Low Concentrations Of Incompatible Elements Research Articles

Geochemistry of the 1989–1990 eruption of redoubt volcano: Part I. Whole-rock major- and trace-element chemistry

The 1989–1990 eruption of Redoubt Volcano produced medium-K calc-alkaline andesite and dacite of limited compositional range (58.2–63.4% SiO 2) and entrained quenched andesitic inclusions (55% SiO 2) which bear chemical similarities to the rest of the ejecta. The earliest (December 15) magmas are pumiceous, often compositionally banded, and the majority is relatively mafic (< 59% SiO 2). The most silicic magmas of the eruption are the late December to early January domes (up to 63.4% SiO 2). Subsequent magmas formed domes and rare pumices which converge on 60% SiO 2. Chemical variations among ejecta comprise tight, linear, two-component arrays for all elements for which the analytical uncertainty is much less than the compositional range. The two-component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent. Additionally, the felsic endmember cannot be easily related to the mafic endmember by normal high-temperature igneous processes (e.g., the silicic endmember has higher Zr yet lower Hf than the mafic endmember). Also relative enrichments of highly incompatible elements are dramatically different across the arrays. The mixing event must have preceded eruption by a significant, yet unspecified amount of time because groundmass glass compositions are homogeneous for all post-December samples (Swanson et al., 1994-this volume), in spite of the whole-rock chemical diversity. This implies time for additional crystallization after the mixing event. Swanson et al. (1994-this volume) discuss evidence for a potentially different mixing event recorded only in December 15 magmas. Cognate cumulate xenoliths composed of pl+cpx+opx+hb+mt+melt were recovered from January and April deposits. These blocks differ from local batholithic country rock in their low concentrations of incompatible elements (e.g., Rb < 5 ppm vs 20–90 ppm, Ba < 150 ppm vs 300–2000 ppm) and low SiO 2 (< 50 wt.% vs > 60 wt.%). They have Mg, Cr, Ni, Sc, and V contents higher than the andesites, but lower than Redoubt basalts and basaltic andesites. Thus, they may be crystallization products of andesites, but do not represent the cumulate residue of basalt fractionation. The xenoliths were probably derived from a shallow or intermediate crustal chamber.

Mantle and crustal contributions to continental flood volcanism

Arndt, N.T., Czamanske, G.K., Wooden, J.L. and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 39–52. Most continental flood basalts are enriched in incompatible elements and have high initial 87Sr/ 86Sr ratios and low ϵ Nd values. Many are depleted in Nb and Ta. The commonly-held view that these characteristics are inherited directly from a source in metasomatized lithospheric mantle is inconsistent with the following arguments: (1) thermomechanical modelling demonstrates that flood basalt magmas come mainly from an asthenospheric or plume source, with minimal direct melting of the continental lithospheric mantle. The low water contents of most flood basalts argue against proposals that hydrous lithosphere was the source. (2) Lithospheric mantle normally has low concentrations of incompatible elements, and chondrite-normalized Nb and Ta contents similar to those of other incompatible elements. Such material cannot be the unmodified source of Nb-Ta-depleted basalts such as those from the Karoo, Ferrar, or Columbia River provinces. We suggest there are two main controls on the compositions of continental flood basalts. The first is lithospheric thickness, which strongly influences the depth and degree of mantle melting of a plume or asthenospheric source, and thus has an important influence on the composition of primary magmas. All liquids formed by partial melting of peridotite at sub-lithosphere depths are highly magnesian (20–25 wt.% MgO) but have variable trace-element contents. Where the lithosphere is thick, the source melts at high pressure, garnet is present, the degree of melting is low, and trace-element concentrations are high. This type of magma evolves to produce the high-Ti type of continental flood basalt. Where the lithosphere is thinner, the source ascends to shallower levels, the degree of melting is greater, garnet may be exhausted, and the magmas have lower trace-element contents; these magmas yield low-Ti basalts. The second control is processing of magmas in chambers that were periodically replenished and tapped, while continuously fractionating and assimilating their wall rocks. The uniform compositions of basalts that evolve in such chambers are far removed from those of their picritic parental magmas. Major elements in continental flood basalts reflect control by olivine, pyroxene, and plagioclase crystallization, and this assemblage places the magma chambers at crustal depth. We believe that trace-element and isotopic compositions are also buffered, and that the erupted basalts represent steady-state liquids tapped from these magma chambers. These processes impose a crustal signature on the magmas, as expressed most strongly in the concentrations of incompatible elements (e.g., Nb-Ta anomalies) and their isotopic characteristics.

Composition of Rift-Related Igneous and Sedimentary Rocks of the Keweenawan Supergroup in the Poersch No. 1, OZ-1, Finn, and Friederich Wells, Northeastern Kansas

A series of wells have been drilled up to a depth of 3,444 m (113,000 ft) into the midcontinent rift system (MRS) in northeastern Kansas. The age of a gabbro sill in the upper portion of the Precambrian rocks was determined to be 1,097.5 ± 3 Ma, and the magnetic polarity of the gabbro correlated to the lower Keweenawan rocks in the northern MRS (Van Schmus et al., 1990). The rocks below 2,259 m (7,411 ft) in the deepest well [Poersch no. 1, total depth 3,435 m (11,270 ft)] consist mostly of arkoses with subordinate amounts of shale, siltstone, and basalt. The rocks above 2,265 m (7,431 ft) in the Poersch well consist of basalt with minor siltstone, arkose, gabbro, and felsite. A proposed high-angle reverse fault could have juxtaposed the upper igneous rocks over the lower sedimentary rocks to produce a reversed stratigraphy. This would make the development of the southern MRS similar to that of the northern MRS. Thus, in the initial extensional phase of the MRS, broad subsidence coincided with abundant volcanism and little sediment production. Grabens formed in the later stages of rift development and were filled with abundant sedimentary rocks along with lesser volcanic rocks. The chemical characteristics of the basalts in the southern MRS are similar to those in the north. The southern basalts are subalkalic to alkalic and follow tholeiitic trends; a number of them are high-alumina basalts. Although there is a lot of scatter, Al2O3, Ni, and Cr concentrations decrease and Fe2O3, TiO2, K2O, rare earth elements, Ba, Hf, and Sc concentrations increase with decreasing Mg number. These trends are consistent with plagioclase, olivine, pyroxene, or spinel fractionation from primary basalts. One basalt could represent a primary magma because it has a high Mg number (0.68), high Ni (638 mg/kg) and Cr (233 mg/kg) concentrations, low incompatible element concentrations (e.g., La = 4.2 mg/kg), and a slight positive Eu anomaly. This possible primary magma could have formed by partial melting (20-25%) of an undepleted spinel peridotite at 30-40 km depth. Most basalts have not been contaminated by crustal rocks or silicic magmas. The mineralogy, chemical composition, and U-Pb geochronology of detrital zircons of the arkoses, siltstones, and shales are consistent with their derivation from the surrounding granitoid highlands with little or no input from the basalts. Even siltstones and arkoses within the mostly basaltic sequences are derived mostly from the granitoids, although the siltstones may have some input from the basalts (e.g., higher Ni concentration than the sandstones).

Basalt magmatism along the passive continental margin of SE Brazil (Campos basin)

The SE-Brazil passive continental margin is characterized by tholeiitic magmatism that is particularly widespread in the marginal Campos basin, facing the inland flood basalts of the adjacent Parana basin. Campos magmatism is represented by Early Cretaceous (EC; 134–122 Ma) flood basalts and minor Upper Cretaceous-Early Tertiary basalt flows and intrusives, which were emplaced in a basin with attenuated crustal thickness (20 km). Petrography, mineral chemistry, wholerock geochemistry and Sr−Nd isotope composition emphasize that the EC-Campos basalts have suffered extensive seawater interaction which caused enrichment in MgO, FeO total, K2O, Rb and Ba, and depletion in SiO2 and CaO, while Zr, Nb, Y and REE remained virtually constant in samples with loss-on-ignition values less than 4 wt%. In general, Campos basalts have bulkrock chemistry similar to those of the inland Parana tholeiites (140–130 Ma) with relatively low concentrations of incompatible elements and TiO2 ( 16) MORB (mid-ocean-ridge basalt) mantle in simple binary mixing models. On the whole, the Early Cretaceous Campos basalts appear as an easterly, younger extension of the northern Parana volcanism and probably erupted during early stages of the major riftingprocesses which caused continental thinning. It is notable that in the Campos marginal basin both the basalt magmatism contemporaneous with the continental break-up, as well as that which occurred after the S. America-Africa separation, appears substantially related to subcontinental lithosphere and a Dupal-like OIB (ocean-island basalt) (e.g. Tristan da Cunha) source components.

Field relations, geochemistry and petrogenesis of the Cretaceous basaltic Jutal dykes, Kohistan, northern Pakistan

A 75 Ma Cretaceous suite of low-medium-K basalt to basaltic andesite dykes intrudes the Kohistan terrane between Jutal and Chalt, Gilgit area, northern Pakistan. Orientational data suggest they intruded in response to NW–SE to N–S tension. The dykes cut the Albian–Aptian Chalt Volcanic Group and the 102 Ma Matum Das trondhjemite pluton. They cut the fold structures and penetrative fabrics associated with the collision between Kohistan and Eurasia and thus constrain the age of formation of the Northern Suture. The dykes define two groups on the basis of trace element geochemistry and geochemical trends. A depleted D-type group has low concentrations of incompatible elements (2–8 x primordial mantle) and (Ce/Yb)N ratios &lt;1. An enriched E-type group is relatively enriched in both low field strength and light Rare Earth elements and exhibits increasing incompatible element concentrations with increasing Zr. Trace element and field data suggest that the dykes are genetically linked and formed by partial melting of a mantle source which has been metasomatized by subduction-related processes. Models involving (i) different degrees of partial melting (ii) two-stage partial melting and (iii) partial melting of a heterogeneous source are considered.

Regional compositional variations of Late Tertiary bimodal rhyolite lavas across the Basin and Range/Colorado Plateau Boundary in western Arizona

Bimodal basalt‐rhyolite fields that erupted since 15 Ma form a NE trending line that crosses the boundary between the Basin and Range and the Colorado Plateau in western Arizona. The bimodal volcanism traverses physiographic province boundaries marked by changes in crustal thickness. Silicic volcanism in these fields postdates detachment faulting in the Basin and Range and generally decreases in age from south to north. We distinguish two rhyolite types that occur in these fields and summarize their geology, petrography, and geochemistry. Both high‐silica (HSR) and low‐silica rhyolites (LSR) occur in these fields. HSR lavas contain &gt;75 wt % SiO2 (normalized to 100% volatile free), have high concentrations of incompatible elements (Rb, U, Th, Cs, Ta), low concentrations of compatible elements (Ba, Sr, Eu, Mg, Cr), and relatively flat rare earth element (REE) patterns (La/LuN &lt;7). In contrast, LSR lavas have 68–74.5 wt % SiO2 (normalized to 100% volatile free), low concentrations of incompatible elements, high concentrations of compatible elements, and negatively sloping REE patterns (La/LuN &gt;8). Further distinction of HSR and LSR lavas is made based on phenocryst assemblages. Trace element and isotopic studies suggest that both the HSR and LSR lavas contain a major crustal component and that the HSR lavas may have formed by extensive crystal fractionation of LSR parent magmas. HSR and LSR lavas display systematic compositional variations along the transect; however, their compositions diverge from south to north. HSR lavas erupted on the Colorado Plateau are more differentiated than those erupted in the Basin and Range, perhaps reflecting more extensive crystal fractionation while ascending the thicker crust of the Colorado Plateau province. In contrast, LSR lavas on the Colorado Plateau are more primitive than those in the Basin and Range. This seeming paradox may be related to lateral changes in source composition, possibly caused by a northerly increase in either metamorphic grade or bulk composition. Alternative models for the LSR compositional variations, such as magma mixing and mantle input into lower crustal sources, cannot be tested with our present data.

Geology and geochemistry of the mafic and ultramafic intrusive rocks, Dadeville belt, Alabama

The majority of mafic complexes within the Dadeville belt of Alabama9s Inner Piedmont contain two plutonic suites: the Doss Mountain suite, consisting of metanorite (DMTN) and metaorthopyroxenite (DMTO); and the Slaughters suite, which is composed of meta-gabbro (SG). All plutonic rocks are pre-regional metamorphism in age but in some places are predominantly free of macroscopic metamorphic fabrics. In the Doss Mountain area, the DMTN and DMTO show progressive mineralogical and textural changes into a poorly foliated and nonlayered amphibolite and an actinolite schist, respectively; such changes are essentially isochemical. The DMTN-DMTO-amphibolite-actinolite schist complex is interpreted to be a layered norite-orthopyroxenite intrusion that has suffered variable degrees of metamorphism. In other areas, the DMTN and DMTO intrude well-lineated and foliated amphibolites of contrasting composition that belong to the Ropes Creek Formation. The SG has well-developed olivine-plagioclase reaction coronas as the result of metamorphism. In all areas examined, the SG appears to intrude the amphibolites. The SG has high-alumina and incompatible-element (Ba, Sr, Ti, Nb, Zr, Y, Pb, Th, Hf, ΣREE) concentrations and high La/Yb ratios. In contrast, the DMTN (and its metamorphosed equivalent) has low alkalies and other incompatible-element contents, has REE patterns that parallel chondritic abundances, and has low La/Yb ratios. On the basis of several geochemical parameters, the DMTN is tholeiitic, whereas the SG is calc-alkaline with some alkalic tendencies. Discriminant diagrams, together with the extremely low Ti concentration in the DMTN and the calc-alkaline nature of the SG, strongly indicate emplacement in an arc environment. High Al 2 O 3 /TiO 2 and CaO/TiO 2 ratios, as well as low incompatible-element concentrations in the DMTN, suggest that its origin was by partial melting of a depleted source. The DMTO represents a cumulate of the norite. Slaughters Gabbro magmatism resulted from partial melting of a less-depleted source.

Areal distribution and age of low-K, high-alumina olivine tholeiite magmatism in the northwestern Great Basin

Field, petrographic, chemical, and geochronologic information has led to the identification and characterization of a widespread low-K, high-alumina olivine tholeiite (HAOT) magma type in the northwestern Great Basin. This basalt covers at least 22,000 km 2 and is estimated to represent a total volume of at least 650 km 3 . The time period over which HAOT lavas were erupted extends from late Miocene to Holocene (10.5–0 m.y. B.P.). This interval overlaps with the timing of Snake River, Cascade, and northwestern Basin and Range volcanism but distinguishes HAOT from the main pulse of Columbia River volcanism (∼15 m.y. B.P.). Furthermore, three major pulses of HAOT magmatism are suggested from the geochronology of this study: 0 to 2.5 m.y. B.P., 3.5 to 6 m.y. B.P., and 7 to 10 m.y. B.P. The distinctive holocrystalline, nonporphyritic, and diktytaxitic texture, the low incompatible-element concentrations, and the high MgO/FeO* of HAOT serve to distinguish this basalt from other basalts of the northwestern United States. The low incompatible-element signature accentuates the similarities between HAOT, mid-ocean–ridge basalts, and back-arc–basin basalts. These similarities, combined with the HAOT chronology, support the idea that the processes giving rise to extensional tectonism and HAOT magmatism in the northwestern Great Basin are similar to those acting in active back-arc–spreading regions.

Island arc character and late Precambrian age of volcanics at Wadi Shwas, Hijaz, Saudi Arabia: geochemical and Sr and Nd isotopic evidence

A summary description of two volcanic sequences named the Surgah and the Shwas Formation, situated in the Wadi Shwas area at long. 42°0′, lat. 20°0′ in the Arabian Shield, is given. Major and trace element concentrations including REE abundances are presented, as well as Sr and some Nd isotope results. One sequence (the Surgah Formation) shows a tholeiitic differentiation trend from basaltic to rhyolitic rocks, whereas the Shwas Formation has calc-alkaline affinities. Both sequences have low concentrations of incompatible elements, notably Rb, Nb, Zr and Ti. The coexistence of a tholeiitic and a calc-alkaline suite, along with these low concentrations, is strongly reminiscent of some present-day island arc environments. An Rb-Sr errorchron age of 721 ± 55Ma was obtained on the Surgah Formation, with high initial 87Sr/ 86Sr ratios around 0.705. The rare earth abundances are overall very low and the patterns show a relative depletion of the light REE in the least evolved rocks, indicating a relatively light-REE-depleted mantle source. A time-integrated light-REE-depleted source is also indicated by the fairly radiogenic 143Nd/ 144Nd ratios (ε Nd values around 8). The Nd “model age” of source depletion ( T CHUR Nd ) is around 2.2 Ga.

High pressure basic inclusions from the Kayrunnera kimberlitic diatreme in New South Wales, Australia

Basic inclusions of two types occur in a kimberlitic diatreme at Kayrunnera in northwestern New South Wales. Type I inclusions comprise assemblages of clinopyroxene+garnet+rutile±plagioclase ±quartz±K-feldspar±scapolite±sphene±apaite. Type II inclusions have assemblages of clinopyroxene +garnet+kyanite+quartz±plagioclase and are lower in Ti, total Fe, and higher in Al and have a higher Mg/Mg+gSFe ratio than the Type I inclusions. Experimental and theoretical data indicate that both inclusion types equilibrated at between 850–900 ° C and 18–23 kb. Due to their low concentrations of incompatible elements, the Type I inclusions are considered to represent a basaltic melt derived from an Fe-rich mantle source rock, and not to be the product of fractionation. The Type II inclusions are believed to represent cumulates which formed from a basaltic magma. The presence of sulphur rich scapolite in the Type I inclusions extends the range of P-T conditions from which this mineral has been reported thus adding further credence to the hypothesis that it may act as a stable repository for S and CO2 in the crust and upper mantle.