Showing papers on "Incompatible element published in 1979"

A geochemical study of magmatism associated with the initial stages of back-arc spreading

Abstract: Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading. Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with CeN/YbN = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although CeN/YbN ratios and 87Sr/86Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher 87Sr/86Sr (0.7039) and CeN/ YbN (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present 87Sr/86Sr ratios. Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.

Vesicularity and CO2 in mid-ocean ridge basalt

Abstract: Vesicles and included CO2are enriched in deep-sea basalts that are also enriched in light rare earth and incompatible elements. This enrichment probably results from a unique deep mantle origin of such melts but may have been modified by CO2 bubbles rising in shallow magma chambers.

Tholeiitic and alkali basalts from the Mid-Atlantic Ridge at 43 ° N

Abstract: Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing.

Open and closed system igneous fractionation within two chilean ophiolites and the tectonic implication

Abstract: The Sarmiento and Tortuga complexes are two mafic complexes located in southern Chile that represent the remnants of the mafic portion of the floor of an Early Cretaceous extensional back-arc basin. Basaltic dikes and lavas within each complex exhibit tholeiite differentiation trends whereby FeO*, FeO*/MgO, TiO2, P2O5, Zr, and Y increase together without significant increases in SiO2. In both complexes, as FeO*/MgO increases, REE abundance increases without significant change in Ce/Yb ratio, but with an increasing negative Eu anomaly. The Sarmiento complex contains intermediate icelandites and silicic dikes and lavas which are conspicuously absent in the Tortuga complex. These non-basaltic compositions have higher Zr, Y and REE contents than the associated basalts, but similar Ce/Yb ratios, suggesting co-genetic origin. Thick cumulate gabbro sequences in both complexes suggest shallow level crystal-liquid fractionation as a major cause of the observed wide range of chemical variations. Significantly, in basalts from the Tortuga complex, incompatible elements (Zr, Y, REE) increase in abundance more rapidly with increasing FeO*/ MgO than in the Sarmiento complex. The rapid increase of incompatible elements relative to FeO*/ MgO in the Tortuga complex is best modeled by fractionation within an open magma chamber steadily replenished with new batches of undifferentiated magma. The observed chemical variations within the Sarmiento complex are best modeled by a magma chamber replenished only a limited number of times by a continuously decreasing volume of undifferentiated magma, followed, subsequent to the last input of new parental magma, by closed system fractionation which results in the formation of ferro-basalts, icelandites and silicic differentiates. Ferro-gabbros (FeO* >20 wt °/00) found within the gabbro unit of the Sarmiento complex closely approximate in composition the calculated crystal extracts required to evolve ferro-basalts into icelandites and the more silicic differentiates. The difference between the nature of the postulated magma chambers within the spreading centers at which the Sarmiento and Tortuga complexes originated suggests that the zone of magma intrusion from the mantle may have been diffuse in the region where the Sarmiento complex formed and more localized in the region where the Tortuga complex formed. This is consistent with other geochemical and field evidence suggesting that the Sarmiento complex represents a less developed stage of evolution than the Tortuga complex of the mafic floor of the Mesozoic back-arc basin in southern Chile. The apparent decoupling of major and trace element variations in ocean floor basalts may be explained by shallow level igneous fractionation without involving large proportions of clinopyroxene if the magma chambers within spreading centers at midocean ridges behave as open systems periodically replenished with batches of undifferentiated parental magma as is inferred for the Tortuga complex in southern Chile.

The Blue Tier Batholith, Northeastern Tasmania

Abstract: The Blue Tier Batholith is one of a number of high-level, essentially postkinematic, composite granitoid bodies occurring at the southern end of the Tasman orogenic belt of Eastern Australia. An integrated study of the structure, texture, and geochemistry of the batholith suggests that it has a cumulate-like character. In particular, the trace element (Ba, Rb, Sr) data, when constrained by textural features of the granitiods, indicate that the batholith formed by fractional crystallization of a single magma which underwent crystallization in situ by progressive nucleation and solidification from the roof, walls, and floor inwards. Progressive changes in liquids (cumulate) mineralogy during crystallization led to the observed sequence of early biotite and/or hornblende granodiorites followed by biotite adamellites and late muscovite biotite granites. Progressive in situ crystallization led in some instances to gradational boundaries between granitoid types whereas periodic tectonic distrubances caused the rest magma to reintrude earlier crystallizates in places: thus emplacement and crystallization sequences are parallel. The ultimate product of fractional crystallization was a water-saturated melt, enriched in incompatible elements, whose crystallization resulted in significant tin mineralization. The chemistry of the rocks comprising the batholith is in many respects analogous to that of basic cumulate rocks, although an origin by outward growth of crystals and expulsion of interstitial melt, coupled with convective mixing, rather than by crystal settling, is favoured for the granitoid suite. It is suggested that the Blue Tier Batholith is not an isolated example of a granitoid body with cumulate-like character, but that such bodies may be more common than is recognized.

Uranium mineralization and granite magmatism in the British Isles

Abstract: Uranium mineralization associated with granites in the Caledonian and Hercynian provinces of the British Isles is shown to be genetically related to the uranium content and distribution, age, and structural setting of these granites. The uranium content of whole rock samples, analysed by the delayed neutron method, is used to demonstrate that mineralization is associated with intrusive complexes with a high mean content of uranium which also exhibit a high concentration of incompatible elements, low K/Rb ratio, low total Sr, low initial $^{87}$Sr/$^{86}$Sr ratio and high geothermal gradient. The standard deviation for uranium is greater where such intrusives are mineralized but the mean value is relatively unaltered. Therefore mineralization is the result of uranium redistribution, and does not involve further introduction of uranium. Fission track studies indicate that the high 'background' uranium content of granites, away from mineralization, is due to the occurrence of uranium in resistate primary phases such as zircon. Uranium is released by the dissolution of these resistate minerals. The processes of greisenization and tourmalinization which have been shown by oxygen isotope studies to involve massive interaction with meteoric water all extract uranium, which is redeposited down the PT gradient as 'primary' uraniferous ore minerals in vein-type mineralization. It is suggested, therefore, that mineralization involves leaching of granite magma enriched in metals and fluorine by water of meteoric origin containing dissolved carbonate. The breakdown of primary zircon is attributed to a phase of short duration of high temperature interaction of granite magma with meteoric water, and uranium mineralization is thought to have occurred at this time. However, the high concentration of uranium, thorium and potassium of the 'background' granite which produce hot rock districts may cause redistribution of uranium by hydrothermal mineralization during periods of high average heat flow from the mantle (as in the Tertiary of southwest England) or during dyke emplacement. An extensive system of channels for heating and circulating water is necessary for this system to function, and faults in granite would be particularly favourable. The regional trend of uranium and incompatible elements shown by late Caledonian (Devonian) and Hercynian granites in Britain is related to dehydration reactions during subduction of oceanic crust. The importance of phlogopite breakdown in accounting for the characteristics of uraniferous granites is discussed in relation to magma generation, with the use of closed and open system models with partial fusion of ocean crust or upper mantle. Uranium enrichment by scavenging of subcontinental lithosphere is considered important, but late stage assimilation of uranium from higher levels in the crust is relatively insignificant. The applications of the models for uranium mineralization to exploration at a regional and local scale are discussed.

Igneous geochemistry of mafic rocks in the Betts Cove ophiolite, Newfoundland

Abstract: The Betts Cove ophiolite, Newfoundland, consists of cumulate ultramafics, gabbro/clinopyroxenites, sheeted dikes and pillow lavas. The pillow lavas are divisible into three compositional groups: lower lavas ( 0.7 wt.% TiO2). The lower and intermediate lavas are very depleted in Ti, Zr, Y, P, and REE and have high Al2O3/TiO2 ratios relative to ‘normal’ oceanic tholeiite. The extreme depletion of these lavas and their dike equivalents (diabase and picrites) suggests they were derived by melting a severely depleted lherzolite. Conversely, the upper lavas, a volumetrically small part of the ophiolite, are compositionally similar to fractionated oceanic tholeiite and thus, their source material may be like that postulated for modern ocean floor basalts. Whereas the majority of basalts in the Betts Cove ophiolite are depleted in ‘incompatible’ elements, most dikes and lavas from the Blow-MeDown ophiolite, western Newfoundland, are not and have incompatible element concentrations similar to modern oceanic tholeiite. The chemical differences between the two ophiolite massifs are related to melting of ultramafic source materials which are in different states of depletion brought about by previous melting episodes.

Regional Geochemistry of Uranium as a Guide to Deposit Formation

Abstract: Geochemical maps of northern Scotland prepared by the Institute of Geological Sciences show the distribution of uranium based on the analysis of stream sediment and water samples by the delayed neutron method. These maps not only indicate the relative concentration of the element in the principal tectonic units but also identify areas of uranium mineralization in the bedrock. Evaluation of the geochemical maps in the light of known geophysical and geological data suggests that both the Galedonides of northern Scotland and the Hebridian craton to the west of the Caledonian orogenic front are underlain by a thick layer of basement depleted in uranium and other incompatible elements. Enrichment of uranium has occurred only where the basement has been disrupted either as a result of magmatism or by deep faulting. Thus, most granites in the area surveyed contain only average abundances of U, but large values characterize certain early Proterozoic and Caledonian granites which appear to have derived at least part of their substance from the mantle. Uranium enrichment in the non-marine Old Red Sandstone overlying the metamorphic Galedonides is most marked in the vicinity of deep faults which are thought to have separated areas of emergence from subsiding sedimentary basins in Old Red Sandstone times. The style of mineralization thus reflects both regional and local factors.

Transitional basalts and tholeiites from the East Pacific Rise, 9°N

Abstract: Basalts from the base of a small seamount on ∼1.5-m.y.-old crust west of the East Pacific Rise (EPR) at 9°N are intermediate in chemical and isotopic composition between light-rare-earth-element-depleted tholeiite (normal midocean ridge basalt (MORB)) and alkali basalt. Like oceanic alkali basalt, these rocks contain significantly more Ba, K, P, Sr, Ti, U, and Zr than normal MORB. Since the absolute abundances of these elements are still well below alkali basalt levels, the label transitional is adopted for these basalts. A series of fractionated MORB also occurs in this area, northwest of the Siqueiros Fracture Zone - Transform Fault. The normal tholeiites are either olivine-plagioclase or plagioclase-clinopyroxene phyric, while the transitional basalts are spinel-olivine phyric. Fractional crystallization quantitatively accounts for the chemical variability of the tholeiitic series but not for the transitional basalts. The tholeiitic series probably evolved in a crustal magma chamber ∼4 km below the crest of the East Pacific Rise. 143Nd/144Nd and other chemical data suggest that the large-ion-lithophile-enriched transitional basalts may represent a hybrid of normal MORB and Siqueiros area alkali basalt. Incompatible element plots of K, P, and U indicate possible derivation of the transitional basalts by magma mixing. Magma mixing of unfractionated normal MORB and Siqueiros alkali basalt has been quantified. Derivation of the transitional basalts from a 1:1 mixture is supported by all available chemical data, including Cr, Cu, Nd, Ni, Sm, Sr, U, and V. This magma mixing apparently occurred at ≲30 km depth within a few tens of kilometers from the EPR axis. These Siqueiros area EPR transitional basalts are compared with Mid-Atlantic Ridge (MAR) transitional basalts from the Iceland and Azores areas. The Siqueiros area basalts reflect a profound chemical and isotopic heterogeneity in the upper mantle, similar to that found along the MAR. Unlike the MAR, the EPR shows no evidence of plumelike bulges and associated large-scale outpourings of nonnormal MORB resulting from these mantle heterogeneities. Siqueiros alkali basalt and MORB, as well as transitional basalt and MORB, were recovered from single dredge hauls. Such close spatial and temporal proximity of the inferred mantle sources places severe constraints on geometric and physicochemical upper mantle models.

Trace elements in igneous processes

Abstract: Several previous chapters have been concerned with the way in which major element compositional data can be put to work in the formulation and testing of petrogenetic hypotheses. Trace elements have not so far been considered in detail because the way in which they are best handled is often substantially different from the treatment for major elements.

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

Abstract: 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.

Geochemistry of Brokeoff volcano, California

Abstract: Brokeoff volcano, a High Cascade stratovolcano located in Lassen Volcanic National Park, California, is composed primarily of andesite, with subordinate amounts of basalt, basaltic andesite, dacite, rhyodacite, and rhyolite. A geochemical study was undertaken to investigate the genetic relationship between members of the basalt-andesite-dacite-rhyolite magma series. Both major-element and trace-element abundances vary regularly with the silica content of the lavas. Relatively small increases in K, Rb, and Ba abundances preclude derivation of most of the rhyodacites by fractional crystallization. A model is proposed for the generation of andesite by partial melting of a garnet peridotite and generation of rhyodacite by partial melting of a similar source at greater depths, followed by hornblende fractionation. A small volume of dacites and rhyodacites have distinctly higher incompatible element abundances and were probably produced by fractional crystallization of the andesite. Andesites and dacites produced by flank eruptions are characterized by complex phenocryst assemblages and were most likely formed by mixing of andesite and rhyodacite magma. It is concluded that members of the basalt-andesite-dacite-rhyolite magma series are not related by a single process, but probably by several processes, even at a single volcanic center.

Geochemistry of the Mantle

Abstract: The degree to which the mantle can be considered chemically uniform depends partly on the scales at which phenomena are analyzed. Many key geophysical observations (e.g., seismic velocities) relate to scales of 10–100 km. A key result discussed in Chapter 1 is that at these scales, the overall pyrolite composition derived for the upper mantle is capable of explaining the essential features of the distribution of seismic velocities and density with depth throughout the transition zone and lower mantle to depths of about 2700 km, when the effects of known phase transformations are taken into account. In particular, there is no convincing evidence requiring the existence of any major radial chemical zoning in the mantle,1 e.g., a substantial increase of the FeO/MgO ratio in the lower mantle. This suggests that the pyrolite upper mantle composition might be applicable throughout the entire mantle. At the very least, this would be a reasonable assumption in attempting to estimate the bulk composition of the mantle.