Peridotite Layer Research Articles

Evidence for melt segregation towards fractures in the Horoman mantle peridotite complex

MELT segregation from a solid matrix in the upper mantle is the first step in the formation of magmas. The mechanisms of melt segregation proposed so far have been based on two different physical models: the percolation of interstitial melt driven by a density difference between melt and matrix, associated with compaction and deformation of the matrix1; or the suction of interstitial melt into fractures from a surrounding porous matrix2–4. Although the latter process was originally invoked to explain the occurrence of dunite and pyroxenite–gabbro dykes with zones depleted in melt component on both sides2,5–7, most of these zones are discordant, small in scale and clearly formed by reaction between peridotite and basaltic melt8. I have suggested9 that thick, concordant dunite zones in the most depleted peridotite layers of the Horoman peridotite complex formed by suction of partial melt towards fractures later filled by dunite. Here I report a detailed mineralogical variation across the dunite which, when set in its geological and petrographic context, provides clearer evidence for melt segregation by dynamic forcing3, rather than passive percolation. This mechanism of melt segregation may play an important role in the formation of primary magmas with low production rate and large geochemical variability.

Across-arc variation of lava chemistry in the Izu-Bonin Arc: identification of subduction components

In order to understand the role of the subducted lithosphere in producing the geochemical characteristics of arc magmas, major- and trace-element along with Sr- and Nd-isotope compositions have been determined for Quaternary volcanic rocks from the Izu-Bonin intra-oceanic arc. 87Sr/ 86Sr and 143Nd/ 144Nd ratios decrease away from the volcanic front of this arc and lie on mixing lines between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted lithosphere and upper-mantle materials in the sub-arc wedge. This across-arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous peridotite at the base of the mantle wedge. This hydrous peridotite is dragged downward with the slab and releases a second-stage metasomatizing fluid beneath the volcanic arc. The higher concentrations of both Sr and Nd in the fluid beneath the volcanic front than those beneath the back-arc side may be a possible cause of the observed across-arc variation in Sr-Nd isotopic ratios. The difference in compositions of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotite layer; amphibole beneath the volcanic front and phlogopite beneath the back-arc side of the volcanic arc. The mineralogically controlled fluid addition may also be responsible for the across-arc variation in Rb/K and Rb/Zr ratios, increasing away from the volcanic front.

Geochemistry of Quaternary lavas from NE Sulawesi: transfer of subduction components into the mantle wedge

Major and trace element, and Sr-Nd isotope compositions were determined for Quaternary volcanic rocks from NE Sulawesi (the Sangihe are), Indonesia, in order to examine the origin of across-arc variation in lava and magma source chemistry. The arc is formed in an intraoceanic tectonic setting and is not associated with a backarc basin, thereby minimizing possible contributions from non-arc geochemical reservoirs. The geochemistry of these arc lavas is likely to provide essential information about the chemical characteristics of subduction components. All incompatible elements, except Pb, increase away from the volcancic front. Major element data for Mg-rich lavas together with available experimental data, suggest that primary magmas are produced at higher pressured by smaller degrees of partial melting beneath the backarc-side volcanoes. Rb/K and Ba/Pb are higher, and 87Sr/86Sr and 143Nd/144Nd are lower in backarc-side lavas. These variations may be attributed to generation of hydrous fluids in the downdragged hydrous peridotite layer at the base of the mantle wedge through the following reactions: decompositions of pargasitic amphibole to form phlogopite and breakdown of phlogopite to crystallize K-richterite, beneath the volcanic front and the backarc-side volcanoes, respectively.

The contact zone of the Rhum ultrabasic intrusion: evidence of peridotite formation from magnesian magmas

The Eastern Layered Series of the Rhum ultrabasic complex formed in situ, and was not tectonically emplaced as previously suggested. Field relations along the contact show no evidence for the existence of a late ‘Marginal Gabbro’, instead peridotite and allivalite layers can be traced to within 2 m of the surrounding partially melted country rocks. Local preservation of a chilled margin to one peridotite layer indicates that the parental magma consisted of an olivine tholeiite liquid carrying approximately 19% (vol.) olivine crystals in suspension.

The structure and petrogenesis of the Trallval and Ruinsival areas of the Rhum ultrabasic complex

ABSTRACTDetailed mapping of the Trallval-Ruinsival area of the ultrabasic complex of Rhum has revealed the relationship between the intrusive peridotites, which form a major part of the complex, and the layered ultrabasic rocks. The layered rocks on Trallval are correlated with Units 8 to 15 of the Eastern Layered Series as seen on Hallival and Askival, and part of an additional unit, Unit 16, has also been identified. The layered rocks of Ruinsival and the western part of Trallval form part of the Central Series and have been subdivided into six cyclic units. They are associated with four sets of intrusive breccia zones which converge towards a central region. Four cylindrical plugs of intrusive peridotite occur within the area of outcrop of the Central Series. Criteria are listed for the distinction of intrusive peridotite from conformable peridotites formed in situ. Distinctive textures and structures are described; these include vertical feldspathic streamers, which are interpreted as direct evidence for the upward expulsion of interstitial fluid, and loading structures at the contact between a peridotite layer and underlying allivalite are ascribed to the intrusion of a peridotite sill. A comparison of the Eastern Layered Series and the Central Series suggests that three stages were involved in the formation of the ultrabasic rocks: (i) the accretion of rhythmically layered unconsolidated olivine cumulates; (ii) the upward expulsion of intercumulus liquid; and (iii) the formation of either allivalite (as in the Eastern Layered Series) or ultrabasic breccia zones (as in the Central Series) from the expelled intercumulus liquid, depending on whether the tectonic environment was quiescent or extensional.

Peridotite sills and metasomatic gabbros in the Eastern Layered Series of the Rhum complex

Mapping of the Eastern Layered Series (ELS) of the Rhum ultrabasic complex on the northern flank of Hallival shows that peridotite and allivalite (troctolite or gabbro) layers are laterally discontinuous and vary both in thickness and lithology. Peridotite generally has sharp upper and lower contacts against the allivalites, which sometimes cut across the layering in the allivalite. Reaction, dissolution and hybridization effects between peridotite and allivalite are developed locally. Some troctolite layers terminate as isolated, fingered blocks in peridotite. There are many small peridotite bodies which are clearly intrusive into allivalite and have previously been identified as distinct peridotite sheets and plugs. They are petrographically almost identical to the major stratiform peridotites and in some cases are apophyses from them. We propose that many of the peridotite layers in the ELS formed as thick sills of picritic magma emplaced into a partly solidified, layered troctolite complex. The stratiform gabbros of the ELS are heterogeneous, layered rocks that commonly contain relicts of troctolite and anorthosite. Wavy (metre-scale) contacts between gabbro and troctolite cut across pre-existing grain-size, modal and rhythmic layering with little disruption. These metasomatic gabbros mimic the textures, grain-size and rhythmic layering of their troctolitic protoliths. We propose that many of the ELS gabbros formed as a result of interaction between porous troctolites and a low-temperature basaltic melt. Residual basaltic melt segregated from solidifying peridotite may have caused this metasomatism.

A Nd and Sr isotopic study of the Ivrea zone, Southern Alps, N-Italy

The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with ɛ 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by “phlogopite metasomatism”. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial ɛ 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic “crustal residence ages” of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.

Olivine‐spinel transitions: Experimental and thermodynamic constraints and implications for the nature of the 400‐km seismic discontinuity

The sequence of high‐pressure phase transitions α→β→γ in olivine is traditionally used as a model for seismic velocity variations in the 200‐ to 650‐km‐depth interval in a mantle of peridotitic bulk composition. It has been proposed that the observed seismic velocity increase at 400‐km depth is too sharp and of too small a magnitude to be attributable to the α→β phase change and that the upper mantle must thus be chemically stratified, with the 400‐km discontinuity being due either to a combination of phase changes in a layer of pyroxene/garnet‐rich “piclogite” composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer. Using available calorimetric, thermoelastic, and synthesis data (and their associated experimental uncertainties), we have derived internally consistent high‐pressure phase relations for the Mg2SiO4‐Fe2SiO4 join. We find that the divariant transition α→α+β→β, which is generally regarded as occurring over a broad depth interval for mantle olivine compositions, is, in fact, extremely sharp. The seismic discontinuity corresponding to the α→α+β→β transition in (Mg0.9Fe0.1)2SiO4 should occur over a depth interval (isothermal) of about 6 km at a depth of approximately 400 km; the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric, thermoelastic, and synthesis data. In addition, we have computed seismic velocity profiles for a model mantle consisting of pure olivine of (Mg0.9Fe0.1)2SiO4 composition. Comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400‐km depth is consistent with a mantle transition zone composed of about 60–70% olivine. We conclude that there is no need to infer the existence of pyroxene/garnet‐rich compositions, such as eclogite or “piclogite,” in the transition zone, since an upper mantle of homogeneous olivine‐rich peridotitic composition is consistent with the available seismic velocity data.

Ophiolite belts in the Koryak Upland, Northeast Asia

In the packets of tectonic slices there are preserved large-size fragments of Late Precambrian-Early Paleozoic, Late Paleozoic-Triassic, Late Jurassic-Valanginian, Albian-Campanian, and Late Cretaceous ophiolite suites developed in both oceanic and marginal (back-arc) basins. Same belts (or zones) of ophiolites often comprise fragments of different-age complexes. In the Paleozoic ophiolites which, judging from the volcanic series compositions, were formed in different paleo-oceanic environments, mafic complexes are the thickest and include gabbro-norite and gabbro-troctolite series; they are associated with less depleted metamorphic peridotite complexes (lherzolite and pyroxene-rich harzburgite). Marginal basin ophiolites of Late Jurassic-Valanginian age incorporate thin mafic rock bodies of gabbro-norite composition; they are associated with lherzolites, pyroxene-rich harzburgites, olivine-rich harzburgites, and major dunite bodies. Petrochemical types of metamorphic peridotite massifs are given and it is assumed that the latter are fragments of an ancient peridotite layer which throughout the Phanerozoic served as the mantle basement for the crust of oceanic or marginal basins occurring in the northwestern Paleo-Pacific.

Mineralogical and chemical variations within layered sills of the Deer Lake Complex, Minnesota

Layered mafic sills, intrusive into an Archean volcano-sedimentary sequence of north-central Minnesota, are the major components of an igneous series known as the Deer Lake Complex. Differentiation in the sills is simple: a basal peridotite layer (highly serpentinized) is successively overlain by pyroxenite, a lower orthopyroxene-bearing gabbro, an upper orthopyroxene-free gabbro, and quartz diorite. Chilled margins of the sills are morphologically complex and exhibit textures characteristic of supercooling. The Complex has undergone greenschist regional metamorphism. Olivine in the Complex ranges only from Fo81 to Fo85. Clinopyroxene in pyroxenite layers ranges from Wo45En45Fs10 to Wo36En53Fs11, whereas that in a lower gabbro sample averages Wo36En44Fs20. Ferroaugite, Wo38En27Fs35, occurs in local iron-rich gabbroic units. Layered sills of the Deer Lake Complex were produced from basaltic parental magmas characterized by high MgO content (average ∼11%), low TiO2 (0.68%) and Al2O3 (12.9%), and Ca/Al<1 (0.97). Differentiation in the sills follows a normal tholeiitic trend, with modest enrichment of iron and alkalis.

Upper mantle structure beneath the Mid-Atlantic Ridge north of the Azores based on observations of compressional waves

Summary. Four seismic refraction profiles have been interpreted which serve to indicate the structure of the lithosphere near the Mid-Atlantic Ridge close to the Azores. An east–west profile which crosses the ridge axis yields a crustal structure. Although energy is propagated across the ridge axis within the crust the axial region marks a clear barrier to propagation within the mantle. A profile parallel to the axis (4 my isochron) shows, below a 7.6 km/s layer, a low-velocity zone underlain by an 8.3 km/s refractor 9 km below the sea bed. On profies normal to the ridge axis higher velocities, which are observed on lines shot towards the ridge, can be attributed to this refractor if it has a dip of several degrees away from the ridge. On another profile parallel to the axis (9 my isochron) a velocity of about 8.3 km/s is only found to exist much deeper at about 30 km depth. These observations are interpreted in the light of seismic refraction results recently obtained by Lewis & Snydsman and of quantitative petrological models, such as that of Bottinga & Allegre. A velocity model based on Bottinga & Allegre's model allows us to understand our results qualitatively. In particular the two 8.3 km/s refractors at 9 and 30 km depth correspond to two different residual peridotite layers. The upper layer contains 1.5–2 wt per cent water and as the lithosphere moves away from the ridge axis the temperature in this layer becomes low enough to start hydration reactions. These cause the low-velocity zone observed at 4 my and the total disappearance of the shallow level refractor before 9 my.

Channel deposits of peridotite, gabbro and chromitite from turbidity currents in the stratiform Fiskenæsset anorthosite complex, southwest Greenland

Trough-shaped layers of peridotite, gabbro and chromitite occur sporadically in a 500 m thick intrusion of mainly anorthosite. The layers are interpreted as deposits in channels which were excavated and filled in by turbidity currents on the floor of the intrusion. They show that these currents were important concentrators of olivine and chromite.

Melting in the deep crust and upper mantle and the nature of the low velocity layer

It is possible that there are minor amounts of water in the upper mantle. This water could significantly lower melting temperatures and may give rise to melts of different composition to those formed by melting under anhydrous conditions. This paper summarizes our experimental study of the influence of water on phase relations in natural gabbro and tonalite samples at pressures from 10 to 25 kb. Our results are used in conjunction with recently published data for other rock-water systems to discuss melting in the upper mantle and lower continental crust. It is suggested that the low velocity zone in the upper mantle could be a layer of peridotite containing interstitial melt, which may be overlain by a layer of peridotite containing interstitial “water”. The minimum possible temperatures for the generation of andesite and basalt liquids are discussed, and it is noted that the initial melt formed from wet peridotite may be andesitic. Finally, granulite facies metamorphism in the lower continental crust and the generation of “granites” are considered.

Interpretations of the low-velocity zone

The physical interpretation of the low-velocity zone of the upper mantle depends critically upon details of the velocity variation which are still uncertain. As examples, Gutenberg's solution is roughly consistent with a homogeneous layer of peridotite affected only by temperature and pressure, while Anderson's is not. Partial melting, relaxation of elasticity and compositional changes are among the suggested explanations. New evidence derived from the free oscillations by Press indicates that the low-velocity layer may also be a high-density layer; this might be eclogite or iron-rich peridotite.

Ultramafic intrusions of the Abitibi area, Ontario

A study has been made of the petrology and geochemistry of a 30-mile (48.3 km) segment of a belt of ultramafic-gabbroic igneous bodies extending past the south side of Lake Abitibi, Ontario. In general, the bodies are sill-like and are differentiated into major layers of peridotite, clinopyroxenite, and gabbro, the layers generally being in that stratigraphic order. In detail, the intrusions fall into four groups: (1) complex sills in which there is a cyclic repetition of layers; (2) simple differentiated sills showing only one sequence of the above rock layers; (3) bodies composed only of peridotite and dunite; and (4) bodies composed wholly of gabbroic rocks.One of the group (2) bodies has a chilled margin equivalent in composition to a tholeiitic basalt. The general structure of the intrusions and their petrographic and chemical features indicate that they are differentiated from basaltic magma by gravity-controlled fractionation. However, it appears that while solidifying, some of the intrusions were open to periodic addition or subtraction of magma. Thus, in the intrusions showing cyclic repetition of layers, it is apparent that the magma was altered prior to the formation of each cyclic unit such that the original order of mineral crystallization was repeated. For other intrusions, it can be inferred that large amounts of partly crystallized liquid were expelled such that each of these intrusions is now largely or wholly represented by ultramafic rocks. The bodies composed wholly of gabbro may be derived from the expelled magma.

Origin of ultrabasic inclusions in basalts in southern Far East

The mode of occurrence, fabric, mineralogical and chemical features of ultrabasic inclusions in basalts of the South Far East are discussed in detail and compared with similar inclusions from various parts of the world. The inclusions occur mostly in alkalic varieties of basalt and commonly have cataclastic fabrics exhibiting preferred orientation of grains. They consist principally of olivine, enstatite or bronzite, chrome-diopside and spinel. Except for spinel, the compositions of the minerals are remarkably similar on a world-wide basis, whereas the enclosing rocks are represented by a variety of alkalic basalts. Chromium content of the inclusions is usually high compared to very low values for enclosing basalt. It is concluded that such inclusions are xenoliths of ultrabasic rocks rather than indigenous crystal differentiates. Their most plausible derivation is from the peridotite layer. This hypothesis explains their consistent composition, their association with deep rifts, the undifferentiated natu...

Development of Dzhungar deep fault

The great Dzhungar fault zone extends across the border between Sinkiang and the U.S.S.R., separating the intermontane depression of the Balkhash region and Dzhungaria from the mountain uplift of Dzhungar Alatau. The length of the fault is about 5OO km and the width is several kilometers. On airphotos the fault appears straight, but on a regional scale it is slightly arcuate. The fault is characterized by prolonged development of both vertical and horizontal (right lateral) displacement starting in middle Paleozoic and continuing intermittently to the present. Also during intervals of time, dislocation penetrated the entire thickness of the earth's crust as far as the peridotite layer permitting ultrabasic intrusion. Acidic intrusions, volcanics, and hydrothermally altered rocks also occur along parts of the fault zone. The fault strongly cuts obliquely across Hercynian folding and also alpine dome-block structures. The fault controlled and localized distinctive formations throughout most of post-Precambrian time. A summary of the geologic history of the fault is presented.—L. T. Grose

CHARACTERISTICS OF VOLCANISM AND SOME GENERAL PROBLEMS IN GEOLOGY - THE SIBERIAN PLATFORM

This paper is a short discussion of the two chief types of igneous activity encountered on the Siberian Platform: trap extrusion and the extrusion of alkalic and ultrabasic rock; the latter type of extrusion involves the emplacement of kimberlites. The paper further develops the hypothesis that the traps and the ultrabasic extrusions came into being when the uppermost portions of the basalt and peridotite layers, respectively, melted. The kimberlite magma came into being in small reservoirs at depths between 70 km and 100 km; these reservoirs lay in high-pressure zones. The presence of eclogite and especially diamond intrusions in the kimberlite confirms the sub-crustal origin of the magma. The layer in which the magma originated, however, is neither very thick nor does it occur everywhere. The Mohorovicic discontinuity takes the place of the sub-crustal basis and ultra-basic reservoirs where the pressure has the normal value. The paper also touches upon diamonds in meteorites. The presence of diamonds in...

Geological Features of Kimberlite-occurring Regions and Some Considerations on the Origin of the Rock

Common geological features of the known kimberlite-occurring regions in the continents are briefly described with special notice to certain tectonic processes that seem to be linked to the eruption of kimberlites.Some investigators suppose that kimberlite magma originated in the peridotite layer just below the Mohorovicic discontinuity. Judging, however, from the tectonic processes prior to the eruption of kimberlites in most regions and the structural model concerning to the crust and the upper mantle supported by the writer, sources of kimberlites might be in more deeper part of the mantle than the peridotite layer.

Origin of cenozoic petrographic provinces of Japan and surrounding areas

Three petrographic provinces can be recognized in the Cenozoic volcanic fields of Japan and surrounding areas. A province of a tholeiite series lies on the Pacific side of the Japanese Islands and includes the Izu Islands, whereas that of an alkali rock series occupies the Japan Sea side of the Islands with a narrow offshoot extending across central Honsyū (Honshū) and a continuation westward to Korea and Manchuria. A province of a calc-alkali rock series is superposed on the two provinces and occupies the greater part of the Japanese Islands exclusive of the Izu Islands and the islands in the Japan Sea southwest of Honsyū and north of Kyūsyū (Kyūshū). The boundary lines between the tholeiite and alkali provinces are located very closely to those between the areas where earthquakes occur at depths shallower than about 200 km and those for deeper ones. It is suggested that the parental tholeiite magma is produced by partial melting of the periodotite layer at depths shallower than 200 km. In the Izu Islands, except Nii-zima(Nii-jima) and Kōzu-sima(Kōzu-shima) close to Honsyū, the magma erupts to the surface without assimilating granitic material because the granitic layer is absent, resulting in volcanoes made up exclusively of the tholeiite series. The parental alkali olivine basalt magma is produced by partial melting of the peridotite layer at depths greater than 200 km. In the Japan Sea region, Korea, and Manchuria, it erupts to the surface without assimilating the granitic material, although it passes through a thick granitic layer, resulting in volcanoes made up exclusively of the alkali series. However, in the Cenozoic orogenic belt of the Japanese Islands, both types of parental magma assimilate granitic material during passage to the surface and erupt to form volcanoes of the calc-alkali series.