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doi:10.2204/iodp.proc.324.109.2010

Volcanology and igneous petrology of Site 1213

Hole 1213B is located ~200 km southwest of Site U1347 on the eastern flank of Tamu Massif and 400 km south of Site U1348 on the northeastern high (Fig. F1). Volcanic rocks recovered at Site 1213 were only sparsely sampled and studied (e.g., Mahoney et al., 2005), yet this site obviously complements the drilling objectives of Expedition 324.

In this report we briefly describe the major lithologies of the basement sequence and their petrography. On board, the core sections were digitally imaged and measurements of point-source susceptibility and optical reflectance were acquired at 1 cm intervals. New lithologic units were assigned, and the basement rocks were described macroscopically for volcanological structures, igneous petrology, and alteration. Microscopic descriptions and digital photomicrographs were made from nine thin sections from Leg 198 available to the scientists onboard. All of this new shipboard data was entered into LIMS. To distinguish our new data and observations from those obtained during Leg 198, Expedition 324(198), Site U1213, and Hole U1213B are the sample identifiers used in the databases according to IODP policy. However, in this report the prefix "U" will not be used. Detailed visual core description reports were automatically generated from the shipboard database entries (see "Site 1213 visual core descriptions" in "Core descriptions"). The major lithologic features of each unit are summarized in Figures F2, F3, and F4.

Physical volcanology

During Leg 198, the igneous basement unit (IV) of Hole 1213B was divided on the basis of lithologic patterns into three stratigraphic subunits (IVa–IVc) separated by intercalated sediments and interpreted as "diabase sills" (Shipboard Scientific Party, 2002). In a similar fashion, we divided the Hole 1213B volcanic basement into nine lithologic units (1–9) based on the presence of sediments (including some displaced dropstones fallen into the hole from the sediment sections above) and variations in the basaltic groundmass grain size (Table T1). We have combined these lithologic units into three stratigraphic subunits representing three individual cooling units as thick as 15 m: Subunit IVa is composed of six lithologic units (1–6), whereas Subunits IVb and IVc correspond to lithologic Units 7 and 9, respectively. Lithologic Unit 8 is a fragment of intercalated sediment between Units 7 and 9. All three cooling units are characterized by a finer grained top and base, indicating they are more likely massive submarine basaltic lava flows (see the "Methods" chapter) and not sills.

First massive basalt flow (stratigraphic Subunit IVa; lithologic Units 1–6)

Subunit IVa is a 14.3 m thick massive basalt flow. This basement unit consists of typically nonvesicular, aphyric to moderately phyric, cryptocrystalline to fine-grained basalts that are slightly to moderately altered. The top of the unit is bounded by overlying sedimentary rocks with "baked" chert (indicated by dark discoloration of the chert near contact with the igneous rocks) at interval 324(198)-1213B-28R-1, 17 cm. Although a glassy rim is not identified at the top of the unit, a cryptocrystalline chill zone ~10 cm thick is present. The base of Unit IVa is defined by a narrow cryptocrystalline chill zone that also is ~10 cm thick and by a lithified/baked sedimentary contact at interval 324(198)-1213B-30R-4, 66 cm. Toward the interior of this massive basalt flow grain sizes increase to fine grained. In the cryptocrystalline and microcrystalline portions, a small amount (<6 vol%) of plagioclase and clinopyroxene microphenocrysts are present, commonly forming glomerocrysts, yet these microphenocrysts could not be identified in the coarser grained interior of the basalt flow where the groundmass crystals have comparable sizes. Phenocrysts are typically subhedral and their size ranges from 0.2 to 0.5 mm.

Sedimentary rocks (lithologic Units 3 and 5) occur at two intervals within Unit IVa (intervals 324(198)-1213B-29R-1, 0–13 cm, and 30R-1, 10–17 cm). These rocks are probably loose pieces of drilling rubble because we could not see either chill zone or grain size variation within the igneous rocks surrounding the sedimentary rocks.

Second massive basalt flow (stratigraphic Subunit IVb; lithologic Unit 7)

This second massive flow unit is 14.6 m thick, nonvesicular, and aphyric to glomeroporphyritic and ranges in grain size from cryptocrystalline to fine-grained basalts. These basalts are only slightly to moderately altered. This unit is bounded at the top and base by sedimentary rocks and has both upper and lower chill zones with cryptocrystalline basalts. The lower chill zone consists of fragmented (and partly brecciated) basalts in interval 324(198)-1213B-32R-4, 62–70 cm. Thicknesses of both upper and lower chill zones are ~10 cm, and altered glass rind up to 1 mm wide is also identified in the lower chill zone. Grain size increases to fine grained away from the chill zones and into the flow interior. Subhedral plagioclase and clinopyroxene phenocrysts are present only as glomerocrysts 5–12 mm in size. Sparse but frequent glomerocrysts are scattered throughout the interior of the massive flow. We observed several vertical fractures that are filled with calcite and clay minerals in the interior, implying that this is indeed a large cooling unit.

Third massive basalt flow (stratigraphic Subunit IVc; lithologic Unit 9)

The third massive basalt flow is at least 11.3 m thick and has a similar nonvesicular, aphyric to glomeroporphyritic character to that of the overlying Subunits IVa and IVb. Both plagioclase and clinopyroxene phenocrysts are subhedral and as large as 10 mm. The top of this unit is defined by an overlying sedimentary rock unit (lithologic Unit 8) and is marked by the presence of a chill zone ~20 cm thick. The top of the chill zone consists of fragmented cryptocrystalline basalts in interval 324(198)-1213B-32R-4, 77–85 cm. Immediately adjacent to the fragmented basalts is a chill zone that has a small amount of vesicles (interval 324(198)-1213B-32R-4, 85–94 cm). The vesicles are now filled with amygdules of calcite and clay minerals. The interior of the massive flow is fine-grained basalt with several fractures, and it extends to the end of the recovered core in interval 324(198)-1213B-33R-3, 130 cm.

Petrography

During Leg 198, nine polished thin sections were prepared from basaltic rocks of Hole 1213B. Seven thin sections came from the interiors of cooling units and two from margins. From the outset of our reappraisal, we addressed the question of whether the cooling units were extrusive (pillows or flows) or, as described in the Leg 198 Scientific Results, intrusive (sills). Our lithologic descriptions suggest the former, and this is confirmed by petrography.

In thin section, the rocks are all nonvesicular plagioclase-clinopyroxene microphyric basalt and show no significant changes in these essential characteristics throughout. Differences in grain size, crystal morphologies, and crystallinity, however, are important depending on distance from the margins in these extrusive units (Fig. F5). These differences depend on the profound differences in cooling rate experienced by individual lava flows (pillow) that erupted under water from their margins, which typically quench to glass, to their interiors, which may have been sufficiently far from the interface with seawater to crystallize to fairly coarse grain size, similar to lava flows on land. Consequently, the procedure of describing pillow lavas from rims to interiors (e.g., Kirkpatrick, 1979; Natland, 1979, 1980) using the terminology for crystal morphologies developed on the basis of controlled cooling experiments (e.g., Lofgren, 1971; Kirkpatrick, 1975) is adopted here. Differences in cooling rate and mineral type, and consequently crystal morphologies, are typically greatest within the outer few centimeters of the extrusive boundaries, changing from spherulitic to fibrous, needle-like, acicular, dendritic and skeletal, and finally tabular with increasing distance from glassy margins, reaching as far away as 30 cm in lava flow and pillow interiors. The rocks of Hole 1213B, however, attain a fairly consistent and relatively coarse grain size in the centers of extrusive units as thick as 12 m, which attests to chemical similarity and perhaps fairly low cooling rates. The rocks can thus be described as a single petrographic type.

Spherulites at the rim

A submarine extrusive origin is confirmed in one instance by the occurrence of coalesced and bow-tie spherulites (pillow Zones 4 and 5 of Kirkpatrick, 1979) at the margin of Thin Section 47 (Sample 324(198)-1213B-31R-1, 2–7 cm), whereas Zones 1–3 (glass, isolated plagioclase spherulites, and partly coalesced spherulites) are missing. The plagioclase fibers in the spherulites become thicker and more distinctive with distance from the rim (Fig. F6), with titanomagnetite crystals also becoming larger. The titanomagnetite is very small and dispersed at the rim (Fig. F6A), partially concentrated at the ends of sheaflike plagioclase spherulites (Fig. F6B), and then scattered between almost randomly oriented acicular and skeletal plagioclase needles at a distance of ~20 mm from the rim (Fig. F6C).

Olivine

Olivine occurs in only one thin section of the pillow rim described above. Several isolated euhedra occur in one portion of the pillow margin and are now completely replaced by dull greenish brown clays.

Plagioclase and pyroxene crystal growth

Within the finer grained rims of the basaltic cooling units, the spherulitic mesostasis contains well-formed crystals (often euhedral) of plagioclase and clinopyroxene microphenocrysts, plus some microglomerophyric intergrowths of the two minerals. Most of the plagioclase is elongate, forming only slightly skeletal laths that do not have dendritic terminations (Fig. F7A, F7B). These crystals were present in the molten liquid prior to eruption and during eruption grew into their present elongate morphologies, yet at these more extreme cooling rates, they did not radiate into sheaf spherulites at their ends. Further into the flow interiors, elongate microphenocrysts like this are not present and plagioclase crystals instead are more equant.

Clinopyroxene ranges from small euhedral crystals (Fig. F8) to somewhat larger grains, some of which are sector zoned (Fig. F8B). Along with plagioclase, these crystals were present before eruption, growing into their present morphologies in the quickly cooled rims, despite extremely high cooling rates and the absence of clinopyroxene dendrites among the spherulitic plagioclase fibers.

Both individual crystals and crystal clumps of clinopyroxene and plagioclase can be traced into the flow interiors. However, the shapes of the crystals become irregular and microphenocrysts virtually disappear against the coarser grained groundmass. This transition can be seen especially well in the standard textural photomicrographs (Fig. F7) and mainly results from the larger crystal sizes in the interior of the cooling units and an increasingly equant tabular form of the groundmass plagioclase. In the flow interiors, plagioclase arrays can be somewhat stellate, with clinopyroxene crystals developed between the arms, but overall these textures represent the development of an interlocking network of plagioclase-clinopyroxene crystal clumps, with dark and now altered intersertal spaces in between.

Coarse crystal clots

A number of rocks have what might be termed "gabbroic" coarse aggregates consisting mainly of interlocking plagioclase crystals (Fig. F9E). Two of these clots occur in Thin Section 51 (Sample 324(198)-1213B-32R-4, 131–134 cm). One clot, consisting of a single large plagioclase crystal, is shown in Figure F9C and F9D, and another (Fig. F9E) is the more exemplary multicrystal intergrowth, which occurs only a few millimeters away. Both have external overgrowths, with skeletal rims of normally zoned plagioclase, that grew from melt in the adjacent finer grained matrix as it crystallized after eruption. Both also have numerous melt inclusions, now altered. Most of the inclusions are very tiny and round, but others are irregular in shape, with outlines suggesting skeletal growth of the crystals. The crystal interiors enclose both altered glass and titanomagnetite (Fig. F9F).

Titanomagnetite

Titanomagnetite is the only magmatic oxide mineral. Its size, from finest to coarsest, and from extrusive margin to flow interior, probably spans three orders of magnitude. In finely spherulitic material, titanomagnetite appears as tiny specks of almost unresolvable morphology when seen in transmitted light (Fig. F7A, F7B), with only the largest of these crystals verging on skeletal in outline. In more coarsely crystalline interiors of the extrusive units, titanomagnetite invariably occurs in intersertal spaces that once were glassy but are now altered. In such locations, it is always distinctly skeletal in morphology, ranging from small hopper-shaped crystals with euhedral outlines (Fig. F10A) to crystals of more irregular shape, including many that are intergrown with plagioclase (Fig. F10B, F10C). Skeletal morphologies persist even in the largest crystals of the mineral (Fig. F10D), whereby titanomagnetite typically began growing together with plagioclase but finished its growth into the differentiating final melts in between crystals. Very long chains like this are rare and only occur in the larger intersertal spaces; a more space-limited elongate skeletal grain that happens to be rooted in clinopyroxene is shown in Figure F10E.

It thus appears that cooling rates in the interior of the extrusive massive lava flows were not very high, allowing large titanomagnetite crystals to grow with coarse skeletal morphologies. At all cooling rates, titanomagnetite joins the crystallization sequence after clinopyroxene and plagioclase coprecipitated, as it occurs either between crystals of plagioclase and clinopyroxene or intergrown with plagioclase. Intergrowths with plagioclase indicate saturation of late-stage differentiating intersertal liquids with titanomagnetite.

Intersertal crystallization

Intersertal spaces between coarse-grained interlocking centers of intergrown plagioclase, clinopyroxene, and titanomagnetite are entirely replaced by clay minerals, which are generally dull orange-green in transmitted light. If this material was once glassy, the volcanic glass is now altered. If this material once contained finely crystalline crystals of silicate and oxide minerals, or sulfide globules, those are now replaced, too. Thin section images in reflected light reveal that a high percentage of the intersertal spaces have been altered to clays, as such materials are simply nonreflective and the polished primary mineral grains stand out against them (Fig. F10E).

Alteration

Basaltic rocks recovered from Hole 1213B have been affected by slight to moderate (5%–25%) low-temperature water-rock interactions resulting in a complete replacement of glassy mesostasis occupying the intersertal spaces (see above) and a slight to moderate replacement of plagioclase and clinopyroxene, present in the groundmass or as phenocrysts. Clay minerals (identified as brown and green clays) are the most abundant secondary phases in Hole 1213B, predominantly replacing the glassy mesostasis (Fig. F11A, F11B), slightly altering plagioclase and pyroxene micro- and phenocrysts, and filling veins. Calcite is a minor secondary phase observed only locally as replacement of the glassy mesostasis (Fig. F11C) and in association with green clays in veins. An example of plagioclase phenocryst alteration is shown in Figure F11D, where the rim of the previous igneous plagioclase has been replaced by a feldspar, possibly richer in Na and/or K in association with sericite. However, this kind of K feldspar after plagioclase alteration is not common in this core. Titanomagnetite, present in the groundmass, has also been slightly to moderately altered. Secondary pyrite is widespread throughout the basalt and present in the groundmass and in veins. No fresh glass was observed on flow margins. Three main types of veins occur in Hole 1213B:

1. Calcite veins (predominant),

2. Green clay veins, and

3. Composite veins of calcite + green clays ± pyrite.

There is an average of ~3 veins/m in the basement lavas, and average vein thickness is ~1 mm. Calcite veins show two main morphologies, blocky and fibrous, which can both be present in a single vein (Fig. F11E). Alteration of basaltic rocks at Site 1213 is interpreted to result from interaction with seawater-derived fluids at relatively low temperature and is similar to what has been observed and described in Hole U1347A (north of Tamu Massif; see the "Site U1347" chapter).

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