Abyssal Basalts Research Articles

In-situ Pb isotopic analysis of sulfides in abyssal peridotites: New insights into heterogeneity and evolution of the oceanic upper mantle

Abyssal peridotites and mid-oceanic ridge basalts (MORBs) represent complementary residue-liquid products of melting and melt migration in the oceanic mantle. Because MORBs are mixtures of melts from different mantle depths, their isotopic signature does not directly describe the isotopic composition of the mantle source, but instead describes the local average composition of different parts of the mantle. In contrast, abyssal peridotites, the residues of fractional melting and melt-rock reaction, should shed more light on the distribution of isotopic heterogeneities. We analyzed Pb isotopic compositions in sulfide grains from the Southwest Indian Ridge and the Gakkel Ridge (Arctic Ocean) using the high-resolution Cameca 1280 ion microprobe. Sulfide Pb isotope ratios show very large variations, with 16 grains from 1 sample covering ∼25% of the entire range observed in the oceanic mantle. Pb isotopes in sulfides preserve a record of mantle compositions not seen in whole-rock MORBs from the same area. Sulfides from the Atlantis II Fracture Zone (Southwest Indian Ridge) confirm the presence of ancient refractory material scatter in the oceanic upper mantle. Gakkel Ridge sulfides define a high degree of isotopic variability, suggesting that oceanic mantle, not subcontinental lithospheric mantle, is the main source of such heterogeneity. Our results confirm that the source of MORBs, as represented by abyssal peridotites, is very heterogeneous and that other mantle end-member components are intimately mixed in. In-situ sulfide analysis is a powerful tool to detect the isotopic diversity of the MORB mantle source.

海洋底玄武岩中の炭素,窒素,ヘリウム,アルゴン同位体比〜マントルの炭素循環の解明にむけて

海洋底玄武岩中の炭素,窒素,ヘリウム,アルゴン同位体比〜マントルの炭素循環の解明にむけて

Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii

The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, contains abundant xenoliths of dunite, wehrlite, and olivine clinopyroxenite with minor gabbro, troctolite, anorthosite, and websterite. The petrography and mineral compositions of 41 dunite, wehrlite, and olivine clinopyTOxenite xenoliths have been studied, and clinopyroxene separates from eight of these have been analyzed for Ba, K, Rb, Sr, rare earth elements, 87Sr/*6Sr, and 143Nd/14*Nd. Temperatures of equilibration obtained by olivine-spinel and pyroxene geothermometry range from 1000 to 1200 °C. Mineralogical data combined with published fluid inclusion data indicate depths of origin in the range of 8-30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine (Fog, _89) and clinopyroxene (Fs3 _,2X and the occurrence of high TiO2 in spinel (0-9-2-8 wt.%) and clinopyroxene (0-35—1-33 wt.%) all indicate that the xenoliths are cumulates, not residues from partial fusion. The separated clinopyroxenes have 87Sr/*6Sr (0-70348-0-70367) and 'Nd/'^Nd (O-51293-O-51299) values that are different from Sr and Nd isotope ratios of Pacific abyssal basalts ( 0-5130, respectively). Also, clinopyroxenes and spinels in the xenoliths have generally higher TiO2 contents (>O-35 and >0-91 wt.%, respectively) than their counterparts in abyssal cumulates (<0-40 and <0-70wt%, respectively). These differences indicate that the xenoliths are not a normal component of oceanic crust Because the xenoliths and alkalic to transitional Hualalai lavas have similar values for Cr/(Cr +Al)

Melting in the oceanic upper mantle: An ion microprobe study of diopsides in abyssal peridotites

A systematic study of rare earth and other trace elements in discrete diopsides from residual abyssal peridotites sampled from 5000 km of ocean ridge demonstrates that they are the residues of variable degrees of melting in the garnet and spinel peridotite fields. Further, the data clearly demonstrate that the peridotites are the residues of near‐fractional melting, not batch melting, and that typical abyssal basalt can evolve from aggregated fractional melts. Ion microprobe analyses of diopsides in abyssal peridotites from fracture zones along the America‐Antarctica and Southwest Indian ridges reveal ubiquitous extreme fractionation of rare earth elements (REE) ([Ce/Yb]n = 0.002–0.05); depletion of Ti (300–1600 ppm), Zr (0.1–10 ppm), and Sr (0.1–10 ppm); and fractionation of Zr relative to Ti (Ti/Zr = 250–4000). Ti and Zr in diopsides decrease with decreasing modal cpx in the peridotites, and samples dredged near hotspots are more depleted in incompatible elements than those dredged away from hotspots, consistent with higher degrees upper mantle melting in the former. All studied samples exhibit marked negative anomalies in Ti and Zr relative to REE. Incompatible element concentrations in peridotite clinopyroxenes are well modeled by repeated melting and segregation in ≤0.1% increments to a total of 5–25% melting, a process very close to Rayleigh (fractional) melting; batch melting of a LREE‐depleted source cannot account for the observed trace element concentrations in abyssal peridotites. The shapes of some REE patterns are consistent with variable degrees of melting initiated within the garnet stability field. Trace element concentrations in calculated integrated fractional liquids approximate the composition of primitive ocean floor basalts, consistent with postsegregation aggregation of small increment melts produced over a depth and melting interval.

Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism

Summary The SW Indian and American-Antarctic Ridges are two of the world’s slowest spreading ocean ridges (less than 1 cm a −1 ), making them the low end-members for rate of ocean ridge magma supply. Two-thirds of the rocks dredged at the numerous large offset transforms along the ridges are residual mantle peridotites. Gabbroic rocks, however, representing layer 3 and possible palaeo-magma chambers are rare. This suggests a highly segmented crustal structure, with anomalously thin crust near fracture zones that may consist of only a thin veneer of pillow basalt erupted over mantle peridotite. The dredged peridotites underwent high degrees of melting, spanning the range believed to produce abyssal basalt. Their depleted compositions show that the melt was almost entirely removed. At the same time, the spatially associated basalts have a large range of compositions, similar to those from the rift valleys, requiring extensive shallow-level fractional crystallization. Since there is little evidence for magma chambers at these fracture zones, it is concluded that melts formed in the underlying mantle flowed laterally through the mantle beneath the crust towards a magmatic centre at the mid-point of an adjacent ridge segment. Magma was then subsequently intruded down the rift valley fissure system from the magmatic centre to erupt onto the fracture zone floor. Alternatively, the melt was drained from a mantle diapir beneath the midpoint of a ridge segment, prior to lateral flow of the residual peridotite beneath the ridge axis to the fracture zone. These processes suggest behaviour of the partially molten layer beneath ocean ridges analogous to Rayleigh-Taylor fluid instability, where a light less viscous fluid layer floating upwards in a denser medium goes unstable and drains at regularly spaced points into protrusions which rise rapidly to the surface. Evidence for such dynamically driven non-uniform melt flow in the mantle is seen in locally-abundant plagioclase peridotites, where the plagioclase crystallized from impregnated trapped melt. These rocks can contain up to 30% trapped melt, contrasting sharply with the typical abyssal peridotite which contains virtually none. Basalts erupted along these ridges provide a classic case of trace- and major-element decoupling during magma genesis. Despite trace-element and isotopic diversity, basalts from individual ridge segments were derived from primary magmas with similar major-element compositions. These observations can be explained if melt flows locally through the depleted mantle at the end of melting towards the midpoint of a ridge segment. This would cause melts originating at different points in an initially heterogeneous mantle to migrate through and equilibrate with the same section of mantle immediately prior to segregation—which, for the most part, would homogenize the melt’s major-element compositions. However, by virtue of the lever rule, this would have little effect on critical incompatible-trace-element or isotopic ratios of the migrating melts because of the very low incompatible-trace-element content of residual peridotite. Ocean ridges, then, appear to be marked by strings of regularly spaced volcanic centres overlying instability points in the partially molten upwelling asthenosphere much as has been postulated for arc volcanism and early continental rifting. Unlike arcs, the asthenosphere upwells to the base of the crust and the magmatic centres undergo continuous extension. Thus, large volcanoes are not constructed, and instead, ribbons of basaltic crust form parallel to the spreading direction. This is most evident at the SW Indian and American-Antarctic Ridges because of their highly attenuated magma supply. Where the magma supply is more robust and the magma chambers are correspondingly larger, the chambers may merge and eliminate the surficial morphological and chemical expression of punctuated magmatism at ocean ridges.

Composition and origin of basaltic glass spherules in pelagic clay from the eastern Pacific

Lower Miocene basaltic glass spherules from DSDP Site 32 pelagic sediments in the eastern Pacific are compositionally diverse, and new analyses and interpretations have been added to those of earlier workers. The spherules are of titanian ferrobasalt which is compositionally similar to highly evolved abyssal basalts and to some oceanic island eruptives, and they were most likely shaped during intense lava fountaining during a number of separate eruptions. These eruptions tapped distinct but related magma batches in terms, for example, of distinctively high TiO 2 and FeO ∗ contents. Their age overlaps that of some of the eruptions of the Columbia River Plateau Basalts, but they are compositionally distinct from most of the latter basalts. Although about 15 m.y. old, they show little alteration. The low chlorine and sulfur contents compared to those of abyssal ferrobasalts are consistent with degassing prior to quenching during subaerial eruptions, and rule out production of the spherules by submarine fountaining. Lava fountaining alone is insufficient to account for the distance of about 100 km from even the closest possible seamount source. Instead, large phreatomagmatic eruption columns reaching at least 15 km and including lava fountaining immediately after the initial explosion are required. Alternatively, and deemed less likely, is their deposition by turbidites derived from Pioneer Seamount.

Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism

Ultramafic xenoliths from Koolau Volcano on the island of Oahu, Hawaii, are divided into spinel lherzolite, pyroxenite, and dunite suites. On the basis of a study of the petrography and mineral compositions of 43 spinel lherzolites, 12 pyroxenites, and 20 dunites, the following characteristics of the dunites in relation to the other nodule types and to Hawaiian lavas emerge. (1) The forsterite content of olivines in the Koolau dunites (Fo82.6-Fo89 7) overlap those of Hawaiian tholeiitic and alkalic lavas and are generally lower than those in abyssal lherzolites and dunites and in Koolau spinel lherzolites. (2) Most of the dunites contain no orthopyroxene, all except two contain chrome spinel, and a few contain interstitial plagioclase and clinopyroxene. (3) Chrome spinels from the Koolau dunites are distinctly higher in Cr/(Cr +Al), lower in Mg/(Mg + Fe2+), and higher in TiO2 than those from abyssal basalts and peridotites. Chrome spinels in the dunites correspond closely in composition to chrome spinels in Hawaiian tholeiitic and alkalic lavas. (4) The abundance of dunite relative to other nodule types decreases outward from the central part of the volcano. The dunites are interpreted not as residues of partial fusion of the mantle but as crystal accumulations stored at shallow depths beneath the central part of Koolau Volcano and derived from picritic magmas parental to the shield-building tholeiitic lavas.

Primary differences in oxygen fugacity and depth of melting in the mantle source regions for oceanic basalts

Most abyssal basalt can be divided into two groups based on major and minor element compositions of the glassy rinds of pillow lavas. Crystal differentiation models cannot relate one group to the other. Instead, depth of partial melting may exert the primary control. The first group (MORB-1) is characterized by low TiO 2 and Na 2O and higher MgO, and is derived from a primary melt originating from the spinel peridotite facies. The higher TiO 2-NaO 2 group (MORB-2) appears to have been derived from the lower-pressure (shallower depth) plagioclase peridotite facies. Mössbauer spectra show that MORB-1 is from a mantle source region with lower oxidation states (between the QFM and NNO buffers) than for MORB-2 (around the NNO buffer).

Upper mantle materials in the oceanic and island arc regions and origin of abyssal basalts(Abstracts of Papers Presented at the Spring Meeting of the Society, 1981)

Upper mantle materials in the oceanic and island arc regions and origin of abyssal basalts(Abstracts of Papers Presented at the Spring Meeting of the Society, 1981)

Isotopic composition of lead in oceanic basalt and its implication to mantle evolution

New data are given in this report for (1) Pb isotopic compositions and U, Th, and Pb concentrations of basalts from the island of Hawaii; (2) redetermined Pb isotopic compositions of some abyssal tholeiites; and (3) U, Th, and Pb concentrations of altered and fresh abyssal basalts, and basalt genesis and mantle evolution are discussed. The Th/U ratios of abyssal and Japanese tholeiites are distinctly lower than those of tholeiites and alkali basalts from other areas. It is thought that these low values reflect a part of the mantle depleted in large ionic lithophile elements. Thus a mantle evolution model is presented, in which Th/U ratios of the depleted zone in the mantle have decreased to ∼2, and U/Pb ratios have increased, showing an apparent ∼1.5-b.y. isochron trend in the 207Pb/ 204Pb vs. 206Pb/ 204Pb plot. The Pb isotopic compositions of basalts from the island of Hawaii are distinct for each of the five volcanoes, and within each volcano, Pb's of tholeiites and alkali basalts are similar. An interaction between partially melted material (hot plume?) of the asthenosphere and the lithosphere is suggested to explain the trend in the Pb isotopic compositions of Hawaiian basalts.

57.Abyssal basalts from IPOD-DSDP Leg 51

57.Abyssal basalts from IPOD-DSDP Leg 51

Monzonitic plutons, California, and a model for generation of alkali-rich, near silica-saturated magmas

Petrochemical data suggest that early Mesozoic alkali-rich monzonite plutons in California were generated by partial melting of a garnet-bearing, virtually feldspar-free source which was rich (compared to abyssal basalt or sub-oceanic mantle peridotites) in LIL elements. These monzonites are characterized by high Na2O+K2O (near 10 wt% total) and Sr (1000 to 2000 ppm), moderate SiO2 (53 to 63 wt%), steep negatively sloping REE patterns (from >100 to <10 × chondritic abundance), and moderate initial 87Sr/86Sr ratios (0.705 to 0.707). Most are very slightly oversaturated in silica, but one pluton includes both oversaturated and undersaturated units.

Tholeiitic basalts from the Tyrrhenian Sea floor

During DSDP Leg 42A, drilling at site 373A in the central Tyrrhenian Sea penetrated Miocene basaltic basement for 190 m and recovered basalt from up to 450 m below the sea floor. Despite mineralogical and chemical alteration due to hydrothermal processes under the high heat flow regime in the Tyrrhenian marginal basin, petrological analogies to high-Al abyssal basalts can be traced. Two interlayered basalt groups have been recognized: (1) the lower-Ti group (core 5/1–2, core 7, 10), and (2) the higher-Ti group (core 5/3, core 6, core 11, core 12 and outer barrel), respectively. We consider a distinction of two magma types: a more primitive magma leading to the plagioclase-bearing, lower-Ti basalts with affinities to high-Al abyssal tholeiites and a more differentiated magma leading to basalts with higher Ti.

Experimental determination of rare earth fractionation patterns in partial melts from peridotite in the upper mantle

Rare earth element patterns of partial melts of a natural peridotite with a chondrite-like rare earth pattern as starting material have been experimentally determined at high pressure. The experimental data are combined with major element data on the same partial melts to test the applicability of models of volatile-free melting of peridotite to rock-forming processes in the upper mantle. Alkali basalts could not have been formed by partial melting of peridotite in the absence of volatiles because the major and trace element abundances produced experimentally are not compatible with those of natural alkali basalts. Abyssal tholeiite without light rare earth element depletion can be derived from volatile-free peridotite by direct partial melting. Suites of abyssal tholeiite and picritic basalt may have been formed by a two-stage melting process from a common peridotite source. The peridotite residue must have lost clinopyroxene and garnet at the time of formation of picritic melt. Abyssal tholeiites with depletion of light rare earth elements could have been derived only from a peridotite source that has been depleted of the same elements prior to the melting event yielding the tholeiite.

Origin of basalt microlapilli in lower Miocene pelagic sediment, northeastern Pacific Ocean

Basalt microlapilli consisting of glassy particles and rock fragments were recovered in lower Miocene pelagic sediment from Deep Sea Drilling Project site 32 in the northeastern Pacific Ocean. Major-element chemistry indicates that the parent tholeiitic magma was high in iron and titanium, similar to many of the abyssal basalts from the eastern Pacific Ocean, and the relatively high K 2 O content suggests additional fractionation before eruption. The microlapilli most likely were erupted subaerially from an island volcano, although a shallow submarine source cannot be discounted. If wind were the only method of dispersal, then the volcano was probably within 100 km of site 32. If surface or density currents played a role in transport, however, the volcano may have been at a greater distance.

Disequilibrium of strontium isotopes between mineral phases of parental rocks during magma genesis — a discussion

Consideration of available diffusion rate data at high temperatures and pressures casts serious doubt on disequilibrium melting as a mechanism by which variable 87Sr/ 86Sr ratios can be generated in basaltic melts whose parental rocks were at or below the low-velocity layer for more than a million years. Diffusion rates are orders of magnitude faster in these regions than in the upper lithosphere and lower crust. Ultramafic fragments with minerals in isotopic disequilibrium are pieces of the upper lithosphere, where elapsed time and comparatively low temperatures have resulted in the observed disequilibrium. Regional isotopic heterogeneity in the mantle is the most reasonable explanation for isotopic variability found in abyssal basalts.

Proterozoic Quench-Texture Basalts from the Labrador Geosyncline

The occurrence of quench-crystal basalts from the Proterozoic Knob Lake and Doublet Groups of the Labrador Geosyncline is reported. The microscopic textures are similar to those in both modern abyssal basalts and Archean basalts from the Superior Province of the Canadian Shield. Textures of quench plagioclase and pyroxene have been well preserved but the plagioclase has been altered.

Magnetic telechemistry is elegant but nature is complex

VOGT and Johnson1 have suggested that the bulk chemistry of in situ abyssal basalts may be diagnosed using sealevel magnetic anomalies. Their suggestion is based on two sets of assumptions and observations.

Basalts and serpentinite from the Puerto Rico Trench, 2. Rare-earth geochemistry

Seven tholeiitic basalts from the Puerto Rico Trench walls (three from the south wall and four from the north wall) were investigated for rare-earth abundances. All these rocks, with the exception of a pigeonite-bearing rock (RC8-RD1–12), were found to possess similar rare-earth features. Their rare-earth patterns resembled those of usual abyssal basalts, and showed no Eu anomalies. There is no difference between the rare-earth abundances of fine-grained and coarse-grained rocks. It is clear that the pigeonite-bearing rock (RC8-RD1 type 2) differs in origin from the abyssal basalts. The rocks with rare-earth patterns similar to that of RC8-RD1–12 are found among the Japanes “geosynclinal” basalts (Tanaka and Sugisaki, 1973). A serpentinite from the same trench was also analyzed for the same elements. The resultant rare-earth pattern suggests that the relevant original rock was separated from a magma having the same relative rare-earth abundances as a chondritic meteorite. The markedly high Eu positive anomaly of this serpentinite is also discussed.

Rare earth elements in a gabbroic body of the Japanese Paleozoic geosyncline

Four gabbros, two diabases and one basalt from a volcanic suite in Japan were analyzed for ten rare earth elements, Ba and Sr. The rare earth patterns normalized against the Leedey chondrite are of solid-type. Europium anomalies, both positive and negative, are found to be correlative with normative anorthite. These chemical features of the volcanic suite have some resemblance to those of abyssal basalts and gabbros. The REE partition coefficients which had prevailed during the formation of source materials of these basic rocks were estimated from these REE data. It can be concluded that (1) the source materials may have been solid-type mantle which was separated from less fractionated liquid-type (rectilinear REE pattern) magma compared with chondrite, and that (2) both groups of gabbros and basalts were derived from different parts of the mantle which had been formed in the cource of sequential magmatic processes at a certain stage of the earth's history.