Geochemical composition and crystallization temperatures of ODP Hole 176-735B gabbroic rocks

The transition from magmatic crystallization to high-temperature metamorphism in deep magma chambers (or lenses) beneath spreading ridges has not been fully described. High-temperature microscopic veins found in olivine gabbros, recovered from Ocean Drilling Program Hole 735B on the Southwest Indian Ridge during Leg 176, yield information on the magmatic-hydrothermal transition beneath spreading ridges. The microscopic veins are composed of high-temperature minerals, (i.e., clinopyroxene, orthopyroxene, brown amphibole, and plagioclase). An important feature of these veins is the 'along-vein variation' in mineralogy, which is correlated with the magmatic minerals that they penetrate. Within grains of magmatic plagioclase, the veins are composed of less calcic plagioclase. In grains of olivine, the veins are composed of orthopyroxene + brown amphibole + plagioclase. In clinopyroxene grains, the veins consist of plagioclase + brown amphibole and are accompanied by an intergrowth of brown amphibole + orthopyroxene. The mode of occurrence of the veins cannot be explained if these veins were crystallized from silicate melts. Consequently, these veins and nearby intergrowths were most likely formed by the reaction of magmatic minerals with fluid phases under the conditions of low fluid/rock ratios. Very similar intergrowths of brown amphibole + orthopyroxene are observed in clinopyroxene grains with 'interfingering' textures. It is believed, in general, that the penetration of seawater does not predate the ductile deformation within Layer 3 gabbros of the slow-spreading ridges. If this is the case, the fluid responsible for the veins did not originate from seawater because the formation of the veins and the interfingering textures preceded ductile deformation and, perhaps, complete solidification of the gabbroic crystal mush. It has been proposed, based on fluid inclusion data, that the exsolution of fluid from the latest-stage magma took place at temperatures >700°C in the slow-spreading Mid-Atlantic Ridge at the Kane Fracture Zone (MARK) area. No obvious mineralogical evidence, however, has been found for these magmatic fluids. The calculated temperatures for the veins and nearby intergrowths found in Hole 735B gabbros are up to 1000°C, and these veins are the most plausible candidate for the mineralogical expression of the migrating magmatic fluids.

Silicate mineralogy of basalts at DSDP Leg 54 Holes

Basalts drilled from the East Pacific Rise, OCP Ridge, and Siqueiros fracture zone during Leg 54 are texturally diverse. Dolerites are equigranular at Sites 422 and 428 and porphyritic, with phenocrysts of plagioclase (An69.73) and Ca-rich clinopyroxene (Ca42Mg48Fe10) at Site 427. The East Pacific Rise lavas and some of those from the OCP Ridge are fine-grained and porphyritic. The majority of the large crystals are clustered skeletal glomerocrysts of plagioclase An64-77), together with olivine (Fo80-87), Ca-rich clinopyroxene, or both. Euhedral phenocrysts of plagioclase, together with olivine, Carich clinopyroxene, and Cr-Al spinel in some cases, occur in most of the fine-grained lavas. These phenocrysts are small (maximum dimension <1 mm in all but one sample), sparse (combined modal amount <1% in all samples), and distinctive from the megacrysts which characterize many ocean-floor lavas. In two East Pacific Rise lavas, zoned plagioclase (An83 cores) is the sole phenocryst phase. In other porphyritic lavas from all the main East Pacific Rise and OCP Ridge units drilled during Leg 54, the plagioclase phenocrysts contain cores of bytownite (An79-87) surrounded by more-sodic feldspar (An67-77). Core/rim relationships vary from continuous normal zoning, through discontinuous zoning, to extensive resorption of the calcic cores in some samples. The compositions of the plagioclase calcic cores are systematically related to those of the glomerophyric plagioclase and olivine in the lavas containing them. Furthermore, only one compositional population of calcic cores occurs in each rock. The possible causes of these relationships are far from clear. Magma mixing, although superficially applicable, is inconsistent with important aspects of the phenocryst mineralogy of these particular lavas. A more satisfactory model to explain both phenocryst zoning and rapid glomerocryst growth immediately before extrusion may be constructed by postulating influx of water into the upwelling magmas within Layer 3 of the oceanic crust beneath the East Pacific Rise, and subsequent loss of part of this water during effervescence within feeder dykes between Layer 3 and the ocean floor. It is shown that this model is fully consistent with published data on water and carbon dioxide contents and ratios in the pillow-margin glasses, vesicles, and phenocryst inclusions of ocean-floor basalts. The evidence for the precipitation of plagioclase- dominated crystalline assemblages from these magmas in the upper part of Layer 3 is concordant with recent geophysically based modeling of the structure of the East Pacific Rise. Calcium-rich clinopyroxenes in dolerites from the OCP Ridge and Siqueiros fracture zone show radial, oscillatory, and sector-zoning. In Sample 428A-5-2 (Piece 5a), the compositional trends resulting from this zoning closely resemble those of the pyroxenes in some lunar lavas. The controls on crystallization of interstitial pigeonite - epitaxial upon augite - in this rock are discussed. Both sector-zoning of the augite and nucleation of pigeonite within microvolumes of magma with a low Ca(Mg + Fe) ratio appear to be important factors.

Geochemical composition and crystallization conditions of melts from the Mid Atlantic Ridge near Ascension Island (7-11°S)

Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7°11°S).The results indicate that midocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100-900MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100-300MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200-900MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 0.4-1wt% H2O in the parental melts.These estimates are in agreement with direct (Fourier transform IR) measurements of H2O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 0.04-0.09 and 0.30-0.55 wt% H2O for depleted and enriched MORBs, respectively. Our results are in general agreement (within ±200MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation