Showing papers on "Incompatible element published in 1989"

Coexisting calcalkaline and high‐niobium basalts from Turrialba Volcano, Costa Rica: Implications for residual titanates in arc magma sources

Abstract: Turrialba volcano, the southeasternmost volcano in the Central American arc, is constructed of medium to high-K calcalkaline basalts, andesites, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high-Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc-tholeiitic basalts. The association of calcalkaline and high-Nb basalts is rare in arc front volcanoes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high-Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab-derived fluid was present during calcalkaline basalt genesis but not during high-Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid-ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end-members for both the calcalkaline and the high-Nb basalts. The calcalkaline basalts represent mixtures of the high-degree melts and oxidized small-degree melts generated by amphibole breakdown in mantle overlying the subducting slab. This small-degree melt has high incompatible element concentrations and is saturated in rutile. Arc-related lamprophyric rocks have compositions that are appropriate for these small-degree melts. High-Nb basalts are mixtures of the high-degree melts and more reduced small-degree melts that are undersaturated in rutile. These reduced melts may migrate around or through the subducting slab into the wedge to become involved in arc magma genesis.

Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanin volcanic chain (39.5° S): an evaluation of source heterogeneity, fractional crystallization and crustal assimilation

Abstract: At 39.5° S in the southern volcanic zone of the Andes three Pleistocene-recent stratovolcanoes, Villarrica, Quetrupillan and Lanin, form a trend perpendicular to the strike of the Andes, 275 to 325 km from the Peru-Chile trench. Basalts from Villarrica and Lanin are geochemically distinct; the latter have higher incompatible element abundances and La/Sm but lower Ba/La and alkali metal/La ratios. These differences are consistent with our previously proposed models involving: a) a west to east decrease in an alkali metal-rich, high Ba/La slab-derived component which causes an across strike decrease in degree of melting; or b) a west to east increase in the contamination of subduction-related magma by enriched subcontinental lithospheric mantle. Silicic and mafic lavas from the stratovolcanoes have overlapping Sr, Nd and O isotopic ratios. Silicic lavas also have geochemical differences that parallel those of their associated basalts, e.g., rhyolite from Villarrica has lower La/Sm and incompatible element contents than high-SiO2 andesite from Lanin. At each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing. Apparently crustal contamination was not an important process in deriving the evolved lavas. Basaltic flows from small scoria cones, 20–35 km from Villarrica volcano have high incompatible element contents and low Ba/La, like Lanin basalts, but trend to higher K/Rb (356–855) and lower 87Sr/ 86Sr (0.70361–0.70400) than basalts from either stratovolcano. However all basalts have similar Nd, Pb and O isotope ratios. The best explanation for the unique features of the cones is that the sources of SVZ magmas, e.g., slab-derived fluids or melts of the subcontinental lithospheric mantle, have varying alkali metal and radiogenic Sr contents. These heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing. This model is preferable to one involving crustal contamination because it can account for variable Sr isotope ratios and uniform Nd and Pb isotope ratios among the basalts, and the divergence of the cones from across-strike geochemical trends defined by the stratovolcanoes.

Volcanological and magmatological evolution of Stromboli volcano (Aeolian Islands): The roles of fractional crystallization, magma mixing, crustal contamination and source heterogeneity

Abstract: The present paper reports the results of a detailed stratigraphical, petrological and geochemical investigation on the island of Stromboli, Aeolian arc, Southern Tyrrhenian sea. Major and trace element data determined on a large quantity of samples from well-established stratigraphic positions indicate that the magmatological evolution of the island through time was more complex than previously known. The activity of the exposed part of Stromboli, which occurred over a time span of about 100 000 years, started with the emission of high-K calc-alkaline (HKCA) volcanics, which were covered by calc-alkaline (CA), shoshonitic (SHO), high-K calc-alkaline (HKCA) and potassic (KS) products. The most recent activity consists of HKCA lavas and the present-day SHO-basaltic volcanics emitted by mildly explosive “strombolian” activity. Most of the products are lavas, with minor amounts of pyroclastic rocks emplaced mainly during the early stages of activity. The transition from the SHO to the KS cycle was associated with the collapse of the upper part of the volcanic apparatus; the transition from KS to the present-day SHO activity has been found to have occurred at the time of the sliding of the western portion of the volcano that generated the “Sciara del Fuoco” depression. The rock series cropping out at Stromboli show variable enrichment in potassium, incompatible trace elements and radiogenic Sr which increase from CA through HKCA, and SHO up to KS rocks. Major, trace element and Sr-isotopic data agree in indicating that the HKCA and SHO series evolved by crystal/liquid fractionation starting from different parental liquids, whereas crustal assimilation appears to have been the leading process during the evolution of KS volcanics. Mixing processes also played a role although they can be well documented only when they occurred between magmas with different isotopic and geochemical characteristics. Geochemical modelling based on trace element and isotopic data indicates that the mafic magmas of the different volcanic series may be generated by melting of an upper mantle heterogeneously enriched in incompatible elements and radiogenic Sr by addition, via subduction, of different amounts of crustal material. Geochemical data, however, are also in agreement with the alternative hypothesis that the most mafic magmas of the different series have been generated by combined processes of fractional crystallization, assimilation and mixing of a CA magma in a deep-sited magma chamber; the mafic magmas formed by these complex processes were successively emplaced in a shallow reservoir where they evolved by simple fractional crystallization (HKCA and SHO series) and by assimilation of crustal material (KS). The occurrence of changes in the geochemical signatures of the magmas at the time of the structural modification of the volcano is believed to favour the hypothesis that the variable composition observed in the volcanic rocks of Stromboli is the result of processes occurring within the volcanic system.

Primitive calc-alkaline and alkaline rock types from the Western Mexican Volcanic Belt

Abstract: Since the early Pliocene, small volumes of alkaline magmas have erupted in the western Mexican Volcanic Belt (MVB) in close association with the volumetrically dominant calc-alkaline magmas. Both suites include relatively rare “primitive” types, characterized by high Mg#, Cr, and Ni contents. Primitive hypersthene-normative basalts, parental to the calc-alkaline suite, are found along the volcanic front but are absent at greater distances from the trench. The volcanic-front calc-alkaline suites are typically hornblende bearing, indicating relatively high water contents. Associated primitive alkaline magmas at volcanic-front locations also contain hydrous minerals. These nepheline-normative suites include basanites, phlogopite-bearing minettes, and other hornblende-bearing lamprophyres. These are probably the youngest and freshest lamprophyres yet discovered on Earth, and ideal samples for addressing the origin of this exotic class of rocks. On extended MORB-normalized elemental plots, all lamprophyres show patterns similar to the calc-alkaline basalts, but have overall enrichments of 2 to 25X in Ti, K, P, Ba, Sr, light rare earth elements, and other incompatible elements. The lamprophyres show the same strong relative enrichments of Ba, K, and Sr, and depletions in Ti and Nb that characterize the calc-alkaline rocks of western Mexico and all subduction zones. These similarities in relative element abundances support a common source region for all primitive magmas of the western MVB. The primitive calc-alkaline and alkaline rocks of western Mexico show narrow, overlapping ranges in Sr, Nd, and Pb isotopic ratios, also consistent with derivation of all magmas from a common source. The presence of the relatively cold subducted Cocos and Rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km. These magmas are enriched in Ba, K, Sr, and other elements, probably transported by hydrous fluids rising from the subducted slab. Several lines of evidence support the presence of phlogopite in the source region of western MVB magmas, probably in the form of phlogopite-rich veins cutting asthenospheric peridotite. The first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres. These lamprophyric melts rarely erupt in arc settings, more commonly forming dikes in the arc crust. Eruption of lamprophyric magmas in the western MVB is favored by through-crustal extensional fracture systems related to active rifting of the Jalisco block from the N. American plate. Larger volumes of calc-alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock.

Shoshonitic volcanism in the Northern Mariana Arc: 2. Large‐ion lithophile and rare earth element abundances: Evidence for the source of incompatible element enrichments in intraoceanic arcs

Abstract: The Mariana-Volcano-Izu arc system extends 2400 km north to south and is an outstanding example of an intraoceanic magmatic arc. In spite of this, the system is poorly known because most of it is submarine. Volcanism is entirely submarine in the northernmost Mariana arc, between 20°40′ and 24°N. This is the Northern Seamount Province (NSP) and was the focus of a detailed marine geologic and geochemical study, with additional data drawn from adjacent arc segments to the north (Volcano arc) and south (Central Island Province (CIP) of the Mariana arc). Samples from 24 submarine volcanoes and three islands were analyzed for concentrations of K, Rb, Sr, Ba, and the rare earth elements (REE). These data show strong variations along the arc, being relatively depleted in the more mature, tholeiitic and low-K calc-alkaline volcanoes of the Volcano arc and the Mariana CIP, containing on average 6100 ppm K, 300 ppm Sr, 200 ppm Ba, and 6 ppm La. All of the NSP is enriched in large ion lithophile (LIL) and light rare earth elements (REE), particularly the northern half (26,000 ppm K, 700 ppm Sr, 900 ppm Ba, 47 ppm La); these lavas have strong shoshonitic affinities. These enrichments do not result from fractional crystallization of CIP-type melts. The source responsible for these enrichments shares some features in common with Mariana CIP and Volcano arc sources: K/Rb and K/Ba in particular are similar (∼500 and ∼30, respectively). However, Ba/La, Sr/Nd, and (Ce/Yb)n change drastically, with Ba/La and Sr/Nd decreasing to mantle values with increasing LIL and LREE enrichment. The origin of the LIL and LREE enrichments in the NSP shoshonites does not result from variations in the behavior or composition of the subducted lithosphere. Melting occurs exclusively within the mantle wedge, and forward modeling of the REE patterns for all Mariana and Volcano arc lavas indicates that melt generation occurs within the stability field of spinel lherzolite, probably within the upper 40–50 km of the subarc asthenosphere. Lavas from the large volcanoes of the Mariana CIP and Volcano arc result from 10–20% melting of spinel lherzolite, followed by varying amounts of low-P fractional crystallization. Inferences based on REE forward models that the NSP shoshonites manifest very low (1%) degrees of partial melting of LIL- and LREE-enriched spinel lherzolite are inconsistent with observed similar concentrations in tholeiites and shoshonites of high field strength cations such as TiO2 and Yb. Some of this inconsistency can be explained as resulting from source or melt mixing, with the NSP shoshonites being derived from a LIL- and LREE-enriched source or melt, with Ba/La and La/Yb indistinguishable from that of ocean island basalts (OIB), while Mariana CIP and Volcano arc melts are derived from a depleted mild-ocean ridge basalt-like mantle that has been recharged with K, Rb, Sr, and Ba by hydrous fluids. These variations are interpreted as reflecting the evolution of the subarc asthenosphere, with a depletion in time resulting from the continuous extraction of basaltic melts.

Petrology and geochemistry of volcanic rocks of the Cerro Galan caldera, northwest Argentina

Abstract: At least 2000 km3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the > 1000km3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO2) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87Sr/86Sr = 0.7108–0.7181; 143Nd/144Nd = 0.51215–0.51225, and δ18O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87Sr/86Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87Sr/86Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material(> 1.5 Ga).Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO2 = 51–57%; K2O = 2–3% and normative plagioclase compositions of An37–48, and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87Sr/86Sr ranges from 0.7055 to 0.7094 and 143Nd/144Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87Sr/86Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites.The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.

Chemical geodynamics in a back arc region around the Sea of Japan: Implications for the genesis of alkaline basalts in Japan, Korea, and China

Abstract: Major and trace element have been analyzed from alkaline basalts from southwestern Japan, Korea, and northeastern China. No significant differences were found in the immobile incompatible element ratios, such as Zr, Y, Hf, Th, and Ti. These ratios, as well as normalized incompatible element patterns, resemble those of continental and oceanic island alkaline basalts. However, southwestern Japanese alkaline basalts show evidence of K, Ba, and Rb enrichment and a slight depletion in Ta relative to La, implying a weak island arc signature. Korean and Chinese alkaline basalts do not have such a signature. Rare earth elements (REE) show near-constant La/Sm ratios and a crossover at the high REE end of patterns for each areas studied. The parallelism in light REE can be derived if the magmas are mixtures formed by (1) relatively large degrees of partial melting of an enriched mantle plume from deeper in the mantle and (2) a small degree of partial melting of a depleted mid-ocean ridge basalt (MORB)-type source. These observations when combined with seismic results suggest that the upper mantle beneath southwestern Japan has been weakly affected by metasomatism caused by dehydration and/or partial melting of subducted Pacific plate (not Philippine Sea plate). The mantle plume may have reacted with weakly metasomatized MORB-type depleted mantle to produce alkaline basalt magmas retaining mild island arc characteristics in southwestern Japan. However, the metasomatism by the subduction of the Pacific plate has not affected the mantle beneath Korea and northeastern China. Here the interaction between plume and MORB-type mantle produced alkaline basalt magma similar to normal continental and oceanic alkaline basalts.

Cumulate gabbros from the Southwest Indian Ridge, 54°S-7° 16′ E: implications for magmatic processes at a slow spreading ridge

Abstract: A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16′ E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO2 contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.

Diapiric transfer of melt in Kilauea Iki lava lake, Hawaii: A quick, efficient process of igneous differentiation

Abstract: Kilauea Iki lava lake, formed in 1959, is a large pond of picritic basalt (average MgO content = 15.34% by weight), which has cooled and crystallized as a small, self-roofed magma chamber. Repeated drilling of the upper crust of the lake, down to its molten core, and more recent (1981) drilling, through the thermal maximum and part way through the lower crust, have made it possible to monitor the differentiation processes in the lake in detail. Differentiation processes recognized as active in the lake include rather inefficient settling of the larger (2-10 mm) olivine phenocrysts, formation of segregation veins, and formation of diapir-like vertical olivine-rich bodies, all processes which occur in one or more of the other Kilauean lava lakes as well. In addition, most of the central part of Kilauea Iki has been affected by diapiric melt transfer. In this process, relatively low-density liquid, present at 1145-1160 °C, rose from within the loose crystal mush at the base of the lens of melt and intruded the equivalent thermal horizon at the top of the lens of melt, passing through the hotter but denser melt in the core of the lake without mixing. The source volume from which the low-density liquid was extracted is depleted in TiO 2 and other incompatible elements and enriched in FeO and CaO. The upper part of the lake shows the opposite effects. The crystalline assemblage present was olivine + augite + minor plagioclase. The crystallinity of both the source and receiving layers was low enough that no obvious textural imprint was left by the transfer process; the principal evidence for its occurrence is the chemical zonation of the lake seen in core from depths of 13 to 80 m. Diapiric melt transfer was active from 1960 to 1971 and has affected most of the central part of the lake from 13 m to at least 80 m. The process ran simultaneously with the other three main differentia tion processes but started and stopped independently of the others. Calculations suggest that between 21 and 42 wt % liquid has been extracted from the depleted zone at 56-78 m in the center of the lake, making this a very efficient process of chemical differentiation.

Formation of platiniferous sulfide horizons by crystal fractionation and magma mixing in the Munni Munni layered intrusion, West Pilbara Block, Western Australia

Abstract: The Munni Munni Complex (T Nd CHUR model age 2.85 Ga), located in the west Pilbara block of Western Australia, is one of the best preserved layered intrusions in Australia. Exposed over an area of 4 X 9 km, it is composed of a lower 1,850-m-thick ultramafic zone and an overlying gabbroic zone which has a minimum thickness of 3,630 m. The ultramafic zone contains rhythmically layered dunite, lherzolite, olivine websterite, clinopyroxenite, and websterite, with orthopyroxenite, norite, chromitite, and platiniferous websterite prominent near the top of the zone. The gabbroic zone consists of gabbronorite, anorthositic gabbro, and minor anorthosite which display a pronounced tholeiitic fractionation trend. The order of appearance of cumulus mineral assemblages in the complex is olivine, olivine + clinopyroxene, clinopyroxene + olivine, clinopyroxene, clinopyroxene + orthopyroxene, orthopyroxene + chromite, and plagioclase + clinopyroxene + ?orthopyroxene. This sequence is at variance with major platinum-group element-bearing intrusions in which crystallization of orthopyroxene generally precedes that of clinopyroxene.Trace-element data, obtained on samples collected across the entire intrusion to investigate the effects of crystal fractionation and S evolution on the distribution of the platinum-group elements, show that in the sulfide-undersaturated ultramafic zone, Pt, Pd, Au, Cu, S, Se, Cs, Rb, St, and Zr behaved incompatibly and were concentrated in the melt during fractionation. The S content of the melt began to increase above the 700-m stratigraphic level of the ultramafic zone, but Pt, Pd, and Au contents increased above background levels of approximately 3 ppb to 3 ppm Pt + Pd only with the attainment of sulfide saturation at approximately the 1,830-m stratigraphic level. The concentration trends of Zr, St, Cs, Rb, and Cu paralleled that of S, but Ir and Ni largely partitioned with early crystallizing olivine and decreased in concentration with increasing fractionation. In contrast to the ultramafic zone, Pt, Pd, It, and Au have depletion trends in the sulfide-saturated gabbroic zone. Hence, the evolution of S largely governed the behavior of the platinum-group elements during the fractionation of the Munni Munni magma(s).The platinum-group element mineralization occurs immediately below the ultramafic-gabbroic contact. It resulted from the combined magmatic processes of crystal fractionation (as evidenced by increasing Cu/(Cu + Ni) ratios and incompatible element trends with stratigraphic height), and magma mixing. Two models are presented. In model 1, a hot, buoyant sulfide-saturated tholeiitic magma (containing 1,700-2,600 ppm S) rose through the density stratified platinum-group element-enriched, sulfide-undersaturated resident ultramafic magma (containing 530 ppm S) until reaching its own density level near the top of the chamber, where it spread out laterally for a distance of at least 12 km. Due to crystallization of plagioclase and subsequent Fe-enrichment [of the melt], the density of the gabbroic melt increased until it overturned and mixed with the platinum-group element-enriched fractionated parts of the ultramafic magma. Model 2 is similar to model 1, except that it involves the fractionation and internal mixing of one magma. In both models, magma mixing triggered sulfide saturation in the hybrid magma and established a high R factor (the silicate/sulfide mass ratio). The chalcophile platinum-group elements, due to their high sulfide/silicate partition coefficients, were scavenged by immiscible sulfide droplets which precipitated slowly in a porphyritic plagioclase websterite in a zone up to 20 m below the ultramafic-gabbroic zone contact. The platinum-group minerals comprising arsenides, sulfarsenides, tellurides, native metals, and mercury-bearing phases form small (<10 mu m) grains now enclosed within silicate minerals or along silicate grain boundaries (78% of occurrences), or are associated with chalcopyrite-pyrrhotite-pentlandite blebs (22%).The Munni Munni mineralized websterite layer is defined by discontinuities in (Pt + Pd)/ Se, (Pt + Pd)/S, (Pt + Pd)/Cs, (Pt + Pd)/Zr, and Cu/Zr ratios and Cu, Cs, Zr, Sr, S, and Se concentrations. Hence, these parameters together with information on the intercumulus component to the cumulates are useful for indicating potentially mineralized units in layered intrusions. Sulfide saturation also occurred along the basal contact of the ultramafic zone, but platinum-group element concentrations (< or =92 ppb Pt + Pd) are low since sulfide precipitation was probably induced by rapid cooling and the sulfides equilibrated with a small volume of silicate melt, implying a low R value. The different settings of sulfide mineralization in the Munni Munni Complex illustrate that timing and physical characteristics of the sulfide-saturation event are critical for the development of platinum-group element-enriched sulfide units in layered intrusions.

Axial and off‐axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35′N–12°51′N and 11°26′N–11°30′N

Abstract: Surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the EPR at 12°35′N–12°51′N and at 11°26′N–11°30′N and from four adjacent seamounts centered at less than 18 km from the rise axis. The basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition. There is no correlation between lava flow morphology, geological settings, and compositions. Based on mineral and compatible element composition, these basalts were classified into three types. The least evolved olivine-basalts and highly phyric plagioclase-basalts (type A) have high Fo87–89, high Mg # (66–70), and high Ni (>100) contents. The most evolved plagioclase-olivine basalts with or without pyroxene (type B and C) have relatively low Fo78–86, low Mg # (55–65), and low Ni (60–100 ppm) contents. Simple crystal fractionation (3–18%) accounts for some of the observed compositional range. However, based on their incompatible element contents, the above basalts are divided into three groups (the various types A to C are found in each group): depleted basalts with low K/Ti (0.04–0.15) ratios, low Zr (<100 ppm) and low Nb (<0.4 ppm) contents, undepleted basalts with the highest K/Ti (0.25–0.46), Zr (150–170 ppm), and Nb (8–16 ppm) contents and the transitional basalts having compositions between these two extremes. These three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3–4 km in width near 12°43′N–12°51′N on the EPR. Similar volcanic diversity in a more limited area (<3 km2) is observed on off-axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis. Depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle. The transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation. Two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting; we suggest that a lherzolitic mantle, with minerals having a variable behavior under partial melting, constitutes this type of composite mantle. Indeed, clinopyroxene, enriched in incompatible elements, will be consumed at an earlier stage of melting than the olivine plus orthopyroxene. Eruption of compositionally distinct magma batches in close proximity on axial and off-axial structures implies periods of quiescence between successive magmatic stages and cycles.

Isotopic and trace element constraints on the origin and evolution of alkaline and calc-alkaline magmas in the Northwestern Mexican Volcanic Belt

Abstract: A wide variety of rock types has been produced by Pliocene to Recent volcanism in the northwestern portion of the Mexican Volcanic Belt. Composite cones erupted rocks typical of calc-alkaline volcanic belts associated with subduction. Monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks. One larger center, Las Navajas, produced basalts, trachytes, and peralkaline rhyolites. Such alkaline rocks are typically associated with crustal extension related to rifting. Both subduction and rifting appear to be taking place in western Mexico. The most basic calc-alkaline rocks, although somewhat fractionated, show depletion in high field strength elements (HFSE) and enrichment in alkaline earth elements relative to other incompatible elements, a characteristic typical of magmas in subduction-related volcanic arcs. The basic alkaline rocks show no depletion in HFSE and show higher concentrations of incompatible elements than the basic calc-alkaline rocks. The calc-alkaline rocks show a tight cluster of 87Sr/86Sr and 143Nd/144Nd ratios, both of which correlate weakly with SiO2. Basic alkaline rocks have lower Sr and higher Nd isotopic ratios, whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc-alkaline rocks. Trace element data indicate that calc-alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas. The evolution of both calc-alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation (AFC) of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite. Assuming a contaminant located in the lower or middle crust, AFC modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc-alkaline magmas. Calc-alkaline and alkaline rocks appear to have resulted from melting of two distinct sources: alkaline from an OIB-type source, and calc-alkaline from a mixed mantle and slab-derived source.

Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico

Abstract: The trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma-mass spectrometer to constrain the composition and evolution of the Mexican lower crust. Most xenoliths found are pyroxeneplagioclase orthogneisses, although quartzo-feldspathic paragneisses form up to 50% of the xenolith population in some areas. Lithologies in the exposed terrains consist of pelitic and carbonate-rich paragneisses, charnockites, and less abundant basic, intermediate, and felsic orthogneisses. Both xenoliths and exposed granulites generally have low abundances of incompatible elements such as K, Rb, Th, and U. The xenoliths, however, often have lower abundances of these elements than the exposed granulites. High K/Rb ratios, coupled with an inverse correlation between K2O and K/Rb, indicate that most samples from the Mexican lower crust are depleted in both K and Rb. Furthermore, Th/U ratios similar to upper crustal materials and high La/Th ratios suggest depletion in both Th and U relative to average crustal material. Removal of a melt phase, either after the crystallization of cumulates or following a partial melting event, can explain many of the chemical features observed and may be responsible for the elemental depletions. Some samples, however, appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths. Available geobarometric data suggest that xenoliths equilibrated near the crust/mantle boundary (∼10 kbar) and represent the lowermost crust, while the exposed granulites equilibrated at shallower crustal levels (∼7 kbar). Thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the Mexican lower crust. The lowermost crust in Mexico consists of crystallized basaltic liquids, cumulates, and/or residue after the removal of a partial melt as well as garnet-rich metasediments.

Spinel‐lherzolite‐bearing quaternary volcanic centers in San Luis Potosí, Mexico: 2. SR and ND Isotopic Systematics

Abstract: We present Sr and Nd isotopic data for 18 basanites, and chemical and isotopic data for five associated kaersutites, from the Ventura and Santo Domingo volcanic fields in the Mexican state of San Luis Potosi. All isotopic data fall in or near the “mantle array.” The kaersutites have the most depleted isotopic compositions (low 87Sr/86Sr and high e143) but are less depleted than mid-ocean ridge basalts (MORB). Basanite isotopic compositions are distributed bimodally, with most samples in the more-depleted mode, which is near the high e143 and low 87Sr/86Sr end of the compositional range observed for ocean island basalts (OIBs). In a companion paper, Luhr et al. [1989] find chemical variations in these volcanic rocks that suggest partial melting could be responsible for the principal chemical trends, but the variability of isotopic compositions precludes any simple one-component model. 87Sr/86Sr, but not e143, is found to correlate with these chemical trends. SiO2 is lowest and normative ne + lc, incompatible element concentrations and light versus heavy rare Earth element ratios are highest in samples with the lowest 87Sr/86Sr values. Crustal contamination may be significant for a few of these samples, but we conclude that the principal isotopic trends arise in mixing of at least three mantle reservoirs: a depleted component analogous to the MORB reservoir, a St. Helena type component, and a third component which affects Sr composition substantially more than Nd composition. The third component carries relatively radiogenic Sr and appears to correlate with degree of partial melting, suggestive of a hydrous component, possibly derived from a subducted slab, such as has previously been invoked in studies of island arcs.

Petrologic and geologic variations along the Southern Explorer Ridge, northeast Pacific Ocean

Abstract: Mid-ocean ridge basalts (MORB) from the well-defined Southern Explorer Ridge segment (SER) in the northeast Pacific Ocean are moderately to strongly enriched in incompatible elements. Enriched MORB were erupted at the highest and widest part of the segment (culmination) where the magma supply may be the greatest and also at the northern and southern ends of the SER. Variations in basalts' incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale. The variations also preclude the existence of a long-lived, well-mixed magma chamber beneath the robust culmination. Instead, magma chambers are probably temporally and spatially isolated, as evidenced by the presence of highly differentiated lavas. Less enriched (transitional) MORB were erupted along the central part of the SER, 11–27 km south of the culmination, and were supplied to the ridge separately from a less enriched pan of the mantle. A more continuous magma chamber cannot be ruled out for this section on geochemical grounds. All of the MORB from the SER have undergone significant amounts of fractional crystallization. Unlike certain segments of the East Pacific Rise, the most evolved MORB were erupted near the culmination of the ridge segment. Aphyric lavas have been recovered only from the vicinity of the culmination, while phyric lavas have erupted along the entire length of the SER. There is no correlation between composition and total crystal abundances. Despite the requirement of multiple parental magmas, most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent, indicating that they formed at similar depths and extents of partial melting. The transitional MORB from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of Na and moderately incompatible elements such as Sm and Zr. Enriched, high-melt fraction lavas at the magma-starved southern end of the SER closely resemble lavas from the culmination. Either the southernmost lavas have flowed laterally within the crust from near the culmination, or there is another melting anomaly that supplies the southern end of the SER. If the latter case is true, correspondence between ridge morphology and mantle thermal state is poor.

Geochemistry of Siluro-Devonian Tobique volcanic belt in northern and central New Brunswick (Canada): tectonic implications

Abstract: Siluro-Devonian volcanic rocks of the northwestern mainland Appalachians are found mainly in the Tobique belt of New Brunswick where they consist predominantly of bimodal mafic–felsic suites erupted in a continental-rift environment. The axis of the Tobique rift trends north-northeast – south-southwest, obliquely to the regional northeast–southwest trend of the Appalachians. These geometric relationships are interpreted as being the result of rifting in a sinistral shear regime produced during emplacement of the Avalon terrene. The basaltic rocks are continental tholeiites and transitional basalts derived from a heterogeneous upper-mantle source that was enriched in incompatible elements relative to the primordial mantle. The mantle source was probably affected by the subduction processes.

Strontium, neodymium and lead isotope constraints on near-ridge seamount production beneath the South Atlantic

Abstract: STUDIES1–7 of seamounts near the East Pacific Rise (EPR) have shown that, although most seamount lavas are petrographically and chemically identical to mid-ocean-ridge basalt, they are chemically and isotopically more diverse than those erupted on the rise axis. They are also generally more primitive (higher MgO content) and, in some cases, more depleted in incompatible elements than the axial basalts. This indicates that although near-ridge sea-mounts and the EPR share a common mantle source, there must be significant differences in their magma-supply processes. Sea-mounts are probably built by small batches of melt that rise rapidly to the surface, preserving evidence of heterogeneity in the mantle source region. By contrast, the processes of melting and melt segregation, storage and extrusion along the fast-spreading (9–13 cm yr–1) EPR are more efficient in mixing and homogenizing basalt compositions1–7. Here we show that the relationship of strontium, neodymium and lead isotope data between seamounts in the South Atlantic and the nearby axis of the slow-spreading Mid-Atlantic Ridge (MAR) is similar to that seen in the Pacific. This indicates that the processes leading to formation of near-ridge seamounts are similar at a wide range of spreading rates. Differences in the specific isotope signatures of lavas from near-ridge seamounts and axes of the EPR and MAR reflect regional differences in the upper-mantle source of mid-ocean-ridge basalts.

Petrology and age of alkalic lava from the Ratak Chain of the Marshall Islands

Abstract: Volcanic rock dredged from the flanks of four volcanic edifices in the Ratak chain of the Marshall Islands consist of alkalic lava that erupted above sea level or in shallow water. Compositions of recovered samples are predominantly differentiated alkalic basalt and hawaiite but include strongly alkalic melilitite. Whole rock 40Ar/39Ar total fusion and incremental heating ages of 87.3±0.6 Ma and 82.2±1.6 Ma determined for samples from Erikub Seamount and Ratak Guyot, respectively, are within the range predicted by plate rotation models but show no age progression consistent with a simple hot spot model. Whole rock major element, and mineral chemistry are similar to those of alkalic lavas from other oceanic islands, but isotopic and certain trace element ratios reveal distinctive mantle source characteristics. Zr/Nb ratios of ∼3.0±0.2 for Bikar and Ratak Guyot, compared to 5.2±0.3 for Majuro Atoll and Erikub Seamount, at comparable degree of differentiation, suggest a more alkalic trend for the northern sites. The 143Nd/144Nd isotopic ratios range from 0.51283 to 0.51289 and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios range from 20.55 to 21.11, 15.71 to 15.77, and 39.93 to 40.75, for samples from Erikub and Ratak. Variations in isotopic and some incompatible element ratios suggest interisland heterogeneity. Similar highly radiogenic lead isotopes, coupled with distinct incompatible element ratios, especially with low Ba/Nb ratios (∼6), are observed for St. Helena in the South Atlantic and for Tubuai and Mangaia in the South Pacific. The similar mantle signature shown by lavas from Tubuai and Mangaia in the Austral-Cook chain, which are located at approximately the position where the Ratak edifices formed during the Late Cretaceous, indicates that this mantle anomaly is not of recent origin but has erupted distinctive lava compositions for at least 87 m.y.

Magma mixing and constitution zone refining in the Lac des Iles Complex, Ontario; genesis of platinum-group element mineralization

Abstract: The Lac des Iles Complex is a 2.74-Ga-old composite intrusion emplaced into a granite and/or granite-gneiss terrane of the Wabigoon subprovince, northwestern Ontario. The complex consists of a gabbroic and ultramafic part; the latter has been subdivided into northern and southern intrusive phases. Both the gabbroic and ultramafic parts contain platinum-group element-bearing Ni-Cu sulfide mineralization, although only the Roby zone in the gabbro is of economic importance.The southern ultramafic complex consists of a wehrlite to olivine-clinopyroxenite core, which is surrounded by websterite and gabbronorite cumulates. Petrographic and geochemical studies suggest that the northern ultramafic complex consists of at least eight cyclic units, which ideally are composed of dunite and wehrlite to olivine-clinopyroxenite cumulates at the base, followed by clinopyroxenite and websterite, locally grading into gabbronorite. This indicates similar crystallization sequences in both ultramafic centers: olivine, olivine + clinopyroxene, clinopyroxene, clinopyroxene + orthopyroxene + or - plagioclase.In the ultramafic complex, platinum-group element-bearing sulfide mineralization occurs at the top of cyclic units in websterite and orthopyroxenite cumulates. Sulfide-bearing samples (up to 1 wt % sulfide) are enriched in Au, Pd, Pt, and Cu relative to Ni, Ir, Os, and Ru, with Pd/Ir ratios on the order of 250. Sulfide immiscibility probably was caused by mixing of fractionated residual melt with more primitive liquid, which periodically intruded the magma chamber. Evidence of mixing is seen in gradational chemical variations across the interface of cyclic units and in the occurrence of inclusions of olivine crystals and crystal aggregates in websterite and gabbronorite cumulates. These inclusions are not in equilibrium with their present host rock and presumably formed before mixing in the more primitive magma. A mineralized orthopyroxenite cumulate at the top of cyclic unit 2 could not have been produced by the normal crystallization sequence. Consideration of the phase relationships in the ternary system olivine-clinopyroxene-plagioclase-quartz suggests that these cumulates are the crystallization product of a hybrid magma formed during magma mixing at the interface of cyclic units.The gabbroic complex consists mainly of largely altered gabbro and gabbronorite cumulates and is intruded by mafic-ultramafic dikes. The center of the gabbroic complex contains the economically important Roby zone, with its platinum-group element-bearing Cu-Ni sulfide mineralization. This is a lithologically and texturally very complex zone in which high-grade platinum-group element mineralization is associated with pegmatite and mafic-ultramaflc dikes. The Roby zone is characterized by strong enrichments of Au, Pd, and Pt relative to Ir and Os, and by variable but extremely high Pd/Ir ratios of greater than 10,000. High Au/Pt and Pd/Pt ratios indicate a fractionation of Pt from Au and Pd.It is suggested that the high volatile content of the residual magma of the gabbroic complex triggered partial remelting of surrounding gabbro cumulates, a process called "constitutional zone refining" by McBirney (1987). During the partial melting event, compatible elements such as Ir and Os are buffered by the residual silicate and oxide mineralogy, whereas incompatible elements such as Au, Pd, and Pt, as well as volatile components such as H 2 O and H 2 S, become highly concentrated in the partial melt phase. Eventually this newly formed magma may become sulfide-saturated, either by cooling and fractional crystallization or due to the continuous dissolution of sulfides when the zone of melting proceeds through the gabbro cumulates. Ratios such as Pd/Ir and Pd/Pt would strongly depend on the degree of remelting and fractional crystallization until the time of sulfide saturation. The matrix of the varitextured zone is interpreted as the residuum of this melting process, whereas pegmatoids and pegmatites, segregationlike gabbro phases, and quartz-bearing plagioclase- and incompatible element-rich dikes are regarded as the final crystallization products of the partial melting.