Showing papers on "Incompatible element published in 1996"

Post-collision, shoshonitic volcanism on the Tibetan Plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts

Abstract: Potassic volcanism has been widespread and semi-continuous on the Tibetan plateau since ~13 Ma, post-dating the orogenic thickening of the India—Asia collision. Volcanism may have commenced slightly earlier (~16—20 Ma) in the southern portion of the plateau arid then ceased around 10 Ma. The dominant lavas are pyroxeneand plagioclase-phyric shoshonites with subordinate occurrences of dacites and rhyolites. Their mineralogy reflects crystallization from high-temperature (^1100°C) magmas which had elevated oxygen and water fugaciti.es. Geochemically, they are characterized by relatively low TiO2, AI2O3 and Fe^Os, and high .A&^Q coupled with variable abundances of compatible trace elements and very high contents of incompatible trace elements. Normalized incompatible element patterns have marked negative Nb, la and Ti anomalies whereas K^O appears to be buffered at ~4% over a wide range of SiO^. Isotope data show a relatively broad and enriched range of^Sr/^Sr (0-7076-0-7106) at more restricted (—5 2 to -8 1). Pb isotopes are characterized by a range of 204 ^Pb/^Pb (38-67-39 SO) /204 pb/20 pb (,5.51_15.72) 207pb/pb (

High field strength element enrichment of Pliocene-Pleistocene island arc basalts, Zamboanga Peninsula, Western Mindanao (Philippines)

Abstract: The Pliocene-Pleistocene magmatic activity of the ^amboanga arc is linked to the southward subduction of the Oligocene-Miocene Sulu Sea back-arc basin along the Sulu Trench. The magmatic products include small amounts ofadakites datedfrom 3-8 to 0-7 Ma, abundant Nb-enriched basalts and basaltic andesites (NEB) datedfrom 2 to 1 Ma and a lone calc-alkaline potassic basaltic andesite dated at 0 4 Ma. Three kinds of NEB are distinguished: nearly primitive Mg-rich (MG) basalts displaying positive or no Nb anomalies with respect to adjacent incompatible elements and more evolved low-K (LK) and calcalkaline (CA) lavas which, despite their Nb enrichment, display negative Nb anomalies. Although the role of OIB-type mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and temporal association of these rocks with adakites suggests a petrogenetic link between them. Trace element characteristics of the NEB imply that amphibole and ilmenite might be present in their source. We suggest that these minerals could be added metasomatically to the mantle through hybridization by percolating slab melts, during which Nb and Ti are preferentially extracted from the adakitic melts. In an early stage (4—3 Ma) of the subduction of the young and hot Sulu Sea basin crust beneath the ^jimboanga peninsula, adakitic liquids formed at depths of 75-85 km. A few of them were emplaced at the surface but most were consumed through slab melt—mantle metasomatic reactions. Adakite production and emplacement continued later (<2 Ma), while the Nb-enriched mantle was brought by convection to depths that allowed its melting and the subsequent emplacement of NEB behind the adakitic front of the ^pmboanga arc

Origin of the recrystallisation front in the Ronda peridotite by km-scale pervasive porous melt flow

Abstract: It is well established that porous melt flow in the upper mantle may significantly affect partial mantle melt compositions. Less well established are the length-scale of porous flow and whether porous melt flow can be a volumetrically important magmatic process. The only source for observations concerning the length-scale and nature of pervasive porous melt flow are peridotite massifs. Here we present such observations in the form of structural, and major and trace element data from peridotites of the Ronda massif, southern Spain. Trace element concentrations were obtained with high analytical precision (ICP-MS) and include trace elements rarely analysed in peridotites, such as Rb, Th, Nb and Ta. The western portion of the Ronda massif can be divided into two structural facies. The first and oldest is composed of deformed, porphyroclastic spinel peridotites, the second of virtually undeformed granular spinel peridotites. They are separated by a recrystallisation front across which grain growth of all phases occurred. The granular domain can be further subdivided into three subfacies: coarse-granular, fine-granular, and layered-granular peridotites. According to structural facies, km-scale spatial variations unrelated to Ca and Al abundances have been recognised for mg-numbers [atomic Mg/(Mg±Fe)] and incompatible elements such as rare earth elements (REE), Th and high-field-strength elements (HFSE; including Ti). Such variations are reminiscent of those commonly ascribed to mantle metasomatism, but have never been documented on the km-scale. The origin of the recrystallisation front is related to km-scale pervasive melt percolation. Feed-back processes between grain growth and melt fraction could have led to important accumulation of melt at the recrystallisation front, accomplished mainly by melting/dissolution. Variation in melt fraction across the front explains the spatial variation in the degree of recrystallisation, mg-numbers, REE fractionation, and HFSE abundances, and could account for many of the classical differences between basalts from convergent and extensional tectonic settings. Whereas the coarse-granular peridotites reflect a stage of steady-state pervasive porous melt flow, the fine- and layered-granular facies probably reflect the terminate stages of porous melt flow. Processes associated with both domains are pyroxene-forming freezing reactions at decreasing melt volumes, and progressive channelling of melt flow associated with olivine-producing reactions. Both processes show complex overprinting relationships in both time and space.

Geochemical stratigraphy of lava flows sampled by the Hawaii Scientific Drilling Project

Abstract: Geochemical discriminants are used to place the boundary between Mauna Loa flows and underlying Mauna Kea flows at a depth of about 280 m. At a given MgO content the Mauna Kea flows are lower in SiO2 and total iron and higher in total alkali, TiO2, and incompatible elements than the Mauna Loa lavas. The uppermost Mauna Kea lavas (280 to 340 m) contain alkali basalts interlayered with tholeiites and correlate with the postshield Hamakua Volcanics. In addition to total alkalis, the alkali basalts have higher TiO2, P2O5, Sr, Ba, Ce, La, Zr, Nb, Y, and V relative to the tholeiites and lower Zr/Nb and Sr/Nb ratios. Some of the alkali basalts are extensively differentiated. Below 340 m all the flows are tholeiitic, with compositions broadly similar to the few “fresh” subaerial shield-building Mauna Kea tholeiites studied to date. High-MgO lavas are unusually abundant, although there is a wide range (7–28%) in MgO content reflecting olivine control. FeO/MgO relationships are used to infer parental picritic magmas with about 15 wt % MgO. Lavas with more MgO than this have accumulated olivine. The Mauna Loa lavas have compositional trends that are controlled by olivine crystallization and accumulation. They compare closely with trends for historical (1843–1984) flows, tending toward the depleted end of the spectrum. They are, though, much more MgO-rich (9–30%) than is typical for most historical and young (<30 ka) prehistoric lavas. The unusual abundance of high-MgO and picritic lavas is attributed to the likelihood that only large-volume, hot, mobile flows will reach Hilo Bay from the northeast rift zone. FeO/MgO relationships are used to infer parental picritic magmas with about 17 wt % MgO. Again, lavas with more MgO than this have accumulated olivine. Systematic changes in incompatible element ratios are used to argue that the magma supply rate has diminished over time. On the other hand, the relatively constant Zr/Nb and Sr/Nb ratios that compare closely with historical and young (<30 kyr) prehistoric flows are used to argue that the source components for these lavas in the Hawaiian plume have remained relatively uniform over the last 100 kyr.

Mantle source heterogeneity and melting processes beneath seafloor spreading centers: The East Pacific Rise, 18°–19°S

Abstract: We present new major and trace element and Nd-Pb-Sr isotopic data on samples from the East Pacific Rise (EPR) axis and nearby seamounts in the Hump area, 18°–19°S. Most samples studied are normal mid-ocean ridge basalt (N-MORB); four samples from the southern seamounts are enriched MORB (E-MORB). Dredge 52 samples from a southern seamount are depleted in incompatible elements yet possess “enriched” isotopic signatures. Except for the dredge 52 samples, all the samples show significant correlations between isotopic ratios and ratios of incompatible elements; that is, incompatible elements and isotopes are coupled. Sr and Nd isotopic ratios correlate significantly better with ratios of moderately incompatible elements than with ratios involving highly incompatible ones (e.g., Rb, Nb, and K) which appear to be “overenriched”. Both isotopic and incompatible element ratios also correlate with the extent of melting calculated from major elements. We interpret these correlations as mixing trends resulting from melting of a heterogeneous source containing enriched (“plume-like”) domains of variable sizes. Overenrichment of highly incompatible elements in E-MORB appears to be recent and is best explained by low-degree-melt infiltration in the source region prior to major melting events. The low-degree melts are primarily derived from isotopically N-MORB mantle. This low-degree melt process also explains the incompatible element-isotope decoupling throughout the EPR between 13° and 23°S. The dredge 52 samples too are consistent with such a process, but their immediate source is a site of low-degree melt generation.

The upper mantle under La Palma, Canary Islands: formation of Si−K−Na-rich melt and its importance as a metasomatic agent

Abstract: Mantle xenoliths hosted by the Historic Volcan de San Antonio, La Palma, Canary Islands, fall into two main group. Group I consists of spinel harzburgites, rare spinel lherzolites and spinel dunites, whereas group II comprises spinel wehrlites, amphibole wehrlites, and amphibole clinopyroxenites. We here present data on group I xenoliths, including veined harzburgites and dunites which provide an excellent basis for detailed studies of metasomatic processes. The spinel harzburgite and lherzolite xenoliths have modal ol−opx−cpx ratios and mineral and whole rock major element chemistry similar to those found in Lanzarote and Hierro, and are interpreted as highly refractory, old oceanic lithospheric mantle. Spinel dunites are interpreted as old oceanic peridotite which has been relatively enriched in olivine and clinopyroxene (and highly incompatible elements) through reactions with basaltic Canarian magmas, with relatively high melt/peridotite ratio. Group I xenoliths from La Palma differ from the Hierro and Lanzarote ones by a frequent presence of minor amounts of phlogopite (and amphibole). Metasomatic processes are also reflected in a marked enrichment of strongly incompatible relative to moderately incompatible trace elements, and in a tendency for Fe−Ti enrichment along grain boundaries in some samples. The veins in the veined xenoliths show a gradual change in phase assemblage and composition of each phase, from Fe−Ti-rich amphibole+augite+Fe−Ti-oxides+apatite+basaltic glass, to Ti-poor phlogopite+Cr-diopside±chromite+ Si−Na−K-rich glass+fluid. Complex reaction zones between veins and peridotite include formation of clinopyroxene±olivine+glass at the expense of orthopyroxene in harzburgite, and clinopyroxene+spinel±amphibole±glass at the expense of olivine in dunite. The dramatic change in glass composition from the broadest to the narrowest veins includes increasing SiO2 from 44 to 67 wt%, decreasing TiO2/Al2O3 ratio from >0.24 to about 0.02, and increasing K2O and Na2O from 1.8 to >7.0 wt% and 3.8 to 6.7 wt%, respectively. The petrographic observations supported by petrographic mixing calculations indicate that the most silicic melts in the veined xenoliths formed as the result of reaction between infiltrating basaltic melt and peridotite wall-rock. The highly silicic, alkaline melt may represent an important metasomatic agent. Pervasive metasomatism by highly silicic melts (and possibly fluids unmixed from these) may account for the enriched trace element patterns and frequent presence of phlogopite in the upper mantle under La Palma.

Mixed Caledonian appinite magmas: implications for lamprophyre fractionation and high Ba-Sr granite genesis

Abstract: Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust.

Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride, N. Italy)

Abstract: Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree (<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (87Sr/86Sr=0.702203–0.702285; 143Nd/144Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions.

Petrology of lavas from the Puu Oo eruption of Kilauea Volcano: III. The Kupaianaha episode (1986-1992)

Abstract: The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume (>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode 30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized (to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle processes.

Pan-African volcanism: petrology and geochemistry of the Dokhan Volcanic Suite in the northern Nubian shield

Abstract: The Late Proterozoic Dokhan volcanic suite (620 Ma) of the northern Nubian shield is the product of Late Pan-African volcanism. The suite covers the entire spectrum from basalt to high-silica rhyolite and occurs as two units: a dark-coloured unit containing basalt-andesite-dacite, and a light-coloured unit encompassing dacite-rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ign-imbrites that are locally interstratified with basalt and andesite lava flows. The suite forms a continuum in composition with a wide range of Si02 (48–77 wt%), CaO (0.1–8.9 wt%), Sr (81–906 ppm), Zr (85–340 ppm) as well as most other elements, and is moderately enriched in incompatible elements, including rare earth elements (REE). The suite exhibits fractionated, subparallel REE patterns that are similar overall to Andean andesites and ignimbrites. Well-defined major and trace element trends and fractionated REE profiles are consistent with a fractionated basalt to rhyolite calc-alkaline magma series. It is a typical calc-alka-line orogenic complex and exhibits mineralogical-geochemical traits of arc-related volcanism. The suite neither resembles products of extensional nor transitional tectonic regimes as previously thought, but was produced in a subduction-related tectonic environment. The mafic nature of the least-evolved rocks of the suite, along with its relatively low initial 87Sr/86Sr ratio (0.7039) are considered to indicate a mantle source. A mantle-derived basaltic magma fractionated, with amphibole and plagioclase dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major and trace element fractionation modelling.

Trace Element and Isotope Geochemistry of the Volcanic Rocks of Bequia, Grenadine Islands, Lesser Antilles Arc: a Study of Subduction Enrichment and Intra-crustal Contamination

Abstract: (~5-5 in clinopyroxene). The isotopically diverse suite (IDS) is characterized by much wider ranges of radiogenic isotopes ,87Sr/86Sr Q-7048-0-7077, Nd/Nd 0-5128-0-5123 and ho6pb/2O4pb 19.7-20-2), in which all of the Sr and Pb ratios are higher and Nd ratios are lower than those of the IHS. The IDS is also characterized by high d0 values, up to 7-6 in clinopyroxene. The Sr and Pb isotope ratios are too high, and the Nd isotope ratios are too low in the IDS for any of these lavas to be derived from unmodified depleted mantle. Both suites are petrologically very similar and their major element compositions and phenocryst contents suggest that they were formed largely by fractional crystallization of a hydrous tholeiitic melt at pressures <3 kbar. The isotopic ratios and enrichments in large ion lithophile elements (LILE), and to some extent light rare earth elements (LREE), as compared with mid-ocean ridge basalts (MORB), of the IHS lavas suggest that they were derived from a depleted mantle source which had been re-enriched by the addition of1-4% of a subduction component. This component probably comprised a mixture of dehydration fluids, and perhaps minor siliceous melts, released from subducting sediments and mafic crust. The extreme isotopic ranges, large enrichments in incompatible elements, more fractionated LREE patterns and higher 5O values of the IDS lavas are interpreted as resulting from ~ 10-55% assimilation—fractional crystallization of sediments, derived from the Guyana Shield, which are present in the arc crust, by IHS type melts.

Incompatible Element Partitioning between Clinopyroxene and Basalt Liquid Revealed by the Study of Melt Inclusions in Minerals from Troodos Lavas, Cyprus

Abstract: Melt inclusions in clinopyroxene from Troodos lavas were studied by the methods of high-temper- ature optical thermometry, secondary ion mass spectrometry, and microprobe analysis to determine the partition coefficients of Ba, Sr, Y, Ti, La, Ce, Nd, Sm, Dy, Er, and Yb between low-Al, high-Mg clinopyroxene and bon- inite-like melt for temperatures of 1100-1190 ° C and a hydrous fluid pressure of 1 kbar. Variations of the parti- tion coefficients were found as a function of the temperature and the Al 2 O 3 content in clinopyroxene. The results proved the study of melt inclusions in minerals to be an efficient tool for determining the coefficients of trace element distribution between the crystalline phase and the liquid.