Showing papers on "Basalt published in 1990"

Reaction Between Ultramafic Rock and Fractionating Basaltic Magma I. Phase Relations, the Origin of Calc-alkaline Magma Series, and the Formation of Discordant Dunite

Abstract: This paper presents results of modelling reaction between peridotite and fractionating tholeiitic basalt in simple and complex silicate systems. Reactions are outlined in appropriate binary and ternary silicate systems. In these simple systems, the result of reactions between 'basalt' and 'peridotite' may be treated as a combination of Fe-Mg exchange and mass transfer reactions at constant Fe/Mg. Fe-Mg exchange in ternary and higher-order systems is nearly isenthalpic, and involves a slight decrease in magma mass at constant temperature. Mass transfer reactions, typically involving dissolution of orthopyroxene and consequent crystallization of olivine, are also nearly isenthalpic in ternary and higher-order silicate systems, and produce a slight increase in the magma mass at constant temperature. The combined reactions are essentially isenthalpic and produce a slight increase in magma mass under conditions of constant temperature or constant enthalpy. Initial liquids saturated in plagioclase-I-olivine will become saturated only in olivine as a result of near-constant-temperature reaction with peridotite, and crystal products of such reactions will be dunite. Liquids saturated in clinopyroxene + olivine will remain on the cpx-ol cotectic during reaction with peridotite, but will crystallize much more olivine than clinopyroxene as a result of reaction, i.e., crystal products will be clinopyroxene-bearing dunite and wehrlite rather than olivine clinopyroxenite, which would be produced by cotectic crystallization. The Mg/Fe ratio of crystal products is 'buffered' by reaction with magnesian peridotite, and dunites so produced will have high, nearly constant Mg/Fe. Production of voluminous magnesian dunite in this manner does not require crystal fractionation of a highly magnesian olivine tholeiite or picrite liquid. Combined reaction with ultramafic wall rock and crystal fractionation due to falling temperature produces a calc-alkaline liquid line of descent from tholeiitic parental liquids under conditions of temperature, pressure, and initial liquid composition which would produce tholeiitic derivative liquids in a closed system. Specifically, closed-system differentiates show iron enrichment at near-constant silica concentration with decreasing temperature, whereas the same initial liquid reacting with peridotite produces silica-enriched derivatives at virtually constant Mg/Fe. Reaction between fractionating basalt and mafic to ultramafic rock is likely to be important in subduction-related magmatic arcs, where tholeiitic primary liquids pass slowly upward through hightemperature wall rock in the lower crust and upper mantle. Although other explanations can account for chemical variation in individual calc-alkaline series, none can account as well for the characteristics snared by all calc-alkaline series. This process, if it is volumetrically important on Earth, has important implications for (Phanerozoic) crustal evolution: sub-arc mantle should be enriched in iron, and depleted in silica and alumina, relative to sub-oceanic mantle, acting as a source for sialic crust It is probable that inter-occanic magmatic arcs have basement similar to alpine peridotite, in which suboceanic mantle has been modified by interaction with slowly ascending basaltic liquids at nearly

Mantle Oxidation State and Its Relationship to Tectonic Environment and Fluid Speciation

Abstract: The earth9s mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo2) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo2 greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo2 of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO2 and H2O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO2 and H2O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe3+ contents are stabilized in the garnet structure and that fo2 deareases with increasing depth.

A major 2.1 Ga event of mafic magmatism in west Africa: An Early stage of crustal accretion

Abstract: Birimian terranes from West Africa (Mauritania, Senegal, Ivory Coast, Burkina Faso, Niger) comprise two major units: a dominantly mafic bimodal volcanic unit and a volcano-detrital unit with mostly felsic to intermediate protolith. Stratigraphic relationships of these units are still a matter of debate but current work suggest that they both formed in a short time interval around 2.1 Ga. Widespread basaltic magmas from the bimodal unit have been analyzed for REE distributions and Sr-Nd isotopes. Three Sm-Nd isochrons on tholeiitic lavas were obtained at 2.229±0.042 Ga and initial eNd = 3.6±1.0 for Mauritania, 2.126±0.024 Ga and initial eNd = 2.9±0.7 for Burkina Faso, 2.063±0.041 Ga and initial eNd = 3.1± .0 for Eastern Senegal, data which compare with the age of 2.11±0.09 Ga and initial eNd = 2.1±1.8 obtained in Guyana by Gruau et al. (1985). Samples from other localities (Ivory Coast, Niger) give generally similar results. Although the variations of Sm/Nd ratios and the scatter of eNd(T) values from +1.2 to +4.3 preclude a single origin for these magmas, initial isotopic heterogeneities are unlikely to bias significantly the ages given by the isochrons which are in good agreement with U-Pb zircon ages (Boher et al., 1989; unpublished data, 1990). Presence of lavas with frequent pillow structures and sediments virtually free of older recycled components suggests that Birimian terranes formed in ocean basins far from continental influence. The isotopic heterogeneities are not consistent with a MORB-like mantle source. Most lavas are slightly depleted in LREE and inversion of the data through a melting model suggests 5–15 percent melting of a slightly depleted Iherzolite. Strong depletion (Burkina Faso) and slight enrichment (Senegal) are occasionally observed. With a noticeable trend of Ti enrichment with differentiation intermediate between that of MORB and IAT, the geochemical signature of Birimian basalts does not fit the best known geodynamic environments. Back-arc or low-Ti continental flood basalts provide a marginally good agreement but still face some difficulties. Oceanic flood basalts similar to those which form oceanic plateaus (e.g. in the Nauru basin) and later accreted to continents as allochtonous terranes represent the most acceptable modern analogue of many Proterozoic basalts. It is suggested that deep plumes piercing young lithosphere can generate huge amounts of tholeiites in a short time. Birimian basalts, like many Early Proterozoic basalts, may also be viewed as recent equivalents of the Archean greenstone belts. The modern komatiite of Gorgona Island is suggested to fit this model of intraplate volcanism. Although the 2.1 Ga magmatic event in West Africa has gone virtually unnoticed in the literature, it extends over several thousand kilometers and compares with the distribution of mantle-derived magmatic activity in other major orogenic provinces (e.g. Superior). It shows that the growth rate of continents cannot be extrapolated from the data obtained solely from the best studied continents (North America, Europe, Australia). If such large crustal segments were overlooked, a spurious pattern of episodic activity of the mantle could arise.

High-field-strength element depletions in arc basalts due to mantle–magma interaction

Abstract: BASALTS, basaltic andesites and andesites in subduction-related magmatic arcs are all depleted in high-field-strength elements (such as Ti, V, Zr, Nb, Hf and Ta) relative to mid-ocean-ridge basalt (MORB). Here we show that these depletions can be produced in liquids by interaction with depleted mantle periodotite, as would occur during the ascent of an arc magma through the mantle lithosphere of the overriding plate. This process involves extensive reaction between liquid, olivine, orthopyroxene and spinel. High-field-strength element depletions are produced because olivine, orthopyroxene and spinel have higher crystal/liquid distribution coefficients for these elements than for other incompatible trace elements. Liquids modified by mantle–magma interaction will also be depleted in heavy rare-earth elements, Cr and Ni, and enriched in light rare-earth and large-ion lithophile elements, relative to MORB. These characteristics are all common in mafic magmas at convergent plate margins1–7.

Compositional Diversity of Late Cenozoic Basalts in a Transect Across the Southern Washington Cascades: Implications for Subduction Zone Magmatism

Abstract: Major volcanoes of the Southern Washington Cascades (SWC) include the large Quaternary stratovolcanoes of Mount St. Helens (MSH) and Mount Adams (MA) and the Indian Heaven (IH) and Simcoe Mountain (SIM) volcanic fields. There are significant differences among these volcanic centers in terms of their composition and evolutionary history. The stratovolcanoes consist largely of andesitic to dacitic lavas and pyroclastics with minor basalt flows. IH consists dominantly of basaltic with minor andesite lavas, all erupted from monogenetic rift and cinder cone vents. SIM has a poorly exposed andesite to rhyolite core but mainly consists of basaltic lavas erupted from numerous widely dispersed vents; it has the morphology of a shield volcano. Distribution of mafic lavas across the SWC is related to north-northwest trending faults and fissure zones that indicate a significant component of east–west extension within the area. There is overlap in eruptive history for the areas studied, but it appears that peak activity was progressively older (MSH (<40 Ka), IH (mostly <0.5 Ma), MA (<0.5 Ma), SIM (1–4 Ma)) and more alkalic toward the east. A variety of compositionally distinct mafic magma types has been identified in the SWC, including low large ion lithophile element (LILE) tholeiitic basalts, moderate LILE calcalkalic basalts, basalts transitional between these two, LILE-enriched mildly alkalic basalts, and basaltic andesites. Compositional diversity among basaltic lavas, both within individual centers as well as across the arc, is an important characteristic of the SWC traverse. The fact that the basaltic magmas either show no correlation between isotopic and trace element components or show trends quite distinct from those of the associated evolved lavas, suggests that their compositional variability is attributable to subcrustal processes. Both the primitive nature of the erupted basalts and the fact that they are relatively common in the SWC sector also imply that such magmas had little residence time in the crust. A majority of the SWC basaltic samples studies are indistinguishable from oceanic island basalts (OIB) in terms of trace element and isotopic compositions, and more importantly, most do not display the typical high field strength element (HFSE) depletion seen in subduction-related magmas in volcanic arcs elsewhere. LILE enrichment and HRSE depletion characteristics of most arc magmas are generally attributed to the role of fluids released by dehydration of subducted oceanic lithosphere and to the effects of sediment subduction. Because most SWC basalts lack these compositional features, we conclude that subducted fluids and sediments do not play an essential role in producing these magmas. Rather, we infer that they formed by variable degree melting of a mixed mantle source consisting mainly of heterogeneously distributed OIB and mid-ocean ridge basalt source domains. Relatively minor occurrences of HFSE-depleted arclike basalts may reflect the presence of a small proportion of slab-metasomatized subarc mantle. The juxtaposition of such different mantle domains within the lithospheric mantle is viewed as a consequence of (1) tectonic mixing associated with accretion of oceanic and island arc terranes along the Pacific margin of North America prior to Neogene time, and possibly (2) a seaward jump in the locus of subduction at about 40 Ma. The Cascades arc is unusual in that the subducting oceanic plate is very young and hot. We suggest that slab dehydration outboard of the volcanic front resulted in a diminished role of aqueous fluids in generating or subsequently modifying SWC magmas compared to the situation at most convergent margins. Furthermore, with low fluid flux conditions, basalt generation is presumably triggered by other processes that increase the temperature of the mantle wedge (e.g., convective mantle flow, shear heating, etc.).

Evolution of Mauna Kea Volcano, Hawaii: Petrologic and geochemical constraints on postshield volcanism

Abstract: All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (∼240–70 ka) and hawaiitic (∼66–4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe-Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugearite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage can not be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene-dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot. Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust-mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low-density, residual hawaiitic melts. Minor assimilation of MORB-related wall rocks, reflected by a trend toward lower 206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low-density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xenoliths during ascent.

Geochemistry of the Siberian Trap of the Noril'sk area, USSR, with implications for the relative contributions of crust and mantle to flood basalt magmatism

Abstract: The sequence investigated of the Siberian Trap at Noril'sk, USSR, consists of at least 45 flows that have been divided into six lava suites. The lower three suites consist of alkalic to subalkalic basalts (the Ivakinsky suite), overlain by nonporphyritic basalts (the Syverminsky suite), and porphyritic and picritic basalts (the Gudchikhinsky suite). The upper three suites are tholeiitic. The uppermost 750 m of dominantly non-porphyritic basalt belong to the Mokulaevsky suite and are characterized by a nearly constant Mg number (0.54–0.56), SiO2 (48.2–49.1 wt%), Ce (12–18 ppm), and Ce/Yb (5–8). The underlying 1100 m of dominantly porphyritic basalt belong to the Morongovsky and Nadezhdinsky suites. There is a continuous increase in SiO2 (48.1–55.2 wt%), Ce (12–41 ppm), and Ce/Yb (5–18) from the top of the Mokulaevsky to the base of the Nadezhdinsky with little change in the Mg number (0.53–0.59). Mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid-ocean ridge basalts. The change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by Mokulaevsky magma. The Nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the Noril'sk-Talnakh mining camp. The Mokulaevsky and Morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the Noril'sk region; these magmas may have risen up out of a different conduit system.

Not so hot "hot spots" in the oceanic mantle.

Abstract: Excess volcanism and crustal swelling associated with hot spots are generally attributed to thermal plumes upwelling from the mantle. This concept has been tested in the portion of the Mid-Atlantic Ridge between 34° and 45° (Azores hot spot). Peridotite and basalt data indicate that the upper mantle in the hot spot has undergone a high degree of melting relative to the mantle elsewhere in the North Atlantic. However, application of various geothermometers suggests that the temperature of equilibration of peridotites in the mantle was lower, or at least not higher, in the hot spot than elsewhere. The presence of H2O-rich metasomatized mantle domains, inferred from peridotite and basalt data, would lower the melting temperature of the hot spot mantle and thereby reconcile its high degree ofmelting with the lack of a mantle temperature anomaly. Thus, some so-called hot spots might be melting anomalies unrelated to abnormally high mantle temperature or thermal plumes.

The timing of crustal extension and the eruption of continental flood basalts

Abstract: THE association between mantle plumes, continental flood basalt eruption and the extension and rifting of the crust is well recognized1–3, but there has always been controversy as to whether the plume or the rifting was the initiating factor4,5. Was rifting triggered by doming above the thermal plume2,3,6, or was rifting the driving force, preceding the associated volcanicity7,8? Here I show that recent flow-by-flow mapping of two continental flood basalt provinces, the Columbia River Basalt Group and the Deccan Basalt Group, allows one to infer the relative timing of volcanicity and crustal extension. In both provinces, eruption of the main tholeiitic phase preceded significant extension and crustal thinning. This suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume, and that the subsequent crustal extension, rifting and decompression was in part a consequence of the initiation of that plume.

Source and Differentiation of Deccan Trap Lavas: Implications of Geochemical and Mineral Chemical Variations

Abstract: The basalt stratigraphy of the Deccan Trap between Mahabaleshwar Ghat and Belgaum over-steps the basement from north to south. Sr-isotope and Zr/Nb ratios, and Sr, Rb, and Ba concentrations correlate portions of the post-Poladpur stratigraphy over 250 km along the Western Ghats, thereby confirming a southerly component of dip of 0·06°. At the southwestern margin, the stratigraphy extends upwards from the compositionally uniform Ambenali Formation (Cox & Hawkesworth, 1984) into a sequence of grossly heterogeneous flow units which have been allocated to the Mahabaleshwar and Panhala Formations (Lightfoot & Hawkesworth, 1988). The Mahabaleshwar Formation is represented only by a sequence of highly fractionated flows (termed the Kolhapur unit) with similar 87Sr/86Sr0 to the Mahabaleshwar (0·7045), but with Sr 2·25%. Succeeding the Kolhapur unit are a series of flows with high 87Sr/86Sr0 (0·7045-0·705), Zr/Nb > 13, and low Sr ( 230), but trace element concentrations similar to the Mahabaleshwar Formation; these have been allocated to the Desur unit of the Panhala. Geochemical variations in flows overlying the Ambenali define two distinct trends: one is attributed to gabbro fractionation, and the other to variations in the compositions of the parental magmas, and arguably their source regions. There is little evidence for significant crustal contamination in these flows, and the degree of fractionation and the composition of the phase extract are shown to vary along strike within the Mahabaleshwar Formation. The high TiO2 content of Kolhapur unit flows is shown to be the result of shallow-level gabbro fractionation, rather than the presence of a primitive high-Ti magma. Mahabaleshwar Formation basalts exhibit a broad negative correlation between the degree of fractionation and Sr-isotopic composition. The endmember with lower 87Sr/86Sr0 has different Zr/Y from the Ambenali basalts, and would appear to have been generated by lower degrees of melting of a similar source. The other endmember has more radiogenic Sr, lower Zr/Nb, similar Zr/Y, but higher mg-number. The simplest interpretation is that these magmas were more primitive and hence hotter and more able to interact with the lithosphere en route to the surface, and that they then mixed to produce the Mahabaleshwar array. The Panhala Formation basalts plot on the Sr-Nd array defined by the Mahabaleshwar Formation, and the Desur unit basalts plot on an extension of this array; this suggests that the source characteristics are also lithospheric. The absolute elemental abundances may then be a function of melting and fractionation. We are impressed by the apparent switch from crustal lithospheric contributions to mantle lithospheric contributions through the stratigraphy, and suggest that this, together with the more protracted fractionation of the magma, reflects a change in the availability of the lithospheric components accompanying the southerly migration of the volcanic edifice.

Discrimination of ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts by the Al/Ti ratio in clinopyroxene

Abstract: In mafic igneous suites typical of anorogenic igneous provinces (mid-ocean ridges, back-arc basins, continental rifts, and "hotspots"), clinopyroxenes of tholeiitic and silica-undersaturated rocks incorporate Al in clinopyroxene chiefly via the charge-coupled substitution viMgivSi2 ⇔ viTiivAl2 as the CaTiAl2O6 molecule. This substitution is promoted by high titania activity relative to silica in the melt, and hence is particularly characteristic of silica-undersaturated mafic rocks. Mafic igneous suites of convergent plate margins (i.e., arc tholeiites and calc-alkalic gabbros and basalts) characteristically incorporate Al in clinopyroxene largely as the CaFe3+AlSiO6 molecule. The viMgivSi ⇔ viFe3+ivAl substitution is promoted by relatively high fugacities of H2O and O2, a characteristic feature of arc-axis magmas. Recognition of cumulates that are complementary to compositions of eruptive rocks in arcs is crucial to development of valid petrogenetic models for arc magmas. Plots of tetrahedrally coordinated Al vs. octahedral Ti in augite of gabbroic and ultramafic cumulates show that augite data arrays for arc-related cumulates have a trend of substantially higher Al/Ti in clinopyroxene than do those from rift-related tholeiites. The Al/Ti ratio in clinopyroxene may be utilized to discriminate ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts.

Trace-element and Sr, Nd, Pb, and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America

Abstract: The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The “cratonic” basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (ɛNd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the “cratonic” basalts. In contrast, the “transitional” basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, ɛNd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the “cratonic” basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The “transitional” basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the “cratonic” and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, δ18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock δ18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock δ18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The “cratonic” basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and δ18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The “transitional” basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the “transitional” basalts earlier in the Cenozoic.

Reaction Between Ultramafic Rock and Fractionating Basaltic Magma II. Experimental Investigation of Reaction Between Olivine Tholeiite and Harzburgite at 1150–1050°C and 5 kb

Abstract: We present results of experiments on mixtures of olivine tholeiite and mantle harzburgite, at 5 kb and 1050-1150°C, under conditions of controlled hydrogen fugacity. The basalt end-member was Kilauea 1921 olivine tholeiite + 3 wt.% H2O, and the harzburgite end-member was a mixture of olivine and orthopyroxene mineral separates made from a mantle-derived lherzolite xenolith. The experiments on mixtures of basalt and harzburgite difl not reach equilibrium in runs ranging from 12 to 200 h duration. Relatively large concentration gradients persisted in both liquid and solid phases in mixed samples, whereas 'control' samples containing only basalt were reasonably homogeneous and were probably close to equilibrium. Compositions of solid phases produced, measured by electron microprobe, show a regular increase in Mg/(Mg + Fe) with increasing proportion of harzburgite at constant temperature, but olivine and clinopyroxene in mixed samples were not in Fe-Mg exchange equilibrium. Modes measured for each sample show that the fraction of liquid relative to the amount of basalt in the sample was constant at constant temperature, and independent of bulk composition: reaction between 1921 basalt and harzburgite does not change the mass of liquid in the system. Average experimental liquid compositions for each sample were obtained by mass balance. Using Kds defined by the 'control' sample for each temperature, and mass balance constraints, phase assemblages (solid- and liquidphase compositions and proportions) were calculated for all mixtures. Whether samples included harzburgite or not, all average experimental liquid compositions, and all predicted liquid compositions, for samples run at 1050°C, are high-alumina basalts by the definition of Kuno (1960). By the criteria of Irvine & Baragar (1971), all but two average experimental liquid compositions in basalt-harzburgite mixtures, and all predicted liquid compositions in basalt-harzburgite mixtures, are calc-alkaline basalts and basaltic andesites, whereas liquids in samples containing only basalt are tholeiitic basalts. Combined crystallization and reaction with harzburgite in the upper mantle will produce calc-alkaline derivative liquids from an olivine tholeiite liquid under conditions of temperature, pressure, water and oxygen fugacity, and initial bulk composition which would produce a tholeiitic liquid line of descent by crystallizati on in a closed system.

Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System

Abstract: The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which the authors call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1,500-1,570C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with the model of passive rifting above an asthenospheric mantle plume.

The Nevados de Payachata volcanic region (18°S/69°W, N. Chile) II. Evidence for widespread crustal involvement in Andean magmatism

Abstract: Volcanism extending over 11 Ma is represented in the rocks of the Nevados de Payachata region, culminating in the formation of two large composite stratocones within the last 500 000 years. Chemically distinct mafic magmas are erupted at a number of parasitic centers. These cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones. Two distinct trends are defined by the intermediate and evolved rocks; a high LILE (large ion lithophile element), TiO2 and Ce/Yb lineage among the youngest rocks (including the two major stratocones), and a more typical calc-alkaline trend among the older (>1 Ma) rock types. Within individual volcanic centers, differentiation involves fractionation of plagioclase, pyroxene and hornblende, with biotite and K-feldspar in the more-evolved rock types. Isotopic compositions (Sr, Pb, Nd, O) vary little with differentiation from basaltic andesite to rhyolite, or with age. Contamination during differentiation from basalt to rhyolite may occur, but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust. The majority of data are similar to “baseline” compositions (Cenozoic parental magmas) from other parts of the central Andes and may reflect a relatively homogeneous magma source (or source mixture) throughout this central volcanic zone (CVZ), which is distinct from the southern and northern Andes, and from island-arc volcanic rocks. The detailed study of Nevados de Payachata serves as a useful reference against which to assess magmatism in general in the CVZ. The possibility that central Andean magmas are generated from an enriched subcontinental-lithosphere mantle wedge is rejected on the basis of: (1) thermal considerations (subcontinental mantle lithosphere is probably cold and refractory); (2) lack of consistency between the tectonic history of the region and geochemical variations through time. Instead, parental magmas in the CVZ are thought to be generated by mixing between normal arc magmas originating in the depleted mantle wedge followed by contamination and homogenization with lower crustal melts. In the central Andes, the extent of contamination increased greatly as the crust thickened due to crustal shortening within the last 20 Ma, the thicker crust providing an effective filter to trap and differentiate magma batches repeatedly during ascent.

Electrical properties of mid-ocean ridge basalt and implications for the structure of the upper oceanic crust in Hole 504B

Abstract: The electrical resistivity, porosity, and cation exchange capacity (CEC) of mid-ocean ridge basalt (MORB) samples from Deep Sea Drilling Project hole 504B have been measured in the laboratory. The presence of chlorites, zeolites and particularly smectites as alteration products of MORB is reflected by high values of CEC, with high and uniform CEC values in the massive units of layers 2A and 2B, and even higher values in the pillows. The porosity and the “intrinsic” formation factor are related, in the massive units, by an inverse power law similar to Archie's formula, with m = 1.0 and a = 10.0. Such a low m value equates to current conduction in cracks and microcracks present at mineral scale throughout the rock. During leg 111 of the Ocean Drilling Program, the Joides Resolution D.V. returned in the equatorial Pacific to deepen hole 504B and to perform a series of downhole experiments. A continuous electrical resistivity profile permitted to discriminate the large-scale seismic layers of the upper oceanic crust and to isolate individual lithologic units. In the extrusive part of the crust, the massive flows (10-m-thick or more) are found to constitute permeability barriers and, subsequently, to constrain fluid circulation. Within layer 2A, the massive flows of unit 2D are associated with the underpressured aquifer located underneath, within Unit 3. In layer 2B, unit 27 is the boundary between low-temperature, seawater alteration facies of basalt, and higher-temperature alteration phases. This relationship between morphology, hydrological regime, and therefore alteration of the basaltic basement is proposed to be related to the accretion process of the upper oceanic crust. The porosity estimate derived from in situ measurements of electrical resistivity is reduced when accounting for surface conduction, with high values computed in layer 2A only. A permeability profile computed on the basis of in situ resistivity measurements reproduces those obtained in situ from packer experiments, and therefore provides a key to the low-permeability high-“apparent”-porosity paradox obtained in the past.