Showing papers on "Incompatible element published in 1982"

Trace element characteristics of lavas from destructive plate boundaries

Abstract: Volcanic are basalts are all characterized by a selective enrichment in incompatible elements of low ionic potential, a feature thought to be due to the input of aqueous fluids from subducted oceanic crust into their mantle source regions. Island arc basalts are additionally characterized by low abundances (for a given degree of fractional crystallization) of incompatible elements of high ionic potential, a feature for which high degrees of melting, stability of minor residual oxide phases, and remelting of depleted mantle are all possible explanations. Calc-alkaline basalts and shoshonites are additionally characterized by enrichment of Th, P, and the light REE in addition to elements of low ionic potential, a feature for which one popular explanation is the contamination of their mantle source regions by a melt derived from subducted sediment. By careful selection of variables, discrimination diagrams can be drawn which highlight these various characteristics and therefore enable volcanic arc basalts to he recognized in cases where geological evidence is ambiguous. Plots of Y against Cr, K[Yb, Ce/Yb, or Th/Yb against Ta/Yb, and Ce/Sr against Cr are all particularly successful and can be modelled in terms of vectors representing different petrogenctic processes. An additional plot of Ti/Y against Nb/Y is useful for identifying 'anomalous' volcanic arc settings such as Grenada and parts of the Aleutian arc. Intermediate and acid rocks from volcanic are settings can also be recognized using a simple plot of Ti against Zr. The lavas from the Oman ophiolite complex provide a good test of the application of these techniques. The results indicate that the complex was made up of back-arc oceanic crust intruded by the products of volcanic arc magmatism.

Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation, assimilation and mixing

Abstract: The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing.

Phase Transformations and Differentiation in Subducted Lithosphere: Implications for Mantle Dynamics, Basalt Petrogenesis, and Crustal Evolution

Abstract: Mantle pyrolite differentiates at mid-oceanic ridges to form a layered lithosphere consisting of a basaltic crust, immediately underlain by harzburgite and further underlain by pyrolite which has experienced depletion only of highly incompatible elements (e.g., Rb, light REE). The body forces driving subduction are concentrated mainly in the upper cool, brittle layer of lithosphere, comprised of basalt and harzburgite. The lower layer of relatively ductile pyrolite is stripped off during subduction and resorbed into the upper mantle. This material, which is depleted in highly incompatible elements, provides a future source region of mid-ocean ridge basalt magmas on a timescale of 10⁹ years. The Nd, Sr, and Pb isotopic characteristics of MORBs are explained on the basis of this model. The slab, which sinks to ~600 km, is comprised mainly of former basalt and harzburgite. These differentiated layers undergo a significantly different series of phase transformations to those experienced by mantle pyrolite. Th...

The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex

Abstract: The volcanic stratigraphy and trace element geochemistry of the Oman ophiolite complex indicate a multistage magmatic origin comprising: (1) magmatism due to sea-floor spreading in a marginal basin; (2) magmatism associated with discrete submarine volcanic centres or seamounts; (3) magmatism associated with crustal uplift and rifting; and (4) magmatism associated with continent-arc collision. Trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event. The petrogenetic pathway for the ‘sea-floor spreading’ lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements (mostly LIL elements and H2O) from an underlying subduction zone; it also requires magma mixing in an ‘open system’ magma chamber prior to eruption. The ‘seamount’ lavas were probably derived by a similar degree of partial melting of a similar source, but fractional crystallization was restricted to smaller high-level magma chambers. The ‘rifting’ lavas were derived from a mantle source that was more depleted than the ‘seamount’ lavas prior to subduction but which was later modified by a larger subduction zone component. The ‘syn-collision’ lavas were however derived from an enriched mantle source, which probably underlay the passive continental margin rather than the marginal basin complex. Results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra-subduction zone setting.

Depletion of light rare-earth elements in felsic magmas

Abstract: Contrary to simple generalizations about their behavior, light rare-earth elements (LREE) do not act as incompatible elements in very felsic magmas. In fact, LREE concentrations typically decrease, often drastically, during differentiation of such magmas. The simplest explanation for this depletion involves the separation of minute, easily overlooked quantities of an extremely LREE-rich accessory mineral, either monazite or allanite. Our data indicate that felsic liquids with < 50 ppm LREE may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids. This accounts for incompatible behavior of LREE even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas. Partitioning of LREE into solid rather than liquid has important implications for trace-element and Nd-isotope modeling of crustal anatexis, as well as for magma differentiation.

The origin and significance of large, tabular dunite bodies in the Trinity peridotite, northern California

Abstract: Kilometer-sized, tabular dunite bodies are contained within harzburgite, lherzolite and plagioclase lherzolite host rocks in the Trinity peridotite, northern California. An igneous origin for the dunite by crystal fractionation of olivine from a melt is suggested by their tabular shapes, clots of poikilitic clinopyroxene grains, chromite pods, and by analogy to dunite bodies in the Samail and Vourinos ophiolites (Hopson et al. 1981; Harkins et al. 1980; Moores 1969). However, structures and systematic variations in mineralogy and mineral chemistry suggest that at least the marginal few meters of the bodies are residues produced by extraction of a basaltic component from a plagioclase lherzolite protolith. A model is suggested in which a picritic melt ascended through the upper mantle in vertically oriented channels. Part of the dunite in the tabular bodies was produced by fractional crystallization of olivine from the melt. Additional dunite at the margins of the bodies was formed by extraction of a basaltic component from plagioclase lherzolite wall-rocks during partial assimilation by the picritic melt. The latter process is similar to the “wall-rock reaction” discussed by Green and Ringwood (1967) and is essentially zone refining of the the mantle wall rocks by the migrating melt. It is significant because it suggests a mechanism in addition to fractional crystallization for enrichment of incompatible elements in basalts.

Galapagos Hot Spot-Spreading Center System: 1. Spatial petrological and geochemical variations (83°W-101°W)

Abstract: We report on the petrology and geochemistry of basalts dredged at 40--50 km intervals along the Galapagos Spreading Center, between 83/sup 0/W and 101/sup 0/W (40 stations). Emphasis is on spatial variations of 'whole rock' major elements, rare earths, trace metals of the first transition series, and the nature of phenocryst assemblages and their abundances. These results provide new constraints on the nature and scale of mantle source heterogeneities, melting conditions, thermal field, and dynamics of crustal formation of the region. We suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading. Basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements, which appears of worldwide extent. We reject Vogt and DeBoer's (1976) model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line, because this model would not explain the gradients in REE observed about the Galapagos Platform. Our preferred model combines the mantle-plume binary mixing model of Schilling (1973) with the concept of recurring rift propagation proposed by Hey (1977a). We further propose that pulsating mantle plume flux, perhaps inmore » the form of a chain of blobs, may initiate the development of new rifts and their propagation. The present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow. A two million year period is suggested for the last 4 m.y. from Wilson and Hey's (1979) information Rigorous testing of our preferred model is possible.« less

Elemental fingerprints of isotopic contamination of hebridean Palaeocene mantle-derived magmas by archaean sial

Abstract: One of the major puzzles presented by the geochemistry of the Palaeocene plateau lavas of Skye and Mull (N.W. Scotland) is that, although a very strong case can be made that the magmas are variably isotopically contaminated by Archaean Lewisian continental crust, little evidence has been gleaned to date from their major- and trace-element compositions to illuminate this hypothetical process. The combined results of published Sr-, Nd- and Pb-isotope studies of these lavas allow the basalts and hawaiites to be divided into three broad groups: essentially uncontaminated; contaminated with granulite-facies Archaean crust; contaminated with amphibolite-facies Archaean crust. Members of each group show distinctive chondrite-normalised incompatible-element patterns. The processes which gave rise to isotopic contamination of these lavas also affected the abundances and ratios of Ba, Rb, Th, K, Sr and light REE in the magmas, whilst having negligible effects on their abundances and ratios of Nb, Ta, P, Zr, Hf, Ti, Y and middle-heavy REE. Because such a wide range of elements were affected by the contamination process, it is postulated that the contaminant was a silicate melt of one or more distinctive crustal rock types, rather than an aqueous or similar fluid causing selective elemental movements from wall rocks into the magmas. As previous experimental and isotopic studies have shown that the Skye and Mull basic magmas were not constrained by cotectic equilibria at the time when they interacted with sial, the compositions of the contaminated lavas have been modelled in terms of simple magma-crust mixtures. Very close approximations to both the abundances and ratios of incompatible elements in the two groups of contaminated basalts may be obtained by adding 15% to 20% of Lewisian leucogneisses to uncontaminated Palaeocene basalt. Nevertheless, major-element constraints suggest that the maximum amount of granitic contaminant which has been added to these magmas lies between 5% and 10%. These estimates may be reconciled by postulating that the contaminants were large-fraction cotectic partial melts of Lewisian leucogneisses, leaving plagioclase residua. A corollary of this hypothesis is that it is necessary to postulate that the “magma chambers” where the sialic contamination occurred were, in fact, dykes or (more probably) sills. The very large surface-to-volume ratios of such magmas bodies would permit the systematic stripping, by partial melting, of the most-easily-fusible leucogneisses and pegmatites from the Lewisian crust, whilst failing to melt its major rock types. A present-day analogue to this situation may be the extensive sill-like magma bodies detected by geophysical methods within the continental crust beneath the Rio Grande Rift, southwestern U.S.A.

Geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica: evidence for mantle metasomatism

Abstract: Two episodes of tholeiite dyke emplacement have been identified in Archaean high-grade metamorphics of the Napier Complex in Enderby Land Middle Proterozoic Amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low-pressure crystal fractionation Group I Amundsen tholeiites were derived from a relatively homogeneous source region 1,190±200 my ago, whereas that of the group II Amundsen tholeiites was chemically and isotopically heterogeneous Group II dykes have various degrees of enrichment in incompatible elements, and commonly show normalised trace element abundance patterns with negative Nb anomalies These features imply variable metasomatism of the source region by a volatile-rich fluid phase (rather than a melt of any observed igneous composition) enriched in K, Rb, Ba, Th, and possibly La and Ce Early Proterozoic (2,350±48 my) tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite-facies metamorphism and include a high-Mg suite of possible komatiitic affinity, ranging in composition from hypersthene-rich tholeiite (norite) to quartz-rich tholeiite They tend to have higher ratios of highly to moderately incompatible elements (eg, K/Zr, K/Ce), and larger Nb anomalies (ie, higher K/Nb) compared with middle Proterozoic tholeiites, suggesting derivation from more enriched source regions Isotopic data are not compatible with significant crustal contamination, but constrain source metasomatism to a time immediately before emplacement Metasomatism of the source region of the much younger group I tholeiites may have been contemporaneous with that of the high-Mg suite

Chemical variation in Hercynian basalts relative to plate tectonics

Abstract: The geochemistry of Hercynian basalts is reviewed with special reference to SW England. Hercynian basalts have the following chemical features: (1) relative enrichment in incompatible elements, (2) fractionated rare earth element (REE) patterns ranging from mild to strongly light-REE enriched, and (3) generally low large-ion-lithophile (LIL)/high field strength (HFS) element ratios. High and variable alkali contents and K/Rb ratios are indicative of low-grade alteration. Basalts are predominantly tholeiites, although alkali basalts may be developed on some trough margins. Basalts from different sedimentary troughs within the main Hercynian tectonic zones are characterized by specific incompatible element abundances and ratios. These variations are interpreted in terms of variable fractional crystallization, partial melting and mantle heterogeneity. Hercynian volcanism was typically bimodal with basic and acid products characterizing the Rheno-Hercynian and Saxo-Thuringian zones. Activity in the Moldanubian zone was more variable with a high proportion of calc-alkali andesites. Two systematic chemical changes throughout the Devonian and Carboniferous have been recognized, with both light/heavy REE and LIL/HFS element ratios increasing with time. These features may relate to the introduction of LIL-enriched fluids derived by dehydration of subducted lithosphere that caused progressive metasomatism of the overlying mantle wedge. Chemical discrimination of tectonic setting demonstrates that most of the basalts typify the continental intra-plate environment, although early Devonian basalts produced in the initial stages of continent rifting have some of the chemical features of incompatible element enriched oceanic basalts. An ensialic back-arc basin model underlain by a shallow northward-dipping subduction zone provides a possible explanation for the spatial and temporal variation in the Hercynian volcanic rocks.

The origin of reversed geochemical zoning in the northern New Hebrides volcanic arc

Abstract: Integration of petrographic and geochemical data on each island in the Banks Group, northern Vanuatu (New Hebrides) has revealed a decrease in K2O (and related incompatible elements) across the islands, away from the New Hebrides trench. The correlation with depth to the Benioff zone is therefore the reverse of that typically found in island arcs. REE evidence and source modelling indicates that this variation represents a progressive depletion in LREE/HREE in the upper mantle, laterally away from the trench and a progressive increase in partial melting of the source, in the same direction. These variations in chemistry are attributed to an earlier west-dipping Miocene subduction system, and the variation in degree of partial melting to the location of the islands with respect to the active back-arc basin. An enrichment of the upper mantle in the Miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements, from the west-dipping Benioff zone, and subsequent reaction with the overlying upper mantle. The model thus envisages approximately contemporaneous development of the Central Chain volcanoes and the backarc basin in the late Pliocene, with partial melting of hydrous, laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back-arc basin. The role of the present subduction regime in magma generation is considered to be very limited.

Geochemistry and Origin of Basaltic Lavas from Society Islands, French Polynesia (South Central Pacific Ocean)

Abstract: The Society Islands of French Polynesia (south-central Pacific Ocean) are formed predominatly by basaltic lavas of alkaline affinities. These intraplate rocks are either Ne- or Hy-normative. Ne-normative basaltic lavas are lower in Si and heavy rare earth elements and higher in Ti, V and Sr than Hy-normative. The differences are attributed to the contrasting behaviour of amphibole during melting of the upper mantle, which, in turn, may be related to the variation in the depth of melting. Compared to pyrolite, the upper mantle source of the lavas was enriched in incompatible elements.

Geochemistry and origin of granitic rocks, Scourian Complex, NW Scotland

Abstract: Concordant granite sheets from the granulite facies Scourian Complex, N.W. Scotland exhibit the following features: 1) a common planar fabric with their host pyroxene granulites; 2) the presence of an exsolved ternary feldspar phase; 3) a low-pressure, water-saturated minimum composition; 4) K/Rb ratios (450–1,350) distinctly higher than most upper crustal granites but similar to the surrounding granulites; 5) low absolute concentrations of the rare earth elements (REEs), light REE enrichment, and large positive Eu anomalies.

Chemical Composition and Evolution of the Mantle

Abstract: A scheme of mantle evolution is proposed that involves extensive (~25%) partial melting of primitive mantle during accretion, followed by cumulate formation in the separated melt and transfer of late-stage fluids, similar to KREEP, from the deeper to the shallower cumulates. Midocean ridge basalts (MORB) form by remelting of incompatible element depleted garnet-rich cumulates; continental and ocean island basalts, including alkali basalts, form by partial melting of a shallow enriched peridotite layer. Forward calculations show that the initial magma and its cumulates have relatively unfractionated Rb/Sr and Sm/Nd and therefore will appear primitive in terms of isotopic ratios. Effective fractionation occurs relatively late in earth history when mantle cooling has reduced the amount of residual fluid in the cumulate layers. The transfer of an intercumulus fluid or partial melt is responsible for depletion of the MO RB reservoir and progressive enrichment of the continental/ocean-island basalt reservoir. The MORB reservoir appears to be an eclogite that earlier had lost a kimberlitic late-state melt. The eclogite, in turn, may have been the result of fractionation of a separated primary melt early in earth history. The large-ion lithophile (LIL) patterns of enriched magmas may be inherited from metasomatic fluids that had been in equilibrium with garnet, rather than indicating a garnet-rich composition for the immediate parent and the residue after partial melting. The composition of the mantle eclogite layer may be picritic. Although the omphacite-pyrope system has not been studied at sufficiently high pressure, results on related systems suggest that the eclogite-garnetite transformation may be responsible for the 400-km discontinuity. The density jump at this discontinuity is about 3%; it seems to be a second-order transition.

Volcanism of shoshonite to high-K andesite affinity in an Archean are environment, Oxford Lake, Manitoba

Abstract: Volcanic rocks of the Archean Oxford Lake Group are characterized by a stratigraphic progression from mafic to felsic compositions, accompanied by a systematic decrease in incompatible element abundances. This, coupled with high abundances of Sr, Rb, K2O, and La, high (La/Yb)n, and unfractionated (flat to concave) heavy-REE (rare earth element) profiles, distinguishes these rocks as an Archean shoshonite to high-K andesite – dacite–rhyolite series, directly comparable to modern analogues formed in convergent tectonic settings. The trace-element data support a model of petrogenesis in which an Archean mantle source region was modified by volatiles rich in large-ion-lithophile elements, the modified mantle was subjected to partial melting forming a parental liquid of shoshonitic character, and this liquid principally underwent amphibole fractionation to form the evolved rock compositions. This process is envisaged as terminal magmatism during the final (senile) stage of activity in an arc setting. The Rb–Sr...

Petrology and petrogenesis of lunar breccia 12013

Abstract: Sample 12013 is extremely rich in incompatible elements (K, P, U, Th, Pb, Ba, Rb, Zr, Nb, and rare earths) relative to other lunar samples, and it contains abundant granitic material composed of silica minerals and K-feldspar The present investigation provides detailed petrologic and mineralogic descriptions of the rock and a discussion of its genesis and history The sample is made up of two lithologies, including a mottled gray and white lithology, and a black lithology A detailed petrology of the gray breccia is presented, taking into account lithic clasts, mineral clasts, deformation textures in clasts, the matrix, and felsite A detailed petrology of the black breccia is also provided Attention is given to a model for the formation of the breccia, an evaluation of the possible genetic relationships between the various components, and some constraints on the nature of the source area

Geochemical constraints on the origin of xenolith-bearing alkali basaltic rocks and megacrysts from the Hoggar, central Sahara.

Abstract: Some xenolith-bearing Hoggar basanites and nephelinites possibly represent primary or near-primary magmas on account of their mg and Ni values. Simple model calculations show that they could be derived by 5–8 percent partial melting of a garnet-bearing pyrolite source. They are characterized by LREE enriched abundance patterns and by highly variable abundances of compatible and incompatible trace elements. These compositional features are compatible with derivation by small degrees of melting of an inhomogeneous mantle source recently enriched in LREE and other incompatible elements. Model calculations involving transition metal and REE contents do not support a cogenetic origin of the ultramafic xenoliths as source or residual material. Megacrysts of kaersutite, clinopyroxene, orthopyroxene, oligoclase, ilmenite and zircon are considered to represent high-pressure near-liquidus to near-solidus phases precipitated from basanitic magmas. Inconsistencies in trace element contents of some kaersutites and clinopyroxenes and host basalts suggest that these megacrysts are not strictly cognate with respect to their present hosts.

The trace element content and petrogenesis of kimberlites in Riley County, Kansas, U. S. A.

Abstract: Kimberlites and associated xenoliths, carbonate veins, and interstitial carbonate material from Riley County, Kansas have been analyzed for certain trace elements and Fe using neutron activation. The non-micaceous kimberlites (Bala and Randolph No. 1) are higher in the incompatible elements (REE, Ba, Th, Hf, and ra) compared to the micaceous kimberlites (Stockdale and Leonardville.) Contamination due to the numerous crustal xenoliths and interstitial carbonate material has probably been small. The xenoliths and interstitial carbonate would generally tend to dilute slightly the trace element concentrations of the kimberlites. The trace element contents (except perhaps the LREE and Th) of the kimberlites can be explained by slightly different degrees of incipient partial melting of average upper mantle peridotite containing about 56%o olivine, 33Vo orthopyroxene, 5Vo clinopyroxene, and,6Vo garnet. Significant fractional crystallization of pyroxene, olivine, or garnet is precluded in the kimberlite melt since the compatible element contents (Co, Cr, and Sc) are quite high and similar to values expected by direct melting of peridotite. However, the LREE and Th contents of the kimberlite are somewhat higher than predicted by direct melting of peridotite possibly the result of volatile transport of these elements in H2O- and CO2-rich fluids. Alternatively, a more LREE- and Th-rich source than used in average peridotite in the above melting model may account for the enrichment of these elements in the kimberlites. None of the carbonate material (veins, xenoliths, and kimberlite groundmass) appears to be igneous carbonate. Rather, it appears to be the result ofvaried amounts ofsecondarv or hydrothermal (H2O- and CO2-rich fluids) activity. The relative importance of these processes cannot be evaluated with the present data.