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doi:10.2204/iodp.proc.345.111.2014

Metamorphic petrology

Background alteration

Hole U1415K

Cores recovered in Hole U1415K contain several lithologies, including sand and fine gravel, as well as gabbroic, doleritic, and basaltic rubble (see METLOGS in “Supplementary material”). The sand and fine gravel are too fine grained to assess alteration but appear to have undergone extensive seafloor weathering based on the light brown sediment color. Ghost Core 345-U1415K-2G includes diverse lithologies with variable alteration: Interval G5 is highly altered diorite, Interval G6 is highly altered disseminated oxide gabbronorite, Interval G7 is moderately altered orthopyroxene-bearing olivine gabbro, and Interval G8 is slightly altered aphyric basalt. In the gabbro, olivine and clinopyroxene are completely replaced, although plagioclase remains only slightly altered. Olivine is replaced by serpentine, magnetite, tremolite, and pyrite. Clinopyroxene is highly to completely altered to green amphibole. In diorite and basalt, matrix plagioclase is only slightly altered. Many of the rocks from this core contain abundant disseminated oxide and pyrite. Relict orthopyroxene is not observed in this core, although chlorite-dominated pseudomorphic aggregates may be alteration products after orthopyroxene. Both clinopyroxene and plagioclase from Interval G7 are more altered than more primitive gabbro or troctolite from other Hess Deep holes.

Clay veins are observed in a few pieces of rubble from Hole U1415K. In Sample 345-U1415K-2G-5, 18–25 cm (Piece 3), a network of dark green veins with a very soft filling of clay minerals is present (see VEIN_HALO.XLSX in METLOGS in “Supplementary material”). The veins lack halos and show a sharp contact with the host rock. Vein shape is irregular, with variable thicknesses of <1–2 mm.

Hole U1415M

Cores recovered in Hole U1415M include fine sediment as well as fine to coarse gravel. The sediment is too fine grained to assess alteration. The gravel appears slightly altered to low-temperature alteration products (i.e., clay minerals). Interval G5 is coarse gravel comprised of aphyric basalt. Alteration in the basalt from this interval is moderate with moderate serpentinization of olivine and abundant sulfide mineralization disseminated within the groundmass and also forming fine sulfide veins.

Hole U1415N

Cores recovered in Hole U1415N are primarily olivine-phyric basalt and olivine dolerite with very fine grained matrixes, with one piece of corona-textured olivine gabbro at the base of the hole (Interval 16). The degree of alteration of the basalt and dolerite is difficult to assess in hand sample but appears to range from 10% to 30%, with some intervals (i.e., Interval 8) more extensively altered as much as 90% (see METLOGS in “Supplementary material”). The fine-grained groundmass includes plagioclase and clinopyroxene that appear fresh to slightly altered. Abundant reddish clay occurs in the groundmass as an alteration product. Clinopyroxene phenocrysts are commonly <10% altered to amphibole. Alteration of olivine phenocrysts ranges from 60% to 100%, with replacement by serpentine in mesh texture, clay minerals, chlorite, and amphibole in addition to pyrite in most of the intervals in Hole U1415N. The olivine phenocrysts are noticeably more altered along the margins of pieces and within vein halos, and in these areas the serpentine mesh is commonly replaced by serpentine + talc ± chlorite (Fig. F8).

Background alteration in gabbro is moderate (10%–30% replacement of primary phases). In olivine-bearing gabbro (Interval 16), olivine is primarily replaced by talc and chlorite (Sample 345-U1415N-4R-1, 48.5–57 cm [Piece 12]). Chlorite rims surround relict olivine where in contact with plagioclase, forming olivine corona textures. Clinopyroxene is 50%–90% altered to green amphibole, secondary clinopyroxene, chlorite, clay minerals, oxides, and sulfides. Orthopyroxene was identified in Thin Section 93 (Sample 345-U1415N-4R-1, 52–54 cm) and is 70% altered to talc and amphibole. Plagioclase is variably altered from <10% to 50%. Plagioclase is primarily altered to chlorite and clay minerals with lesser amounts of secondary plagioclase, zeolite, and oxide. Chlorite rims are weakly developed around olivine and orthopyroxene. Pyrite is abundant as a disseminated phase in the groundmass and is present in the alteration products after olivine.

The major type of vein is composed of clay minerals that replace former vein-filling chlorite or prehnite (see VEIN_HALO.XLSX in METLOGS in “Supplementary material”). An amphibole vein observed in Interval 2 (Sample 345-U1415N-1R-1, 11–12 cm [Piece 3]) is partially altered to chlorite. Chlorite and prehnite in veins are commonly replaced by clay minerals. Prehnite in veins often shows radiating texture, whereas chlorite veins typically have cross-fiber texture. These original textures are preserved in spite of partial replacement by clays (Fig. F9).

Metamorphic conditions and degrees of alteration

The dominant alteration of the rock recovered in Holes U1415K, U1415M, and U1415N ranged from greenschist facies to subgreenschist facies conditions. Alteration in the basaltic and doleritic rocks is more intense along the edges of these pieces, suggesting that hydration occurred primarily along fractures. These samples lack compelling evidence for higher temperature alteration. Alteration in the gabbro at the base of Hole U1415N shows corona textures between olivine and plagioclase comparable to many samples in Hole U1415J, indicating partial hydration in the lower amphibolite facies.

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