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doi:10.2204/iodp.proc.324.105.2010

Alteration and metamorphic petrology

Volcaniclastic rocks recovered from Hole U1348A (Cores 324-U1348A-14R through 26R) have been extensively altered, resulting in a near-complete replacement of primary glass in the vitric clasts and almost complete replacement of primary phases present in the rare lithic fragments. Fresh glass was observed only in one 6 cm thick layer in interval 324-U1348A-23R-1, 116–122 cm. Clay minerals, together with calcite and zeolites, are the predominant secondary minerals in Hole U1348A cores, replacing glass and primary minerals, filling voids, and cementing volcanic clasts.

Alteration mineralogy

Alteration degree and mineralogy of Hole U1348A cores is based on thin section description and X-ray diffraction (XRD) analysis on bulk rock powder (Table T5). The fine-grained nature of the volcanic clasts causes difficulties in identifying the alteration mineralogy in hand specimen.

Secondary mineralogy

Vitric and lithic clasts forming the volcaniclastic rocks in Hole U1348A are all extensively altered, except in one interval (324-U1348A-23R-1, 116–122 cm; see "Igneous petrology" for further description). Rims of the vitric clasts are mainly altered to green-brown palagonite (Fig. F38), preserving the outline of the clasts. Based on XRD spectra, minerals forming the palagonite are mainly nontronite and montmorillonite (Fig. F39). The inner part of the vitric clasts shows two different textures: (1) alteration of glass to palagonite and clays and (2) complete dissolution of glass and replacement by calcite and/or zeolites (Fig. F40), with zeolites showing a fibrous and/or blocky morphology. Lithic clasts also show a high degree of alteration with almost complete replacement of glassy mesostasis to palagonite and brown clays (e.g., nontronite and montmorillonite, based on XRD spectra), whereas plagioclase microliths remain relatively unaltered (Fig. F41). Lithic fragments in Thin Section 158 (Sample 324-U1348A-14R-3, 54–56 cm) show a similar texture and alteration degree to basaltic rocks recovered from the top of Hole U1347A (Fig. F41).

The vitric and/or lithic clasts are cemented by calcite and/or zeolites (Figs. F40, F42) with variations in their proportions and occurrences downhole (Fig. F43). Zeolites form a corona alteration around the palagonite rims of the vitric and lithic clasts (Fig. F44). Two types of zeolites were observed in the corona alteration: (1) a fibrous zeolite and (2) a tabular blueish zeolite identified as phillipsite. This is consistent with XRD spectra, which indicate the presence of phillipsite in bulk rock samples (Fig. F39). Calcite cement occurred at a late stage, filling the empty space between the highly altered clasts.

Variations in mineralogy downhole

Based on identification of secondary minerals on XRD spectra and observations of thin sections, alteration mineral assemblages appear to vary downhole (Fig. F43; Table T5). Phillipsite and calcite are the most abundant secondary minerals and are associated with montmorillonite, nontronite, and sepiolite, with variations in their occurrences with depth in Hole U1348A. Celadonite and palygorskite were observed in the sedimentary cores in the upper part of the hole (stratigraphic Unit II; Section 324-U1348A-12R-CC at 180.77 mbsf and interval 324-U1348A-12R-1, 17–18 cm, at 180.5 mbsf), and further details and interpretations are given in "Sedimentology" Palygorskite has also been identified in the volcaniclastic rocks at the bottom of Hole U1348A, from Section 324-U1348A-20R-2 (258.8 mbsf) to 22R-4 (Fig. F43).

Interpretations

The volcaniclastic rocks recovered from Hole U1348A are all highly altered to various smectites and zeolites, which are the most common minerals to form during alteration and palagonitization of volcanic glass. The smectites identified to form the palagonite in Hole U1348A are predominantly montmorillonite (Ca smectite) and nontronite (Ca and Fe smectite). Occurrences of these two minerals indicate palagonitization in open marine systems (see Fisher and Schmincke, 1984, for a review). Phillipsite is present in the entire volcaniclastic section in Hole U1348A and is usually one the most common and first zeolite to form during palagonitization.

Palygorskite has been identified in the sediments at the top of the hole as well as in some sections at the bottom (e.g., interval 324-U1348A-20R-2, 36–39 cm; Figs. F39, F43). Palygorskite is often observed in deep marine sediments and commonly forms at the interface between sediments and seawater, where Al and Si may be derived from volcanic ashes and Mg is released during basalt weathering. Palygorskite has also been observed in fractures of basaltic rocks from the Mariana forearc region (Natland and Mahoney, 1982) in association with Fe hydroxides. This secondary mineral assemblage suggests relative oxidizing conditions and moderately elevated temperatures. Occurrences of palygorskite at the bottom of Hole U1348A may similarly suggest interactions of the volcaniclastic rocks with relatively warm hydrothermal fluids. Further shore-based studies will be conducted to better constrain the alteration of these volcaniclastic rocks and the formation of palygorskite.

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