Showing papers on "Basalt published in 1979"

Elemental and Sr isotope variations in basic lavas from Iceland and the surrounding ocean floor

Abstract: Major and trace element data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous.

Geochemistry of basalts drilled in the North Atlantic by IPOD Leg 49: Implications for mantle heterogeneity

Abstract: IPOD Leg 49 recovered basalts from 9 holes at 7 sites along 3 transects across the Mid-Atlantic Ridge: 63°N (Reykjanes), 45°N and 36°N (FAMOUS area). This has provided further information on the nature of mantle heterogeneity in the North Atlantic by enabling studies to be made of the variation of basalt composition with depth and with time near critical areas (Iceland and the Azores) where deep mantle plumes are thought to exist. Over 150 samples have been analysed for up to 40 major and trace elements and the results used to place constraints on the petrogenesis of the erupted basalts and hence on the geochemical nature of their source regions. It is apparent that few of the recovered basalts have the geochemical characteristics of typical “depleted” midocean ridge basalts (MORB). An unusually wide range of basalt compositions may be erupted at a single site: the range of rare earth patterns within the short section cored at Site 413, for instance, encompasses the total variation of REE patterns previously reported from the FAMOUS area. Nevertheless it is possible to account for most of the compositional variation at a single site by partial melting processes (including dynamic melting) and fractional crystallization. Partial melting mechanisms seem to be the dominant processes relating basalt compositions, particularly at 36°N and 45°N, suggesting that long-lived sub-axial magma chambers may not be a consistent feature of the slow-spreading Mid-Atlantic Ridge. Comparisons of basalts erupted at the same ridge segment for periods of the order of 35 m.y. (now lying along the same mantle flow line) do show some significant inter-site differences in Rb/Sr, Ce/Yb, 87 Sr/ 86 Sr, etc., which cannot be accounted for by fractionation mechanisms and which must reflect heterogeneities in the mantle source. However when hygromagmatophile (HYG) trace element levels and ratios are considered, it is the constancy or consistency of these HYG ratios which is the more remarkable, implying that the mantle source feeding a particular ridge segment was uniform with respect to these elements for periods of the order of 35 m.y. and probably since the opening of the Atlantic. Yet these HYG element ratios at 63°N are very different from those at 45°N and 36°N and significantly different from the values at 22°N and in “MORB”. The observed variations are difficult to reconcile with current concepts of mantle plumes and binary mixing models. The mantle is certainly heterogeneous, but there is not simply an “enriched” and a “depleted” source, but rather a range of sources heterogeneous on different scales for different elements — to an extent and volume depending on previous depletion/enrichment events. HYG element ratios offer the best method of defining compositionally different mantle segments since they are little modified by the fractionation processes associated with basalt generation.

Abyssal tholeiites from the Oceanographer Fracture Zone

Abstract: Fresh basalts from the Oceanographer Fracture Zone are petrographically and chemically similar to typical abyssal tholeiites, but are somewhat enriched in large ion lithophile elements, with consistent differences among separate dredge hauls. Olivine compositions are in equilibrium with host basalt for reasonable KDvalues, but some plagioclases are anomalously calcic (e.g., a rock highly differentiated in Mg/Mg+Fe bears some plagioclase of An86). Ti/Al in clinopyroxene is approximately constant at 1/6, but Ti+Al abundance decreases in the sequence basalt groundmass cpx, basalt phenocryst+microphenocryst cpx, gabbro cpx (samples from adjacent dredges), an effect that may be related to decrease in cooling rate. Least-squares calculations indicate that 45% of magma with the composition of the more primitive sample must be removed as plagioclase, clinopyroxene and olivine (in the ratio 4.4∶2.7∶1, respectively) to obtain the composition of the more differentiated sample. Both samples have excess plagioclase on the liquidus, however, which should not be the case for the differentiated composition if it is produced by fractionation of all three phases. The excess of plagioclase and the anomalous plagioclase compositions indicate that the samples cannot be related solely by fractional crystallization. Additional processes such as magma mixing or plagioclase accumulation (or both) must have effected these differences.

Alteration of Basaltic Rocks by Hydrothermal-Activity at 100-300°C

Abstract: Six high-temperature geothermal areas in Iceland have been investigated by deep drilling. The underground rocks down to 2 km depth are basaltic hyaloclastites and lavas. Olivine-tholeiites are dominant in some areas, but tholeiites in others. Hydrothermal alteration has taken place at 100-300°C. In all the fields the same groups of alteration minerals are formed at the same rock temperature. However, minor differences are observed due to the varying composition of the basaltic rocks. Smectites (iron-rich saponites), zeolites, calcium silicates, calcite, pyrite and quartz are formed at rock temperatures below 200°C. The smectites have transformed into mixed-layer clay minerals and swelling chlorites at 200-230°C. Most zeolites and the calcium silicates disappear in this temperature interval. Chlorites become the dominant sheet-silicates when the rock temperature exceeds 240°C. Epidote and prehnite are formed at slightly higher temperature. Actinolite appears near to 300°C. The clay minerals are quantitatively the most significant alteration minerals. Further they respond quickly to temperature changes in the geothermal system. An examination of the clay minerals and their relation to other alteration minerals is very useful for interpretation of the thermal history of a geothermal field.

Nd and Sr isotopic study of the Bay of Islands Ophiolite Complex and the evolution of the source of midocean ridge basalts

Abstract: Two Sm-Nd internal isochrons for pyroxene gabbros of the Bay of Islands Ophiolite Complex give well-defined ages of 508 ± 6 m.y. and 501 ± 13 m.y. with initial ^(143)Nd/^(144)Nd of e_(Nd) = +7.7 ∓ 0.1 and e_(Nd) = +7.5 ∓ 0.2, respectively. Total rock samples from pillow basalts, sheeted dikes, trondhjemites, hornblende gabbros, pyroxene gabbros, and an orthopyroxenite layer from the harzburgite give initial e_(Nd) in the range from +6.5 to +8.1 with an average value of +7.6. The initial ^(87)Sr/^(86)Sr obtained on a pyroxenegabbro is e_(Sr) = −19.3 ± 0.3, which is typical of oceanic samples. However, the initial ^(87)Sr/^(86)Sr within the different phases of the complex is found to be highly variable (∼52 e units) and shows the effect of sea water alteration. The e_(Nd) values demonstrate a clear oceanic affinity for the Bay of Islands complex and support earlier interpretations made on the basis of structure and geochemistry. The magnitudes of the initial e_(Nd) values (+7.6) are somewhat smaller than for typical present-day midocean ridge basalts (MORB) (+10). This is most likely due to differential evolution over the past 0.5 aeon of the oceanic mantle relative to the bulk earth. The observed shift is quantitatively what should be expected for a simple single-stage evolution. For a model with a single differentiation event at time T_D to produce the depleted mantle, both Sm-Nd and Rb-Sr data for MORB and the Bay of Islands Complex give T_D ≈ 1.8 aeons. This age is, however, interpreted as the mean age of the MORB source and does not refer to a unique event. The total time for producing this source by a uniform process would be of the order of 3.6 aeons. The Nd isotopic signature of oceanic crust is clearly present in this Paleozoic ophiolite and suggests that typical high-e_(Nd) reservoirs are sources of oceanic crust through the Phanerozoic. This implies relatively rapid turnover between oceanic crust and mantle sources and a good mixing of oceanic mantle for the past 1.0 aeon, including contributions from recycled continental materials. These data indicate that the major distinction between oceanic basalts and continental basalts observed in recent rocks is also preserved through the Phanerozoic. These isotopic differences clearly imply a long time distinction between the magma sources of these basalt types. It should be possible to apply the distinctive e_(Nd) values of oceanic crust and mantle to identify old obducted oceanic segments on continental blocks.

Vertical movement in mare basins: Relation to mare emplacement, basin tectonics, and lunar thermal history

Abstract: A hypothesis is developed that the spatial and temporal distributions of linear rilles and mare ridges in the mare regions of the moon are the products of two superposed stress systems: a local stress due to lithospheric loading by basalt fill in the mare basin, and a global thermal stress due to the thermal history of the lunar interior. Quantitative models for stress in the Serenitatis basin area, including the global thermal stress associated with lunar thermal history, are presented and used to account for the spatial distribution, the orientations, and the formation times of rilles and ridges in that region. The models provide a simple explanation for the localization of the most recent eruption sites of mare basalt magmas to mare basin edges.

The Galapagos Rift at 86° W: 3. Sheet flows, collapse pits, and lava lakes of the Rift Valley

Abstract: It has long been known that pillow basalts are a dominant feature of oceanic volcanism on midocean ridges. Recent studies in the Cayman Trough, East Pacific Rise, and Galapagos Rift valley have shown that sheet flows are also an important component, especially on ridges with intermediate and fast opening rates. This paper describes the surface forms of such sheet flows observed in the Galapagos Rift valley. The principal types are lobate, smooth, rippled, and wrinkled, as well as hackly and jumpled forms. Collapse pits of a wide range of sizes are common; they appear to result from subsidence of the lava as a consequence of distal spreading of the flow and/or headward drainback into the subsurface plumbing system. Lava pillars are numerous around collapse pits, and current evidence suggests that they most likely are spiracles produced by water trapped below an advancing flow and rising through it to the surface. From Hawaiian analogs we conclude that sheet flows can be considered a submarine equivalent of surface-fed pahoehoe, while the pillow basalts are analogous to subaerial tube-fed pahoehoe. We view the difference between sheet flows and pillow basalts to be the result of different degrees of channelization and rates of deliverymore » of lava to flow fronts, associated with differences in duration and rate of erruption. In this model the sheet flows and pillowed ridges of the Galapagos Rift are the analogs of the early and the late products of an Hawaiian erruptive event, respectively. The sheet represent early, brief but voluminous eruptions. These are followed by more sustained, slower but steadier eruptive phases that produce pillow basalts after an internal plumbing system has been well established.e of pillows on slow spreading ridges and the sheet flow and pillow complexes on faster spreading ridges.« less

Revisions in Stratigraphic Nomenclature of the Columbia River Basalt Group

Abstract: From abstract: New stratigraphic nomenclature for units within the Columbia River Basalt Group is introduced to revise and expand that currently in use; it is based largely on subdivisions made informally by T. L. Wright, M. J. Grolier, and D. A. Swanson in 1973. The Yakima Basalt is raised to subgroup status, and three formations-the Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt, in order of decreasing age-are defined within it.

Boninites, komatiites and ophiolitic basalts

Abstract: The rare Phanerozoic rock type boninite petrographically resembles Archaean basaltic komatiites and the range of boninite compositions overlaps that of basaltic komatiites. Quench amphibole and hydrous glass in the groundmass of boninites confirm earlier ideas that they may be the products of partial melting of peridotite in hydrous conditions, whereas basaltic komatiites have been assumed to result from fractionation of dry melts. Where field relationships are known, boninites are found either in ophiolites or a fore-arc tectonic environment.

A geochemical study of magmatism associated with the initial stages of back-arc spreading

Abstract: Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading. Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with CeN/YbN = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although CeN/YbN ratios and 87Sr/86Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher 87Sr/86Sr (0.7039) and CeN/ YbN (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present 87Sr/86Sr ratios. Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.

Geochemistry of Mesozoic marginal basin floor igneous rocks from southern Chile

Abstract: Extension behind a Late Jurassic continental margin volcanic arc in southern Chile caused rifting and the development of a narrow marginal basin floored by oceanic crust. Extension ceased and the basin was closed and uplifted in mid-Cretaceous time, so the basin floor is now exposed as the upper part of an autochthonous ophiolite complex composed of gabbros, sheeted dikes, and pillow lavas, with minor plagiogranite and associated siliceous dikes. Many of the rocks are altered. The metamorphic grade increases from zeolite or greenschist facies in the pillow lavas to amphibolite facies in the gabbros, but the maximum intensity of recrystallization occurs in the sheeted dike unit and is associated with loss of Rb and K and increasing K/Rb ratio, contrasting with the effects produced by low-temperature alteration of basalts by sea water. Metamorphic effects seem to be related to hydrothermal convective systems operating at the spreading axis at the time of basin formation. Geochemically, the rocks have affinities with mid-oceanic ridge basalts, but K, Rb, and Ba contents and Ba/Sr and Ce/Yb ratios are higher and K/Rb ratios are lower in the least altered rocks than in mid-oceanic ridge basalts. Similar features are apparent in some other marginal basin basalts. Fractionation trends are tholeiitic, the mafic rocks displaying a wide range of Fe/Mg ratios (0.9 to 5.2) but without any concomitant silica enrichment. Rare-earth elements, TiO 2 , and Zr correlate positively and Cr and Ni negatively with Fe/Mg, while the gabbros have lower contents of some incompatible elements as a result of their cumulate nature. The leucocratic rocks within the mafic complex have been derived from two distinct sources. Some trondhjemites and granophyres have compositions indicating derivation by refusion of continental material bordering the mafic complex. The plagiogranites, however, have a distinctive geochemistry, consistent with an origin by high-level differentiation of the mafic magmas. Such rocks, normally lying in or just below the sheeted dike unit, may be a common if minor component of oceanic crust.

Oceanic Plagiogranite Revisited

Abstract: Leucocratic rocks called plagiogranites are found in the upper parts of gabbros and in sheeted complexes of ophiolites, and represent small localized differentiates of subalkaline tholeiitic basalt. These rocks are composed of quartz and plagioclase, with accessory ferromagnesian minerals. Low-temperature hydrothermal alteration is evidenced by secondary minerals including epidote, chlorite, and actinolite. Plagiogranites are characterized by unique low k 2 O contents; Na 2 O and CaO values are higher, and total iron contents are lower than most granites and rhyolites of continental affinities. Plagiogranites can be distinguished from continental leucocratic rocks by these and other chemical characteristics, including K/Rb, Rb/Sr, and rare-earth element values. Normal compositional zoning of plagioclase and compositions of granophyric intergrowths indicate that these rocks are the product of igneous processes. Their low K 2 O character is not solely the result of post-crystallization hydrothermal alteration. However, strontium and oxygen isotope data show that interaction with seawater or meteoric water has taken place at some stage during the formation of plagiogranite.

Petrology and chemistry of recent lavas in the northern Marianas: Implications for the origin of island arc basalts

Abstract: Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (87Sr/86Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites.

143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios from the Azores and their significance in LIL-element enriched mantle

Abstract: Basalts from Sao Miguel are displaced to higher 87Sr/86Sr ratios, and have a significantly shallower slope, compared with the main correlation between 143Nd/144Nd and 87Sr/86Sr for most mantle-derived volcanic rocks. Available data on young magmatic rocks indicate that large ionic lithophile (LIL)-element enrichment in the upper mantle involves two readily distinguishable components which reflect different histories.

Magma genesis in the New Britain island-arc: Constraints from Nd and Sr isotopes and trace-element patterns

Abstract: A selected suite of fresh volcanic rocks from the New Britain island arc has been analyzed for 143Nd/144Nd, 87Sr/86Sr, major and trace elements to investigate relationships between isotopes, trace elements and petrology, and depth to the underlying Benioff zone. From these relationships inferences about magma generation are made utilizing Nd and Sr isotope systematics in possible source materials. Lavas ranging in composition from basalt to rhyolite show minimal variation of 143Nd/144Nd. Small variations in 87Sr/86Sr do not correlate with depth to the Benioff zone, but are related to magma type. Nd-Sr isotopes suggest that island arc lavas in general are derived from a mixture of suboceanic mantle and hydrothermally altered mid-ocean ridge-type basalt, but the New Britain magma source appears homogeneous with little indication of either the involvement of oceanic crust or mantle inhomogeneity. Trace element patterns in New Britain lavas are not consistent with Nd isotope data for currently accepted petrologic and trace element models of magma genesis. Mafic lavas from New Britain and other island arcs have anomalously high Sr/Nd, possibly due to components derived from subducted oceanic crust.

Fission track dating, thermal histories and tectonics of igneous intrusions in East Greenland

Abstract: Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks. The sphene and zircon ages of Caledonian rocks agree with other radiometric ages but apatite is strongly discordant indicating that these rocks cooled very slowly over a 200 m.y. period. It was not until the Permian/Lower Jurassic that they finally cooled below 100 ° C, possibly as a consequence of uplift and erosion at this time in connection with extensive rifting. No evidence of a Tertiary imprint has been found in these rocks. Layered gabbros, such as Skaergaard, were emplaced at about the same time (ca. 54 m.y.) as the latest plateau basalts. Some evidence of syenitic activity from this period occurs in the Angmagssalik area ca. 400 km to the south but the syenites of Kangerdlugssuaq cluster around 50 m.y. The Gardiner ultramafic alkaline complex and some of the offshore gabbros apparently also were emplaced at about 50 m.y. Late dykes in the Kangerdlugssuaq area were emplaced over a considerable time span (43-34 m.y.) in keeping with their variable petrographic character, and the Kialineq centre was formed at 36.2±0.4 m.y. Intrusions of the Masters Vig area differ in age. Kap Simpson and Kap Parry to the northeast were emplaced around 40 m.y. whereas the Werner Bjerge complex is the youngest igneous activity so far identified in Greenland with an age of 30.3±1.3 m.y. Many apatites give strongly discordant ages of about 36 m.y. and these are concentrated in the area of a major domal uplift centred on Kangerdlugssuaq. The uplift is older than these ages but on field evidence post-dates the basalts. It probably formed in conjunction with alkaline magmatism at ca. 50 m.y. Cooling below ca. 200 ° was slow for these intrusions and was probably controlled by a number of factors including erosion of the dome, high heat flow caused by continuing dyke injection and regional plateau uplift. The last is believed to have taken place about 35 m.y. ago at the time of emplacement of the Kialineq plutons and last dykes. Renewed rapid erosion and declining heat flow at this time led to rapid cooling of the rocks now at the surface to below 100 °.

The Petrology and Geochemistry of the Azores Islands

Abstract: Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria Differentiated lavas include both typical trachytes and comenditic trachytes and comendites Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks Systematic interisland differences are also apparent in these variation diagrams LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 070332 to 070354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 070394 and ratios in Sao Miguel lavas range up to 070525, suggesting basalts from these islands are derived from a chemically distinct source Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy The alkali basalts are derived by roughly half as much melting as are the tholeiites Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent

Chemical stratification of the mantle

Abstract: Differentiation of the mantle results in a thick layer of basalt which cannot sink below about 670 km even after it converts to eclogite and garnetite. The mantle is therefore chemically stratified. The upper mantle stratification is possibly garnet peridotite overlying eclogite.