Showing papers on "Basalt published in 2003"

Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

Abstract: The phase relations of primitive magnesian andesites and basaltic andesites from the Mt Shasta region, N California have been determined over a range of pressure and temperature conditions and H2O contents The experimental results are used to explore the influence of H2O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments At 200-MPa H2O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt Shasta region lavas This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO2 We propose a two-stage process for the origin of these lavas (1) Extensive hydrous mantle melting produces H2O-rich (>45--6 wt% H2O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue Trace elements and H2O are contributed from a slab-derived fluid and/or melt (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization Crustal-level differentiation occurs under near-H2O saturated conditions producing the distinctive high SiO2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas In a subset of Mt Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H2O contents (>10 wt% H2O) and lower crustal crystallization pressures (800 MPa) Igneous rocks that possess major and trace element characteristics similar to those of the Mt Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites) We propose that these magmas also form by hydrous mantle melting

Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction

Abstract: Geochronological (K---Ar or Ar/Ar), major and trace element, Sr---Nd---Pb isotopic and mineral chemical data are presented for newly discovered Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes of Tibet. Alkali basalts of 65---45Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers [100 Mg/(Mg‡Fe)](54---65),Cr (204---839 ppm) and Ni (94---218 ppm) contents, and relatively low ratios of Sr/Sr (0 7046---0 7061), Pb/Pb (18 21---18 89), Pb/Pb (15 49---15 61) and Pb/Pb (38 42---38 89), and high ratios of Nd/Nd (0 5124---0 5127). In contrast, younger volcanic rocks in the western Qiangtang terrane ( 30Ma) and the central Lhasa terrane ( 23, 13 and 8Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are characterized by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (Pb/Pb ˆ 18 43---19 10; Pb/Pb ˆ 15 64---15 83; Pb/Pb ˆ 39 14---39 67). Sr/Sr (0 7088---0 7092) and Nd/Nd ( 0 5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 45---29Ma potassic volcanic rocks in the north---central Qiangtang terrane, whereas Sr/Sr (0 7167---0 7243) and Nd/Nd ( 0 5119) ratios of central Lhasa terrane lavas are similar to those of 25---16Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 65---45Ma alkali basalts in the western Qiangtang terrane, along with widespread calc-alkaline volcanic rocks of this age in the Lhasa terrane, may be related to roll-back of a previously shallow north-dipping slab of Tethyan oceanic lithosphere beneath Tibet. Subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in Tibet because of its occurrence in distinct, east---west-trending belts (45---29Ma in the Qiangtang terrane; 25---17Ma in the northern Lhasa terrane; 16---8Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems.

Relationships between mafic and peralkaline silicic magmatism in continental rift settings: A petrological, geochemical and isotopic study of the Gedemsa volcano, Central Ethiopian rift

Abstract: Petrological and geochemical data are reported for basalts and silicic peralkaline rocks from the Quaternary Gedemsa volcano, northern Ethiopian rift, with the aim of discussing the petrogenesis of peralkaline magmas and the significance of the Daly Gap occurring at local and regional scales. Incompatible element vs incompatible element diagrams display smooth positive trends; the isotope ratios of the silicic rocks (Sr/Sr ˆ 0 70406---0 70719; Nd/Nd ˆ 0 51274---0 51279) encompass those of the mafic rocks. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modelling of incompatible vs compatible elements excludes the possibility that peralkaline rhyolites are generated by melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks (AFC) starting from trachytes; the latter have exceedingly low contents of compatible elements, which precludes a derivation by basalt melting. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. This process generated a zoned magma chamber whose silicic upper part acted as a density filter for mafic magmas and was preferentially tapped; mafic magmas, ponding at the bottom, were erupted only during post-caldera stages, intensively mingled with silicic melts. The large number of caldera depressions found in the northern Ethiopian rift and their coincidence with zones of positive gravity anomalies suggest the occurrence of numerous magma chambers where evolutionary processes generated silicic peralkaline melts starting from mafic parental magmas. This suggests that the petrological and volcanological model proposed for Gedemsa may have regional significance, thus furnishing an explanation for the large-volume peralkaline ignimbrites in the Ethiopian rift.

Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean

Abstract: A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

Partial melting experiments on a MORB‐like pyroxenite between 2 and 3 GPa: Constraints on the presence of pyroxenite in basalt source regions from solidus location and melting rate

Abstract: [1] We present partial melting experiments at 2–3 GPa on a basaltic pyroxenite (G2) similar in composition to typical oceanic crust. The 3.0 GPa solidus is located at 1310 ± 12°C and the liquidus is 1500–1525°C. Clinopyroxene, garnet, quartz, and rutile are subsolidus phases. Garnet, quartz, and rutile are absent above 1475°C, 1365°C, and 1335°C, respectively. At the solidus, the garnet mode is low (18 wt.%) because clinopyroxene is unusually aluminous (13.8–15.5 wt.% Al2O3). In adiabatically upwelling mantle near 2–3 GPa, G2-like pyroxenite begins melting 35–50 km deeper than peridotite. The calculated near-solidus adiabatic productivity for G2 is ∼13%/GPa and averages ∼59%/GPa through the melting interval, suggesting substantial partial melting deep in basalt source regions: G2 is ∼60% molten at the 3 GPa peridotite solidus. Small percentages of pyroxenite in the source significantly affect oceanic crust production and composition, as the proportion of pyroxenite-derived melt contributed to oceanic crust formation is 5 to >10 times the pyroxenite proportion in the source. Given the overall depleted isotopic character of mid-ocean ridge basalt (MORB), oversampling of fertile G2-like pyroxenite limits the abundance of such lithologies to ∼<2% of the MORB source. Owing to high extents of partial melting, the effect of modest amounts of pyroxenite on Sm/Yb ratios of aggregated basalts is limited and depends largely on the average bulk composition of the pyroxenite source. Low near-solidus adiabatic productivities could allow small (∼1–2%) proportions of basaltic pyroxenite to enhance (230Th)/(238U) in oceanic basalts without requiring marked shifts in other indicators of heterogeneity, such as Sr or Pb isotopes.

High 3He/4He ratios in picritic basalts from Baffin Island and the role of a mixed reservoir in mantle plumes.

Abstract: The high 3He/4He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic 3He/4He ratios yet recorded. A strong correlation between 3He/4He and 87Sr/86Sr, 143Nd/144Nd and trace element ratios demonstrate that the 3He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest 3He/4He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a 3He-recharged depleted mantle (HRDM) reservoir may be the principal source of high 3He/4He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.

Geochemistry of Mesozoic mafic rocks adjacent to the Chenzhou-Linwu Fault, South China; implications for the lithospheric boundary between the Yangtze and Cathaysia blocks

Abstract: To constrain the Mesozoic tectonic evolution and the lithospheric boundary between the Yangtze and Cathaysia blocks in South China, we present geochronological and geochemical data for Mesozoic basaltic lavas and related mafic dikes west (Group 1) and east (Group 2) of the Chenzhou-Linwu fault. Three episodes of mafic magmatism around the Chenzhou-Linwu fault were identified: ca.175 Ma, 125-150 Ma, and 80-95 Ma, respectively. Group 1 rocks (alkaline basanite and trachybasalt), with ages of >125 Ma, have a wide range of 87Sr/86Sr(t) values (0.7035-0.7069), and eNd(t) values (-3.75 to + 6.10). In contrast, Group 2 rocks (subalkaline basalt and basaltic andesite), with ages of > 125 Ma, exhibit 87Sr/86Sr(t) values of 0.7075-0.7087 and eNd(t) values of -2.04 to + 1.05. Both groups are strongly enriched in incompatible elements, with variable negative Nb-Ta anomalies. However, Group 1 rocks commonly have higher LREE and Ba/Nb, Rb/Nb, Ba/Th, and Ba/La ratios and lower Th/Nb, Th/La, and Zr/Nb ratios than Group 2...

Does depleted mantle form an intrinsic part of the Iceland plume

Abstract: [1] Icelandic basalt ranges in composition from voluminous tholeiite, erupted in the rift zones, to small-volume, mildly alkaline basalt erupted off-axis. In addition, small-volume flows of primitive basalt, highly depleted in incompatible elements, are sometimes found in the actively spreading rift axes. Relative incompatible-element depletion or enrichment in Icelandic basalt is correlated with variation in radiogenic isotope ratios, implying that the mantle beneath Iceland is heterogeneous and that the relative contribution of the various mantle components relates to eruption environment (on- or off-axis) and hence to degree of melting. Thus small-degree off-axis melting preferentially samples an enriched and more fusible mantle component, whereas more extensive melting beneath the rift axes produces magma that more closely represents the bulk Iceland plume mantle composition. The small-volume flows of depleted basalt represent melts that have preferentially sampled a depleted and more refractory mantle component. A debate has arisen over the nature of the depleted component in the Iceland plume. Some authors [e.g., Hanan and Schilling, 1997] argue that the depleted component is ambient upper mantle, the source of normal mid-ocean ridge basalt (NMORB) in this region. Others [e.g., Thirlwall, 1995; Kerr et al., 1995; Fitton et al., 1997], however, have used various lines of evidence to suggest that the plume contains an intrinsic depleted component that is distinct from the NMORB source. Hanan et al. [2000] attempt to refute the existence of a depleted Iceland plume (DIP) component through a critical evaluation of the Nb-Zr-Y arguments advanced by Fitton et al. [1997] and the Hf-Nd-isotopic evidence presented by Kempton et al. [1998]. In this paper we examine the case presented by Hanan et al. [2000] and conclude that their arguments are flawed. Firstly, their trace-element data set excludes data from depleted Icelandic basalt samples and so it is not surprising that they find no evidence for a DIP component. Secondly, they present two new Hf-isotope analyses of a single depleted Icelandic basalt sample and show that the data plot in their NMORB field on an eHf versus eNd diagram. However, new data allow the resolution of distinct NMORB and depleted Icelandic basalt fields on this diagram. We conclude that trace-element and radiogenic isotope data from Iceland require the existence of a DIP component.

Mesozoic crust-mantle interaction beneath the North China craton: A consequence of the dispersal of Gondwanaland and accretion of Asia

Abstract: We present evidence from zircons entrained within lower-crustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.

Incompatible element ratios in oceanic basalts and komatiites: Tracking deep mantle sources and continental growth rates with time

Abstract: [1] Ratios of elements with similar incompatibilities in the mantle can be used to characterize magma sources through time. Nb/Y and Zr/Y distributions in oceanic basalts support the existence of a long-lived, deep depleted source in mantle. Zr/Y, Nb/Y, Zr/Nb, and Nb/Th ratios in oceanic basalts and komatiites suggest that depleted and recycled components, together probably with an enriched component, were present in the deep mantle by 3.5 Ga. Low Zr/Nb and Hf/Sm ratios and high La/Yb and Nb/Y ratios in some plume basalts and Al-depleted komatiites may reflect majorite fractionation. High Zr/Nb ratios and low Nb/Y ratios in Archean Al-undepleted komatiites may record partial melting of a Mg-perovskite source in deep mantle plumes in which Mg-perovskite crystallizes and accumulates in komatiite melts during ascent. Oceanic greenstone basalts show a gradual increase in the Nb/Th ratio with time with a relatively sudden increase at about 2 Ga. This trend is consistent with gradual continental growth and with a major episode of continental growth at 2.7 Ga. Nb/Th ratios in some Early Archean basalts may record extraction of up to 25% of the present volume of continental crust from the early upper mantle. An alternative explanation for the rapid increase in Nb/Th in oceanic basalts at 2 Ga is that a catastrophic 2.7-Ga event in the mantle changed the composition or/and location of the primary volume of mantle from which continental crust was extracted.

Sr‐Nd‐Pb composition of Mesozoic Pacific oceanic crust (Site 1149 and 801, ODP Leg 185): Implications for alteration of ocean crust and the input into the Izu‐Bonin‐Mariana subduction system

Abstract: We report Sr, Nd and Pb isotopic compositions of sediments and variably altered igneous rocks from ODP Site 801 (Marianas) and ODP Site 1149 (Izu-Bonin). These Sites provide the most complete drilled ocean crust sections located in front of the Mariana and Izu-Bonin trenches and characterize the unmodified isotopic input into these subduction zones. The subducted ocean crust belongs to the oldest (130–167 Ma) in situ Pacific Ocean crust and thus has end-member character with respect to alteration and sediment load. The lithostratigraphic division of sedimentary units at Site 1149 into clays, cherts, lower clays and carbonates with clay is reflected on isotope correlation diagrams. The Pb isotope data of the sediments show much greater variation than previously reported from this region. Particularly noteworthy are zeolite-bearing clays and clay bearing carbonates from the lower Units that have Pb isotopic compositions identical to the Izu Volcanic Front. The basaltic basement samples display variable 87Sr/86Sr ratios at near constant 143Nd/144Nd ratios, indicating mixing with seawater derived Sr. Most basaltic samples from Site 1149 and 801 exhibit highly variable 206Pb/204Pb (17.88–20.00) at near constant 207Pb/204Pb and 208Pb/204Pb ratios. Three samples from Site 801 display the most extreme 206Pb/204Pb (23.70–26.86) and 207Pb/204Pb (15.73–15.83) ratios ever measured in altered MORB reflecting an increase of 238U/204Pb ratios (μ), most likely through addition of seawater derived U. Initial Pb isotopes of most samples overlap with the age corrected field of the Pacific MORB source, thus the increase in μ took place shortly after formation of the crust in most samples. According to our new isotope data the radiogenic end-member of the Izu arc volcanic rocks could either represent Pb from the lower sediment column released from the slab by delayed dewatering or an integrated slab fluid in which 90–95% of the Pb comes from the basaltic crust and 5–10% of the Pb from the sediments. The Pb isotope systematics of the Mariana arc output suggest two component mixing. Both components appear to be input derived with the radiogenic component represented by average Site 801 sediment and the unradiogenic component generated by mixing of ∼80% unaltered crust with ∼20% highly altered crust.

Petrogenesis of the Largest Intraplate Volcanic Field on the Arabian Plate (Jordan): a Mixed Lithosphere-Asthenosphere Source Activated by Lithospheric Extension

Abstract: Miocene to Recent volcanism in northwestern Arabia produced the largest intraplate volcanic field on the Arabian plate (Harrat Ash Shaam, Jordan). The chemically and isotopically diverse volcanic field comprises mafic alkali basalts and basanites. The magmas underwent limited fractional crystallization of ol cpx plag and rare samples have assimilated up to 20% of Late Proterozoic crust en route to the surface. However, there are subtle Sr±Nd±Pb isotopic variations (Sr/Sr ˆ 0 70305±0 70377, Nd/Nd ˆ 0 51297±0 51285, Pb/ Pb ˆ 18 8±19 2), which exhibit marked correlations with major elements, incompatible trace element ratios and abundances in relatively primitive basalts (MgO 48 5 wt %), and cannot be explained by fractional crystallization and crustal contamination alone. Instead, the data require polybaric melting of heterogeneous sources. Semi-quantitative melt modelling suggests that this heterogeneity is the result of small degree melts (2±5%) from spineland garnet-facies mantle, inferred to be shallowArabian lithosphere, thatmixedwith smaller degreemelts (51%) from a predominantly deep garnet-bearing asthenospheric(?) source with ocean island basalt characteristics. The latter may be a ubiquitous part of the asthenosphere but is preferentially tapped at small degrees of partial melting. Volcanism in Jordan appears to be the result of melting lithospheric mantle in response to lithospheric extension. With time, thinning of the lithosphere allowed progressively deeper mantle (asthenosphere?) to be activated and melts from this to mix with the shallower lithospheric mantle melts. Although Jordanian intraplate volcanism is isotopically similar to examples of Late Cenozoic volcanism throughout the Arabian peninsula (Israel, Saudi Arabia), subtle chemical and isotopic differences between Yemen and Jordan intraplate volcanism suggest that the Afar plume has not been channelled northwestwards beneath the Arabian plate and played no role in producing the northern Saudi Arabian and Jordan intraplate volcanic fields.

A petrogenetic model for the origin and compositional variation of the martian basaltic meteorites

Abstract: The major element, trace element, and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean. The models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate Mars from the literature. These models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic CaO/Al2O3 ratios and their source regions obtained their parent/daughter ( 87 Rb/ 86 Sr, 147 Sm/ 144 Nd, and 176 Lu/ 177 Hf) ratios calculated from the initial Sr, Nd, and Hf isotopic compositions of the meteorites. High pressure experiments suggest that majoritic garnet is the liquidus phase for Mars relevant compositions at or above 12 GPa. Early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated CaO/Al2O3 ratio and a high Mg#. Olivine-pyroxene-garnet-dominated cumulates that crystallize subsequently will also be characterized by superchondritic CaO/Al2O3 ratios. Melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites. Furthermore, crystallization models demonstrate that some of these cumulates have parent/daughter ratios that are similar to those calculated for the most incompatible-element-depleted source region (i.e., that of the meteorite Queen Alexandra (QUE) 94201). The incompatible-element abundances of the most depleted (QUE 94201-like) source region have also been calculated and provide an estimate of the composition of depleted martian mantle. The incompatible-element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted Earth's mantle. Melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of QUE 94201 (e.g., Ba, Th, K, P, Hf, Zr, and heavy rare earth elements) fairly well but does not reproduce the abundances of Rb, U, Ta and light rare earth elements. The source regions for meteorites such as Shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile. Melting of this hybrid source yields liquids with major element abundances and incompatible-element patterns that are very similar to the Shergotty bulk rock.

3.13 – Geochemistry of the Igneous Oceanic Crust

Abstract: 3.13.4 WORLDWIDE GEOCHEMICAL VARIATIONS AMONG OCEAN RIDGE BASALTS 437 3.13.4.1 Crystallization 438 3.13.4.1.1 Major elements 438 3.13.4.1.2 Trace elements 438 3.13.4.1.3 Correcting for crystallization 443 3.13.4.2 Melting 444 3.13.4.2.1 Major elements 444 3.13.4.2.2 Trace elements 446 3.13.4.2.3 The shape of the melting regime and the generation of diverse melt compositions 447 3.13.4.3 Mantle Heterogeneity 447 3.13.4.3.1 Pyroxenite melting 448 3.13.4.3.2 Assimilation of altered crust 449 3.13.4.3.3 Local trends in basalt composition 450 3.13.4.4 Spatial Variations in Lava Compositions 451 3.13.4.4.1 Along-axis chemical variations 451 3.13.4.4.2 Temporal variations on lava composition 454

Magma Genesis and Mantle Heterogeneity in the Manus Back-Arc Basin, Papua New Guinea

Abstract: Geochemical data from back-arc volcanic zones in the Manus Basin INTRODUCTION are used to define five magma types. Closest to the New Britain arc It has long been known that the diversity of rock types are medium-K lavas of the island arc association and back-arc and chemical lineages in extensional back-arc basins basin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB greatly exceeds that of mid-ocean ridges. Although lavas and mildly enriched T-MORB (transitional MORB) occur along generally similar to mid-ocean ridge basalts (MORB) are the Manus Spreading Center (MSC) and Extensional Transform known from several back-arc basins, a broad range of Zone (ETZ). The MSC also erupted extreme back-arc basin other rock types also have been found. Several workers basalts (XBABB), enriched in light rare earth elements, P, and Zr. have emphasized enrichments in certain volatile and Compared with normal MORB, Manus MORB are even more lithophile elements compared with MORB (e.g. Tarney depleted in high field strength elements and slightly enriched in fluidet al., 1977; Garcia et al., 1979; Fryer et al., 1981; Sinton mobile elements, indicating slight, prior enrichment of their source & Fryer, 1987). Fryer et al. (1981) argued that the chemical with subduction-related components. Chemical variations and modsignature of lavas erupted in back-arcs is unique to that eling suggest systematic, coupled relationships between extent of setting and proposed the term back-arc basin basalts mantle melting, prior depletion of the mantle source, and enrichment (BABB) to denote this magma series. However, the exact in subduction-related components. Closest to the arc, the greatest nature and cause of the mantle source enrichment has addition of subduction-related components has occurred in the mantle led to various competing models for the petrogenesis of with the greatest amount of prior depletion, which has melted the lavas in back-arc basins. Most of these models involve most. Variations in K2O/H2O indicate that the subduction-related source component mixing, with one component generally component is best described as a phlogopite and/or K-amphibolebearing hybridized peridotite. Magmas from the East Manus Rifts accepted to be depleted asthenosphere similar to, or even are enriched in Na and Zr with radiogenic Sr/Sr, possibly more depleted than, the source for typical MORB. The indicating crustal interaction in a zone of incipient rifting. The nature of the other components varies with author and/ source for XBABB and lavas from the Witu Islands requires a or basin. Several workers have argued that BABB have mantle component similar to carbonatite melt. concentrations of incompatible and volatile elements that are intermediate between MORB and those in island arcs lavas, and hence, have called on the subducted slab for the source of a component contributing to the

Trace Element and Sr, Nd, Pb and O Isotope Variations in Medium-K and High-K Volcanic Rocks from Merapi Volcano, Central Java, Indonesia: Evidence for the Involvement of Subducted Sediments in Sunda Arc Magma Genesis

Abstract: Merapi volcano (Central Java), located within the Quaternary volcanic front of the Sunda arc, is one of the most active volcanoes of the Indonesian archipelago. During the Holocene, Merapi erupted basalts and basaltic andesites of medium-K affinity during its earlier stages of activity and high-K compositions over the past 1900 years. Merapi lavas and pyroclastic rocks are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and higher Sr/Sr, Pb/Pb, Pb/Pb and Pb/Pb ratios and lower Nd/Nd ratios compared with Indian Ocean mid-ocean ridge basalt (MORB). Merapi high-K series rocks are enriched in LILE and LREE and slightly depleted in heavy REE (HREE) and HFSE compared with rocks from the medium-K series. The increase in K2O is accompanied by a marked increase in Sr/Sr and a general decrease in Nd/ Nd, but not by systematic variations in dO values. The low dO nature of the Merapi magmas, and the lack of any major shift in isotopic compositions along the evolutionary trend of the two individual series, precludes extensive crustal assimilation during magma ascent and differentiation, emphasizing the importance of subducted continental material in the genesis of Merapi magmas. The compositional contrast between medium-K and high-K series volcanics largely reflects variable contributions from subducted sediment to the mantle wedge, which was similar to a MORB-source mantle before any subduction-related modification. The temporal change in the K2O content of the magmas reflects compositional variation in the mantle wedge rather than intracrustal processes occurring within the shallow volcanic system.