholeiites from Kauai, Hawaii

Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (87Sr/86Sr= 0.7035−0.7058,eNd = −5to+3, and206Pb/204Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated238U/204Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.

Basic Volcanism Associated with Intraplate Linear Features [and Discussion]

The population density of olivine phenocrysts of the tholeiites display an exponential variation, which is typical of igneous as well as contact metamorphic rocks. The exponential variation is explained by a new growth probability model, which is consistent with experimental work. The forsterite content of the olivine phenocrysts decreases with decreasing size. Various phenocryst features suggest that the tholeiites first crystallized slowly in a magma chamber, after which they underwent crystallization for a short period of time in a feeder dyke before eruption took place.

Delayed fractionation of basaltic lavas

Intraplate volcanic lineaments include ocean island chains and continental rift systems. Basic lavas erupted in such lineaments form a continuum from tholeiitic basalt in the basements of ocean islands to nephelinites and melilitites in continental rifts and as a capping on ocean islands. All these magma types are enriched in large-ion lithophile elements (l.i.l.e.) compared with mid-ocean ridge basalts (m.o.r.b.), although isotopic data suggest that their mantle sources had been depleted in l.i.l.e. for long periods. In this paper we present a comparison of geochemical data from several suites of basic volcanic rocks ranging from Hawaiian tholeiite to Ugandan melilitite. L.i.l.e. abundance patterns can, in most cases, be explained by variable degrees of melting of a l.i.l.e.-depleted m.o.r.b. mantle source containing l.i.l.e.-rich streaks. Metasomatic enrichment of the mantle source is not a necessary precursor to magmatism.

Trace element characteristics of Upper Cenozoic basaltic rocks of Thailand, Kampuchea and Vietnam

The phenocryst cores of the basaltic lavas from Jan Mayen and Hawaii display a range in compositions. The textural features of the phenocrysts also vary, both euhedral and skeletal phenocrysts are present in the same thin section. Apparently the basaltic magmas underwent crystallization within a temperature interval of 50–200° C before they became fractionated. The fractionates of basaltic lavas are therefore average compositions of the phenocryst assemblages rather than liquidus compositions. This type of fractionation is called delayed fractionation. It is considered that most tholeiitic and alkalic basaltic lavas undergo delayed fractionation.

The Role of Lithospheric Mantle in the Generation of Late Cenozoic Basic Magmas in the Western United States

Abstract A suite of 57 samples of Upper Cenozoic basaltic rocks of Southeast Asia are classified as nephelinite, basanite, trachybasalt, alkali basalt, basaltic trachyandesite, sub-alkali (tholeiitic) basalt and basaltic andesite, using a combination of petrographic criteria and major element geochemistry. The nephelinites are characterized by enrichment in most trace elements, including Ba, Sr, Zr, Y, Nb, V, La, Ce and Nd. Basanites and trachybasalts are more varied but tend to display more moderate enrichment in most of the same elements compared to the alkali basalts and basaltic trachyandesites. The sub-alkali basalts and basaltic andesites generally contain low abundances of these elements. The chemical data are consistent with origin of the above groups by increasing amounts of partial melting of a garnet peridotite source combined with crystal fractionation involving mainly olivine, pyroxene and (except in the nephelinite group) plagioclase. Commonly employed discrimination diagrams for chemical affinity and tectonic setting correctly classify most of these samples as continental (within-plate) alkalic or tholeiitic.