Primary silicate mineral chemistry of a 1.5-km section of very slow spreading lower ocean crust: ODP Hole 735B, Southwest Indian Ridge

Abstract

We present a synthesis of some 20,504 mineral analyses of ~500 Hole 735B gabbros, including 10,236 new analyses conducted for this paper. These are used to construct a mineral stratigraphy for 1.5-km-deep Hole 735B, the only long section of the lower crust drilled in situ in the oceans. At long wavelengths, generally >200 m, there is a good chemical correlation among the principal silicate phases, consistent with the in situ crystallization of three or four distinct olivine gabbro bodies, representing at least two major cycles of intrusion. Initial cooling and crystallization of these bodies must have been fairly rapid to form a crystal mush, followed by subsequent compaction and migration of late iron-titanium-rich liquids into shear zones and fractures through which they were emplaced to higher levels in the lower crust where they crystallized and reacted with the olivine gabbro host rock to form a wide variety of ferrogabbros. At the wave lengths of the individual intrusions, as represented by the several olivine gabbro sequences, there is a general upward trend of iron and sodium enrichment but a poor correlation between the compositions of the major silicate phases. This, together with a wide range in minor incompatible and compatible element concentrations in olivine and pyroxene at a given Mg#, is con-., 2002.Primary silicate mineral chemistry of a 1.5-km section of very slow spreading lower ocean crust: ODP Hole 735B, Southwest Indian Ridge. In Natland, H.J.B. DICK ET AL. PRIMARY SILICATE MINERAL CHEMISTRY 2 sistent with widespread permeable flow of late melt through these intrusions, in contrast to what has been documented for a 600-m section of reputedly fast-spreading ocean crust in the Oman Ophiolite. This unexpected finding could be related to enhanced compaction and deformation-controlled late-stage melt migration at the scale of intrusion at a slow-spreading ocean ridge, compared to the relatively static environment in the lower crust at fast-spreading ridges. H.J.B. DICK ET AL. PRIMARY SILICATE MINERAL CHEMISTRY 3basis of clear textural and mineralogic differences. This breakdown did not generally include intervals of less than ~4 cm, and further subdivision of the core could be made. With the analytical data set estimated to represent ~500 discrete samples, it remains inadequate to describe the core in anywhere near its entirety. ANALYTICAL METHODSThe data sets reported in Tables T1, T2, T3, T4, and T5, other than those collected by the first author and his colleagues at Massachusetts Institute of Technology (MIT), were collected by investigators using different analytical schemes, techniques, and standards. They describe these in papers in the Leg 118 and Leg 176 volumes Hebert et al., 1991;Natland et al., 1991; Niu et al., Chap. 8, this volume;Ozawa et al., 1991; Robinson et al., Chap. 9, this volume). The new data collected at the MIT Electron Microprobe Facility used a JEOL JXA-733 Superprobe. The operating conditions included a 15-keV accelerating voltage, 10-nA probe current, 10-µm spot si...

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“…The oxide-rich section of Hole 1105A is both thicker and more enriched in oxides than the principal oxiderich portions of Hole 735B (Units III and IV; 170.2-274.1 mbsf). Deformation grade and alteration patterns are also different, and a downhole differentiation trend similar to that observed for Hole 735B oxide gabbro is questionable for Hole 1105A whole-rock compositions but may be more evident when considering mineral compositions Dick et al, 2002;Pettigrew, Casey, Miller, et al, 1999;Casey et al, 2007). If the Hole 1105A section is equivalent to any portion of Hole 735B, the most likely candidate would be Units III and IV (170.2-274.1 mbsf; Casey et al, 2007).…”

The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U–Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

“…The bulk composition of Hole 735B, and samples from the surface of Atlantis Bank, have lower Mg# than primary mantle melts, suggesting that complementary primitive cumulates are preserved out of the sampled section, either along axis, deeper in the crust, or in the upper mantle (Coogan et al, 2001a;Dick et al, 2000). Based on the Dick et al (2002). The blue dashed line shows a proposed correlation between Holes 735B and 1105A based on oxide gabbro geochemistry .…”

Plagiogranitic Rocks of Evros Ophiolite, Ne Greece

“…However, plagiogranitic rocks are studied in many cases to show extreme differentiation procedures, which involve highly acidic magmas to have been generated from mafic precursors and to date with great accuracy the formation of the whole ophiolite (e.g. Coleman and Donato 1979, Bébien 1991, Floyd et al 1998, Hatzipanagiotou and Tsikouras 1999, Dick et al 2002, Koepke et al 2007.…”