Showing papers on "Incompatible element published in 1997"

Element transport from slab to volcanic front at the Mariana arc

Abstract: We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near-constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid-oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U-230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.

A mantle-derived bimodal suite in the Hercynian Belt: Nd isotope and trace element evidence for a subduction-related rift origin of the Late Devonian Brévenne metavolcanics, Massif Central (France)

Abstract: In the Brevenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed, together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma, respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive ɛNdi (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean ridge basalts and an enriched end-member with ɛNdi > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNdi > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced by their high ɛNdi (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically distinct from the metarhyolites. Their ɛNdi values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts.

Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)

Abstract: Petrological and geochemical data are reported for volcanic rocks from Vulcano island. The subaerial volcanism (120 ka to present) built up a NW-SE elongated composite structure, affected by two intersecting multistage calderas. Volcanics older than 20 ka consist mostly of high-K calc-alkaline (HKCA) to shoshonitic (SHO) mafic rocks. These magmas interacted significantly with the continental crust, which generated variable Sr isotopic ratios (0.70412–0.70520). However, a major role was also played by input of parental liquids into the magma chamber, which prevented further evolution of the magmas. HKCA, SHO, and potassic (KS) rocks formed from 20 to 8 ka, display a much larger range of SiO2 (from shoshonites to rhyolites) and higher concentrations of incompatible elements with respect to the previous stage. Sr isotopic ratios show small variations (0.70448–0.70486). Mixing of silicic and mafic liquids and fractional crystallization processes (FC) were the main evolutionary processes during this stage. Volcanics younger than 8 ka consist of SHO and leucite-bearing KS mafic rocks, with abundant intermediate and silicic products. Mafic and intermediate rocks display similar incompatible element abundances and Sr isotopic ratios as the previous stage volcanics, whereas higher 87Sr/86Sr (0.70494–0.70583) are observed in some rhyolites. These products originated from a complex interplay of FC, crustal assimilation, and magma mixing processes. The most mafic rocks show increasing incompatible element abundances, Rb/Sr, Rb/Ba, Mg/Al, Mg/Ca, and a decrease in large ion lithophile to high field strength element ratios, passing from older HKCA-SHO to the younger SHO-KS volcanics. These variations suggest a shifting of magma sources from a slightly metasomatized asthenosphere (fertile peridotite) to a more strongly metasomatized lithospheric mantle (residual peridotite). Time-related petrological and geochemical variations have been used to develop a model for the evolution of the Vulcano plumbing system.

Enclaves in the Cadillac Mountain Granite (Coastal Maine): Samples of Hybrid Magma from the Base of the Chamber

Abstract: The Cadillac Mountain intrusive complex is dominated by the INTRODUCTION Cadillac Mountain granite and a 2–3 km thick section of interlayered Enclaves occur in nearly all granites and have recently gabbroic, dioritic and granitic rocks which occurs near the base of been the focus of many studies [see reviews by Vernon the granite. The layered rocks record hundreds of injections of basaltic (1983) and Didier & Barbarin (1991)]. Although enclaves magma that ponded on the chamber floor and variably interacted may have originated in many different ways and from with the overlying silicic magma. Magmatic enclaves, ranging in different sources, there has been a growing consensus composition from 55 to 78 wt % SiO2, are abundant in granite that there are textural criteria to recognize a class of above the layered mafic rocks. The most mafic enclaves are highly enclaves that formed from different magmas that were enriched in incompatible elements and depleted in compatible elemingled into the granitic magma while it was still mobile ments. Their compositions can be best explained by periodic (Vernon, 1984, 1990); these can appropriately be termed replenishment, mixing and fractional crystallization of basaltic magmatic enclaves. Their compositions are commonly magma at the base of the chamber. The intermediate to silicic intermediate but can vary from basaltic to highly silicic. enclaves formed by hybridization between the evolved basaltic magma Magmatic enclaves occur in all types of granite and are and resident silicic magma. There is little evidence for significant probably the dominant type of enclave in most calcexchange between enclaves and the enclosing granite. Instead, alkaline I-type granites. In spite of partial reequilibration, hybridization apparently occurred between stratified mafic and silicic enclaves of this type commonly have Nd isotopic commagmas at the base of the chamber. Enclaves in a restricted area positions that are distinct from the enclosing granite and commonly show distinctive compositional characteristics, suggesting support their formation from mantle-derived magmas they were derived from a discrete batch of hybrid magma. Enclaves (Holden et al., 1987). were probably dispersed into a localized portion of the granitic The compositions of magmatic enclaves can vary magma when replenishment or eruption disrupted the intermediate widely even within a single pluton, and they can only layer. rarely be interpreted as simple mixtures between an original basaltic magma and the host granitic magma. There is ample petrographic and chemical evidence in many enclaves for hybridization (mixing of liquids and

The variable role of slab-derived fluids in the generation of a suite of primitive calc-alkaline lavas from the Southernmost Cascades, California

Abstract: The compositional continuum observed in primitive calc-alkaline lavas erupted from small volcanoes across t}te southemmost Cascade arc is lnoduced by the introduction of a variable proportion of slab-derived fluid itrto the superjacent peridotite layer of the mantle wedge. Magnas derived from fluid-rich sources are erupted primarily in the foremc and are characterized by Sr and Pb enrichment (prinitive mantle-normalized Sr/P > 5.5), depletions of Ta and Nb, low incompatible-element abundances, and MORB-like Sr and Pb isotopic ratios. Magnas derived from fluid-poor sources are erupted primarily in the arc axis and behind the arc, and are characterized by weak enrichment in Sr [l.0 < (Sr/P),v < 1.3], weak depletions in Ta aad M, highsr incompatible-element abundances, and OIB-like Sr, Nd, and Pb isotopic ratios. Fluxing the mantle wedge above the subducting slab with Hzo-rich fluid stabilizes amphibole and emiches the wedge peridotites in incompatible elements, particularly unradiogenic Sr and Pb. The hydrated amphibole-bearing portion ofthe mantle wedge is downdragged beneath the forearc, where its solidus is exceeded yielding melts that are enriched in Sr and Pb, and depleted in Ta and Nb (reflecting both high Sr and Pb relative to Ta and Nb in the fluid, and the greater compatibility ofTa and Nb in amphibole compared to other silicate phases in the wedge). A steady decrease ofthe fluid-contributed geochemical signature away from the trench is produced by the progressive dehydration of tle downdragged portion of the mantle wedge witl depth, resulting from melt extraction and increased temperature at tle slab-wedge interface. lnverse correlation between incompatible-element abundances and the size of the fluid-confiibuted geochemical signature is generated by melting of more depleted peridotites, rather than by significant differences in the degree of melting. High-(Sr/P)ri' lavas of the forearc are generated by melting of a MORB-source-like peridotite that has been fluxed with a greater proportion of slab-derived flui{ and low (Sr/P)rv lavas of the arc axis are produced by melting of an OlB-source-like peridotite in the presence of a smaller proportion of slab-derived fluid. This study documents the contol that a slab-derived fluid can have on incompatible element and isotopic systematics of arc magmas by 1) the addition of incompatible elements to the wedge, 2) the stabilization of hydrous phases in the wedge, and 3) the lowering of peridotite solidi. Kewords: Cascade arc, calc-alkaline, primitive lav4 slab-derived fluid, Sr enrichmen! Califomia.

High μ (HIMU) ocean island basalts in southern Polynesia: New evidence for whole mantle scale recycling of subducted oceanic crust

Abstract: Major elements, trace elements, and Pb isotopic compositions were determined for ocean island basalts (OIBs) from Polynesia in the southern Pacific in order to document the chemical characteristics of OIB sources and to understand their origin. High μ (HIMU: μ=238U/204Pb) basalts, which have distinctly high Pb isotopic ratios, have systematically different compositions from non-HIMU basalts; HIMU basalts are more enriched in Fe2O3*, MnO, and CaO and more depleted in SiO2, K2O, P2O5, Ni and incompatible trace elements than non-HIMU, except for Nb. Major element characteristics of HIMU basalts suggest that the HIMU source is more fertile, i.e., more enriched in a basaltic component, than non-HIMU sources. This is consistent with the suggestion that subducted oceanic crust may contribute to the formation of the HIMU reservoir. Relative depletion of incompatible trace elements in HIMU is consistent with involvement of sedimentary components in non-HIMU sources. However, enrichment of Nb relative to other incompatible elements in HIMU cannot be explained by simple addition of the crustal component nor partial melting processes in the upper mantle, implying that lower mantle processes may contribute to the formation of the HIMU source.

Petrogenesis of a Phonolite-Trachyte Succession at Mount Sidley, Marie Byrd Land, Antarctica

Abstract: The 1.5 Ma evolution of the Late Pliocene (5.7 to 4.2 Ma) Mt Sidley volcano, Marie Byrd Land, is examined using major and trace elements, Sr, Nd, O and Pb isotopic data. A large (5 km × 5 km) breached caldera exposes lavas and tephras, deep within Mt Sidley, and allows its magmatic evolution to be elucidated. Two alkaline rock series are distinguished: (a) a strongly silica-under-saturated basanite to phonolite series; (b) a more silica-saturated to -oversaturated alkali basalt to trachyte series. Rock compositions in both series fall within a narrow range of 77Sr/86Sri (0.7028–0.7032), 143Nd/144Ndi (0.51285–0.51290) and δ18O (5.0–6.0‰), and with 206Pb/204Pb (>19.5), suggest an asthenospheric source containing a strong mantle plume component. Partial melting models require ≤2% melting to produce primary basanite and ≤5% melting to produce alkali basalt from the same mantle source. The differentiation of the phonolitic series is modeled by fractionation of diopside, olivine, plagioclase, titaniferous magnetite, nepheline and/or apatite from basanite to derive 35% mugearite, 25% benmoreite and 20% phonolite as residual liquids. Fractional crystallization of a similar mineral assemblage from alkali basalt is modeled for compositions in the trachyte series. However, many trachytes have variable 87Sr/86Sri (0.7033–0.7042), low 143Nd/144Ndi (0.51280–0.51283), high δ18O (6.5–8.4‰) and are silica oversaturated, suggesting they are contaminated by crust. The trachytes evolved by a two-step assimilation–fractional crystallization process (AFC). The first step involved contamination of alkali basalt by calc-alkaline granitoids within the middle crust where high assimilation to crystallization rates (high-r AFC) produced trachytic magmas characterized by depletions in Ta and Nb relative to K and Rb. The second step involved further fractionation of these magmas by low-r AFC within the upper crust to produce another suite of trachytes showing extreme incompatible element enrichment (e.g. Zr>1000 p.p.m/ and Th>100 p.p.m.).

Comparison of major- and trace-element geochemistry of abyssal peridotites and mafic plutonic rocks with basalts from the MARK region of the Mid-Atlantic Ridge

Abstract: Holes 920B, 920D, 921A, 921B, 921C, 921D, 921E, 922A, 922B, and 923A were drilled into crust of the Mid-Atlantic Ridge near the Kane Fracture Zone (MARK) area during Leg 153 of the Ocean Drilling Program. Holes 920B and 920D were drilled into an ultramafic massif and Sites 921, 922, and 923 were drilled into a gabbroic massif, both of which are located on the western rift valley wall of the Mid-Atlantic Ridge south of the Kane Transform. Bulk-rock major-, trace-, and rare-earth element (REE) analyses of diabasic, gabbroic, and ultramafic rocks recovered from these holes are reported here. These bulk analyses are augmented by the results of mineral chemistry studies on a subset of the same samples. Large ranges in bulk-rock and mineral chemistry are documented from all rock types. Ultramafic rocks in Holes 920B and 920D are interpreted to be dominantly residual mantle, but they include variably fractionated ultramafic and mafic cumulates that have intrusive contacts with the residual mantle harzburgites. Bulk-rock major- and compatible trace-element abundances, as well as petrographic data for residual harzburgites, indicate that a fertile MORB mantle was depleted by -15% to 20% partial melting or 10%-15% if a more depleted mantle source, such as Tinaquillo Lherzolite, is chosen. The mean extent of melting is likely to have been approximately half of the maximum value computed based on the residuum. Incompatible trace-element data show, however, that this residuum may have been part of an open system and refertilized at late stages by melts flowing through a locally porous matrix and later by more channelized melts (veins) as the residuum became part of the mechanical lithosphere. The crystallization products of these late melts include disseminated magmatic clinopyroxene and narrow veins or composite veins of dunite, wehrlite, pyroxenite, and gabbroic rocks. Ultramafic vein samples are variably depleted to enriched in incompatible elements and span a wide range of fractionation extents based on bulk-rock and mineral chemistry. Melts calculated to have been in equilibrium with clinopyroxene in ultramafic and mafic samples from Site 920 vary widely. They are dominantly ultradepleted, but include some samples that are enriched in incompatible elements (Na and Ti) with respect to MARK basalts, glasses, and Leg 153 diabases. The range in composition cannot simply be explained by crystal fractionation of a single parental magma, but requires a broad range of parental melts or their derivatives to be in equilibrium with clinopyroxene. Bulk-rock and mineral chemistry studies of residual and cumulate ultramafic rocks support the notion of an open-system, near-fractional mantle melting column. The residual peridotites were also cut by late-stage, variably altered, high-MgO (13-15 wt%) diabase dikes with quenched margins. Gabbroic samples from Sites 921, 922, and 923 drilled within the gabbroic massif likewise cover a broad spectrum of lithologies and compositions, and include troctolites, olivine gabbros, gabbros, oxide gabbros, felsic diorites, and quartz diorites. Melt compositions calculated to be in equilibrium with gabbroic clinopyroxene include melts that range from those that are significantly more fractionated to less fractionated than basaltic glasses from the MARK area, but also show a smaller range of parental melts in gabbroic samples when compared to the range documented in Site 920 ultramafic and mafic samples. Hole 923A, in which recovery was high, shows clear evidence of downhole cryptic chemical variation consistent with recharge and magma mixing within subaxial magma chambers. In addition, bulk-rock REE abundances in gabbroic samples show both enriched and depleted light REE (LREE) patterns. The LREE abundances range from less than 1 X chondrite to >100 X chondrite in gabbroic samples. MARK basaltic rocks cover a much narrower range, from 6 to 24 x chondrite. The chondrite-normalized La/Yb ratios of plutonic rocks vary

Geochemistry of Mesozoic Pacific mid-ocean ridge basalt: Constraints on melt generation and the evolution of the Pacific upper mantle

Abstract: We present major and trace element and Sr-Nd-Pb isotope results on Mesozoic (130-151 Ma) mid-ocean ridge basalt (MORB) recovered from five Deep Sea Drilling Project sites in the central and northwestern Pacific Ocean. Seawater alteration is responsible for much of the major element variability in these basalts, but magmatic variations are still discernible. Major element modeling of the least altered samples indicates that the basalts were generated by degrees and pressures of melting identical to those of modern Pacific MORB, and this, in addition to the similarity in spreading rates between the East Pacific Rise and Mesozoic Pacific ridges, suggests that the style of mantle upwelling and melting at spreading centers is spreading rate dependent. In general, the five Mesozoic MORB units, like Jurassic Pacific MORB from Ocean Drilling Program Site 801, are depleted in highly incompatible elements relative to average N-MORB and display a wide range in Nd and Pb isotopic ratios (e Nd (T) = 8.4-11.6; 206 Pb/ 204 Pb i = 17.9-18.6) but have a low and uniform Sr isotopic composition ( 87 Sr/ 86 Sr i = 0.7023-0.7026). This isotopic variation can be explained by mixing a depleted mantle source with small amounts of recycled oceanic crust (HIMU). In contrast to the older MORB, mid-Cretaceous Pacific MORB ( 115-100 Ma) are moderately to strongly enriched in highly incompatible elements with an enriched mantle isotopic affinity. The shift in MORB composition coincides with the onset of effusive mid-Cretaceous intraplate volcanism in the Pacific and reflects widespread contamination of the Pacific upper mantle with materials derived from the plumes or plume heads responsible for mid-Cretaceous oceanic plateaus and seamount chains.

Postshield volcanism and catastrophic mass wasting of the Waianae Volcano, Oahu, Hawaii

Abstract: The 3.9- to 2.9-Ma Waianae Volcano is the older of two volcanoes making up the island of Oahu, Hawaii. Exposed on the volcanic edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas. The postshield "alkalic cap" consists of aphyric hawaiite of the Palehua Member of the Waianae Volcanics, overlain unconformably by a small volume of alkalic basalt of the Kolekole Volcanics. Kolekole Volcanics mantle erosional topography, including the uppermost slopes of the great Lualualei Valley on the lee side of the Waianae Range. Twenty new K–Ar dates, combined with magnetic polarity data and geologic relationships, constrain the ages of lavas of the Palehua member to 3.06–2.98 Ma and lavas of the Kolekole Volcanics to 2.97–2.90 Ma. The geochemical data and the nearly contemporaneous ages suggest that the Kolekole Volcanics do not represent a completely independent or separate volcanic event from earlier postshield activity; thus, the Kolekole Volcanics are reduced in rank, becoming the Kolekole Member of the Waianae Volcanics. Magmas of the Palehua and Kolekole Members have similar incompatible element ratios, and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below the edifice. However, lavas of the Kolekole Member are less fractionated and appear to have evolved at greater depths than the earlier Palehua hawaiites. Postshield primary magma compositions of the Palehua and Kolekole Members are consistent with formation by partial melting of mantle material of less than 5–10% relative to Waianae shield lavas. Within the section of Palehua Member lavas, an increase with respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial melting by approximately 1–2%. This trend is reversed with the onset of eruption of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval between the eruption of Palehua and Kolekole Member lavas appears to date the initial formation of Lualualei Valley, which was accompanied by a marked change in magmatic conditions. We speculate that the mass-wasting event separating lavas of the Palehua and Kolekole Members may be related to the formation of a large submarine landslide west and southwest of Waianae Volcano. Enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1–2% in extent of melting between the youngest Palehua magmas and the posterosional magmas of the Kolekole Member. The association between magmatic change and a giant landsliding event suggests that there may be a general relationship between large mass-wasting events and subsequent magmatism in Hawaiian volcano evolution.

Petrological systematics of the Mid-Atlantic Ridge south of Kane: Implications for ocean crust formation

Abstract: Models of ridge segmentation, mantle flow and melt focusing predict how the chemical compositions of mantle melts should vary along a mid-ocean ridge axis. The compositions of basaltic lavas can be compared to these predictions to test the models. Such tests have been carried out using basalts from the neovolcanic zone south of the Kane fracture zone (the MARK area), where there are both a large transform and nontransform offsets. Before evaluating mantle models, the effects of differentiation must be accounted for. Fractional crystallization at low pressures (constrained by new melting experiments on these samples) does not account for the data. High pressure or in situ crystallization better account for the differentiation trends; however, these two processes imply different relationships between magmatic differentiation and position within a segment. Irrespective of the differentiation model, significant differences exist among parental magmas. Magmas near the transform have much lower levels of highly incompatible trace elements but higher levels of moderately incompatible trace elements, suggesting both lower extents of melting and a more depleted source. These two characteristics may be natural consequences of the truncation of a melting regime by a large-offset transform: depleted mantle from across the transform may contribute to the melting regime, while the cooler thermal environment produces less melt. Quantitative modeling of these geochemical characteristics produces thin crust near the transform, consistent with seismic and gravity studies. In contrast, thin crust adjacent to nontransform offsets is associated with no reduction in extent of mantle melting. These results, along with data from other regions, suggest that nontransform offsets overlie a continuous melting regime, and melt focusing creates the variations in crustal thickness. Focused flow may also lead to incompatible element enrichment at segment centers, and relative depletion at segment margins. Only offsets that truncate the melting regime, such as large transforms, are associated with diminished extents of melting within the mantle. Petrological evidence obtained thus far is not consistent with active upwelling to explain crustal thickness variations along nontransform offset bounded segments.

Geologic and geochemical relationships between the contact sublayer, inclusions, and the main mass of the Sudbury Igneous Complex; a case study of the Whistle Mine Embayment

Abstract: More than half of the giant Ni-Cu-platinum-group element (PGE) sulfide ores of the Sudbury Igneous Complex are associated with a discontinuous unit at the base of the main mass known as the "sublayer." The sublayer is comprised of two fragment-rich members: (1) a metamorphic-textured footwall breccia, and (2) an igneous-textured contact sublayer. The contact sublayer occurs as a thick unit in depressions at the base of the Sudbury Igneous Complex termed "embayments"; it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase, melanorite, olivine melanorite, metamorphosed melanorite, wehrlite, and dunite. We present data for a case study of the Whistle mine embayment and show that the igneous-textured sublayer matrix is geochemically unlike the main mass norites, quartz gabbros, and granophyres. For example, the igneous-textured sublayer matrix in the Whistle mine has La/Sm = 4.0, La/ Nb = 5.1, and Th/Zr = 0.02, and the main mass norites, quartz gabbros, and granophyres have ratios of 5.5 to 7, 2.8 to 4.2, and 0.04 to 0.05, respectively. This matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase. The igneous-textured sublayer matrix at the Whistle mine can be modeled with small amounts of assimilation of local country-rock granitoids ( approximately 10%), large degrees of assimilation of diabase that are not derived from the immediate country rocks ( approximately 70%), and small contributions from the main mass magma type ( approximately 20% mafic norite). The amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma; even with a superheated magma, 70 percent assimilation is too large. A model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments. These fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer. There are significant differences in the composition of the igneous-textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments. The melanorite inclusions in the sublayer at the Whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous-textured sublayer matrix, but they have similar high incompatible element concentrations (e.g., olivine melanorites have 20-65 ppm Ce in rocks with 15-21 wt % MgO), 1 to 10 percent interstitial sulfide, up to 0.5 percent apatite, 1 to 15 percent biotite, and 1.85 Ga age zircon and baddeleyite. The mafic-ultramafic inclusions are interpreted to be the broken-up remnants of an earlier cumulate formed at depth from a main mass magma; this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase. Olivine compositional data for the mafic inclusions (Fo (sub 71-79) ; 500-3,800 ppm Ni), in the absence of reequilibration, indicate that olivine crystallization both predated and postdated sulfur saturation of the magma. Cr-rich spinels from these rocks confirm that the parental magma was especially Cr rich, and based on the olivine compositional data, had a tholeiitic Mg/Fe ratio.

Olivine and chromian spinel in primitive calc-alkaline and tholeiitic lavas from the southernmost Cascade Range, California; a reflection of relative fertility of the source

Abstract: Chromian spinel and coexis '.9 olivine phenocrysts from a geochemically diverse suite of primitive tholeiitic and calc-alkaline basalts and maguesian andesites from the Lassen region, in the southernmost Cascade Range, in California, show that the sub-arc mantle is zoned. Depleted calc-alkaline basalts aad magnesian andesites erupt in the forearc region, and calc-alkaline basalts contain increasing abundances of incompatible elements toward the backmc. High-alumina olivine tholeiites erupt from the arc and backarc areas. Olivine from all these lavas displays a limited compositional range, from Fo86 to Fo9l, and crystallized at high temperature, generally 1225-Ln5"C. Chromian spinel trapped in the olivine phenocrysts displays a large raage of composition: Cr# values span the range 9J6. Excess Al in the spinel relative to that in l-atm spinel suggests that it crystallized at elevated pressure. The phenocrysts in these lavas are in equilibrium with their host liquids. The fulI range of Cr# of the spinel compositions cannot be explained by differentiation or variable pressure, variations in.t(Oz), subsolidus equilibration or variations in degree of partial melting of a single peridotitic source. Rather, the systematic compositional differences among phenocrysts in these primitive lavas result from bulk chemical variability in tlefu mantle sources. Correlations between spinel and host-rock compositions support the assertion that the geochemical diversity of Lassen basalts reflects tle relative fertility of their mantle sources.

Hafnium isotope evidence for the origin of Cenozoic basaltic lavas from the southwestern United States

Abstract: Hafnium isotope Hafnium analyses of 52 Cenozoic basalts from the southwestern United States document the differences and similarities of the mantle beneath the continents, as compared to the suboceanic mantle. One of the major conclusions of this work is documentation of a widespread Nd-Hf isotope array that is oblique to that of the oceanic island basalt (OIB) array, indicating that the subcontinental mantle forms an important component to the Nd-Hf isotope balance of the Earth. Basalts from the Basin and Range province have Sr, Nd, and Pb isotope compositions that overlap those of OIB, and have been interpreted by many workers to have been derived from an OIB-like mantle plume [e.g., Fitton et al, 1991; Kempton et al., 1991]. However, the Hf isotope compositions of Basin and Range basalts do not overlap those of OIB, and are instead more similar to mid-ocean ridge basalt (MORB) mantle. Recognition of a MORB source for high-eNd Basin and Range lavas is possible only with addition of Hf isotope data. The Pb, Sr, Nd, and Hf isotope compositions and trace element contents of Basin and Range basalts can be matched by mixing melts that were derived from small degrees of partial melting of a depleted MORB mantle, with partial melts of incompatible element enriched pyroxenite veins. In order to match the low Lu/Hf ratios measured in these basalts, one of these components (pyroxenite veins or depleted peridotite) must have been garnet bearing. Moreover, to match the positive eNd and eHf values of Basin and Range basalts, these lithophile element-enriched pyroxenite veins must be relatively young; we suggest they reflect mantle veining during earlier widespread Mesozoic or Cenozoic magmatism. Basalts from the Rocky Mountains and western Great Basin have similar trace element contents, high lithophile element contents and high large-ion lithophile element (LILE) to high-field-strength element (HFSE) ratios. The Sr, Nd, and Pb isotope compositions of most western Great Basin samples plot along the enriched mantle (EM) II array of Zindler and Hart [1986], but basalts from the Rocky Mountains plot along the EM I array. The Hf isotope compositions of western Great Basin and Rocky Mountain samples overlap. However, these Hf isotope compositions are anomalous relative to the OIB array; most samples have higher 8nf values at a given eNd value as compared to OIB. The lead isotope compositions of both the Rocky Mountain and western Great Basin samples plot significantly above the northern hemisphere reference line in terms of 207Pb/204Pb ratios, supporting models for input of crustal material into the mantle. Crustal recycling into the mantle can produce a mantle source region that has high LILE/HFSE ratios, which are a characteristic of the source of basalts from the western Great Basin and Rocky Mountains. To produce the negative eNd and eHf values, but high eHf values at a given eNd value (as compared to the OIB mantle), the crustal material input into the mantle must be a blend of pelagic and turbidite sediments. Ancient subduction and storage of pelagic sediments in the mantle will produce a source region with negative eNd values but positive eHf values. In contrast, subduction of turbidite sediments will produce a mantle that has negative eHf and eNd values, but low eHf values at a given eNd value as compared to the OIB mantle. A mixture of pelagic to turbidite sediments that has a ratio greater than 1:1.2 can produce the negative eHf and eNd values, and high eHf values at a given eNd value (relative to OIB).

The Petrogenetic Evolution of Lavas from Easter Island and Neighbouring Seamounts, Near-ridge Hotspot Volcanoes in the SE Pacific

Abstract: but also the volcanism of non-plume near-ridge seamounts at the Major and trace element, mineralogical and petrographical data are East Pacific Rise. We suggest that the development of each magmatic presented for submarine and subaerial lavas from Easter Island stage depends on the age of the overlying plate, which determines and two neighbouring seamounts. The samples can be divided (1) the amount of depleted mid-ocean ridge material mixed into the into three groups based on their major element composition and magma source and (2) the degree of partial melting. incompatible element enrichment, typified by their (La/Sm) N ratios. Tholeiitic samples with (La/Sm) N of ~1·2 are comparable with lavas constituting three young volcanic fields closer to the spreading axis. The other two lava series from Easter Island and the two seamounts are transitional to slightly alkaline, one having an intermediate enrichment with (La/Sm) N of 1·5–2, the other being

Petrogenetic models for magmatism on the eucrite parent body: Evidence from orthopyroxene in diogenites

Abstract: Diogenites are recognized as a major constituent of the howardite, eucrite and diogenite (HED) meteorite group. Recently, several papers (Mittlefehldt, 1994; Fowler et al., 1994, 1995) have identified trace- element systematics in diogenites that appeared to mimic simple magmatic processes that involved large degrees of crystallization (up to 95% orthopyroxene) of basalt with extremely high normative hypersthene. Such a crystallization scenario linking all the diogenites is highly unlikely. The purpose of this study is to explore other possible models relating the diogenites. Computational major-element melting models of a variety of different potential bulk compositions for the eucrite parent body (EPB) mantle indicate that these compositions show a similar sequence in residuum min- eral assemblage with increasing degrees of partial melting. Numerous bulk compositions would produce melts with Mg# appropriate for diogenitic parent magmas at low to moderate degrees of partial melting (1 5% to 30%). These calculations also show that melts with similar Mg# and variable incompatible element con- centrations may be produced during small to moderate degrees of EPB mantle melting. The trace-element characteristic of the orthopyroxene in diogenites does not support a model for large amounts of fractional crystallization of a single "hypersthene normative" basaltic magma following either small-scale or large-scale EPB mantle melting. Small degrees of fractional crystallization of a series of dis- tinct basaltic magmas are much more likely. Only two melting models that we considered hold any promise for producing different batches of "diogenitic magmas." The first model involves the fractional melting of a homogeneous source that produces parental magmas to diogenites with an extensive range of incompatible elements and limited variations in Mg#. There are several requirements for this model to work. The first requirement of this model is that the DorthoPYroxene/melt must change during melting or crystallization to compress the range of incompatible elements in the calculated diogenitic magmas. The second prerequisite is that either some of the calculated diogenitic magmas are parental to eucrites or the Mg# in diogenitic magmas are influenced by slight changes in oxygen fugacity during partial melting. The second model in- volves batch melting of a source that reflects accretional heterogeneities capable of generating diogenitic magmas with the calculated Mg# and incompatible element contents. Both of these models require small to moderate degrees of partial melting that may limit the efficiency of core separation.