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doi:10.2204/iodp.proc.324.107.2010

Igneous petrology

Stratigraphic unit description and volcanology

In this site report the stratigraphic units are given Roman numbering (stratigraphic Units I–IV), whereas the small-scale lithologic units identified during core description are given Arabic numbering (lithologic Units 1–183). We have identified these lithologic units on the basis of sedimentological changes or, in the case of volcanological units, criteria including the presence of chilled margins or contacts, identifiable flow tops, vesicle distribution, and the occurrence of intercalated volcaniclastic or sedimentary horizons. When considered together, these criteria define the various sizes of individual volcanic inflation units. Accordingly, in the volcanic context, the term "stratigraphic unit" is used to combine smaller consecutive inflation units of similar character into lava packages or single eruptive sequences (see "Igneous petrology" in the "Methods" chapter).

Drilling at Site U1350 penetrated ~143 m of sediment (Unit I) and ~173 m of volcanic basement (Units II–IV). Volcanic basement was drilled beginning with Core 324-U1349A-6R at 143.1 mbsf (Figs. F12, F13). Recovery was relatively high in the volcanic basement (15%–96%), but the contact between the overlying sediment and basement was not recovered. In order of increasing depth, the volcanic basement consists of a series of massive basalt flows intervening with a subordinate proportion of smaller inflation units that passes down first into a transitional zone of roughly equal proportions of massive flows and smaller inflation units, and then into a package dominated by aphyric to sparsely plagioclase phyric pillow lavas (Unit II). Below this lies a layer of hyaloclastite and brecciated basalt (Unit III) and then a short succession of well-preserved plagioclase-phyric pillow lavas set in a matrix of micritic limestone (Unit IV). Coring in the hole and for Expedition 324 terminated at ~315.8 mbsf (Core 324-U1349A-26R). Because of technical difficulties, it was not possible to obtain information using downhole logging tools. Therefore, interpretation of the cored succession is based upon observational and analytical techniques. Unlike other holes drilled during Expedition 324, it was not possible to corroborate stratigraphical interpretation with downhole data.

The basalts of Unit II (Fig. F12) consist of aphyric basaltic lavas ranging in thickness from a few large 3–6 m units with homogeneous nonvesicular cores to smaller 1–2 m pillowlike units readily identifiable from their glassy rinds, thick chilled upper and lower margins, and rare instances of flow-to-flow contacts. Overall, Unit II is a progression from predominantly thick flow units with repeated occurrences of intercalated sedimentary horizons in the top of this succession to a densely packed pillow lava stack at the base of this unit lacking any sedimentary horizons. In the absence of other information, including downhole logging data, this volcanic succession is arbitrarily divided into three subunits based on the type and relative importance of different flow units and the associated frequency of sedimentary intercalations. The topmost 77 m (Subunit IIa) consists predominantly of larger, internally homogeneous inflation units, the middle ~21 m is of both massive and pillowed inflation units (Subunit IIb), and below this is a ~50 m stack of 0.2–0.9 m thick, aphyric to sparsely plagioclase-phyric pillow lava units (Subunit IIc). These pillow units lie above ~6 m of hyaloclastite (Unit III) with a black, fine-grained matrix containing gravel- to sand-sized glassy material, hyalobasalt fragments, and/or small pods (~5–15 cm) of aphyric basalt (Sections 324-U1350A-24R-3 through 25R-1). The basalt in the small pods of Unit III is very similar to that of Unit II above, but occurs in a hyaloclastite facies interpreted as having formed by cooling-contraction granulation, mass wasting, and/or autobrecciation. The contact between the base of Unit III and the volcanic succession beneath was not recovered. The lowermost ~19 m is composed of a thick succession (Unit IV) in two high-recovery (>90%) cores (Cores 324-U1350A-25R and 26R) that preserve, in great detail, a stack of ~0.1–0.5 m thick plagioclase-phyric pillow lavas. This pillow stack includes many good examples of thick chilled margins, internal vesiculation patterns, pillow/pillow contacts, and pillow/sediment contacts with intervening zones of thermal alteration. The sediment consists of a micritic limestone that in many instances encase pillow units, which, together with a lack of bedding structure, indicates that these pillow lavas were extruded onto, or into, an unconsolidated or fluidized micritic sediment substrate.

Upper massive basalt flows and lower pillow lavas (stratigraphic Unit II, lithologic Units 2–90)

The upper part of the volcanic basement (Subunit IIa) consists of a ~77 m thick sequence of massive aphyric basalt inflation units (Cores 324-U1350A-6R through 16R) (Fig. F14). Recovery through this sequence was 15%–66%, with recovered sections of these massive units ranging between ~1.5 and 4 m thick. These massive basalt flows have glassy subhorizontal margins, horizons of vesicle accumulation in the upper parts of the flow, pipe vesicles, associated vertical and horizontal segregation features within the homogeneous flow cores, and little evidence of internal jointing or fracturing. These features are best preserved in the 3.0 and 5.4 m thick flows of lithologic Units 10 and 11 (recovered in Sections 324-U1350A-9R-1 through 10R-2). In addition, this upper part of the volcanic basement is sparsely intercalated with thin limestones containing recrystallized microfossils, comminuted shell debris, and altered volcaniclastic (glassy) particles. The lowermost of the major lavas in Subunit IIa is a 2.5 m thick lava flow (lithologic Unit 27), below which the first readily identifiable pillow lava occurs (lithologic Unit 29) in Section 324-U1350A-17R-1. This pillow lava comprises the top of the transition succession of Subunit IIb.

Subunit IIb is a ~21 m thick zone representing a changeover from thicker basalt units to dominantly pillow-type basalts (Sections 324-U1350A-17R-1 through 18R-3). It contains five larger lava units (~1–2.5 m thick) intercalated with smaller pillowlike inflation units (<0.5–1 m thick) or intervals of fragmentary pillow lava (i.e., hyaloclastite) material. The smaller inflation units display chilled margins, glassy contacts, and typical pillowlike radial vesicle distribution patterns, together with sparse sedimentary intercalations similar in composition and content to those fragments in Subunit IIa. The limited recovery of these materials makes any further interpretation regarding the original thickness or internal structure of these sediment intercalations difficult.

Subunit IIc is a ~50 m succession of 0.2–0.9 m thick aphyric and sparsely phyric pillow lavas (Sections 324-U1350A-19R-1 through 24R-2) with dimensions and internal structures (e.g., subvertical pipe vesicles and segregation pipes) similar to those observed in cores from Holes U1346A and U1347A. The first well-preserved evidence of a more continuous pillow lava stack occurs in Section 324-U1350A-19R-2 (lithologic Unit 41) and contains several folds in glassy crust and small lava pods interspersed with brecciated pillow material. The top of this ~50 m succession contains three ~1–1.5 m thick, more homogeneous lava flow units and minor sedimentary material typically preserved as a lava/sediment contact.

The lowermost ~23 m of Subunit IIc contains no sedimentary material and is a continuous sequence of pillow lavas displaying millimeter-sized, sparsely scattered amygdules, curvilinear glassy rinds with microvesicular bands with tube- and drop-shaped vesicles, and interpillow tension cracks with chilled margins and jointing patterns (Fig. F15). Plagioclase phenocrysts (up to 2%) are sparsely distributed throughout the basaltic material within this pillow stack. This lowermost pillow lava stack correlates well with an interval of low magnetic susceptibility (see "Physical properties"), which may itself be a result of the presence of plagioclase phenocrysts, a finer grain size, and/or an increased degree of alteration.

Hyaloclastite and pillow breccia (stratigraphic Unit III, lithologic Unit 91)

A distinct horizon of hyaloclastite and pillow breccia was recovered in Sections 324-U1350A-24R-3 through 25R-1 (Fig. F16); it is ~6 m thick and consists of a mixture of poorly sorted angular glass shards and spalled pillow material (0.2–5.0 cm), plagioclase crystal fragments, and globular basaltic lava inclusions (or pods) (5–15 cm). No internal bedding or grading can be discerned in this unit. A dull black glassy matrix constitutes the bulk (>50%) of the deposit, and the greenish glass shard fragments themselves consist of partially devitrified material, vitric black clasts, and sparse plagioclase phenocrysts (<2–5 mm in size). The breccia is monomict (i.e., it only contains fragments of quenched basaltic material) and is clast or matrix supported, depending upon the relative proportions of fine to coarse constituents. The isolated globular inclusions of lava are internally highly fractured, moderately vesicular, and also contain sparse plagioclase phenocrysts. The larger pods of basaltic material have rudimentary chilled margins and glassy rims, indicating that they were incorporated or intruded into the deposit while in a near molten state. Like the immediately overlying pillow lava unit, the fine-grained basaltic inclusions and glass fragments are sparsely plagioclase-phyric, sparsely vesicular basalt, suggesting that the two are petrogenetically related. The hyaloclastite succession is likely to represent the onset of the magmatism that initially produced the overlying pillow lava stack of Subunit IIc.

Lower pillow lavas with interpillow sediment (stratigraphic Unit IV, lithologic Units 92–183)

The lowermost ~19 m thick volcanic succession (Unit IV) consists of two consecutive cores (324-U1350A-25R and 26R). These cores preserve a stack of ~0.1–0.5 m thick plagioclase-phyric pillow lavas in great detail, including the sedimentary material between the individual small inflation units. Exceptional recovery of drill core in the lower pillow lava Unit IV (93%–96%) provides a nearly complete sequence of pillow lavas. Accordingly, the lower unit of pillow lavas is remarkable for its preservation of pillow lava structures and glassy chilled margins and their relationships with surrounding sediment and associated alteration effects on the sediment. The details of some of the best preserved features are shown in Figures F17, F18, and F19.

Almost all of the glassy rinds and chilled margins are preserved intact, and pillow/pillow contacts as well as pillow/sediment contacts are abundant throughout these cores. For instance, the cores show examples of otherwise rarely preserved delicate features, such as in situ spalling of pillow rim material, fractured and fragmented jigsaw-fit textures of chilled basalt, and marginal cracking of the pillow. Cracking patterns resulting from cooling and contraction can been seen to have ruptured the crust and penetrated through the inner chill boundaries into the pillows cores themselves, which would have permitted further cooling through the penetration of water along the neoformed joints (McPhie et al., 1993). Many of these preserved features are likely to have formed as a result of in situ cooling, cracking, and contraction and indicate that relatively little transport or compaction occurred after emplacement.

The topmost sedimentary intercalation within this pillow stack is a single, thin (~15–25 cm) horizon, which, in its upper part consists of fine-grained hyaloclastite sand (interval 324-U1350A-25R-1, 126–149 cm) and preserves complex structural detail. The finer grained portion of this hyaloclastite comprises densely packed glass shards that have been compacted and lithified before being deformed; the laminations within it are fractured and intruded by fingers of melt. The coarser beds are composed of more porous glass shard material with voids filled by carbonate cement. The remainder of the sedimentary intercalations in the pillow stack below this horizon is fine-grained micritic limestone containing common radiolarian tests entirely replaced by carbonate and shell debris. The limestone is homogeneous and contains no discernable sedimentary structure, with the exception in some instances of fine laminations of volcanic particles that may indicate post-eruptive sedimentary draping of the lava pillows (interval 324-U1350A-26R-5, 90–130 cm).

Preliminary assessment

The cored volcanic basement at Site U1350 is an entirely marine volcanic succession but exhibits two different styles of seafloor basaltic volcanism. The initial (basal) lava succession (Unit IV) consists of small but complete pillow lavas erupted into a marine environment in which there was an abundant supply of micritic sediment. The second lava succession (Unit III followed by Unit II) begins with the production of hyaloclastite before building into a series of larger pillowlike inflation units. These latter units were either erupted sufficiently rapidly to prevent any significant accumulation of interpillow sediment, or else they were erupted into a sediment-starved environment. The presence of pillow/pillow contacts (Sections 324-U1350A-22R-4 and 22R-5) could indicate merging of adjacent eruptive pods. It may also indicate a multiple-pod inflation mechanism that produces eruptive units that are laterally more extensive and which can exhibit relatively slow cooling characteristics. The slow cooling, for example, may explain their internally homogeneous texture, together with well-developed pipe vesicles and degassed cores observed in many of the inflation units.

The different types of pillow lavas that form Unit IV as compared with those at the base of Unit II require further explanation. The lower pillow stack of Unit IV consists of small, rounded pillow lavas displaying most of the typical structures (e.g., chilled zones, vesicle patterns, and internal structure). The thick, well-formed glassy rinds indicate each pillow was erupted and cooled as an individual entity. However, those pillow lavas that constitute Subunit IIc differ in that they are larger, the glassy contact margins are thinner, and their chilled margins are less well developed. In Subunit IIc, the tube- and drop-shaped vesicles characteristic of the marginal chill zones are often restricted to a narrow, sparsely vesicular zone immediately inward of the contact, and the well-developed concentric vesicle bands indicative of both significant internal degassing and progressive cooling are absent. Moreover, internal cooling cracks and associated radiating vesiculation patterns are also noticeably absent in the larger pillowlike eruptive units. It is reasonable to conclude that these two pillow stacks represent subtly different eruptive styles and, presumably, different magma gas contents.

In general, only a small proportion of pillow units that form in typical pillow lava fields and stacks consists of self-contained and separate erupted entities. Most pillow lavas form through a processes of internal inflation, expansion, and rupture; eventually lava will break out from the chilled crust through a process of "budding" to form a new, magmatically interconnected lobe or pillow. As this process continues it eventually forms networks of interconnected tubes and lobes that together make up the pillow lava field (Moore, 1975; Yamagishi, 1985; Walker, 1992). Walker (1992) suggested that the nature of the lobe expansion (i.e., the production of multiple or discrete entities) is influenced by lava viscosity. Crust rupturing is more common in the more viscous basalt lavas, and hence internal cooling is augmented through penetration of water along fractures; more viscous basalt leads to the development of discrete pods or pillows rather than an interconnected network. Low-viscosity lavas have more elastic, hence largely unbroken, crusts and together with slower cooling rates produce pillows with smoother surfaces and an anastomosing, interconnected magmatic plumbing between the inflation units. Other variables that control the morphology of submarine lava flows include effusion rate, flow velocity, and local or regional slope of the seafloor (e.g., Batiza and White, 2000; Gregg and Smith, 2003). The relatively thin glassy rind and chill zones, together with the absence of interpillow sedimentary material in Subunit IIc, could be interpreted as rapid accumulation of interconnected low-viscosity lava pods, each remaining hot while the adjacent one budded and erupted. Pillowed lava units of Unit IV display characteristics of higher viscosity lavas and a more rapid cooling, including the complex internal fracture patterns and thick well-developed glassy rinds and chilled zones. Moreover, the pillow lavas from Unit IV are distinct in that they are distinctly more plagioclase-phyric, a factor which is known to significantly increase lava viscosity (Jerram et al., 2003).

Finally, during the development of pillowed lava flows, interaction with preexisting sediments can occur under a wide range of physical conditions. If the eruption encroaches in an area of previous seafloor sedimentary deposition, the nature of the interaction of the pillows with these sediments will depend upon the physical nature of the substrate and its grain size (i.e., whether they are muds, sands, or pebbles/breccia and whether it is consolidated, cemented, or unlithified). The contact between (wet) sediment and lava in Unit IV is, in most cases, well defined and marked by 1–2 cm zones of baking/alteration. In addition, the contacts between the pillow lavas and sediment also exhibit more complex relationships. Interpenetration and occurrences of fine-grained carbonate sediment mixed with basaltic lava, or irregular enclaves of deformed and altered sediment, can be observed in the contacts between some pillows (Fig. F19). There is a lack of any bedding or lamination in the sediment, and some of the pillows are entirely encased or "suspended" within a sediment envelope. A likely explanation of these pillow lava-sediment relationships is fluidization of the fine-grained sediment occurred, attributable to flash heating and vaporization of sediment pore water during emplacement of the pillow lava (Kokelaar, 1982). Such momentary fluidization causes sediment reconstitution, the destruction of any inherent bedding structures, and localized transport of the material. It thus is likely that the pillow lavas of Unit IV were emplaced into unconsolidated fine-grained micritic sediment that was fluidized, mobilized, and either injected, squeezed, or otherwise redeposited between individual close-packed pillows to produce the observed intricate pillow lava/sediment contacts.

Petrography and igneous petrology

The petrography in Hole U1350A is described according to the volcanological divisions described above (Units II–IV). The glassy and finely spherulitic margins occurring around both pillows and flows show that plagioclase and clinopyroxene, both separately and intergrown, occur in tiny stellate aggregates as quench minerals. Olivine is present as a microphenocryst. No spinel occurs. Some of the rocks have moderate abundances of large tabular plagioclase microphenocrysts, phenocrysts, and glomerocrysts. The glomerocrysts reveal complex crystallization and cooling histories for the plagioclase and can have skeletal interiors with inclusions that were formerly glass but that are now darkened by spherulitic crystallization and alteration. With the exception of Unit IV, the fluctuations of phenocrysts do not appear to delimit different compositions of basalt (Table T3). The rocks of Units II and III pertrographically resemble the moderately to strongly differentiated rocks of Hole U1347A. A summary table of thin section descriptions of the volcaniclastic rocks is given in 324GLASS.XLS and the contacts between lithologic units are described in 324UNIT.XLS in LOGS in "Supplementary material."

Upper massive flow succession (stratigraphic Subunit IIa, lithologic Units 2–27)

The petrography of the upper massive basalt lava flow units of Subunit IIa (Cores 324-U1350A-6R through 16R) is described on the basis of observations on 24 thin sections from 18 inflation units. These lavas are typically sparsely vesicular (~1%–15%) and aphyric to sparsely plagioclase phyric (<1%), with individual phenocrysts as large as 2.0 mm. Plagioclase microphenocrysts are usually present as glomerocrysts. Plagioclase phenocryst and microphenocryst abundances (Fig. F20) within units of the lower part of Subunit IIa are slightly elevated (~1%) with respect to those of upper part (trace). In addition, traces of olivine and clinopyroxene occur as small phenocrysts and/or microphenocrysts near the bottom of Subunit IIa. Olivine phenocrysts sometimes occur in skeletal forms (<1.2 mm) and are typically pseudomorphed by brown clay (see "Alteration and metamorphic petrology"). Clinopyroxene phenocrysts or microphenocrysts usually subophitically enclose acicular microcrysts of plagioclase or form glomerocrystic aggregates with tabular plagioclase. Groundmass textures in the interiors of the upper massive flows are predominantly intersertal, intergranular, spherulitic, and variolitic. The size of groundmass minerals ranges from microcrystalline near margins of the flow units to very fine grained at the interior part of relatively thick flows (>2 m), where crystallinity is also high (up to ~70%). Occasionally, fresh glass is preserved as patches within spherulites in some chilled margins.

Transitional succession (stratigraphic Subunit IIb, lithologic Units 28–38)

The petrography of massive flows and pillow lavas in the transitional succession (Subunit IIb) is based on seven thin sections from seven inflation units (Sections 324-U1350A-17R-1 through 18R-3). They are of sparse vesicularity (2%–5%) and aphyric to sparsely plagioclase-phyric (<2%). Plagioclase phenocrysts and microphenocrysts are the predominant minerals, present as glomerocrysts forming aggregates as large as 2 mm. Rare clinopyroxene microphenocrysts observed at the top of Subunit IIb subophitically enclose acicular microcrysts of plagioclase and form glomerocrystic aggregates with tabular plagioclase. Olivine phenocrysts are not observed in Subunit IIb. Groundmass textures of the lava units in the transitional succession are predominantly intersertal, hyalophytic, spherulitic, and variolitic. Groundmass sizes range from microcrystalline near the margins of flows or thin-flow interiors to very fine grained at the interior part of relatively thick flows (>1 m). Crystallinity is as high as ~50% in the cores of the thicker flows and is lower than that of Subunit IIa.

Aphyric pillow succession and hyaloclastite (stratigraphic Subunit IIc and Unit III, lithologic Units 39–91)

The petrography of Subunit IIc and Unit III is based on 14 thin sections from 14 inflation units in Subunit IIc and 1 thin section from Unit III (Sections 324-U1350A-19R-1 through 25R-1). These basalt units are sparsely vesicular (1%–5%), aphyric at the upper part of Subunit IIc (240–270 mbsf), and sparsely phyric (<2%) at the base of Subunit IIc (270–290 mbsf) (Figs. F12, F20). In both Subunit IIa and Unit III, plagioclase phenocrysts and microphenocrysts are the main minerals, usually present as glomerocrysts forming aggregates as large as 2 mm. Olivine and clinopyroxene are rare as microphenocrysts. Olivine microphenocrysts (<0.5 mm) are pseudomorphed by brown clay (see "Alteration and metamorphic petrology"). Clinopyroxene microphenocrysts form glomerocrystic aggregates as large as 0.5 mm. Groundmass textures in the aphyric pillow lavas (Subunit IIc) are dominantly intersertal, hyalophytic, and spherulitic. Groundmass size is mostly microcrystalline within the flow interiors and cryptocrystalline nearer the unit margins. Fresh glass showing spherulitic textures is preserved as glassy rinds at the lowermost chilled margins of some flows (Thin Section 312; Sample 324-U1350A-24R-1, 128–131 cm). The crystallinity of the aphyric pillow succession (Subunit IIc) reaches a maximum of ~50%, which, although low, is comparable to that of the transitional succession (Subunit IIb). The hyaloclastite of Unit III has a cryptocrystalline groundmass with hyalophytic fragments.

Plagioclase-phyric pillow succession (stratigraphic Unit IV, lithologic Units 92–183)

The petrography of Unit IV is based on 15 thin sections from 14 inflation units (Cores 324-U1350A-25R and 26R). They are sparsely vesicular (3%–5%) and sparsely to moderately plagioclase-phyric (2%–5%). Plagioclase phenocrysts and microphenocrysts are the predominant minerals, usually present as glomerocrysts forming aggregates as large as 5 mm. Some individual plagioclase phenocrysts (<5 mm) are anhedral and have sieved textures, honeycomb textures, and rim-resorption features. Rare olivine microphenocrysts are observed at the top of Unit IV but are pseudomorphed by brown clay and calcite (see "Alteration and metamorphic petrology"). Groundmasses in the plagioclase-phyric pillow succession have hyalophytic textures with phenocrysts of plagioclase and pyroxene. The groundmass is dominantly cryptocrystalline at flow interiors and near the flow margins. Chilled margins of the pillows consist mainly of spherulite or fully altered glass to clay minerals (see "Alteration and metamorphic petrology"). Crystallinity of the pillow succession of Unit IV is <30%.

Two types of groundmass crystal morphologies

Since many of the rocks are comparatively unaltered, differences in texture are immediately apparent in reflected light. These differences are not always obvious when dark, microcrystalline material is observed in transmitted light because of the "clouding effect" of superimposed crystal interference(s) through the thickness of the thin section. Reflected light, instead, provides a single surface for examination, and crystal shapes are sharply outlined. Using this technique, two principal textural types of rock become apparent in Hole U1350A (Fig. F21):

Type 1 can be termed the "plagioclase network" type (Fig. F21A, F21B). In this type, the largest plagioclases are usually acicular crystals with bifurcating arms projecting only short distances outward from the ends, but appearing as a loose skein across the thin section. Smaller, but irregular and dendritic, clinopyroxene probably occurs between the plagioclase crystals, as does formerly glassy intersertal material that is darker brown. In reflected light, the faceted crystal outlines of plagioclase (darker gray) are prominent, yet the clinopyroxene (brighter gray) is a smaller proportion of the modal abundance and does not stand out, being almost hidden between the feldspars (Fig. F21B). Titanomagnetite (bright white, in the same image) is skeletal and occurs mainly within the dark intersertal material. Some Type 1 basalt has what appear to be irregular segregation patches as long 1 cm in some thin sections (Fig. F21E, F21F).

Type 2 can be termed the "plagioclase-clinopyroxene network" type (Fig. F21C, F21D). In this type, the basalts have dozens of fairly coarse grained intergrowths of tabular plagioclase and subhedral clinopyroxene forming a network of "islands" within a finer grained dark matrix (Fig. F21C, F21D). Plagioclase in the islands is tabular and euhedral, although it may project skeletally into the matrix. The clinopyroxene has, at most, only tiny dendritic projections into the matrix, whereas the matrix itself consists of elongate skeletal to feathery plagioclase of various sizes that do not form a network skein. Small amounts of clinopyroxene cluster within the interstices of the feldspars. Titanomagnetite cannot be distinguished at this magnification.

Figure F22 shows, sequentially through the core, a series of reflected light photomicrographs. From the first five of these (Fig. F22A–F22D), differences in average grain size and crystallinity (highlighted in reflected light) are mainly the result of placement of the thin sections in different portions of the basalt flow interiors that cooled at different rates. But the general similarity of Type 1 plagioclase skeining is evident in all of them. Type 2 networking is only present in the thin sections shown in Figure F22E, F22F and F22G. In the finer grained rock of Figure F22F, the bulk of the rock is occupied by finely crystalline plagioclase-clinopyroxene intergrowths that clump between small, dark intersertal patches. Figure F22G also shows a groundmass Type 2 network, again plagioclase-clinopyroxene intergrowths set in a matrix of the sample previously shown in Figure F21C and F21D.

The photomicrographs in Figure F22 are arranged by core depth. From this, all of Subunit IIa consists of Type 1 basalt except for one sample (Fig. F22E; Sample 324-U1350A-16R-3, 35–39 cm), whereas Subunit IIb is Type 2. Figure F22H depicts the uppermost thin section of the lower Subunit IIc, and it shows a Type 1 networking again, after which the Type 1 plagioclase skeining prevails to the bottom of the hole. The thin section from Unit III (Thin Section 311; Sample 324-U1350A-24R-3, 35–38 cm) has a cryptocrystalline texture, which precludes identification of groundmass texture. Petrographically, then, there are at least three (and probably four) different rock types among the basalt in Hole U1350A. In sequence upward, they correspond to Units IV and III combined and although Subunits IIc and IIa are very similar petrographically, they are separated by something that is petrographically distinct in Subunit IIb.

Finally, Figure F23 shows representative phenocryst and glomerocrystic clusters. Figure F23A–F23D is from a single thin section of Type 1 basalt. In it plagioclase and clinopyroxene occur both as clumps or clusters and as separate phenocrysts. Plagioclase also occurs in monomineralogic glomerocrysts. Figure F23E shows a single but complex Type 2 networking intergrowth of plagioclase and clinopyroxene. The plagioclase is skeletal but fairly well faceted; it is arranged like randomly projecting nails held in a clinopyroxene fist. In Figure F23F, a reflected-light image of the left portion of the intergrowth in the previous photomicrograph, the outlines of clinopyroxenes are the straight boundaries of plagioclase crystals on one side but are very irregular at the edges of intersertal patches on the other. The pyroxenes are also full of unpolished specks, an indication of the bladelike or dendritic tendency of its crystal morphology.

The tendency might be to discern the arrangements of these intergrowths in Hole U1350A basalts as microphyric clusters or even phenocrysts. However, in the same manner as the ophimottle clots in Hole U1349A are not phenocrysts, the intergrowths in Hole U1350A perhaps are derived from a deep-seated magma chamber. Type 1 and 2 crystal networks also occur in the basalts in Hole U1347A, but in Hole U1350A we see a more explicit development of their features. Further evaluation along these lines will depend on assessment of crystal morphologies at quenched pillow margins in shore-based thin section studies, the detailed development of a chemical stratigraphy, and comparison with basalts of other holes drilled during Expedition 324.

Preliminary assessment

Comparing all basalt drilled at Sites 1213, U1346, U1347, U1349, and U1350, it becomes evident that Shatsky Rise presents a basaltic province in which plagioclase-clinopyroxene intergrowths, from the stage of earlier crystallization of mottles with ophitic textures (Hole U1349A) to the formation of phenocrysts and glomerocrysts (Holes 1213B, U1347A, and U1350A) to the quench stage and to the stage of coarser grained crystallization in the interiors of flows and pillows, are well developed in all holes. This petrographic attribute is evident even among the rare and tiny quench crystals in the one glassy sample recovered among the hyaloclastites of Hole U1348A. Basalt in Hole U1346A and in particular Hole U1349A is more primitive than the others, since it contains fairly abundant olivine and Cr spinel, whereby even plagioclase-clinopyroxene intergrowths occur in remnants of altered glass margins. Petrologically, this attribute indicates that at all stages of crystallization the temperature interval between onset of plagioclase crystallization and that of clinopyroxene was negligible. Experimentally, this is not a usual feature of mid-ocean-ridge basalt (MORB) at low pressure, with the exception of the melt containing significant dissolved water, since these conditions would allow a basalt to crystallize plagioclase and clinopyroxene together.

Although some of the basalt in Hole U1349A can be termed picritic, they differ from the only well-documented picrite found on modern spreading ridges, such as that of Siqueiros Fracture Zone on the East Pacific Rise (Natland, 1980). More evolved olivine tholeiite from drill sites on the Mid-Atlantic Ridge still has an obvious gap in crystallization temperature between the onset of crystallization of plagioclase and later clinopyroxene (e.g., Kirkpatrick, 1979). As MORB-like as Shatsky Rise basalt may be in its geochemistry, in this one respect, it is different.

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