Cumulate Layers Research Articles

The Shetland Ophiolite Complex: field evidence for the intrusive emplacement of the ‘cumulate’ layers

Synopsis Unlike many other ophiolites, the lower-crustal sequence in the Shetland ophiolite has not been been formed in situ as a series of cumulate layers in the base of a magma chamber overlying the harzburgitic mantle. Field evidence shows that the layers were intruded sequentially above the mantle. Dunite rose as a fluid through mantle conduits, relics of which are preserved as invasive pods and sheets within the harzburgite. It formed an intrusive layer several kilometres thick between the mantle and an overlying banded wehrlite–clinopyroxenite layer, relics of which are preserved in places in the roof of the dunite intrusion as partially assimilated xenoliths and screens. A similarly uniform and thick intrusive layer of gabbro was emplaced immediately above the dunite layer and contains xenoliths and screens of the wehrlite–clinopyroxenite layer in both its base and its roof. A mixed gabbro–microgabbro layer lies immediately above the gabbro and has been cut repeatedly by parallel basic sheets giving the layer a quasi-sheeted-dyke appearance. However, the sheets are more nearly parallel than normal to the gabbro and dunite layers and have been intruded from outside the exposed layered sequence. The succession listed is lying on its side forming a nappe, in which both the layers and the sheets are vertical. It is not clear whether the succession had been rotated into this position before or after the sheets were emplaced. It is suggested that the dunite in the mantle was formed in the manner proposed by Kelemen et al. in 1995 , that is that MORB produced by adiabatic melting in the mantle was undersaturated in orthopyroxene and reached the top of the mantle by intergranular flow through the mantle during which the porosity along the flow path was increased by dissolution of the pyroxenes, thus concentrating the flow into dunite conduits. Under these circumstances the flow of liquid MORB through the solid dunite conduits, because of increased porosity and decreased grain adhesion, could possibly lead to fluidization of the dunite in the conduits. The fluidized dunite would be forced upwards out of the mantle to form the overlying intrusive dunite layer.

Petrology of the gabbro and sheeted basaltic intrusives at North Cape, New Zealand

Abstract The North Cape massif consists of a semi‐conformable sequence of serpentinite, gabbro, sheeted sill and dike units, and pillow lavas. Although structurally disrupted, they can be interpreted in terms of an idealised ophiolite sequence and represent the most complete sequence in the Northland Ophiolite. Their age is considered to be Late Cretaceous ‐ Paleocene on the basis of microfossils in associated sediments. Early Miocene K‐Ar ages from igneous rocks are thought to reflect the time of emplacement as a thrust sheet of oceanic crust and upper mantle. The gabbros are divided into a lower unit characterised by well‐developed cumulate layering and an upper unit which is massive; the sheeted sills and dikes are quartz‐diorite and microgabbro interleaved with minor pillow lava. Two phases of alteration are observed, a pervasive low‐grade greenschist metamorphism attributed to sea‐water interaction after formation as oceanic crust, and an overprinting zeolitic alteration which is possibly post‐emplac...

The role of ilmenite in the source region for mare basalts: Evidence from niobium, zirconium, and cerium in picritic glasses

To investigate models for the generation of lunar high Ti-basalts, we have analyzed lunar picritic glasses for Zr, Nb, Ce, and Ti at high precision using ion microprobe techniques. The picritic magmas represented by these glasses have experienced minor crystallization, which has allowed us to partially eliminate the effects of post-melting processes commonly experienced by crystalline high-Ti mare basalts. The Nb Zr for these glasses ranges from .05-.11. The high-Ti glasses generally tend to have higher values of Nb Zr (.072-.109) than the very low-Ti glasses (.048-.085). The crystalline mare basalts tend to have slightly higher Nb Zr than glasses with similar Ti from the same site. For example, the Apollo 17 (A17) high-Ti basalts have Nb Zr of approximately .09. whereas, the A17 high-Ti glasses have Nb Zr of .07. KREEP has Nb Zr of approximately .06. Thus Zr is fractionated from Nb to different degrees in the various picritic magmas. The concentrations of Zr, Nb, and Ce increase from the very low-Ti glasses to the high-Ti glasses, and along the trajectory the Nb Ce and Zr Ce increase. Nb Ce (.25-1.6) and Zr Ce (4–15) for the picritic glasses overlap with KREEP ( Nb Ce = .36 and Zr Ce =5 ). Zr Ti and Nb Ti show a wide range of variation in these glasses. Both Zr Ti and Nb Ti in the glasses range from approximately .0014 to slightly less than .0003. The Zr Ti and Nb Ti for these glasses overlap with that of the crystalline mare basalts. Generally, with increasing Ti, Nb, and Zr, Zr Ti and Nb Ti decrease. The exceptions to this are the Apollo 14 (A14) glasses that exhibit an increase in Nb Ti and Zr Ti . Based on these data for the picritic glasses and experimentally determined partition coefficients for Nb, Zr, and Ce, the mantle sources for these picritic magmas are slightly to moderately fractionated from C1 chondrite and previous estimates of the bulk silicate Moon. Our best fit model for our data and this observation is that both the very low-Ti and high-Ti picritic magmas were derived through small to moderate degrees of nonmodal melting of lunar mantle sources consisting of a mixture of late-stage LMO cumulates (derived after >95% crystallization of the LMO) and early to intermediate LMO cumulates (derived prior to 80% crystallization of the LMO). The early LMO cumulates had Nb Zr , Zr Ce , and Nb Ce ratios near C1 chondrite, whereas these ratios were fractionated in the late-stage LMO cumulates. This hybridization of mantle sources occurred during large scale overturning of the LMO cumulate pile. The source for the low-Ti picritic magmas had very minor amounts of ilmenite, whereas the source for the high-Ti picritic magmas probably contained less than 6% ilmenite. For all the picritic magmas, ilmenite was exhausted from the residua during melting. Models suggesting that the high-Ti magmas are derived through the assimilation of an ilmenite-bearing cumulate layer or preferential assimilation of ilmenite by low-Ti primary magmas are not consistent with the Nb, Zr, Ce, and Ti data magmas (Hubbard and Minear, 1975; Wagner and Grove, 1993, 1995). In particular, the preferential assimilation by very low-Ti picritic magmas of ilmenite with expected Nb Ce (20,000–22,000) and Nb Zr (55) signatures would displace the resulting high-Ti magma too far from our observed data. Large scale overturning of the LMO cumulate pile also accounts for the trace element signatures found in the A14 picritic glasses. The evolved signature found in these primitive very low-Ti picritic glasses is most likely a product of KREEP incorporation into the LMO cumulate source rather than either contamination by evolved ilmenite-bearing cumulates or incorporation of higher proportions of locally derived intercumulus melt.

A model for the thermal and chemical evolution of the Moon's interior: implications for the onset of mare volcanism

Crystallization of the lunar magma ocean creates a chemically stratified Moon consisting of an anorthositic crust and magma ocean cumulates overlying the primitive lunar interior. Within the magma ocean cumulates the last liquids to crystallize form dense, ilmenite-rich cumulates that contain high concentrations of incompatible radioactive elements. The underlying olivine-orthopyroxene cumulates are also stratified with later crystallized, denser, more Fe-rich compositions at the top. This paper explores the chemical and thermal consequences of an internal evolution model accounting for the possible role of these sources of chemical buoyancy. Rayleigh-Taylor instability causes the dense ilmenite-rich cumulate layer and underlying Fe-rich cumulates to sink toward the center of the Moon, forming a dense lunar core. After this overturn, radioactive heating within the ilmenite-rich cumulate core heats the overlying mantle, causing it to melt. In this model, the source region for high-TiO 2 mare basalts is a convectively mixed layer above the core-mantle boundary which would contain small and variable amounts of admixed ilmenite and KREEP. This deep high-pressure melting, as required for the generation of mare basalts, occurs after a reasonable time interval to explain the onset of mare basalt volcanism if the content of radioactive elements in the core and the chemical density gradients above the core are sufficiently high but within a range of values that might have been present in the Moon. Regardless of details implied by particular model parameters, gravitational overturn driven by the high density of magma ocean Fe-rich cumulates should concentrate high-TiO 2 mare basalt sources, and probably a significant fraction of radioactive heating, toward the center of the Moon. This will have important implications for both the thermal evolution of the Moon and for mare basalt genesis.

The reflectivity of magmatic underplating using the layered mafic intrusion analog

The reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates. One-dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0.18. A seismic profile across the Bjerkreim intrusion in southern Norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0.05 produces detectable reflections. Two-dimensional models demonstrate that the subhorizontal cumulate layering in typical mid- to upper-crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image. Reflection coefficients predicted for granulite-facies mineral assemblages are similar to those predicted for the igneous assemblages. Reflection coefficients predicted for cumulates intruded or re-equilibrated at eclogite-facies conditions may be either decreased or enhanced with respect to the igneous assemblages, depending on the abundance of plagioclase in the cumulate layers and the pressure. Results suggest that magmatic underplating should not be invoked as an explanation for non-reflective crust except in cases where eclogitization has occurred.

特集 火山活動と地殻応力場 中央海嶺のマグマ供給システム

Recent progress in studies on magma plubming system beneath mid-ocean ridges is reviewed. Mid-ocean ridges are not continuous, homogeneous series of crests, but are segmented by various topographical offsets of several orders of magnitude. Such topographical ridge segmentation is a surface manifestation of along-strike variations in the pattern of mantle convection and the supply of magma.Upwelling of the mantle material beneath the spreading center takes the form of a diapir, rather than sheet-like flow. Such diapiric upwelling has an along-axis dimension of several hundred to several tens of kilometers, which corresponds to first-and second-order segments, but some are as small as the scale of third-order segments. Where magma upwelling is intense, hot lithosphere and high magmatic productivity produces thick crusts, resulting in low gravity anomaly.When the spreading rate is low, mantle flow is cooled on route to the bottom of the lithosphere and ceases to melt at depths. Thus magmas produced beneath slow spreading ridges have low degrees of melting. On the contrary, fast spreading ridges are followed by intense mantle upwelling which melts significantly the mantle column from deeper part of the upper mantle up to just beneath the crust.Magmatic budget is so large at fast spreading ridges as to maintain large, steady-state magma chambers along the ridge axes with extensions comparable to second-and third-order segments. Such magma chambers have a very small melt pocket one to two kilometers wide and up to several hundred meters thick, underlain by a large mush of crystals and melt. Cumulate layers on the top of the mush sink and deform to S-shaped or downwarping layers as seen in large layered plutonic bodies in some ophiolites. However, low spreading ridges do not have enough supply of magma but only possess short-lived small magma chambers consisting of crystal mush.Volcanic landforms consist of major topographic features associated with small volcanic edifices : the former is a large shieled volcano several tens of kilometers long and is probably formed by episodic eruptions every several tens of thousand years ; the latter is a small conical lava cone comprising pillow flows which is constructed during a short-period eruption but some are active as long as 1.8 million years which are unequivocally central volcanoes.Rifting episodes at spreading axes depend on the spreading rate and the width of the volcanic zone. Slow spreading ridges such as Iceland have rifting every 100 years, but fast spreading ridges such as East Pacific Rise rift every year or two. Such rifting episodes continue for several years and are associated with multiple dikes with an injection interval of a month up to a year.Diapiric mantle upwelling, structure of magma chambers and volcanic landforms seen in mid-ocean ridges resemble those in some large ophiolites such as the Troodos, Cyprus and the Semail, Oman, and post-Teritary volcanism in Iceland.

Constraints on the formation of cyclic units in ultramafic zones of large basaltic chambers

Petrological models for the formation of cyclic units in ultramafic zones at the base of large mafic layered intrusions are still few and simple. In this study, we develop simple physical constraints, such as the volume balance and density relationships between the various liquids involved. We consider the formation of an entire ultramafic zone, made of N cyclic units due to N successive reinjections. We tackle the following problems. Are all the injections of the same chemical composition? Are the N injections responsible for the formation of the whole magma chamber? Two end-member models are examined. In the first one, the chamber grows with each injection and does not lose any magma (variable volume model, VV). In the second one, the chamber initially forms with a large volume of magma and remains at a constant volume with subsequent reinjections of small volumes balanced by eruptions of equal volume (fixed volume model, FV). Several scenarios for the formation of a cyclic unit are envisioned. Petrological data such as thicknesses of the cumulate layers and their compositions, together with the fractionation density of the cumulate phases, allow the calculation of the density evolution, as a function of the initial density only. Thus, the model yields constraints on the density of the initial liquid, and hence on its chemical composition. The volumes of the injected magmas are calculated as well as the evolution of the densities of the different involved liquids. We apply the calculation to the specific case of the Ultramafic Series of the Stillwater complex. The multiple reinjection models require that the initial liquids densities vary between 2.71 and 2.72 g/cm3, corresponding to MgO-rich liquids for which there is no field evidence, either in dikes or in chilled margins. They also require that the source produces liquids of different chemical composition. Thus, the evolution of the igneous system requires another, larger reservoir where differentiation takes place prior to injection into the shallower one. Contrary to the multiple reinjection models, a closed system model where crystallization proceeds in an isolated bottom layer does not require any specific value of density for the liquids, provided, of course, that it is of basaltic composition. On the negative side, such a model is not supported by any physical processes which would account for the formation of cyclic units.

Axial segmentation at a fossil oceanic spreading centre in the Haylayn block (Semail nappe, Oman): off-axis mantle diapir and advancing ridge tip

The Haylayn block, located in the central part of the Oman ophiolite, is characterized by a number of special features, in its south-eastern end, that previously led to various interpretations. These characteristics are: • -an important change in the strike of the primary dykes at the level of the sheeted dyke complex, crosscut by a dense pattern of ridge-parallel secondary dykes; • -a steepening and rotation of the petrological Moho and the cumulate layering in places cut by numerous ridge-parallel diabase dykes; • -intrusive planar laminated gabbronorites, associated with important magmatic breccias; • -large bodies of intrusive wehrlites related to a dome of mantle harzburgites outcropping within the plutonic sequence. The dome of harzburgites is interpreted as an off-axis diapir placed approximately 5 km south of the spreading axis, generating the magma of the huge wehrlite bodies. Some of the gabbronorites (Haymiliyah area) might result from the ultimate differentiation, in a closed fractionation system. of the old rift system, whereas the structural relationships of the others (SE of the studied area) require the existence of a new propagating rift.

Geology of the Zambales ophiolite, Luzon, Philippines

The Zambales ophiolite of western Luzon, Philippines, exposes a typical succession of basalt flows, diabasic dikes, gabbro and tectonized harzburgite. The age established by limiting strata is late Eocene. Lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift. Highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the Zambales ophiolite may be one of the least disturbed (by emplacement) ophiolitic masses known. The exposed mass trends north and the upper surface plunges at low angles (a few degrees) to the north and south. The chemistry and composition of the rocks in the northwest part of the Zambales area (Acoje block) is distinct from that in the southeastern segment (Coto block). The Acoje block, according to Evans (1983) and Hawkins and Evans (1983), resembles (on a chemical basis) arc-tholeiite series rocks from intra-island arcs and the rocks in the Coto block are typical back-arc basin rock series. The present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation. The gabbro probably formed below a spreading center in an elongate, in cross section, V-shaped, magma chamber. The gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places. Numerous erosional windows through the gabbro in the northern and eastern side of the Zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick. Harzburgite is exposed to a depth of about 800 m in the Bagsit River area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth. A transitional zone, about 200 m thick lying between the gabbro and harzburgite, is composed of serpentinized dunite. Commonly the dunite contains disseminated sulfide minerals and at the Acoje Mines, platinum-group elements. A compositional layering within the gabbro is in places cumulate in the lower part of the unit but may have formed by nucleation higher up on the relatively steep sides of the magma chamber. A widespread gneissic banding in the gabbro forms large mappable structures which are many times more complex than is the disposition of the major rock units. These structures are believed to be the result of extensive slumping in the magma chamber. The structure produced by the cumulate layering merges with the gneissic banding, commonly without discernible change in attitude. This tectonic layered structure crosses the gabbro-peridotite boundary at any angle without seeming to disturb the original rock distribution. At greater depths below the boundary (ca. 800 m), the harzburgite contains low dipping banding, which probably reflects the result of differential movement within the mantle. Chromite occurs almost exclusively in a zone that generally lies no more than 200–300 m below the gabbro-peridotite boundary. Refractory-grade chromite is found in this zone below the olivine gabbro in the Goto block and as low-grade metallurgical grade chromite below norite in the Acoje block. At Acoje Mines the chromite is present in layers in dunite, which the writer interprets as being distributed in a zone along the gently dipping (ca. 25°) gabbro-peridotite boundary. The steeply dipping (ca. 60–80 ° ) individual layers lie en echelon along the boundary at an angle (ca. 50 ° ) to the contact. At Coto the chromite forms large discontinuous masses in the lowest dunite and in the uppermost harzburgite. Except for the chromite present as layers at Acoje, the regional tectonic layering crosses the chromite deposits without structural deviation. The chromite deposits and associated peridotite may be cumulate in origin, but have been modified to such an extent that cumulate textures are generally obliterated. The angle of repose of cumulate layers in the Acoje area and in the Coto block dip towards each other raising the possibility that the Zambales area may contain the relics of a spreading center. Initial emplacement of the Zambales ophiolite took place by uplift and the ultramafic portion was exposed to erosion in the earliest Miocene or late Oligocene. Submergence of some of the ophiolite followed the previous uplift and on the west side of the Zambales Range submergence of several kilometers is indicated. Final emergence appears to have taken place in Pliocene or Pleistocene time by block uplift and areas of greatest uplift closely conform to the present topographic surface.

Spectral mapping of Alaskan ophiolites using landsat thematic mapper data

Ophiolites are sections of oceanic lithosphere that have been exposed by thrusting in mountain belts. Thematic Mapper three-band color composites have been used to differentiate the main rock types of the ophiolites of the western Brooks Range, Alaska, and to map mineralogic variations produced by processes that affect oceanic lithosphere, such as mantle partial melting, hydration of ultramafic rock, formation of cumulate layers, and hydrothermal alteration and metamorphism of basalt. Scaled composites of TM Bands 7,3,1 and 5,4,2, and 3,2,1, displayed in red, green, and blue are most useful for visually differentiating rocks of the ophiolites. Ternary diagrams of digital number valves are used to evaluate color variations observed in the three-band composites. Geologic maps, interpreted from TM images and reconnaissance field studies, are presented for the Asik Mountain, Maiyumerak Mountains, Avan Hills, and Misheguk Mountain ophiolites. The mapping results are being used to guide additional studies of the tectonics of the Brooks Range ophiolites.

Nd and Sr isotope systematics of clastic metasediments from Isua, West Greenland: Identification of pre‐3.8 Ga Differentiated Crustal Components

Recent field work on the Isua metasediments has established that there are two lithological sequences (A and B), which are divided into a number of distinct formations. Each clastic metasedimentary unit has a relatively unique trace element signature. The depleted mantle Sm‐Nd model ages of samples from all major units range from 3.59 to 3.98 Ga, whereas Rb‐Sr model ages cover a very wide range (1.9–4.6 Ga). The samples from the sequence A felsic gneiss formation (A6) yield a relatively good Rb‐Sr isochron age of T = 3.71±0.07 Ga. The sequence B samples do not yield a Rb‐Sr isochron; however, some of the samples plot close to the A6 Rb‐Sr isochron, and sequences A and B appear to have essentially the same deposition age. The Sm‐Nd data for both sequences yield initial εNd values in the range −1 to +3 for a deposition age TSTRAT = 3.64 Ga (if TSTRAT = 3.81 Ga, the range is 0–4). This range appears to reflect real variability of initial εNd values corresponding to the time of deposition. Data trends in εNd; versus 1/Nd diagrams suggest at least three distinct components in each of sequences A and B. Model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as ≈0.4 b.y. Other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition. The very high initial εNd values implied for early depleted mantle sources (+2 to +4), suggest a very large pre‐3.8 Ga continental crust. Alternatively, these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a ≈4.5 Ga magma ocean.

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ∼1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. On the basis of its position within the section and its laterally discontinuous nature, we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ∼20°-40° eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Several lines of evidence argue against these features being either artifacts or out-of-the-plane events. Instead, their presence may be ascribed either to crustal-penetrating fault zones or to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. Because of the laminated character of these events and their typical occurrence within 1.0 to 1.5 s of the reflection Moho, we prefer a compositional versus a structural interpretation for their origin. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred; although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities, significant departures are recognized elsewhere. Similarly, the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the Blake Spur fracture zone. An interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others.

Metasomatism of cumulus magnesian olivine by iron-rich postcumulus liquids in the upper Critical Zone of the Bushveld Complex

AbstractHybrid olivine grains from locally concordant contact zones between bodies of iron-rich ultramafic pegmatite (postcumulate) and layered harzburgite (cumulate) are distinguished from cumulus olivine grains by petrographic features and compositional differences. The hybrid olivines, which in comparison with the cumulus crystals are Fe-Mn-rich and Mg-Ni-poor, represent an arrested stage of replacement and exhibit unusually high NiO/MgO ratios. These features are explained by a disequilibrium process of cation-for-cation exchange between crystals and silicate liquid i.e. magmatic metasomatism. Plots of cations across a contact zone give straight line relationships, a function of the extent to which the metasomatizing liquid infiltrated the cumulate layer. A plot of NiO against MgO enables a distinction to be made between metasomatic olivine and olivine that has fractionally crystallized from a tholeiitic magma. These metasomatic olivines all formed by the replacement of pre-existing cumulus olivine and no chemical evidence has been found to support the formation of olivine in iron-rich ultramafic pegmatite bodies by metasomatism of other phases.

Geochemistry, Petrogenesis and Tectonic Environment of Circum-Superior Belt Basalts, Canada

Summary The Circum-Superior Belt of Proterozoic basalts and sedimentary rocks surrounds, and in some places, unconformably overlies the Archaean Superior Craton in northern Quebec, Canada. Four main groups of volcanic rocks are recognized. The oldest unit, the Eskimo Formation on the Belcher Islands, consists of subaerial massive continental tholeiites with 4–7% MgO, relatively high SiO 2 and K 2 O, enriched LREE and other incompatible elements, negative Nb and Ta anomalies, and ɛ Nd (T = 1.8 Ga) of −7 to −8. The overlying Flaherty Formation contains largely subaqueous, massive or pillowed, mildly alkaline (transitional) basalts with relatively low SiO 2 , high alkalis and TiO 2 , enriched incompatible trace elements (including Nb and Ta), and ɛ Nd of + 3. Stratigraphically higher units (on the Ottawa Islands and the Chukotat Formation, Cape Smith Belt) contain subaqueous, massive or pillowed flows, and layered gabbro-peridotite flows with compositions ranging from MgO-rich ol-phyric basalt and evolved cpx-plag-phyric basalt to ol-cpx cumulate. Maximum MgO in noncumulate samples reaches 17 wt%, and trace element characteristics are MORB-like: LREE and other incompatible elements are depleted, and ɛ Nd is + 4 to + 5. The highest units, exposed on the Ottawa Islands, consist of komatiitic basalts and include unusual spinifex-textured layered flows. Although maximum noncumulate MgO contents are about 16 wt%, olivine xenocrysts in cumulate layers have the composition Fo 92 , and provide evidence of a komatiite (MgO = ∼ 22%) precursor magma. Komatiitic basalts are less depleted than the underlying Mg-basalts, with fiat to enriched LREE, negative Nb and Ta anomalies, and ɛ Nd = 0. The chemical features of Eskimo Formation basalts and spinifex basalts can be explained by crustal contamination of parental magmas derived from depleted mantle with ɛ Nd = + 5: the continental tholeiites by 20–30% contamination of parental basalt and the spinifex basalts by about 7% contamination of parental komatiite. The Mg-basalt series results from partial melting of depleted mantle followed by low pressure fractional crystallization, while Flaherty Formation basalts come from a more enriched mantle source. The stratigraphic sequence—(1) early contaminated continental basalt, (2) transitional basalt, (3) MORB-like basalt, (4) crustally-contaminated komatiite—is considered to have arisen during continental rifting, spreading, and then closure of an ocean basin surrounding the Superior Craton.

Magmas and magma chamber evolution, Troodos ophiolite, Cyprus

The Troodos ophiolite traditionally has been considered to have formed by constructive processes at an oceanic spreading center. New information on the phase layering and cryptic variation in gabbros and ultramafic cumulates overlying tectonized harzburgites, however, does not support the existence of a large, steady-state, frequently replenished, axial magma chamber. Compositional evidence suggests that the Troodos ophiolite was formed by two distinct and chemically unrelated magmatic systems. A stratigraphically lower websteritic and gabbroic cumulate sequence exhibits only one major magma-chamber replenishment, suggesting a nonspreading environment. An upper gabbroic cumulate sequence suggests frequent magma-chamber replenishments, pointing to a relatively fast-spreading environment. This suggests that the alleged Troodos axial magma chamber formed in an off-axis, nonspreading tectonic environment, probably by magmatic underplating of a thin oceanic crust. Spreading-related magmatic activity in the Troodos ophiolite was confined to the upper gabbros and dike complex.

The parental magma of the Nakhla achondrite: Ultrabasic volcanism on the shergottite parent body

The Nakhla meteorite is an augite-rich igneous cumulate rock; magma trapped among the cumulus grains is now represented by fine-grained mesostasis and post-cumulus overgrowths on cumulus grains. The composition of the intercumulus magma, calculated by mass balance, is ultrabasic (SiO2 < 45%) and enriched in incompatible elements (LaLu = 6 × CI). The cumulus augite and intercumulus magma were in chemical equilibrium, implying that the intercumulus magma is a sample of Nakhla's parental magma. Complex processes like infiltration metasomatism were therefore unimportant. Nakhla contains olivine crystals which are as large or larger than the cumulus augite crystals. The largest olivine crystals, probably xenocrysts, preserve core compositions too Fe-rich to have been in equilibrium with the intercumulus magma. Other smaller olivine grains may have been phenocrysts. The closest terrestrial analogs to Nakhla (and the other nakhlites) are pyroxene-rich cumulate layers in differentiated picrite and komatiite flows and shallow sills. Spinel or garnet was fractionated from the parental magmas of the nakhlites or their parental mantle, based on the superchondritic CaOAl2O3 ratio of Nakhla's intercumulus magma and its lack of an Eu anomaly. The three nakhlites and Chassigny are half of the known samples of the Shergottite Parent Body (SPB). One may tentatively infer that ultrabasic volcanism was much more important on the SPB than on the moon, Eucrite Parent Body, or present-day Earth.

Postcumulus processes in layered intrusions

AbstractDuring the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (&lt; 1%) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.

Morphologies of Gypsum on a Modern Sabkha: Clues to Depositional Conditions: ABSTRACT

Gypsum is the dominant evaporite mineral across the Holocene marine sabkha of the northwest Gulf of California, Baja California, Mexico. This sabkha is a complex of mud flats and saline pans in which gypsum occurs as layers and isolated intrasediment crystals. Six distinct morphologies of gypsum were found: flat plates, acicular prisms, bladed prisms, blocky prisms, blocky hemipyramids, discoidal hemipyramids. Flat plates (< 0.5 mm wide) and acicular prisms (< 1 mm long), in the form of radiating clusters and interpenetrating twins, crystallize at the brine-air interface during the temporary saline lake stage of the pan. These tiny crystals settle out on the bottom of the pan as a cumulate layer. Bladed prisms (0.5-1.5 mm long) nucleate on this cumulate layer and gr w upward as vertically oriented prisms (commonly swallowtail twins) making a grass-like layer. Blocky prisms (up to 2 cm long), barrel-like in shape with swallowtail twin terminations, are found as pendant cements in shelter vugs formed by buckling of the gypsum layers of the pan. Blocky hemipyramids (up to 1 cm across) are found as a loose mush making a diapir-like mound beneath m-scale polygonal cracks in the layered gypsum of the pan. These clear, blocky hemipyramids grow diagenetically in the vadose zone during the dry stage of the pan when evaporative pumping draws subsurface brine up along the polygonal cracks. Discoidal hemipyramids (up to 2 cm long) occur as isolated crystals. clusters of crystals, and rosettes within the siliciclastic sediment of the mud flats surrounding the an. They are of vadose and phreatic origin. Gypsum morphologies clearly vary with depositional conditions across this sabkha and this finding should allow us to use morphological variation in gypsum as a sensitive interpretative tool for ancient evaporites and evaporitive sediments. End_of_Article - Last_Page 460------------

Au Boys de dueil and the Grief-Decalogue Relationship in Sixteenth-Century Chansons

Research Article| January 01 1984 Au Boys de dueil and the Grief-Decalogue Relationship in Sixteenth-Century Chansons Dorothy S. Packer Dorothy S. Packer Search for other works by this author on: This Site PubMed Google Scholar Journal of Musicology (1984) 3 (1): 19–54. https://doi.org/10.2307/763660 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Cite Icon Cite Search Site Citation Dorothy S. Packer; Au Boys de dueil and the Grief-Decalogue Relationship in Sixteenth-Century Chansons. Journal of Musicology 1 January 1984; 3 (1): 19–54. doi: https://doi.org/10.2307/763660 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentJournal of Musicology Search This content is only available via PDF. Copyright 1984 Imperial Printing Company, Inc. Article PDF first page preview Close Modal You do not currently have access to this content.

Isotopic evolution of the mantle: a model

We investigate a model of mantle evolution which acknowledges the importances, in early Earth history, of large degrees of partial melting and crystal fractionation rather than, as in recent models, small degrees of partial melting operating on primary mantle. The major source regions are in the upper mantle and evolve from a primitive mantle melt by crystal fractionation and the formation of chemically distinct cumulate layers. The Rb/Sr and Sm/Nd ratios in melts are relatively unfractionated in processes involving large (> 15%) degrees of partial melting. The ratios are even less fractionated in orthocumulates forming from these melts. Small degrees of partial melting, or removal of residual fluids after nearly complete crystalization of cumulate layers, are the processes that lead to significant fractionation. The importance of these processes increases as the upper mantle cools. The observation of nearly primitive isotopic ratios does not necessarily imply the presence of ancient primitive undifferential reservoirs. The transfer of intercumulus fluids from a crystalizing cumulate layer gives time-dependent Rb/Sr and Sm/Nd ratios in both the donor and receiving reservoirs since the late-stage fluids become more enriched as crystalization proceeds. Enrichment in such a system is progressive with time. Time-integrated Nd/Sm and Rb/Sr ratios, therefore, need not depart much from primitive ratios even though current values for LILE concentration, Rb/Sr, Nd/Sm, etc., may be large. The time constant for LILE enrichment appears to be 2 Gyr, about the same as required for the U/Pb system and the same as the characteristics time associated with mantle cooling. The mantle reservoirs may be much more ancient than generally assumed.