https://www2.le.ac.uk/departments/geology/people/jenkin-grt/personal/publication-abstracts/Coogan%20et%20al%202002.pdf

Constraining the cooling rate of the lower oceanic crust: a new approach applied to the Oman ophiolite

Drilling at Site 332 recovered a wide variety of relatively fresh, chemically primitive, low-titanium, olivine or olivine-plagioclase phyric tholeiitic basalts. Apparent crystallization sequences are either olivine + chromite, plagioclase, clinopyroxene, titanomagnetite or olivine + plagioclase (± chromite), clinopyroxene, titanomagnetite. Cr in pyroxenes falls off rapidly with crystallization, followed by a decrease in Al and an increase in Ti, reflecting chemical changes in the host melts with advancing crystallization. Coarse-grained rocks recovered at Site 334 are serpentinized lherzolitic peridotites and two pyroxene gabbros ± olivine. Textures and primary pyroxene compositions indicate crystallization from a low-alkali, olivine tholeiite magma. Complex exsolution features and a high degree of equilibration within pyroxenes indicates a long cooling and annealing time within the oceanic crust, probably within layer 3.

http://www.deepseadrilling.org/37/Volume/dsdp37_55.pdf

55. petrology of basalts, gabbros, and peridotites from dsdp leg 37

Pyroxene analyses of superior quality, from DSDP Legs 11, 15, 17, 34 and 37 have been studied statistically using cluster and factor analysis. These techniques are used to identify the various basalt groups using pyroxene chemistry, and to gain information about pyroxene crystal chemistry. The tholeiitic and alkalic pyroxene groups differ primarily in Cr2O3, TiO2, and Na2O contents. Relative to an average deep sea pyroxene, the tholeiitic pyroxenes are enriched in Cr2O3 and depleted in TiO2 and Na2O, while the alkalic pyroxenes are depleted in Cr2O3and enriched in TiO2 and Na2O. The primary substitutional couples for the two groups are: VITi-IVAl2 and VIFe3+-IVAl.

Clinopyroxenes from deep sea basalts: A statistical analysis

Fractionation and mantle heterogeneity in basalts from the Peru-Chile Trench

Petrology of basaltic rocks from the São Paulo ridge, southwestern Atlantic Ocean

Three sites drilled on the Nazca plate on DSDP Leg 34 recovered fresh basalts quite unlike those recovered from previous legs. The textures of these rocks range from vitrophyric to ophitic, with variolitic intergrowths and altered glass present even in the groundmass of the coarsest unit sampled, and are suggestive of fairly rapid cooling histories. Site 321 basalts are all uniformly fine grained, vesicular, and intersertal. Many samples from all sites are Plagioclase phyric, and Hole 319A basalts appear to be largely Plagioclase and olivine phyric. Fresh relicts of olivine phenocrysts were recognized only in Hole 319A basalts, and olivine crystallization may have persisted into the groundmass in some Hole 319A basalts. The feldspars are K-poor (mainly <1% Or) plagioclases in all Leg 34 samples, and feldspar apparently crystallized in all samples during two or three major stages. The pyroxenes are aluminous augites and pigeonites which crystallized in one or two major stages. The rather strong chemical zonation in both these phases is also indicative of fairly rapid cooling. Site 321 pyroxenes are distinctive with respect to changes in the amounts of components other than Wo-En-Fs with crystallization. Phase chemical and textural data allow the delineation of at least two major cooling units within the depth range of Hole 319A samples, two cooling units represented by Hole 320B samples (out of many reported), and at least two cooling units within the depth range of Site 321 samples, even in the absence of textural evidence for such delineations. INTRODUCTION DSDP Leg 34 sampled parts of the Nazca plate off the western coast of South America (Figure la). Three sites were drilled in an attempt to core deeply into basement rock, but technical problems prevented the desired extensive penetration. The basalts which were recovered from the Nazca plate, however, are among the freshest samples to date from the DSDP expeditions, and their relative freshness facilitates comprehensive petrographic and phase chemical studies. Stratigraphic sections for each of the three sites sampled are presented in Figure lb and show the locations of the samples available for this study. The amount of basalt recovered from each basalt section is indicated by the dark patterns. Hole 319A is located on the western flank of the extinct Galapagos Rise on crustal rocks dated by paleontological study of overlying sediments at about 15 m.y. (Hart et al., 1974). The first basalt was encountered at approximately 98 meters below the water-sediment interface and was continously cored for 59 meters before the hole was abandoned (Figure lb); however, only 25% of the basalt was recovered. Shipboard measurements of densities, seismic velocities, and water contents all suggest rather fresh basalt throughout the cored section (Chapter 3, this volume). On the basis of textural changes, the shipboard scientists divided the basalt section (319A) into six definite cooling units and at least five additional thinner units. The major cooling units are 15.6 and 12.5 meters thick (Figure lb). Rapid cooling of the margins of flow units is suggested by the finegrained variolitic nature of some samples and the abundance of glass fragments recovered from the core catchers; however, no contact zones were actually recovered (Chapter 3, this volume). Preliminary observations by the shipboard scientists suggest that the basalts at this site represent a suite of rapidly cooled, pillowed to massive flows, erupted within a short period of time. Hole 320B cored a series of about nine pillows and thin flows beneath sediment dated at approximately 30 m.y. old. Although the basalts are more altered than those in Hole 319A, they nevertheless are less altered than expected for crustal rocks of this age. Basalt was encountered at about 154 meters below mudline and was cored for about 28 meters with poor recovery (Figure lb). Site 321 penetrated approximately 10 meters of basalt, 5 meters of which was recovered. The basalt section covers at least two cooling units (M. Bass, personal communication, 1974). The oldest overlying sediment is dated at 40 m.y. (Hart et al., 1974). The basalt, which was encountered 125 meters below mudline, was not quite as fresh as that from Hole 319A. The unusual freshness of both Hole 319A and Site 321 basalts (relative to other deep-sea basalts) is one line of evidence which suggests that these rocks may be

/pdf/petrography-and-phase-chemistry-of-basalts-from-dsdp-leg-34-qqz1e084n1.pdf

L.J. Mazzullo

Papers

Petrography and Phase Chemistry of Basalts from DSDP Leg 34: Nazca Plate