Submarine Basalts Research Articles

Depths and temperatures of mid-ocean-ridge magma chambers and the composition of their source magmas

Summary Electron-microprobe analyses of submarine basalt glass and coexisting olivine from the Galapagos Rise and the Mid-Atlantic Ridge (60–63°N) reveal that olivine phenocrysts are virtually unzoned and olivine compositions are identical to those predicted by assuming that the olivine crystallized at the liquidus temperature of the host glass. The absence of chemical zoning of olivine indicates that their host magmas must have cooled by less than 15°C and the MgO/FeO of the magma must have been nearly constant (less than 3 wt% olivine removed) during the time that olivine phenocrysts existed in the magma. Olivine phenocrysts re-equilibrate with their host magma relatively quickly and any changes in temperature or composition that occurred in the 24 h previous to eruption would be recorded in the olivine composition. The settling rates of 1-mm olivine phenocrysts are too slow compared to magma ascent rates to allow early-formed olivine to settle out. There, many magmas that erupt at mid-ocean-ridge crests must ascend from their final reservoir nearly isothermally and isochemically, or alternatively they ascend at superliquidus temperatures and cool below the liquidus just prior to eruption. In either case the magmas identify the temperature (±15°C) and the composition of the magma reservoir immediately prior to eruption. The depth of these magma reservoirs may be estimated through experimental and empirical determination of the pressure-temperature phase diagrams of oceanic basalts. For Galapagos Rise and Mid-Atlantic Ridge basalts these reservoirs are at 3–25 km beneath the ridge crust and in some cases individual reservoirs may be located within ±3 km.

Role of the Eastern Mediterranean ophiolites (Turkey, Syria, Cyprus) in the history of the Neo-Tethys

Summary The internal organization and structure of the Taurus, Troodos, Hatay and Baër-Bassit ophiolite massifs are used, along with K-Ar geochronological data for the different parts of the assemblages, to outline stages in the evolution of the Neo-Tethys ocean during Mesozoic times. The authors distinguish: (1) an intra-continental stage, during Middle and Upper Triassic, when alkaline submarine basalts were erupted during rifting of the Arabian platform; (2) a Lower Cretaceous to Campanian phase, marked by active spreading for at least 40 Ma, along east-west trending palaeo-spreading axes, offset by north-south transform fracture zones. During that period, the oldest oceanic lithosphere (Tauride ophiolites and the Baër-Bassit complex) began to be sliced up with the development of metamorphic soles (104 to 88 Ma), later intruded by swarms of tholeiitic diabase dykes of Campanian age (ca. 75 Ma). At that time, the latest spreading zones, now represented by the Troodos and Hatay ophiolites, were still active in the Neo-Tethys; and (3) during the Maastrichtian (70–65 Ma), all ophiolitic massifs were thrust over the Arabo-African continental margin from a single Neo-Tethys oceanic basin.

Magnetic properties of submarine basalts from Site 443 of DSDP Leg 58, the Shikoku Basin : Takahashi, Shinji, 1983. J. Geomagn. Geoelect., 35(8):281–303

Magnetic properties of submarine basalts from Site 443 of DSDP Leg 58, the Shikoku Basin : Takahashi, Shinji, 1983. J. Geomagn. Geoelect., 35(8):281–303

Systematics of modal phenocryst assemblages in submarine basalts: Petrologic implications

Phenocryst assemblages in ocean-ridge basalts generally show an increasing proportion of plagioclase as the total amount of phenocrysts increases. The variations in phase assemblages, as well as most crystal-liquid Kd's, are similar to variations (equimodal trends) predicted by low-pressure laboratory experiments, suggesting that many of these basalts have experienced varying degrees of low-pressure cyrstallization prior to quenching, with little sorting of crystals and liquid. Important exceptions include moderately to highly phyric basalts enriched either in plagioclase or olivine which lie well off the experimental trends. In these basalts, megacrysts and xenocrysts usually cited as evidence for magma mixing commonly represent a small proportion of the total crystalline phase assemblage. However, phase proportions for many of these basalts lie well outside the range that could be produced by simple mixing; selective gravitative sorting either prior or subsequent to mixing appears to be the likely explanation for these phyric basalts. A relation between spreading rate and phase proportions is neither supported nor refuted by the data, which as yet do not adequately represent fast-spreading ridges.

Concentrations, sources, and losses of H 2O, CO 2, and S in Kilauean basalt

Basaltic glasses included in olivine phenocrysts from Kilauea volcano contain concentrations of H 2O, CO 2, and S similar to glassy Kilauean basalt dredged from the deep sea floor and greater than vesicular, subaerial Kilauean basalt. Our result contrasts with earlier reports that inclusions of basaltic glass in phenocrysts have little or no H 2O and large ratios of CO 2 H 2O . Our analysed inclusions of glass are larger than 100 micrometers thick and similar in chemical composition to the host glass surrounding the olivine crystals indicating that the trapped melts are representative of the bulk liquid from which the crystals grew. Crystallization of about 2–8% of olivine from the melts after they were trapped is indicated by slight departures from the experimentally established equilibrium distribution of Mg and Fe between olivine and liquid. The measured concentrations of CO 2 correspond to phenocryst crystallization pressures of about 1.3 kbar for a subaerial basalt and about 5 kbar for a submarine basalt, consistent with geophysical models of Kilauea volcano. The compositions of volcanic gas predicted from our analyses are consistent with restored compositions of actual Kilauean gases. The rate of sulfur emission predicted from our analyses is greater than the sulfur dioxide emission rate observed during repose, but probably consistent with total degassing including eruptive episodes. The concentrations of H 2O, K 2O, Cl, and P in parental Kilauean basalt can be derived from upper mantle phlogopitic mica, pargasitic amphibole and apatite with compositions close to those of natural primary minerals in ultramafic xenoliths from continental kimberlites, or solely from apatite and phlogopitic mica with H 2O K 2O near 0.47 ± 0.03, slightly higher than the range of values reported. The amounts of phlogopitic mica and pargasitic amphibole contributing volatiles to Kilauean tholeiite is about 10 percent by mass of the parental liquid, or about 5% if the source does not include amphibole. In view of an estimated 20% of partial melting of mantle source rock to produce Kilauean tholeiites, there may be about 2 weight percent of mica plus amphibole in part of the mantle beneath Kilauea, or about 1 weight percent of phlogopitic mica if amphibole is absent.

The identification of titanomaghemites: Model mechanisms for the maghemitization and inversion processes and their magnetic consequences

A model mechanism has been derived for the evolution of the cation distribution in titanomaghemites, which have formed by the progressive removal of iron from the spinel lattice. According to the model, the cation distribution of a titanomaghemite is not a unique function of composition but depends on the path by which maghemitization has taken place. Cation distributions (and consequent magnetic properties) have been calculated and compared for titanomaghemite produced by the “removal of iron” and “addition of oxygen” mechanisms. For a given composition, the Curie point temperature is almost invariant, being, according to molecular field theory, more sensitive to composition than cation distribution. Therefore, the model cannot explain the variations observed in synthetic titanomaghemite of different provenances. According to the model, the spontaneous magnetization is significantly lower for the removal of iron mechanism, which may be helpful in explaining a decrease in the remanence of submarine basalts with age. Structurally metastable titanomaghemites invert to an intergrowth of phases on heating above approximately 350°C and it is expected that the inversion product would be independent of the mechanism by which maghemitization had previously taken place. Available X-ray, and other data, have been used to produce a simple model of the inversion process, and values of the Curie point of the inversion product and the ratio of the magnetization before and after the inversion derived. These model values compare favourably with experimental values for a synthetic titanomaghemite in which inversion was induced by progressive slow heating to approximately 600°C. The inversion process therefore has potential in determining the compositions of naturally occurring titanomaghemites.

Magnetic properties of submarine basalts from Site 443 of DSDP Leg 58, the Shikoku Basin.

Magnetic properties of submarine basalts from Site 443 of DSDP Leg 58, the Shikoku Basin.

Hydrothermal alteration of submarine basalts : from zeolitic to spilitic facies in the upper Triassic pillow-lavas of Antalya, Turkey

The upper Triassic alkalic basalts of the Alakir Çay valley (Antalya nappes, Turkey) have been hydrothermally metamorphosed under the zeolite facies conditions. Two of these submarine volcanic massifs, mainly made of pillow-lavas flows, offer good field sections over more than 650 m thick. In the Çalbali Dag massif, the zeolite and associated minerals have been analyzed inside and outside the pillow-lavas (inter-pillow matrix), from bottom to top of the volcanic pile. Natrolite, gonnardite, thomsonite, analcite, gmelinite, stilbite, heulandite and laumontite have been determined, associated with calcite, K-feldspar, celadonite, smectites and chlorite. No evolution of the zeolite assemblages with depth has been detected : this fact implies a rather smooth thermal gradient in the volcanic pile during the hydrothermal circulation. In the Sayrun massif , the same widespread development of zeolites, calcite and smectite is observed, with the addition of true albite-chlorite spilitic paragenesis, well developed in some areas. The reality of a progressive transition from zeolitic assemblages to spilitic ones is an important conclusion of this study.

Volcanic evolution in eastern Papua

Late Mesozoic and Cenozoic volcanic rocks in eastern Papua record a complex series of volcanotectonic events which reflect interaction between the Indo-Australian and Solomon sea plates. Basement formations of Upper Cretaceous and Eocene submarine basalt are comparable to those characteristic of sea floor spreading centers and are thought to have originated during volcanic activity associated with spreading in the Coral Sea basin. Arc-trench type andesitic volcanism was prominent during the late Cenozoic but shows no clear relationship to a subduction event. An alternative explanation links the development of thickened crust and consequent crust/mantle interaction with the generation of andesitic magmas. The tectonic environment of eastern Papua during the late Cenozoic was one of block faulting and uplift associated with crustal tension. The presence of Quaternary peralkaline rhyolites suggests that this environment is now being replaced by active rifting.

Correlated Nd, Sr and Pb isotope variation in Walvis Ridge basalts and implications for the evolution of their mantle source

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/ 144Nd (0.51238), 206Pb/ 204Pb (17.54), 208Pb/ 204Pb (15.47), 208Pb/ 204Pb (38.14) and high 87Sr/ 86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/ 144Nd (0.51270), 206Pb/ 204Pb (18.32), 207Pb/ 204Pb (15.52), 208Pb/ 204Pb (38.77) and lower 87Sr/ 86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe Zr Ba Nb Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.

An experimental study of the intensity and stability of thermoremanent magnetization acquired by synthetic monodomain titanomagnetite substituted by aluminium

Summary. Titanomagnetites of composition Fe2.4-δAlδTi0.6O4 and Fe2.6-δAlδTi0–4O4(δ=0, 0.1 and 0.2 in both cases) were prepared in the monodomain state by pulverization of sintered synthetic material. In low fields, the thermoremanence (TRM) was found to be linear with inducing field and of high enough intensity to account for typical natural remanent magnetizations of fresh submarine basalts. The higher field TRM acquisition curves follow the Neel model curve for an assemblage of non-interacting identical particles in a general way only, the differences being due to interactions, or the range of particle blocking temperatures and volumes or other features of the samples not included in the model. The unblocking temperatures of low field TRM lie in a narrow range below the Curie point. The low field TRM is very resistant to alternating field demagnetization and provides a very striking illustration of the strength of the TRM mechanism in preserving a stable record of a weak magnetic field. The result of a Lowrie-Fuller test on the material is consistent with the monodomain state.

Basin Evolution of the Lower Proterozoic Kaniapiskau Supergroup, Central Labrador Miogeocline (Trough), Quebec

ABSTRACT The Labrador Trough is the eroded remnant of a formerly much more extensive Early Proterozoic (Aphebian) geosyncline. The trough axis lies along the boundary between ultrastable Archean craton to the west (Superior Province) and metastable Archean craton to the east (Churchill Province), and defines an eastward-thickening miogeocline. The Labrador Trough sequence (Kaniapiskau Supergroup) consists of two distinct depositional cycles of sediment and volcanic deposition. An erosional unconformity and a possible episode of tectonic disturbance divide the depositional cycles. Smaller-scale cycles within each major depositional cycle represent shallowing-upward siliciclastic-carbonate or iron formation sequences of an early shelf phase. Both depositional cycles are terminated by a basin phase made up of black shale and shelf-derived clastics. The occurrence of platform-margin facies along the deformed eastern trough edge indicates the development of a shelf margin toward the east. A complex of submarine basalt flows and gabbro sills intercalated with basinal sediments represents a relatively static zone of rifting active throughout the deposition of cycles I and II. This igneous zone is preserved mainly by thrusting along the eastern trough edge. The geosyncline was deformed and metamorphosed during the Hudsonian orogeny (1800 Ma). Extensive thrusting produced a fold-thrust structural pattern and the present trough shape.

Chemistry and isotope ratios of sulfur in basalts and volcanic gases at Kilauea volcano, Hawaii

Eighteen basalts and some volcanic gases from the submarine and subaerial parts of Kilauea volcano were analyzed for the concentration and isotope ratios of sulfur. By means of a newly developed technique, sulfide and sulfate sulfur in the basalts were separately but simultaneously determined. The submarine basalt has 700 ± 100 ppm total sulfur with δ 34 S Σs of 0.7 ± 0.1 ‰. The sulfate/sulfide molar ratio ranges from 0.15 to 0.56 and the fractionation factor between sulfate and sulfide is +7.5 ± 1.5‰. On the other hand, the concentration and δ 34 S Σs values of the total sulfur in the subaerial basalt are reduced to 150 ± 50 ppm and −0.8 ± 0.2‰, respectively. The sulfate to sulfide ratio and the fractionation factor between them are also smaller, 0.01 to 0.25 and +3.0‰, respectively. Chemical and isotopic evidence strongly suggests that sulfate and sulfide in the submarine basalt are in chemical and isotopic equilibria with each other at magmatic conditions. Their relative abundance and the isotope fractionation factors may be used to estimate the ƒo 2 and temperature of these basalts at the time of their extrusion onto the sea floor. The observed change in sulfur chemistry and isotopic ratios from the submarine to subaerial basalts can be interpreted as degassing of the SO 2 from basalt thereby depleting sulfate and 34S in basalt. The volcanic sulfur gases, predominantly SO 2, from the 1971 and 1974 fissures in Kilauea Crater have δ 34 S values of 0.8 to 0.9%., slightly heavier than the total sulfur in the submarine basalts and definitely heavier than the subaerial basalts, in accord with the above model. However, the δ 34 S value of sulfur gases (largely SO 2) from Sulfur Bank is 8.0%., implying a secondary origin of the sulfur. The δ 34 S values of native sulfur deposits at various sites of Kilauea and Mauna Loa volcanos, sulfate ions of four deep wells and hydrogen sulfide from a geothermal well along the east rift zone are also reported. The high δ 34 S values (+5 to +6%. o) found for the hydrogen sulfide might be an indication of hot basaltseawater reaction beneath the east rift zone.

Contrasted abyssal basalt liquidus trends: evidence for mantle major element heterogeneity

Natural submarine basalt glasses define liquidus trends which may be summarized as curvilinear regression lines in the normative plagioclase-pyroxene-olivine ternary. Individual basalt suites separated in time or space may differ in major element composition; these differences translate into systematic differences in normative plagioclase, leading to efficient discrimination of individual trends in this ternary. Comparison of two contrasted sets of trends to available experimental and petrographic data confirms the similarity of the natural basalt trends to those predicted by 1-atm experiments, in general agreement with earlier studies based on whole-rock data. These contasted liquidus trends are believed to reflect major element heterogeneity and varying degrees of melting of the mantle source. This major element heterogeneity is not simply correlated with heterogeneity in incompatible trace elements, and like incompatible trace element heterogeneity, there is growing evidence that it may vary abruptly in space and time. The existence of these compositionally contrasted basalt suites must be considered in petrogenetic modelling; specifically, they will introduce scatter in most generalized variation diagrams, and will increase the likelihood of “misfits” in fractionation calculations utilizing parent and residual compositions drawn from different suites.

An experimental study of thermoremanent magnetization acquired by synthetic monodomain titanomaghemites.

In this investigation the intensity and stability of thermoremanence (TRM) acquired by synthetic titanomaghemites has been studied for the first time. The titanomaghemites have compositions derived by oxidation from the titanomagnetite compositions, Fe2.4Ti0.6O4, Fe2.3Ti0.6Al0.1O4 and Fe2.2Ti0.6Al0.2O4 and are in the monodomain state. Compositions over the complete oxidation range cannot normally acquire TRM as the Curie temperatures of the more highly oxidized compositions exceed the “inversion” temperature (-300°C) at which the titanomaghemites transform to a multiphase intergrowth. In the composition range studied, the intensity of TRM falls and stability against both alternating field and thermal demagnetization rises after maghemitization. In the laboratory experiments, maghemitization preceeds TRM acquisition. In submarine basalts TRM acquisition will preceed maghemitization. The monodomain TRM model suggests that the TRM intensity in the latter case will be higher than in the former. The fall in TRM (-80%) with increasing degree of maghemitization of the laboratory experiments is greater therefore than that which would be expected from the TRM-maghemitization sequence in nature. It would seem that other mechanisms than change in chemical composition (e.g. changes in microstructure) are needed to provide an explanation of any cases in which the intensities of remanence of deep sea basalts are believed to fall by amounts similar to those found in the present experiments.

Anomalous magnetic directions recorded by laboratory-induced chemical remanent magnetization

It has been generally accepted that chemical remanent magnetization (CRM) is acquired either in the direction of the ambient field at the time of chemical alteration1 or in the direction of a pre-existing natural remanent magnetization (CRM) 2–4. We report here our experiments in which CRM is carried by magnetite formed during laboratory heating of titanomaghaemite in submarine basalts. This CRM is a stable, single component remanence, acquired in an intermediate direction between the ambient field and pre-existing NRM direction. The role of prior NRM in controlling the direction of CRM is suppressed by externally applied fields of ≈50 µT. (0.5 Oe) but is prominent in fields of ≈20 µT. Thus, stable, intermediate-direction CRMs acquired in past geomagnetic fields are a possibility.

The nature and distribution of carbon in submarine basalts and peridotite nodules

Primary carbonaceous material has been identified in submarine basaltic glasses and mantle-derived peridotite nodules from alkali basalts using electron microprobe techniques. In the submarine rocks carbon occurs (1) in quench-produced microcracks in glasses and phenocrysts, (2) in vesicles, where it is preferentially concentrated on the sulfide spherules attached to vesicle walls, and (3) in microcracks and CO 2-rich bubbles in inclusions of glass completely enclosed by phenocrysts. In peridotite nodules carbon exists in intergrain cracks, along grain boundaries, and on the walls of fluid inclusions disposed in two dimensional arrays. The carbonaceous material is believed to consist of a mixture of graphite, other forms of elemental carbon, and possibly small amounts of organic matter. It is suggested that carbon precipitates by disproportionation of CO according to the reaction 2 CO→C+CO 2 and that this reaction is catalyzed by sulfide-oxide surfaces in vesicles. Once deposition has begun, the reaction continues on carbon surfaces as well. Based on the large amounts of condensed carbon observed in some vapor inclusions and the apparent lack of oxidation features associated with them, it is proposed that carbon condensed from a magmatic vapor in which CO was a significant constituent. This implies that oxygen fugacities of undegassed basaltic melts under confining pressures of the shallow crust are typically lower than those of the QFM buffer at equivalent temperatures. This is in agreement with some intrinsic oxygen fugacity measurements on similar undegassed materials. Regardless of the mechanism of its formation, the presence of carbon in CO 2-rich vesicles and inclusions in basaltic glasses and mantle nodules adds uncertainty to estimates of minimum pressures of entrapment based on measurements of fluid densities. Condensed carbon also accounts for some of the carbon isotopic characteristics of these rocks.

The Concentration of $$CO_{2}$$ in Submarine Tholeiitic Basalts

The concentrations of and in glass separates from submarine tholeiitic basalts have been determined. The presence of vesicles in samples precludes accurate determination of the concentration in glass unless steps are taken to analyze only uncontaminated vesicle-free glass. Special care has been exercised during sample preparation to avoid these problems. A new analytical technique for micro-determination of and has been applied to five mid-ocean ridge and two Kilauea submarine east rift zone basalts quenched at pressures as great as 0.5 kb. The uncertainty of quenching pressure due to flow of lava subsequent to surface quenching, global sea-level variations, and local deformation of the sea-floor are discussed. Non-tectonic uncertainties of quenching pressure have been estimated. The concentration of in glass separates from submarine tholeiitic basalts increases linearly with quenching pressure. The apparent solubility of in tholeiitic basalts (containing ) at temperatures of quenching on the sea-floor is approximately . The pressure-dependent concentration of in basaltic liquids permits its use as a geobarometer. Potential applications include geobarometry of individual crystals, measurements of absolute vertical displacements of the sea-floor, and determination of pressures of emplacement for basaltic dikes. The saturation of submarine basalts with a -rich vapor phase and the uncertainty as to how much has been exsolved prior to quenching may require new interpretations of the trace-element variations of submarine basalts.

Samail Ophiolite plutonic suite: Field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge magma chamber

Geologic mapping of an intact plutonic sequence within the Samail ophiolite in the Ibra area, southeastern Oman Mountains, reveals stratigraphic, structural, and petrologic details of oceanic layer 3. Four measured stratigraphic sections, each spaced about 5 km apart across the southern flank of Jabal Dimh, define a time‐transgressive progression within the ophiolite and reveal geometric and petrologic features of a spreading‐ridge magma chamber. The sections show the following vertical sequence: (1) dunite (chr‐ol cumulates ± harzburgite xenoliths) 0–200 m thick, grading up from a transition zone with harzburgite tectonite, (2) interlayered wehrlite‐melagabbro‐gabbro (cpx‐ol and ol‐cpx‐pl cumulates) 0–100 m thick, (3) layered gabbro (chiefly ol‐cpx‐pl cumulates but including recurrent intervals of cumulus wehrlite and melagabbro) 2.6–5.5 km thick, (4) planar laminated nonlayered gabbro (chiefly ol‐cpx‐pl cumulates) 100–400 m thick, (5) hypidiomorphic (ol)‐hb‐cpx gabbro (high‐level gabbro) 200–800 m thick, (6) small, discontinous diorite to plagiogranite bodies at or near the top of the gabbro. Cumulus textures (adcumulus > mesocumulus), planar lamination, and cumulus layering (phase, ratio, and grain size layers at mm to 10‐m scale, commonly graded) within this sequence show that crystals accumulated from the base of the magma chamber upward to within a few hundred meters of the top; downward solidification from the roof was minor. Cyclicity within the cumulus sequence is represented by the recurrence of olivine‐rich intervals (melagabbo and wehrlite) up to high stratigraphic levels and by hundreds of phase‐graded layers (ol‐rich at the bases to pl‐rich at the tops), individually up to 5 m thick. Limited cryptic variation relative to closed‐system layered intrusions and the limited range in solid‐solution components of olivine (Fo69–90), plagioclase (An62–95) and clinopyroxene (En40–54, Fs4–16, Wo37–49) from the cumulus suite require replenishment of the magma during its crystallization history. Zig‐zag normal and reverse cryptic variation differentiation trends (both in major solid‐solution components and in minor element concentration) indicate that the replenishment took place in pulses followed by periods of magma mixing and crystal fractionation. Recurrent olivine‐rich intervals commonly (but not always) coincide with reverse cryptic variation trends. They are considered to be the products of fresh draughts of primitive magma, with olivine and chromite as the only liquidus phases prior to extensive mixing with the more fractionated resident magma within the chamber. The sequence of crystallization is explained using the simplified tholeiitic basalt tetrahedron of Presnall et al. (1979). The plutonic sequence is roofed by sheeted dikes that are overlain by submarine basalt, indicating that the magma crystallized beneath a spreading ocean ridge. Major and trace element geochemistry of the dike complex is similar in many respects to that of mid‐ocean ridge basalt (MORB) and yield Mg/Mg + Fe+2 ratios that overlap with ratios predicted for parent liquids of the cumulus suite based on crystal/liquid equilibria. Field evidence, including continuity of layering over large areas, distinctive layer sets that are mapped for several km along strike, and lack of chamber edge contacts indicates that crystallization of Jabal Dimh plutonics took place in a single, large, long‐lived magma chamber rather than in small transitory chambers. Similarity of mineral compositions and tentative correlation of cryptic variation trends between stratigraphic sections favors a large, single‐chamber model. The chamber shape, controlled by floor growth upward greatly exceeding roof growth downward as the chamber halves diverged from the paleo‐spreading axis, must have been funnel‐shaped in cross section; i.e., the floor sloped inward to the center from a sandwich horizon that developed just beneath the roof at both sides. This conclusion is based on the occurrence of cumulates up to a very high level in the stratigraphic sections and on geologic mapping which shows that cumulus layering locally approaches the base of the cumulus section at an angle. This vergence angle is <20°, well below inferred angles of repose for cumulates. Tentative correlation of cryptic variation between sections yields similar vergence angles. The scarcity of slump structures and of layer disruption within the main layered sequence verifies that the floor sloped rather gently, surely much less than 45°. Thus, the half‐width of the chamber must have exceeded its vertical dimension, which averages ∼5 km. A half‐width of ∼18 km is calculated from a preferred vergence angle of 15°. The cumulates crystallized within this spreading chamber from a magma body that was repeatedly replenished by primitive magma from a central feeder zone at the spreading axis. High‐level gabbro and diorite through plagiogranite crystallized from stagnant residual magma at the outer edges of the chamber, beyond reach of replenishment from the center.

Methods for estimating Curie temperatures of titanomaghemites from experimental J s-T data

Methods for determining the Curie temperature ( T c ) of titanomaghemites from experimental saturation magnetization-temperature ( J s-T ) data are reviewed. J s-T curves for many submarine basalts and synthetic titanomaghemites are irreversible and determining Curie temperatures from these curves is not a straightforward procedure. Subsequently, differences of sometimes over 100°C in the values of T c may result just from the method of calculation. Two methods for determining T c will be discussed: (1) the graphical method, and (2) the extrapolation method. The graphical method is the most common method employed for determining Curie temperatures of submarine basalts and synthetic titanomaghemites. The extrapolation method based on the quantum mechanical and thermodynamic aspects of the temperature variation of saturation magnetization near T c , although not new to solid state physics, has not been used for estimating Curie temperatures of submarine basalts. The extrapolation method is more objective than the graphical method and uses the actual magnetization data in estimating T c .