Basalt Chemistry Research Articles

An evaluation of the global variations in the major element chemistry of arc basalts

Arc volcanoes occur at convergent margins with a wide range in subduction parameters, and variations in these parameters might be expected to lead to variations in the chemistry of magmas parental to arcs. Major element analyses from approximately 100 volcanic centers within 30 arcs, normalized to 6% MgO to minimize the effects of crystal fractionation, display wide variations. Na 2O and CaO at 6% MgO (Na 6.0 and Ca 6.0) correlate remarkably well with the thickness of the overlying crust. These systematics are consistent with two possible models. In the first model, the crust behaves as a chemical filter; where the crust is thick, magmas crystallize at higher pressure and interact more extensively with the arc crust. Modeling of high-pressure crystallization and assimilation, however, does not reproduce the associated variations in Na 6.0 and Ca 6.0 without calling upon complicated combinations of fractionating phases and assimilants. In the second model, crustal thickness determines the height of the mantle column available for melting beneath arc volcanoes. If melting begins beneath arcs at similar depths, then the column of mantle that undergoes decompression melting is much shorter beneath the thickest arc crust. The shorter mantle column for arcs built on thick crust will lead to smaller extents of melting in the mantle, and hence higher Na 6.0 and lower Ca 6.0 in the parental magmas. Modeling shows that variations in the extent of melting in the mantle can easily account for the associated variations in Ca 6.0 and Na 6.0. The abundances of the other major elements at 6% MgO do not correlate well with crustal thickness, or any other subduction parameter. Co-variation of some of these other major elements (e.g., Si 6.0 and Fe 6.0) within individual arcs suggests that they are strongly influenced by local crustal level processes that obscure partial melting systematics. Correction for the crustal processes improves the relationship between Na 6.0 and Ca 6.0 that is so readily explained by partial melting. The extents of melting in the mantle beneath arc volcanoes estimated from the ranges in Na 6.0 and Ca 6.0 are remarkably similar to those estimated beneath mid-ocean ridges. This observation provides further evidence that the mantle wedge, and not the slab, melts beneath arc volcanic fronts.

Hydrothermal alteration processes at midocean ridges: Experimental and theoretical constraints from Ca and Sr exchange reactions and Sr isotopic ratios

Diabase and basalt were reacted with Na‐Ca‐K‐Cl fluids of seawater chlorinity at 375–425°C, 375–400 bars, and fluid/rock mass ratios of 0.5–1.0 to assess the role of basalt chemistry and texture on Sr and Ca mobility during high‐temperature hydrothermal alteration. An additional experiment, utilizing an 84Sr spiked fluid, was performed to help quantify reaction rates of processes affecting Sr mobility. The experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges. Alteration of basalt and diabase is characterized by formation of tremolite‐actinolite‐smectite‐chlorite and clinozoisitic epidote‐smectite‐chlorite, respectively. Diabase alteration produced dissolved Sr/Ca ratios similar to those observed for ridge crest hot spring fluids, whereas alteration of a mostly crystalline basalt produced significantly lower ratios. This observation supports the premise that the Sr/Ca ratios observed in vent fluids may be produced during deep‐seated reaction of the hydrothermal fluids with diabase dikes and/or gabbro at relatively low fluid/rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures (400°C) leads to formation of a mineral with low DSr/Ca, Possibly hydrothermal plagioclase. Results of the 84Sr spiked experiment indicated that only 4% of the Sr in basalt was mobilized after 800 hours of reaction despite the fact that B and Li were nearly quantitatively leached. It is thus suggested that B and Li are good indicators of the amount of fresh rock encountered by fluids, while Sr concentration and isotopic data can be used to estimate the degree of alteration that the Sr‐bearing primary minerals have undergone. The degree of alteration of subsurface reservoirs may be estimated by dividing the fluid/rock ratio obtained for highly mobile elements by the fluid/rock ratio obtained from path‐integrated Sr isotope and concentration data. Based on this method, for example, it can be estimated that only 5% of the primary Sr‐bearing minerals encountered by fluids in the subsurface at EPR 21°N are actually converted to secondary phases.

Major Elemental Chemistry of Quaternary Arc Basalts in the Cricum-Pacific Region and its Tectonic Implications

Major Elemental Chemistry of Quaternary Arc Basalts in the Cricum-Pacific Region and its Tectonic Implications

Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness

Regional averages of the major element chemistry of ocean ridge basalts, corrected for low‐pressure fractionation, correlate with regional averages of axial depth for the global system of ocean ridges, including hot spots, cold spots, and back arc basins, as well as “normal” ocean ridges. Quantitative consideration of the variations of each major element during melting of the mantle suggests that the global major element variations can be accounted for by ∼8–20% melting of the mantle at associated mean pressures of 5–16 kbar. The lowest extents of melting occur at shallowest depths in the mantle and are associated with the deepest ocean ridges. Calculated mean primary magmas show a range in composition from 10 to 15 wt % MgO, and the primary magma compositions correlate with depth. Data for Sm, Yb, Sc, and Ni are consistent with the major elements, but highly incompatible elements show more complicated behavior. In addition, some hot spots have anomalous chemistry, suggesting major element heterogeneity. Thermal modeling of mantle ascending adiabatically beneath the ridge is consistent with the chemical data and melting calculations, provided the melt is tapped from throughout the ascending mantle column. The thermal modeling independently predicts the observed relationships among basalt chemistry, ridge depth, and crustal thickness resulting from temperature variations in the mantle. Beneath the shallowest and deepest ridge axes, temperature differences of approximately 250°C in the subsolidus mantle are required to account for the global systematics.

Michael O'Hara Wins VGP Award

To many, the study of volcanism is a window into the condition of the mantle. Subtle (and not so subtle) variations in time and space of basalt chemistry may simply reflect variations in the mantle source regions of basalt. To some, however, it is not so simple.Our awardee today, Michael J. O'Hara, has been an early leader in exploring the “information content” of basaltic volcanism by using the methods of experimental petrology and geochemical variation. Significantly, he was among the earliest to reject the notion that basalts tell us very much directly about the mantle. Mike has not dismissed basalts as being devoid of information about the evolution of the earth. Rather, he has presented us with a comprehensive view of basalts as agents in a complex crust‐building process. Gone is the easy connection between a basalt and its source region. Instead, this connection is replaced by a melodrama of geochemical adventures leading to eruption or intrusion within the crust. Indeed, there is now more and more field and petrographic evidence for some of the many complicating activities to which Mike has drawn our attention. Mike has shown that the processes responsible for crust building are interesting and important in their own right, even though they may cloud some interpretations of the significance of basalts.

フィリピン海のテクトニクス

The models on the tectonic evolution of the Philippine Sea can be divided mainly into two:(1) entrapment of a normal ocean at 43Ma for the West Philippine Basin formation and back-arc basin formations caused by successive landward retreats of the upper Philippine Sea plate, and (2) back-arc basin formations caused by successive seaward retreats of the trench axis. These models are discussed on the basis of the presently available data.The island arc type volcanism at the northern half of the Palau-Kyushu Ridge since 48Ma, and the age-depth relation and chemistry of basalts at Sites 291 and 292 in the West Philippine Basin favor the back-arc spreading origin for this basin. In contrast, the chemistry of basalts at Site 447 and the large distance between the Central Basin Ridge and the northern Palau-Kyushu Ridge favor the entrapment origin for this basin. It is likely that the West Philippine Basin has formed under the unique situation of the ocean basin formation, i.e., with subduction at its northern margin and a spreading center far from this subduction zone.Subduction or collision of the aseismic ridges in and around the Philippine Sea is discussed in terms of the crustal structure of the ridges, seismicity, and onland geology. It is found that all these aseismic ridges except for Izu which has a thick 20-30km crust have been subducted, although many of them have formed a cusp or discontinuity at the trench. Elucidating the times of the bending of the Cretaceous-Paleogene terranes in central Honshu and in Kyushu is critical to discriminate the two models for the Shikoku Basin formation, i. e., the fixed trench model and the trench retreating model.

A Grenville Sm–Nd age for the Glenelg eclogite in north-west Scotland

Eclogite, a garnet–pyroxene rock with basaltic chemistry, is a product of unusually high-pressure metamorphism. In Scotland, it occurs within the Caledonian orogenic belt close to the Moine thrust, where it forms partially amphibolitized, easterly-dipping layers, associated with metasediments in the eastern Glenelg Lewisian inlier1–4. In spite of its potential importance for an understanding of the geological history of north-west Scotland, its age has never been established more precisely than ‘probably Proterozoic’4. A K–Ar omphacite age of 1,515 Myr5 is rendered unreliable by the possible presence of excess argon. We report here the results of an attempt to date the eclogite metamorphism by the Sm–Nd method. Two samples of eclogite yielded mineral isochrons of just over 1,000 Myr. The results represent clear evidence of Grenville metamorphism in Scotland, and imply close proximity of its front to the Moine thrust near Glenelg. Also, as the local Moine cover has not apparently reached eclogite facies, it was presumably deposited on an eroded Grenville basement.

The geochemistry of oceanic basalts in the vicinity of transform faults: Observations and implications

Consideration of geochemical data from basalts formed near major Atlantic and Pacific transform faults reveals two significant sets of observations. First, compared to basalts formed far from the transform, basalts near the ridge/transform intersection have, for the same MgO contents, higher abundances of TiO 2 and other incompatible elements, higher La/Sm and La/Yb ratios, and often higher FeO. These enrichments are distinct from and occur in addition to the more variable and fractionated compositions which have been frequently noted [10–13]. Modeling of this “ transform fault effect” using data from the Tamayo/EPR intersection suggests the chemical systematics are caused by decreasing extents of melting as the transform is approached. Second, there are chemical discontinuities in the major element, trace element and isotopic chemistry of basalts across many transforms. These “ transform discontinuities” occur in normal ocean crust as well as around hot spots. Consideration of the melting zone in the mantle suggests that the transform fault effect is a natural consequence of the ridge/transform plate boundary. The melting zone beneath a ridge segment must terminate across the transform, leading to lower extents of melting at the transform edge. The surface manifestation of the change in the melting zone may be affected by the age of the transform offset, the spreading rate, the transform spacing and the interaction of mantle flow with the local thermal structure; it may be obscured by episodic magma chamber processes and mantle heterogeneity. The significance of transform discontinuities depends on whether they persist with age. If they do not, then temporally variable crust-forming processes may produce changes along a flow line similar to those at zero age across a transform. If, on the other hand, a discontinuity persists with age, then the transform may be related to a fundamental discontinuity in the underlying mantle. Long-lived transform discontinuities would have profound implications for the nature of plate motions, mantle convection and mantle heterogeneity.

Alteration of the oceanic crust: Implications for geochemical cycles of lithium and boron

Fresh tholeiitic basalt glass has been reacted with seawater at 150°C, (water/rock mass ratio of 10), and fresh diabase has been reacted with a Na-K-Ca-Cl fluid at 375°C (water/rock mass ratios of 1, 2, and 5) to understand better the role of temperature, basalt composition, and water/rock mass ratio on the direction and magnitude of B and Li exchange during basalt alteration. At 150°C, slight but nevertheless significant amounts of B and Li were removed from seawater and incorporated into a dominantly smectite alteration phase. At 375°C, however, B and Li were leached from basalt. B behaved as a “soluble” element and attained concentrations in solution limited only by the B concentration in basalt and the water/rock mass ratio. Li, however, was less mobile. For example, at water/rock mass ratios of 1, 2, and 5, the percent of Li leached from basalt was 58, 70, and 92% respectively. This suggests some mineralogic control on Li mobility during hydrothermal alteration of basalt, especially at low-water/rock mass ratios. In general, these results, as well as those for B, are consistent with the temperature-dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids.Based on the distribution and chemistry of products of seafloor weathering, low (≤ 150°C) and high-temperature hydrothermal alteration of basalt, and the chemistry of ridge crest hydrothermal fluids, it was estimated that alteration of the oceanic crust is a Li source for seawater. This is not true for B, however, since the hot spring flux estimated for B is balanced by low-temperature basalt alteration. These data, coupled with B and Li flux estimates for other processes (e.g., continental weathering, clay mineral adsorption, authigenic silicate formation and formation of siliceous skeletal material) yield new insight into the B and Li geochemical cycles. Calculations performed here indicate relatively good agreement between the magnitude of B and Li sources and sinks. The geochemical cycle of B, however, may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones. Serpentinites are conspicuously enriched in B and if the B source for these rocks is seawater, then an additional B sink exists which must be integrated into the B geochemical cycle. However, until more data are available in terms of areal extent of serpentinization, serpentite chemistry and isotopic composition, the importance of B in these rocks with respect to the B geochemical cycle remains speculative at best.

Crystallization of plagioclase, augite, and olivine in synthetic systems and in tholeiites

AbstractExperimental data from the system CaO-MgO-Na2O-Al2O3-SiO2 and from the system CaO-MgO-FeO-Al2O3-SiO2 are used to show the nature of the changes in composition, with temperature and with pressure, of liquids in equilibrium with augite, plagioclase, and olivine as MgO in the system CaO-MgO-Al2O3-SiO2 is partially replaced by FeO and as Na2O replaces CaO. As differentiation of tholeiitic basalts proceeds, Fe/Mg increases, Na/Ca increases, and normative hypersthene increases and these effects alter the solubilities, and hence the proportions of augite, plagioclase, and olivine which co-precipitate. In particular olivine becomes more soluble (less is precipitated) as Fe/Mg increases; plagioclase becomes more soluble but augite less soluble as Na/(Na + Ca) increases; augite also becomes less soluble as normative hypersthene increases, and as pressure is reduced during ascent.Experimental data from remelted ocean tholeiites encounter the same equilibrium and are presented and compared with the data from synthetic systems.Some natural examples of daughter liquids related to parents by crystallization of augite, plagioclase, olivine, are combined with the experimental data and with some model calculations to demonstrate that only a very limited range of proportions of augite : plagioclase: olivine (approximately 30 : 50 : 15 to 20 : 60: 20) produces daughter liquids which lie on the major element variation of the tholeiitic basalt series. Other proportions lead to daughter liquids which are not basalts. In the light of these restrictions, several recent publications are reinterpreted as examples of fractionation of augite, plagioclase, and olivine, rather than examples of partial melting. The recognition of the small effect that massive (50 to 83%) crystallization has on basaltic chemistry by contrast with the large effect that small accumulations of phenocrysts can have is particularly emphasized in this reinterpretation.Comparisons and possible relationships are suggested between some low-K2O, low-TiO2 tholeiites; some calcic-, aphyric-, dykes of some tholeiitic provinces; some chilled margins of some layered intrusions; some basalts from Iceland, and some basalts from the ocean ridges.

Perspectives on the ethiopian volcanic province

Published major-element analyses of Ethiopian volcanic rocks have been subjected to a systematic discriminant analysis. The plateau regions can be subdivided according to the proportions of alkaline and tholeiitic basalts. In northern Ethiopia, these subprovinces show increasing basalt alkalinity with time. The most voluminous basalts have lowest magnesium values, independent of the degree of alkalinity. Rift and Afar basalt chemistry falls within the spectrum observed for the plateau basalts, with no perceptible difference resulting from lithospheric attenuation beneath Afar. However, silicic volcanics of the Rift-Afar floor differ in bulk terms from those of the plateau margins in showing a stronger bias towards peralkalinity, and having higher Na/K values. Two particularly voluminous volcanic episodes have occurred in Ethiopia, dated at ?30–19 m.y. and 4.5–0 m.y. and which link well with one model for seafloor spreading in the Red Sea and Gulf of Aden. Evidence for a mantle hotspot under Ethiopia remains ambiguous.

Chemical and isotopic diversity in basalts dredged from the East Pacific Rise at 10°S, the fossil Galapagos Rise and the Nazca plate

We present petrographic, chemical and isotopic data for fresh lava samples dredged from three regions: (1) the fossil Galapagos Rise; (2) an elongate volcano near this extinct spreading center; and (3) the East Pacific Rise at 10°S. The samples from the Galapagos Rise are among the first samples from any fossil spreading center to be analyzed. Alkalic picrites from the elongate seamount and transitional basalts from the East Pacific Rise are both somewhat unusual rock types considering their respective tectonic environments. The dredges from the East Pacific Rise at about 10°S recovered unusual transitional, light rare-earth element (LREE) enriched basalts which show a range of fractionation. On the basis of their chemical and isotopic abundances, it is unlikely that the lavas are related by a single simple process of magmatic differentiation. We suggest that the mantle source region of these basalts was chemically and isotopically heterogeneous. The chemistry of LREE-depleted tholeiitic basalt dredged from near the axis of the extinct Galapagos Rise indicates complex petrogenesis and differentiation. The presence of tholeiitic basalts here indicates that unlike the Guadalupe and Mathematician fossil ridges, the Galapagos Rise has not been the site of voluminous post-abandonment alkalic volcanism. Alkalic basalts of picritic bulk composition dredged from an elongate seamount near the Galapagos Rise do not represent liquid compositions. Instead, we suggest that these alkalic liquids contain added olivine and plagioclase xenocrysts. Although most of the samples analyzed are very fresh, a few have been altered. The latter exhibit characteristic chemical and isotopic effects of seawater alteration.

Thermal and kinematic control on ocean-ridge magma fractionation: contrasts between Atlantic and Pacific spreading axes

Differences in the lithology and chemistry of mid-ocean ridge basalts generated at Pacific and Atlantic spreading axes reflect a simple correlation to the spreading rate. On the basis of experimental data and thermal models, these differences are attributed to variation in the nature of magma supply and fractionation processes as a function of spreading rate, rather than to processess in the upper mantle. Thermal and kinematic models for ocean ridges predict stable steady-state magma reservoirs at faster-spreading axes (>5 cm/yr) and more restricted, transient systems at slower-spreading (<5 cm/yr) axes. The chemical and lithological character of basalts confirms this pattern and suggests that slow-spreading axes(e.g. the Mid-Atlantic Ridge) are characterized by polybaric magma fractionation in complex reservoir systems, reflected by the widespread accumulation of calcic plagioclase in low viscosity melt, phenocryst reaction morphologies and pyroxene-dominated fractionation ‘extracts’. Fast-spreading axes (e.g. East Pacific Rise) show low-pressure basaltic fractionation trends to iron-rich compositions with little evidence for plagioclase accumulation or crystal-liquid reaction, in comparatively stable shallow-level chambers. Spreading rates of 5–6 cm/yr in the Gulf of California may be reflected by ‘intermediate’ fractionation regimes influenced by some magma crystallization at depth with incipient accumulation of plagioclase followed by further fractionation at low pressure. The major effect of spreading rate differences appears to be on the lithological and chemical patterns of basalts induced after partial melting, with minimal influence on the composition of primary melts.

Nature of mantle heterogeneity in the North Atlantic: evidence from deep sea drilling

Studies of dredged and drilled samples from the North Atlantic ocean have revealed that basalts with a wide range of major and trace element compositions have been generated at the Mid-Atlantic Ridge (M.A.R.). Many of the basalts erupted between latitudes 30° and 70° N do not have the geochemical characteristics of normal mid-ocean ridge basalts (m.o.r.b.) depleted in the more-hygromagmatophile (hyg.) elements. Drilling along mantle flow lines transverse to the ridge has shown that different segments of the M.A.R. have produced basalts with a distinct compositional range for tens of millions of years. As more data have become available, the nature and scale of this variation have been established and tighter constraints can now be placed on the petrogenetic processes involved. The rare earth elements are used to test quantitatively the effects of open and closed system fractional crystallization, equilibrium partial melting (including continuous melting), zone refining and mantle mixing processes on basalt chemistry. When evaluated in terms of the more-hyg. elements, the results show that major heterogeneities must exist in the mantle sources feeding the M.A.R. Ratios of many of the more-hyg. elements remain consistent in space and time in basalts erupted at a particular ridge segment, but vary widely between different ridge segments. These ratios are not significantly modified by the processes of basalt generation. The hyg. element relations provide a major constraint on the nature of heterogeneity in the Earth’s mantle and the processes producing it. The mantle sources of anomalous ridge segments can be best explained in terms of variable veining of a hyg. element depleted host by a hyg. element enriched liquid or fluid generated by very small degrees of partial melting. Such incipient melting, as well as subduction zone processes, may be viable mechanisms for changing hyg. element ratios in the mantle source regions on the scale observed. These processes can be integrated into a model for mantle evolution which involves (1) upward migration of incipient melts to provide a hyg. element enriched source for alkali basalts and a hyg. element depleted source for normal m.o.r.b., and (2) extraction of continental crust and recycling of the depleted residue into the mantle at subduction zones. Also, some recycling of continental material into the mantle may be required to explain Pb isotope patterns.

Age, palaeomagnetism and chemistry of melilite basalts in the Southern Cape, South Africa

SummaryPost-Karroo melilite basalts intrude Cretaceous and older sedimentary units in the Cape Province, South Africa. K-Ar ages determined from 4 occurrences range from 63–75 Ma b.p. (Late Cretaceous to earliest Tertiary) and are significantly older than alkaline intrusions to the 5, on the Alphard Bank. These igneous intrusions may be part of a linear trend that connects young oceanic volcanism at Bouvet Island with Cretaceous kimberlitic activity in southern Africa. This lineament nearly parallels the Walvis Ridge and a hot spot model for its origin is examined.

Skagi and western neovolcanic zones in Iceland: 2. Geochemical variations

Rare‐earth, Sc, V, Cr, Fe, Co, and Ni contents of basalts from the Skagi and western neovolcanic (Langjökull, Thingvellir, and Reykjanes peninsula) zones of Iceland are reported. In comparison to the Langjökull zone, which is dominated by olivine tholeiites, the Skagi zone is characterized by a uniform tholeiitic basalt province significantly higher in Fe, Na, Ti, P, K, Fe*/(Fe* + Mg), La/Sm, and large‐ion lithophile trace elements with bulk crystal‐melt partition coefficients D ≪ 1 (La, K, and P) and lower in Mg, Ca, and trace elements, with D > 1 (Ni and Cr); however, Co, with D ≃ 1, remains practically constant throughout the zones. Basalts from the Reykjanes peninsula are generally intermediate between those from the Skagi and the Langjökull zone, and those from the Thingvellir zone resemble mostly those from Langjökull. The petrologic and geochemical discontinuity between Skagi and Langjökull is abrupt and apparently coincides with steepening of existing geothermal gradients and thickening of the Iceland crust from south to north, as well as with an age discontinuity. On the other hand, variation along the neovolcanic zone from the Langjökull‐Thingvellir zone to the Reykjanes peninsula and the Reykjanes Ridge are gradational. Fractionation processes involving crystal and melt are primarily responsible for the compositional variations observed along these zones. Modeling suggests that the most primitive melts of the four zones can be derived by variable degrees of melting of a relatively homogeneous Iherzolite mantle source with an approximately chondritic relative RE pattern but enriched 4–6 times, thus clearly distinct from the LRE‐depleted mantle source of ‘normal’ ridge basalts south and north of Iceland. Estimated degrees of melting range 8–10% for Skagi, 18–20% for the Reykjanes peninsula, and 30–50% for the Thingvellir and Langjökull zones. The estimated transition metal abundance pattern of the Iceland Iherzolite mantle is strongly fractionated in relation to Cl chondrites (Sc, Ti, and V are enriched, and Cr, Mn, Fe, Co, and Ni are depleted). In addition, chemical dispersion within the four zones requires shallow depth fractional crystallization of olivine, plagioclase, and clinopyroxene, up to 50% for Skagi and to a lesser extent for the western neovolcanic zone. Both geochemical and geophysical data suggest intense melting beneath central Iceland, decreasing in intensity southward along the western neovolcanic zone. The model is consistent with a mantle plume upwelling beneath Iceland and with published rheological and thermal analyses of such a phenomenon which suggest a smaller half width for the thermal anomaly than for the upward motion of the plume. Independently, geochemical evidence suggests that the melting anomaly reaches a minimum on the southwest Iceland shelf (∼63°20′N), whereas binary mantle plume‐LIL‐depleted asthenosphere mixing extends to 61°N, judging from La/Sm, Sr, and Pb isotopic gradients along the Reykjanes Ridge. Finally, the similarity in chemistry of basalts from Skagi and the southern part of the eastern neovolcanic zone, the low heat flow, and the small degree of melting required for Skagi basalts suggest a transient stage of rifting for both zones. Rifting along Skagi, 0.5–2.5 m.y. ago, appears to have been only ephemeral and never seems to have reached thermal maturity.

Trace elements in ocean ridge basalts

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.

The Sudbury dikes of the Grenville Front region: paleomagnetism, petrochemistry, and K–Ar age studies

Northwest trending diabase dikes having alkali olivine basalt chemistry occur abundantly in the Southern Structural Province of the Canadian Shield of Ontario. In several subregions these dikes can be traced across the Grenville Front for a short distance into the Grenville Province. Within the Southern Province the dikes possess a westerly directed primary magnetization yielding a paleomagnelie pole of high precision and accuracy (168° W. 21/2°S, A95 = 21/2°). The geological age of these dikes is probably about 1250 Ma. At distances ranging from 2–8 km (northwest) of the Grenville Front these dikes are re magnetized and are characterized by anomalously high apparent K–Ar ages. The acquisition of this later magnetization having a direction of 111°, +261/2° with a corresponding pole at 161/2° W, 4° S. A95 = 61/2° takes place without any general mineralogical expression of metamorphism within the dikes. A few dikes sharing the above direction of magnetization were intruded during this time of reheating at about 1000 Ma. A short dike was located in the Grenville Front region southwest of Sudbury which possesses tholeiitic chemistry, early Paleozoic apparent K–Ar ages, and a cleaned remanence direction of 125°, +59° with a corresponding pole at 42° W, 10° N. These early Paleozoic dikes may be more extensive within the Grenville Province.

Geodynamics of Iceland and the North Atlantic Area. Proceedings of the NATO Advanced Study Institute Held in Reykjavik, Iceland, 1-7 July 1974

Continental Drift in the Atlantic and the Arctic.- The Reykjanes Ridge - a Summary of Geophysical Data.- Deep Structure, Evolution and Origin of the Icelandic Transverse Ridge.- Morphology of the Mid-Ocean Ridge between Iceland and the Arctic.- The Azores-Gibraltar Plate Boundary.- Deep Drill Investigations of the Oceanic Crust in the North Atlantic.- The Iceland Phenomenon: Imprints of a Hot Spot on the Ocean Crust, and Implications for Flow Below the Plates.- Tertiary Basalts of Baffin Bay: Geochemical Evidence for a Fossil Hot-Spot.- Petrochemistry of the Volcanic Rocks of the North Atlantic Ridge System.- Chemistry of Tholeiitic Basalts from Iceland, and their Relation to the Kverkfjoell Hot Spot.- The Petrologic Nature of the Lower Oceanic Crust and Upper Mantle.- The Structure of Eastern Iceland.- Eruptive Mechanisms in Iceland.- On the Topography of the Volcanic Zones in Iceland (Abstract).- Focal Mechanism Solutions of Intraplate Earthquakes and Stresses in the Lithosphere.- Seismicity of Iceland.- Vertical Crustal Movement in Iceland.- Crustal Movements in the Myvatn- and in the Thingvallavatn-Area.- Marine Heat Flow Measurements in the Norwegian-Greenland Sea and in the Vicinity of Iceland.- Heat Flow and Hydrothermal Activity in Iceland.- The Composition of Thermal Fluids in Iceland and Geological Features Related to the Thermal Activity.

Geochronology and petrochemistry of Easter and Sala y Gomez Islands: implications for the origin of the Sala y Gomez Ridge

Abstract Easter and Sala y Gomez Islands lie along the Sala y Gomez Ridge, a broad zone of high topography and scattered seamounts extending east-southeast from the East Pacific Rise. K-Ar ages and major element abundances of volcanic rocks from these islands are used to test the fixed melting spot hypothesis for the origin of this feature. Poike volcano, the oldest center on Easter Island, was constructed in two episodes, occurring at 2.5 and 0.9 m.y. ago. Eruptive activity on Sala y Gomez was nearly contemporaneous with the earliest volcanism on Easter Island. No migration of volcanism with time is apparent along the Sala y Gomez Ridge. Basaltic rocks from Easter Island have tholeiitic affinities, while those dredged from the base of Sala y Gomez belong to an alkali olivine basalt series. Differences in basalt chemistry suggest that the volcanic rocks from the two islands formed from magmas that equilibrated at different pressures, and a model is presented which relates the character of the erupted liquids to magma segregation at the base of a progressively thickening lithosphere. The islands and seamounts comprising the Sala y Gomez Ridge do not fall on a small circle about the Nazca-mantle pole of absolute motion. Available evidence, therefore, does not support a fixed melting spot origin for Easter Island, Sala y Gomez, and the Sala y Gomez Ridge. Rather, it appears that the Sala y Gomez Ridge formed along a leaky fracture zone, and in response to a major re-orientation of spreading centers in this area.