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doi:10.2204/iodp.proc.324.101.2010

Results

Site U1346

Site U1346 is situated at the north edge of the Shirshov Massif summit where acoustic basement is nearly flat, implying a subaerially eroded summit platform (Sager et al., 1999) (Figs. F6, F7). A single hole was drilled (Hole U1346A), penetrating 191.8 m below the seafloor, with 139.2 m of sedimentary cover and 52.6 m of igneous rock (Table T1).

Because the primary goal of drilling Hole U1346A was to recover the igneous basement, sediment coring began only ~70 m above the suspected basement/sediment interface. Despite difficult drilling conditions, ~4 m of sediments were recovered in the first six cores prior to entering into basaltic basement. The recovered sedimentary material represents various lithologies and depositional environments, with an average recovery of 10.2% between 100.5 and 141.7 m core depth below seafloor (mbsf). The uppermost sedimentary interval, stratigraphic Unit I (Cores 324-U1346A-1W through 3R) yielded only small, isolated pieces of dark chert fragments (Table T2; Fig. F8). Recovery in Cores 4R through 6R improved because of a reduction in the amount of chert in the formation. This short sequence of lithified sediments included an intriguing sequence of intermingled basalt and limestone in Section 324-U1346A-4R-1, interpreted as a debris flow (stratigraphic Unit II). In this unit, soft-sediment deformation occurred around the larger volcanic clasts, indicating that the clasts impacted the sediment, either rolling downslope from the eruption source or as a mass flow deposit generated through posteruptive erosion of the volcanic edifice. Section 4R-2 contained a series of laminated volcaniclastic sequences, grading from very coarse sand to clay, which are interpreted as turbiditic in origin (Unit III). The remaining sediments (stratigraphic Unit IV), from the base of Section 4R-2 to the top of 6R-1, are composed of clay-bearing limestones and calcareous mudstones containing abundant shell fragments and other biogenic components, along with glauconite and altered volcaniclastics. Taken together, these components are suggestive of a relatively shallow marine depositional environment in close proximity to a source of volcanic material.

Calcareous nannofossils in the recovered sediments are rare to abundant and moderately to poorly preserved. The age of four samples from Cores 324-U1346A-4R and 5R is assignable to the Berriasian to Hauterivian. Within the foraminifer assemblage obtained from Section 324-U1346A-4R-CC, the planktonic group is completely absent. Benthic foraminifers are well-preserved, diverse, and representative of the neritic–upper bathyal assemblage (estimated paleodepth ≤ 500 m). Various other biogenic sedimentary components were observed in the samples examined for foraminifer analyses; these were dominantly radiolarians with lesser amounts of ostracodes, inoceramid prisms, echinoid plates, sponge spicules, bryozoans, and calcareous fragments.

Basement coring at Site U1346 on Shirshov Massif documented a ~53 m thick stack of highly vesicular basaltic pillow lavas or lava "inflation units" (stratigraphic Unit V) beneath the succession of pelagic nannofossil–bearing chalks and cherts, volcanogenic silts and sands, and volcaniclastic debris (Table T2; Fig. F9). Within the lava stack, individual pillow or inflation units were readily identified by the presence of chilled glassy margins, upper and lower chill zones (Fig. F10), characteristic pillow vesicle patterns, and crystal grain-size variations. In total, 40 individual "lava cooling units" were recognized in the cores from Unit V, but the entire unit was interpreted to represent a single eruptive event.

The pillow basalts are generally vesicular in nature and have zones that are moderately vesicular (30%–50% vesicles). Although they appear macroscopically aphyric, a closer inspection reveals that microphenocrysts of olivine and pyroxene were originally present but are now totally replaced by calcite. All samples contain large proportions of less altered, very fine grained plagioclase laths set in a variolitic matrix.

Extensive low-temperature water-rock alteration has left a marked impression on all igneous rocks recovered at this site, resulting in near complete replacement of pyroxene, olivine, and glassy pillow rinds and complete replacement of glassy mesostasis. In contrast, plagioclase shows only slight to moderate alteration. Based on rock color and mineralogy, three types of alteration were determined in Hole U1346A: (1) a green alteration, recovered only in volcaniclastic debris interspersed with sediment at the top of the hole (Unit II; interval 324-U1346A-4R-1, 0–39 cm); (2) a dark gray alteration; and (3) a brown alteration. In the basement pillow lavas, the dark gray alteration is most abundant and is interspersed with the brown alteration throughout the hole. The most abundant secondary minerals observed in the basaltic rocks are clay minerals, and the nature of clay minerals changes with alteration type, from predominantly green clays, including nontronite, in the green alteration, to green and brown clays, including saponite, in the gray alteration, to mainly brown clays in the brown alteration. Calcite is also abundant in all alteration types, replacing pyroxene and olivine, rarely the groundmass, and filling vesicles and veins. Sulfide minerals are present only in stratigraphic Unit II and the upper portions of basement pillow lavas in Sections 324-U1346A-7R-1 and 8R-1. These mineral assemblages suggest that the basaltic rocks from Hole U1346A have extensively interacted with seawater-derived and CO₂-rich fluids at low temperature and more locally with S-rich hydrothermal fluids.

Despite the moderate to complete alteration, shipboard analysis of major and several trace elements by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) in lava samples recovered from stratigraphic Unit V reveals that the rocks are tholeiitic basalt (Fig. F11). The concentrations of many elements, including K, Si, Ca, P, Sr, Ba, and Ni, were modified significantly by the alteration. However, several elements, including Ti, Zr, Y, Cr, V, and Sc, appear to have been affected relatively little. Relationships among these elements indicate a strong similarity with the older basalts recovered from Site 1213, which lies ~870 km southwest of Site U1346.

Both syn- and postmagmatic structures can be recognized within the Hole U1346A igneous complex. The main synmagmatic structural features are amygdules, pillow structures, irregular vein networks or curved veins, and breccias. Postmagmatic structures include conjugate veins and joints. Dip angles of the veins in the hole from top to bottom become gradually steeper; however, joint dips are generally low. The structures observed throughout Unit V are consistent with the interpretation that the rocks are a pile of stacked pillows whose sizes differ from ~20 to 200 cm.

Gamma ray attenuation (GRA) measurements of the basement sections reveal that there is low variability in bulk density despite changes in the style and degree of alteration. Magnetic susceptibility appeared to be a more sensitive tracer of alteration, and several regions were identified where magnetic susceptibility covaried with changes in alteration style. Thermal conductivity measurements averaged 1.52 W/(m·K), with no significant relationship with depth.

P-wave velocity measurements of 24 discrete samples showed no appreciable anisotropy or depth relationship and averaged 4.5 km/s throughout the core with a maximum of ~5.6 km/s and a minimum of ~3.5 km/s. Moisture and density measurements of bulk, dry, and grain density of the discrete samples confirmed indications from the GRA density (measured on whole cores) that there was little systematic change in the density of the basaltic sequence with depth. Porosity was high and ranged from 8% to 33%, with the highest values closer to the top of the hole.

In total, 20 discrete samples obtained from Hole U1346A were measured to investigate paleomagnetic remanence of the upper part of the Shirshov Massif basement. Most of samples show a stable component between 300° and 475°C and have low unblocking temperatures (400°–450°C), which is characteristic of titanomagnetite (or titanomaghemite). Alternating-field demagnetizations show that the magnetization is stable after 10–15 mT demagnetization. The samples from Hole U1346A are characterized by shallow negative inclinations with an arithmetic mean of –20.3° ± 5.3° (1σ).

Downhole logging data obtained from Hole U1346A included natural and spectral gamma ray, density, photoelectric factor (PEF), and electrical resistivity measurements from three depths of investigation. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 14 logging units in Hole U1346A, with three in the section covered by the bottom-hole assembly (BHA), four in the sedimentary sequences in the open hole interval, and seven in the basaltic basement. The sedimentary sequence shows several prominent gamma ray anomalies associated with U enrichment. The most prominent anomaly is found at the sediment/basement interface and may be indicative of focused hydrothermal fluid flow. Shallower anomalies recorded through the BHA may represent oceanic anoxic events previously interpreted in this area. Electrical resistivity measurements in the basaltic basement show four distinctive massive zones characterized by higher resistivity values, which may represent individual thick lava flows. Relatively high K content in the basement section also indicates a high degree of hydrothermal alteration.

Site U1347

Site U1347 is situated on the upper flank, east of the summit of Tamu Massif (Fig. F12). The site location was chosen at a spot where sediments are thin and the "layered basement" signature seen elsewhere on the southern Tamu Massif is also thin (Fig. F13). Drilling in a single hole (U1347A) penetrated 317.5 m below the seafloor, including a 157.6 m sedimentary section and a 159.9 m igneous section (Table T1).

Coring recovered 17.7 m of sediment (stratigraphic Units I–III) in Cores 324-U1347A-1W through 11R over a stratigraphic interval of ~158 m before entering basaltic basement (Table T2). The recovered sediments are dominated by radiolarian-rich volcaniclastic siltstones, with varying proportions of glauconite (Fig. F8). There are also minor intervals of chert and claystone. Bioturbation is often pervasive in the silty facies, with rip-up clasts and erosional contacts as common features, suggestive of turbulent and transient depositional events. Sediments are also present as relatively thin interbeds between the massive basaltic flows and pillow basalt units within the igneous complex. These sediments are similar in character and composition (e.g., predominantly radiolarian-bearing siltstones) to the sediments above basement, although some show features consistent with thermal alteration as a result of subsequent basalt emplacement.

Calcareous nannofossils and foraminifers from the sediments of Site U1347 are generally moderate to poor in preservation and low in abundance and diversity. Intercalated sediments in the underlying basement section are almost barren of both microfossil taxa. Fortunately, the age of four samples from Cores 324-U1347A-2R and 10R is assignable to the Berriasian to Valanginian based on calcareous nannofossils. The foraminifer assemblage is marked by the absence of planktonic forms. Benthic foraminifers, although the total number of specimens is limited, are characterized by a neritic assemblage in the lower part of the sediment section (Cores 6R through 8R) and a bathyal assemblage upsection (Cores 1R through 2R). Therefore, an overall deepening trend from <200 m to 200–2500 m is inferred. Samples processed for foraminifer analyses are to a large extent dominated by volcanogenic lithic fragments and minerals; the major biogenic component is radiolarians with peaks in size, abundance, and diversity in Cores 6R through 8R.

After penetrating the sediment section, a 159.9 m thick volcanic basement succession (Units IV–XVI) (Table T2; Fig. F9) was encountered, consisting of "packages" of massive basalt flows and pillow inflation units intercalated by five sedimentary successions up to ~5 m thick. Based on the dominant type of eruptive unit, the volcanic basement can be described in three main packages or groups. From top to bottom, these are: Group 1, an upper "layered" series of four massive lava flows (~8–19 m thick) (Fig. F14) intercalated with two ~5 m thick sediment packages and totaling ~60 m in thickness (Units IV, V, VII, and IX); Group 2, a ~75 m lava stack consisting for the most part of pillow basalts (each ~0.2–1.0 m thick) (Fig. F15) and medium-size inflation units (1–2 m thick), interspersed by relatively thin sedimentary intercalations and three larger (~3–6 m thick) basalt flows (Units X, XII, and XIV); and Group 3, a lower set of two particularly massive basaltic lava flows consisting of a very thick upper (~23 m) homogeneous lava flow that overlies a unit of similar character at the bottom of the hole (Units XV and XVI). In many instances, the high recovery (average = 64.2%) for Hole U1347A igneous basement yielded well-preserved lower- and upper-contact zones with chilled margins, baked sediment contacts, and folded pahoehoe-type upper crusts. The frequent recovery of thick (often fresh) glassy rinds within the pillow-unit stack indicates that alteration was essentially buffered in these rocks.

In Group 1 the three uppermost massive flows are aphyric (stratigraphic Units IV, V, and VII) and petrographically different from the sparsely to moderately pyroxene-plagioclase phyric basalt in the fourth flow (Unit IX) and the lavas lower in the succession. Examination of unit thicknesses within Group 2 reveals a pattern of repetition, beginning with massive inflation units, passing upward into predominantly medium-size units, and then into a sequence of closely packed pillow lavas, before the cycle is repeated. These repetitions may represent repeated eruptive pulses during which the lava effusion rate diminished. Group 3 includes the lowest two lava flows recovered (Units XV and XVI); they have well-developed chilled margin zones at their tops (glassy to microcrystalline in the topmost ~2 m) and thinner ones (~0.5 m) at their bases. Thick glassy rinds were not recovered in these flows and vesiculation is confined to the upper 2–3 m, below which the flows become very homogeneous and massive and largely nonvesicular. It may be deduced from their large thickness that these flows are likely to be laterally extensive "tabular flow" units, possibly similar in dimension to those described in flood basalt provinces.

The majority of the volcanic units are plagioclase-pyroxene phyric basalts. Toward the cores of the lava flows and pillows, crystallinity increases to 50% and in some cases to 95%. Most notable are the appearance of plagioclase phenocrysts in the fourth massive flow in the upper volcanic group (stratigraphic Unit IX), the rarity or absence of clinopyroxene phenocrysts in the lower pillow stack (Units XII and XIV), and their absence in the lowermost massive flows (Units XV and XVI). In all cases the basalts were saturated in both clinopyroxene and plagioclase; these minerals always seem to occur together as clusters or glomerocrysts intergrown in the matrix between the larger microcrysts and phenocrysts. The rocks appear, therefore, to have been in a condition of low-pressure plagioclase-clinopyroxene cotectic crystallization during all stages of cooling and differentiation. Olivine phenocrysts occur rarely as early-coprecipitating liquidus minerals and now are completely replaced by clays and calcite. Remnants of fresh olivine with melt inclusions or spinel were discovered only in a few thin sections. Titanomagnetite is highly variable throughout the lava succession, as also reflected in highly variable magnetic susceptibility. For instance, glass in the chilled margins has no discernible titanomagnetite and consequently low magnetic susceptibilities. However, spherulitic overgrowths on plagioclases and larger (sometimes elongate skeletal) titanomagnetite grains increase significantly toward the cores of the (thicker) lava flows, and are correlatable to increased magnetic susceptibility readings.

Overall, basalts of Hole U1347A appear to have more differentiated compositions compared with basalts from basement at several other sites cored on Shatsky Rise (Sites 1213, U1346, and U1349). This is evident from the predominance of plagioclase and clinopyroxene intergrowths at all stages of crystallization, the scarcity of olivine, and the almost complete absence of Cr spinel. This phenocryst assemblage and character of intergrowths compare well to those of rather evolved low-temperature gabbros formed in "shallow" crustal magma chambers beneath fast- and superfast-spreading ridges.

Alteration within the volcanic section varies from slight to moderate (estimated to range from 5% to 50%). Generally, both the primary mineralogy (with the exception of extensive olivine replacement) and finer spherulitic textures in the interstices between phenocrysts and microcrysts are well preserved, especially close to pillow margins. Sometimes fresh glass is still present in the groundmass. In contrast, away from the pillow margins, the glassy mesostasis in the groundmass is almost completely replaced. Plagioclase and clinopyroxene are generally well preserved throughout the hole, both in the groundmass and as phenocrysts. Clay minerals, together with calcite, are the most abundant secondary minerals in Hole U1347A, replacing primary phases and glassy mesostasis and filling vesicles and veins. Pyrite is widespread throughout the hole, being present in the groundmass, vesicles, and veins. Three main types of veins were observed: (1) calcite veins, which predominate; (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. No significant variation in alteration mineralogy was observed in Hole U1347A. Alteration of the basaltic rocks is interpreted to result from interaction with seawater-derived fluids at relatively low temperature (<100°C).

Two kinds of structures were distinguished within the igneous complex: pillow structures and sheet flow structures. The typical sheet flow structure is normally characterized by three parts: (1) an upper lava crust, (2) a middle lava core, and (3) a basal zone. Dip angles of both veins and joints in the hole become gradually steeper downhole. The entire structure of the Hole U1347A igneous complex is consistent with stacked layers of pillows and massive basalt sheet flows. Taking the horizontal sedimentary bedding into account, structural observations indicate that the site has undergone no tilting or horizontal-axis block rotation.

Onboard measurements of major and several trace elements by ICP-AES reveal that the Site U1347 lavas are tholeiitic basalts (Fig. F11). The chemical effects of posteruptive alteration are smaller than for samples of other Expedition 324 sites, especially the highly altered rocks cored at Sites U1346 and U1349. In general, both the major and trace element compositions are consistent with the Site U1347 basalts being variably more evolved relatives of the basalts at Site 1213 on the southern flank of Tamu Massif. Compared to normal ocean-ridge basalts, the Site U1347 lavas exhibit modest relative enrichment in the more highly incompatible elements, qualitatively similar to that seen in the Site 1213 basalts. This result suggests a mantle source slightly richer in the more incompatible elements than normal ocean-ridge source mantle and/or that the Site U1347 magmas formed by slightly smaller amounts of partial melting and possibly in the presence of residual garnet.

Magnetic susceptibility in the igneous rocks is typically ~2000 x 10^–5 SI and GRA density is from 2.4 to 2.6 g/cm³. The massive flow (stratigraphic Unit XV) below ~290 mbsf notably reaches magnetic susceptibilities as high as 3800 x 10^–5 SI and also has the highest measured densities. Total counts of natural radiation measured in igneous rocks were between 2 and 4 cps throughout the hole, which is an order of magnitude less than observed at Site U1346. Fifty-eight thermal conductivity measurements of igneous rocks were performed, yielding an average of 1.59 ± 0.200 W/(m·K) (2σ). One sedimentary measurement of 1.007 ± 0.017 W/(m·K) (2σ) was also obtained. In general, the massive igneous units have slightly higher thermal conductivity than the pillow units. This is seen particularly well in the lowermost massive flow unit below ~290 mbsf, which has a thermal conductivity of 1.733 ± 0.092 W/(m·K) (2σ; N = 11).

P-wave velocity measured on discrete samples showed no appreciable anisotropy but appeared to vary with stratigraphic units. The Unit XV massive flow and upper pillow basalts (Unit X) have high compressional wave velocities (up to 7.04 km/s). The high velocities are coincident with low porosity (<5%) and high bulk densities (>2.75 g/cm³) measured by discrete sampling.

A total of 61 discrete samples were measured to investigate paleomagnetic remanence of the upper part of the Site U1347 basement. Only about half of the samples demagnetized by thermal demagnetization show a stable component, whereas most samples demagnetized by alternating-field demagnetization show a fairly stable component between 15 and 80 mT. Most samples have low unblocking temperatures of ~300°–400°C, which is characteristic of titanomagnetite (or titanomaghemite). We suggest four magnetic zones downhole: (1) Section 324-U1347A-12R-1, with shallow negative inclination (–6° ± 7°; 1σ); (2) Sections 12R-2 through 16R-5 (lower part of the massive basalt Flows 1–3; stratigraphic Units IV, V, and VII), with an average inclination of 28° ± 13°; (3) Sections 17R-2 through 26R-1 (basalt Flow 4 and pillow lava section; stratigraphic Units IX–XIV), with an average inclination of 20° ± 14°; and (4) Sections 26R-2 through 29R-4 (basalt Flows 5 and 6; stratigraphic Units XV and XVI), with an average inclination of 54° ± 27°. Whereas results in the upper three zones appear reasonably reliable, detailed rock magnetic analyses are necessary to interpret the erratic magnetic behavior of the lowermost zone.

Downhole logging data obtained from Hole U1347A included natural and spectral gamma ray, density, neutron porosity, PEF, and electrical resistivity measurements from three depths of investigation. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 15 logging units, with 3 in the sediment sequences and 12 in the basaltic basement. These units correlate well with those defined by core material logging. Electrical resistivity measurements in the basaltic basement show distinctive high-resistivity zones that likely represent massive flows and pillow flow units interspersed with low-resistivity zones that mark sediment interbeds. Natural gamma ray measurements show five intervals of higher readings that indicate interbedded sediments within the basaltic basement. These sedimentary intervals also display higher K values. FMS images show zones of distinctive pillow lavas, zones with high fracture density, and intervals that seem to represent massive lava flows.

Site U1348

Site U1348 is located on the north flank of Tamu Massif (Fig. F12). The target for drilling was the upper part of a basement high where sediments are thin (Fig. F16). A single hole (U1348A) was drilled at the site, with 324.1 m of penetration below the seafloor (Table T1).

A thick sequence (~120 m stratigraphically) of volcaniclastic sediments, topped with shallow water calcareous sandstones, greenish clays, nannofossil ooze, and chert, was recovered from Hole U1348A (Fig. F8). The uppermost cores (stratigraphic Unit I) contained red chert interbedded with a remarkably well preserved, section of Cenozoic–Late Cretaceous nannofossil ooze over a meter long (Core 2R). Yellow brecciated cherts were also recovered stratigraphically below the red cherts and above highly silicified, altered sandstones. Below this, a sequence of shallow-water bioclastic sandstones (Unit II) with volcanic clasts was found (Cores 10R through 13R). This sequence includes an interval of bright green zeolitic clays. Although basaltic basement was not reached at this site, Cores 14R through 26R recovered a unique sequence of highly altered marine volcaniclastic rocks (Units III–VI). Based on the marine fossil content and bedding structures, these have been interpreted to represent a mixture of in situ and re-deposited material that was erupted in a submarine environment.

Micropaleontological investigations revealed that the sediments from the upper cores of stratigraphic Unit I (Cores 324-U1348A-1W through 10R) are pelagic in origin and, although recovery was dominated by chert-rich lithologies, are generally suited for calcareous microfossil studies. A few centimeters of gray ooze in the wash barrel's core catcher (Core 1W) and in the uppermost centimeters of the undisrupted nannofossil ooze are rich in Cenozoic (Miocene) calcareous and siliceous microfossils (Core 2R). Microfossils of mid- to Late Cretaceous age occur in Cores 324-U1348A-2R through 10R. Calcareous nannofossils are subject to progressive reduction in abundance, diversity, and preservation state with increasing burial depth. Planktonic foraminifers contained in this interval are generally well preserved and abundant. A series of primary/secondary zonal marker species that correlate well to the standard biochronology are recorded by planktonic foraminifers, ranging from the early Aptian to early Campanian (120–80 Ma), and they are used for construction of the age model for this site. Therefore, the underlying units are considered to be >120 Ma in age. Benthic foraminifers show remarkable changes in abundance and diversity throughout the hole, and the subsidence history of Tamu Massif from upper to lower bathyal depth can be discerned.

The ~120 m thick volcaniclastic succession of stratigraphic Units III–VI (Cores 324-U1348A-14R and below) consists predominantly of hyaloclastite (Figs. F9, F17). The lithology ranges from matrix-supported clay- or sand-size hyaloclastite to dominantly heavily compacted granule- or pebble-size clast-supported hyaloclastite in the bottom of the hole. Petrography reveals that the volcanic glass shards and larger vitric clasts are thoroughly altered, with minor admixtures of bioclastic materials, fossil debris, and well-preserved calcitic fossils at some horizons. In contrast to other Expedition 324 drill sites, no massive or pillow lava flow successions were encountered.

The hyaloclastite succession proved difficult to interpret because of pervasive, and often complete, alteration of the volcanogenic constituents, which masks both original composition and structure. A minority of sparsely vesicular hyalobasalt fragments occur, especially in the upper horizons (stratigraphic Units III–IV), yet high proportions of altered volcanic glass shards and clasts occur throughout, increasing to almost completely form the hyaloclastitic rocks in the clast-supported packages of Unit VI. Under the microscope these fragments appear to have originally been sparsely vesicular vitric clasts, broken down into abundant glass shards, which were subsequently exposed to intense alteration and compaction. The predominance of altered glass shards throughout Units III–VI is indicative of substantial submarine volcanism. Sedimentary reworking of these primary hyaloclastite constituents is evident in some units.

With only one exception, the original hyaloclastite composition has been entirely transformed to secondary palagonite, zeolite, and calcite. This replacement commences with the alteration of volcanic glass shards to palagonite, which is mainly composed of montmorillonite and nontronite. This process starts along the rims and continues with the development of alteration spheroides within the glass shards. Further progression of this process resulted in complete replacement of the edges of these shards, followed by replacement of the glass cores by a combination of zeolite and calcite. Some rare lithic fragments show similar texture and degree of alteration to basaltic rocks recovered at the top of Hole U1347A. However, even these clasts show almost complete replacement of primary phases and transformation to brown clays. Clay minerals, together with calcite and zeolites, are the predominant secondary minerals in Hole U1348A. Vitric and lithic clasts are cemented by calcite and/or zeolites, with variations in their occurrences and proportions downhole. Fibrous and tabular zeolites (i.e., phillipsite) commonly form a corona of alteration around the palagonite particle rims and cement, with or without calcite, the volcanic clasts.

In one unique horizon, cuspate, sparsely vesicular fragments of volcanic glass, consisting of numerous unaltered contiguous glass shards, were recognized in one continuous ~26 cm interval (324-U1348A-23R-1, 110–126 cm, and 23R-2, 1–8 cm). In the rest of the core hyaloclastite fragments are identifiable only from their pseudomorphed glass shard outlines. However, toward the bottom of the hole (in stratigraphic Unit VI) even these pseudomorphed forms have been mostly obliterated through alteration and by a large amount of compaction of this hyaloclastite material by the overburden. The preservation of the few glass fragments in Sections 324-U1348A-23R-1 and 23R-2 seems to result from an early "armoring" of these vitric clasts by impermeable calcite. In these samples, the glass clasts contain no phenocrysts and preserve only a sparsely microcrystic primary mineralogy of fresh olivine and intergrown plagioclase and pyroxene. These clasts are only sparsely vesicular and typically show the incipient and/or partial transformation to palagonite, seen so pervasively throughout this succession.

Two kinds of structures, primary structure and postdepositional structure, are observed in the volcaniclastic stratigraphic Units III–VI. Bedding as primary structure is often displayed by layered fine and granular hyaloclastite sequences that are interbedded in coarser hyaloclastitic breccia. Sedimentary stratification, including graded bedding and cross bedding, is observed particularly in the fine-grained hyaloclastites. Most strata, however, are parallel and show only shallow dips (<30°). Postdepositional structures cut through the bedding, including microfaults and veins. Veins are typically 0.1 to 1.5 cm thick. Both microfaults and veins show steep dip angles >50°.

Two samples from clay-rich layers in sedimentary stratigraphic Unit II have relatively high concentrations of SiO₂, K₂O, and Zr and very low concentrations of CaO, P₂O₅, and TiO₂, as determined by ICP-AES. The clay-rich layers may contain a large proportion of material derived through wind or water transport from continental crust and/or one or more magmatic arcs. However, some of the SiO₂ in these two layers may have come from circulating solutions that originated in overlying or underlying beds.

Clasts and bulk samples of volcaniclastic material from stratigraphic Units III, V, and VI show the chemical effects of complete alteration, greater even than that at Site U1346, on average. However, the Zr-Ti relationship among the samples is similar to that seen for the tholeiitic basalts of Site U1347 (Fig. F11) and suggests that the Site U1348 volcaniclastic rocks were derived from magmas broadly similar in composition to those that fed the Site U1347 lavas.

Physical properties (magnetic susceptibility and bulk density) of clastic stratigraphic Units III–VI correlate well with varying concentrations of volcaniclastic material (versus more calcite rich intervals). Average magnetic susceptibility generally decreases downhole from ~40–45 x 10^–5 SI near the top of Core 324-U1348A-15R to less than 20 x 10^–5 SI near the bottom of the hole (Core 26R). A few excursions to 100 x 10^–5 SI occur in intervals 324-U1348A-17R-1, 0–10 cm, and 58–60 cm; 19R-1, 106–108 cm; 20R-1, 18–118 cm; and 20R-4, 73–92 cm. GRA bulk density and bulk density of discrete samples are between 1.8 and 2.4 g/cm³. Natural gamma radiation averages between 10 and 30 cps, with highest spectral peaks corresponding to ⁴⁰K. Porosity of discrete samples is high (27%–55%). P-wave velocity of discrete samples, from 2.0 to 3.3 km/s, varies inversely with porosity and directly with bulk density.

Because no basaltic basement was recovered and hyaloclastites typically have weak magnetism, paleomagnetic measurements were only made on eight discrete samples of volcaniclastic sediment. These samples likely carry a depositional remanent magnetization instead of a thermoremanent magnetization. Therefore, any directional result will be more complicated to interpret. The 2G cryomagnetometer was used for these measurements because the natural remanent magetization of these samples was too weak to be measured on the Molspin Minispin magnetometer (a few tens of mA/m). Only alternating-field (AF) demagnetizations were carried out, using the DTech degausser. The low magnetic susceptibilities (between 4 × 10^–4 SI and 8 × 10^–4 SI) indicate that magnetic minerals are not abundant in these samples. Compared to the basalt samples from other Expedition 324 holes, the samples have a higher median destructive field (between 10 and 25 mT), which suggests that the magnetization carriers are single-domain grains. In five cases, once the low-coercivity overprint is removed, it is possible to isolate a stable component pointing toward the origin. Inclinations are mostly shallow and positive, between 4° and 23°, although one sample gives a negative inclination (–9°).

Downhole logging data obtained from Hole U1348A included natural and spectral gamma ray, density, PEF, and electrical resistivity measurements from three depths. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 15 logging units with one in the section covered by the BHA, five in the sedimentary sequences in the open hole interval, and nine in the volcaniclastic section. Electrical resistivity measurements show distinctive higher resistivity zones that likely represent less altered intervals interspersed with low-resistivity zones that mark sediment interbeds and more altered sequences. Natural gamma ray measurements show several intervals of higher readings that indicate interbedded sediments and greater amounts alteration. These intervals also display higher K, U, and Th values. FMS images show zones with distinct horizontal layering, dipping beds, and vesicular or brecciated intervals. Preliminary structural analyses of dipping beds show features striking northeast–southwest and dipping mostly 20°–30° to the southeast.

Site U1349

Site U1349 is located at the summit of Ori Massif (Fig. F18) on a flat-topped basement ridge (Fig. F19) that may have been planed flat by wave erosion. Hole U1349A penetrated 250.4 m beneath the seafloor, through 165.1 m of sediment and 85.3 m of igneous basement (Table T1).

In total, 10.1 m of sediment was obtained from Cores 324-U1347A-1W through 7R over a stratigraphic interval of 49 m (average recovery = ~20%) (Fig. F8). Cores 1W through 4R recovered predominately red chert with occasional ooze and porcellanite intervals (stratigraphic Unit I). Below this, a friable sand-silt-claystone sequence was found that also contains granules of highly weathered volcanic material (Units II and III). This interval, however, is so disturbed by drilling that further interpretation of the rock is difficult. Most of Cores 5R and 6R contain greenish gray volcaniclastic sandstones and lapillistones, likely deposited by turbiditic flows. A thin, yellowish red clay-rich horizon in Section 324-U1349A-6R-CC is interpreted as a paleosol within the deepest purely sedimentary section recovered. In the underlying basement, several other thin sedimentary beds are found between basaltic lava flows. The most significant of these is a piece of oolitic limestone in Section 9R-1 at 173.7 mbsf, which logging data suggest is ~6 m thick.

Calcareous microfossils occur in chalk that encrusts reddish cherts and in pinkish nannofossil ooze in Cores 324-U1349A-1W through 4R, whereas the underlying siliciclastic and volcaniclastic sediments of Cores 4R through 7R are barren. Nannofossils are rare to common in abundance but poorly preserved throughout. Planktonic foraminifers are represented in exceptionally high abundance and diversity with good preservation in the nannofossil ooze recovered in Core 2R. Zonal marker and other age-diagnostic species strongly indicate a narrowly constrained age at the middle–late Albian transition. Although a foraminifer material is scarce, other examined levels above and below are also marked by the occurrence of some age-diagnostic species, allowing the entire age range of Cores 1W through 4R to be estimated as Albian–Cenomanian. Core 2R also yielded a diverse benthic foraminifer assemblage; the estimated bathymetric range is middle bathyal.

Volcanic basement was encountered in Core 324-U1349-7R at 165.1 mbsf (Table T2; Fig. F9). The cored basaltic succession of stratigraphic Unit IV is ~55 m thick (Sections 324-U1349A-7R-2 through 13R-4) and consists of 25 lava units characterized by high (40%–75%) vesicularity, the presence of magma mixing features (Fig. F20), and, in all but the interiors of the thickest inflation units, a pervasive reddish brown Fe oxyhydroxide hematite alteration. This succession also contains 10 recovered weathered flow tops readily identified by a reddening over ~5–10 cm intervals, tentatively interpreted as subaerial alteration occurring between the emplacement of successive eruptive units. In the top ~10 m of this lava succession, the flow inflation units are relatively thin (~0.3–1.1 m). Whereas many have highly vesicular reddened flow tops, a number of examples preserve interflow carbonate sediment. The most significant of these is an oolitic limestone that is preserved at the top of a highly vesicular flow. Other instances of this interflow carbonate sedimentation are preserved only as carbonate infilling of vesicle voids within flow tops. In addition to the subaerial exposure inferred elsewhere in Unit IV, these filled-vesicle flow tops record contrasting periods of shallow marine inundation of the lava field. The lowermost part of Unit IV contains a series of much thicker (1–6 m) massive flows. Preliminary comparisons to degassing models for basalt in Hawaii, and assuming 0.05–0.5 wt% H₂O concentrations in the source, imply pressures below 15–20 bar in order to produce the high (40–70 vol%) vesicularity in the lavas of Unit IV, corresponding to <100 m water depth.

Beneath the lava succession of stratigraphic Unit IV is a transition to flow breccia at 221.7 mbsf (Table T2; Fig. F9). This section consists of ~29 m of an alternating assemblage of massive lava pods and fragmental basalt. The lava pods are vesicular, though to a lesser degree than the flow tops in Unit IV, on average ~1 m thick and range in thickness between 0.5 and 2 m. The assemblage is interpreted as a series of autobrecciated inflation units (Unit V).

Petrographic examination reveals that the basalts of both stratigraphic Units IV and V are all Cr spinel–bearing and olivine-phyric. Based on ICP-AES analyses, these basalts have >8%–15% MgO and thus may be termed picritic. Olivine phenocrysts are sparsely to moderately abundant (up to 4%) but are usually entirely altered to iddingsite, clays, and Fe oxyhydroxides. The groundmass of some rocks contains abundant "ophimottle" crystal aggregates up to ~5 mm in size and consisting of well-preserved clinopyroxene ophitically intergrown with plagioclase. Iron oxyhydroxides and secondary hematite are present in the groundmass and responsible for the brownish red color of the rocks in the lower part of Units IV.

Extensive water-rock interactions under variable temperature conditions resulted in complete replacement of glassy mesostasis and olivine phenocrysts in both stratigraphic Units IV and V. Both plagioclase and clinopyroxene are generally well preserved in Unit IV, whereas below ~218 mbsf (bottom of Unit IV and Unit V, from Section 324-U1349A-13R-5 to 16R-7), alteration is more pronounced, with nearly complete replacement of clinopyroxene and almost complete replacement of plagioclase microliths. The overall alteration of the basaltic rocks ranges from high to complete. Three types of alteration were determined in Hole U1349A (from top to the bottom): (1) a red-brown alteration (upper and middle of Unit IV), (2) a transitional alteration (bottom of Unit IV), and (3) a green alteration (Unit V). In addition, subaerially weathered flow tops have been identified within Unit IV. Clay minerals (saponite, nontronite, and montmorillonite) are the predominant secondary minerals in Hole U1349A samples, replacing glassy mesostasis and primary phases (olivine, plagioclase, and pyroxene) and filling vesicles and veins. Calcite and hematite are also abundant secondary minerals, especially in Unit IV. Secondary mineral assemblages change with depth from clay minerals + hematite + calcite in the red-brown alteration to clay minerals + chlorite + zeolite ± hematite in the transition zone to saponite ± serpentine in the green alteration. This suggests variations in alteration conditions starting with subaerial oxidative tropical weathering within Unit IV. Upon submergence of the lavas, relatively low temperature, oxidizing seawater alteration occurred at the top of the hole (producing the red-brown alteration) and transitioned to more reducing and higher temperature alteration toward the bottom of the hole (resulting in the green alteration).

The severe alteration has affected and modified the bulk-rock chemical compositions of the basement rocks significantly. In addition, chemical evidence is present for accumulation of olivine, and possibly clinopyroxene, crystals in the melt. The combined effects of crystal accumulation and alteration hinder interpretation of magmatic liquid compositions. However, shipboard measurements indicate that the Site U1349 samples are the least differentiated of any Shatsky Rise basement rocks analyzed. Both the lava section of Unit IV and the autobreccia of Unit V appear to be low-TiO₂, low-Zr tholeiitic basalts with similarities to primitive ocean-ridge and OJP basalts (Fig. F11).

Aspects of the mineralogy, especially the compositions of clinopyroxenes and intergrown plagioclase in the ophimottled rock, and of Cr spinel throughout, might further help in constraining the primary composition of the Hole U1349A lavas (i.e., prior to alteration). Perhaps the most pressing question concerning the pervasive alteration of stratigraphic Units IV and V has to do with the high alteration temperature inferred for some portions of these units. Oxy-exsolution is observed in all titanomagnetite crystals and may indicate that alteration approached temperatures up to 600°C. However, no secondary minerals indicative of such high temperatures (epidote, actinolite, etc.) were observed.

Structural data are consistent with the interpretation that Hole U1349A penetrated several stacked sheet flow units, some of which are intercalated with limestone. Evidence for apparently subaerial features (including pahoehoe structures) and submarine structures (including pillow lava and brecciated pillow lava) is clear. The typical sheet flows at this site have three parts: (1) an upper lava crust with horizontal zones of hollow vesicles, (2) a middle lava core with quite regular, well-developed veins in a fine-grained groundmass and low vesicularity, and (3) a basal zone with fewer vesicles and few veins. In this hole, no joints are observed, but veins are well developed. Vein dips become steeper downhole in the upper part of the igneous succession, become shallower again, and then steepen downhole again.

Physical property measurements can be used to distinguish four of the five units (II–V) of Hole U1349A. No discrete samples were taken from the chert fragments of Unit I, nor was the material continuous enough for whole-round track measurements. Volcaniclastic sandstones and claystones of Unit II and volcaniclastic sandstones and lapillistones of Subunit IIIa yielded magnetic susceptibilities <1000 x 10^–5 SI, GRA densities <2.0 g/cm³ and natural gamma radiation (NGR) counts between 10 and 40 cps. Three discrete measurements of Unit II and Subunit IIIa revealed high porosities (20%–40%) accompanied by P-wave velocities <4.0 km/s. Thermal conductivity measurements yielded low values between 1.01 and 1.57 W/(m·K), typical of sedimentary material.

Beneath the sandstones of Unit II and Subunit IIIa, a volcaniclastic conglomerate (Subunit IIIb) can be distinguished clearly by excursions to extremely elevated magnetic susceptibility. A distinct spike reaching almost 4000 x 10^–5 SI, the highest value measured during Expedition 324, is derived from an individual volcanic clast in Section 324-U1349-7R-1 containing abundant groundmass magnetite. The upper oxidized portion of the vesicular basalts of Unit IV, lying below the conglomerate, has low magnetic susceptibility (<1000 x 10^–5 SI) and low total NGR counts (<20 cps) similar to sedimentary Unit II and Subunit IIIb, but is distinguishable by higher GRA density (>2.0 g/cm³). Discrete sample measurements confirm higher bulk density (>2.5 g/cm³), lower porosity (<10%), and higher P-wave velocities (4.0–6.0 km/s) compared to the sedimentary units. Core 324-U1349-11R in Unit IV marks the end of the oxidized zone and shows an increase in magnetic susceptibility, averaging ~1000 x 10^–5 SI, with excursions to values >2000 x 10^–5 SI. The increase in magnetic susceptibility is not accompanied by any significant difference in GRA density, porosity, or P-wave velocity compared to the oxidized material above. Thermal conductivity measurements throughout Unit IV, irrespective of relative oxidation, yielded some of the highest values recorded during Expedition 324 and averaged 1.799 W/(m·K) ± 0.357 (2σ; N = 22).

The autobrecciated basalt of Unit V is clearly distinguishable from the vesicular basalts above by a decrease in magnetic susceptibility (average = 100 x 10^–5 SI). Total counts from the NGR show a muted decrease to 5–10 cps, whereas GRA density remains similar to that of the basalts above. Discrete sampling reflects the heterogeneous distribution of clasts within Unit V, showing large variations in porosity, P-wave velocity, and, to a lesser extent, bulk density. Thermal conductivity measurements reflect the higher porosities measured in the unit, yielding values lower than the basalts, with an average of 1.364 W/(m·K) ± 0.305 (2σ; N = 10).

A total of 32 basement samples were demagnetized for paleomagnetic studies (18 AF and 14 thermal demagnetizations). As expected, samples from volcaniclastic units show demagnetization results in which a characteristic remanent magnetization is difficult to isolate. These samples likely carry a depositional remanent magnetization, which makes the interpretation more complicated. Samples from olivine-phyric amygdaloidal basalt flows were characterized by a high-coercivity, high unblocking–temperature magnetization carrier. It is still unclear whether this magnetization is carried by titanomagnetite, in which case it is a true thermoremanent magnetization, or by hematite, in which case it is a chemical remanent magnetization. Both minerals have been identified in thin section. In any case, additional rock magnetic measurements and interpretations are necessary in order to interpret these results. The average inclination in Hole U1349A is –4.3 ± 5.9° (1σ), suggesting that the lavas in Hole U1349A were formed near the magnetic equator.

Downhole logging data obtained from Hole U1349A included natural and spectral gamma ray, density, and electrical resistivity measurements from three depths of investigation. Interpretations of gamma ray and electrical resistivity downhole logs were used to identify 19 logging units, 1 in the section covered by the BHA, 5 in the sedimentary sequences in the open hole interval, and 13 in the basaltic basement section. Electrical resistivity measurements show distinctive higher resistivity zones that likely represent less altered intervals interspersed with low-resistivity zones that mark more altered sequences. NGR measurements show a large peak just below the sediment/basement interface that may indicate a zone of concentrated hydrothermal alteration. This interval also displays very high U values and a smaller peak in K values. FMS images show zones with highly fractured intervals, potential veins, and vesicular or brecciated intervals.

Site U1350

Site U1350 is located on the lower east flank of Ori Massif (Fig. F18) where sediments are thin and the underlying acoustic basement appears to be relatively strong and smooth (Fig. F21). The site was chosen because it has a setting similar to Site U1347, which yielded relatively fresh basalts, and because it would provide a valuable comparison with the summit of Ori Massif (Site U1349). A single hole (U1350A) was drilled at the site, penetrating 143.1 m of sediment and 172.7 m of igneous basement to a total depth of 315.8 m (Table T1).

Because recovery of the soft sediments overlying the igneous basement was poor, only one purely sedimentary unit was defined at Site U1350 (Unit I); this unit spans Cores 324-U1350A-1W through 6R (Fig. F8). Although it is likely Unit I is composed of both chert and soft calcareous ooze or chalk, only chert (predominantly black and less commonly reddish) with minor amounts of occluded porcellanite was recovered. Well-preserved radiolarians, concentrated around relict burrow features, are common in the cherts of Unit I. Igneous basement was reached in Core 6R at 143.1 mbsf (Table T1), but additional sediments were encountered interbedded with the basaltic strata in Units II and IV. These sediments were predominantly fine-grained carbonates with a persistent radiolarian component and volcaniclastics. Varying quantities of assorted bivalve and brachiopod fossils were also present. Sedimentary interbeds were especially abundant in Unit IV (Cores 25R and 26R), where carbonate-rich oozes have been intruded by small pillow basalts prior to lithification (Fig. F22).

Calcareous nannofossils sampled from chert-rich lithologies of Unit I and Subunit IIa (Cores 324-U1350A-1W through 9R) are common to high in abundance and moderately to poorly preserved. The assemblage is indicative of Early Cretaceous age. However, these two units are almost barren of both planktonic and benthic foraminifers, with only a few silica-replaced specimens. The intercalated limestones of Unit IV are severely recrystallized and do not yield any calcareous microfossils. A low-diversity, poorly preserved assemblage of radiolarians is present throughout the examined Site U1350 sediments. No contact between the sediment of Unit I and the volcanic basement was recovered.

The upper part of the igneous basement (Unit II) (Fig. F9) contains both massive basalt flows and pillows and larger, pillowlike inflation units, with the percentage of massive flows decreasing with depth. Below this is a ~6 m thick layer of hyaloclastite and brecciated basalt (Unit III), followed by a succession of well-preserved plagioclase-phyric pillow lavas set in a matrix of intercalated micritic limestone (Unit IV), as described above. Unit II may be divided into an upper ~77 m thick succession (Cores 324-U1350A-6R through 16R) predominantly consisting of a series of larger inflation units, yielding recovered thicknesses of 1–2 m, with two larger (3–5 m thick) flows. This succession is sparsely intercalated with thin layers of volcanogenic limestone (Subunit IIa) (Fig. F23). Below is a middle (~19 m thick) transitional succession (Sections 324-U1350A-17R-1 through 19R-1) in which larger inflation units of 1–2 m thickness are intercalated with smaller pillow lava units (Subunit IIb). The pillows display chilled margins, glassy contacts, and vesicle distribution patterns similar to those of pillow units observed in Holes U1346A and U1347A. At the base of Unit II is a ~50 m stack of 0.2–0.9 m thick pillow lava units (Sections 324-U1350A-19R-1 through 24R-2) with well-preserved glassy contacts and chilled margins (Subunit IIc) (Fig. F24). Within Subunit IIc, the lowermost ~23 m consists of a well-preserved stack of units displaying pillow/pillow contacts, relatively thin glass rims, and chilled margins but lacking intercalated sediment layers. In this lowermost part, the pillow units are sparsely plagioclase phyric (up to 2%), and magnetic susceptibility abruptly diminishes to lower values characteristic of Units III and IV below.

The pillows of Unit II lie atop ~6 m of hyaloclastite (Unit III) (Figs. F9, F25) consisting of gravel- to sand-size glassy material, larger hyaloclastite fragments, and/or small pods (~5–15 cm) of aphyric basalt (Sections 324-U1350A-24R-3 through 24R-4. The aphyric basalt pods appear to be petrographically similar to Unit II above, but on the whole the hyaloclastite is a sufficiently different facies to merit is own division and represents a thin series of autobrecciated inflation units. The lowermost ~1 m of Unit III in Section 324-U1350A-24R-4 consists of gravel and pebble-size fragments only, preventing identification of the contact between Unit III and the underlying volcanic succession.

The lowermost, ~19 m thick sequence (Unit IV) (Fig. F9) consists of Cores 324-U1350A-25R and 26R with extremely high recovery (>90%). These cores preserve a stack of ~0.1–0.5 m thick plagioclase-phyric pillow lavas (~6% plagioclase), including the sedimentary material that occurs between individual pillow lavas (Fig. F22). Glassy rinds and thick, chilled margins can be examined readily, and pillow/pillow and pillow/sediment contacts are abundant throughout. Thermal alteration effects can be observed at most basalt/sediment contacts, especially in examples where the pillow lava was injected into soft sediment. The encasement of some pillow units within sediment indicates that the substrate onto and into which the pillow lavas were extruded was unconsolidated and/or fluidized by the entry of hot lava.

Thin section examination shows that crystallites in well-preserved glass rims are mainly plagioclase and clinopyroxene; spinel is not present in the basalts of Hole U1350A. Groundmass textures consist of networks of interlocking acicular plagioclase around which both clinopyroxene and titanomagnetite have crystallized to varying proportions, depending on grain size and rate of cooling. Only in the transitional succession of Subunit IIb do some well-developed networks of interlocking clinopyroxene and plagioclase occur as clumps or aggregates at all grain sizes.

The entire basement section (massive lava flows, pillow lavas, and hyaloclastites) has been affected by slight to high amounts of low-temperature water-rock interaction, resulting in complete replacement of glassy mesostasis and olivine phenocrysts and slight to almost complete replacement of groundmass minerals (plagioclase and clinopyroxene). In contrast, plagioclase phenocrysts are generally well preserved throughout the hole, except in Unit IV. Fresh glass is commonly preserved on the margins of flows and pillows in Subunits IIa and IIb, rarely preserved in Subunit IIc, and not preserved in Units III and IV. One type of gray alteration was identified, with significant variation in alteration degree, from slight to moderate in the upper flow succession and hyaloclastites (Units II and III), to moderate to high in the plagioclase-phyric pillow succession (Unit IV). Clay minerals, together with calcite, are the predominant secondary minerals in Hole U1350A, replacing primary phases and glassy mesostasis and filling vesicles and veins. Other alteration minerals observed in the basaltic rocks are pyrite and zeolites, present as filling vesicles and veins, and possible sanidine, replacing plagioclase phenocrysts in Unit IV. Four main vein types were identified in the basaltic rocks of Units II to IV: (1) calcite veins (± pyrite), (2) saponite veins, (3) calcite and saponite veins (± pyrite), and (4) pyrite veins. A total of 461 veins and vein networks were recorded in the recovered 75 m, corresponding to an average of 6.1 veins/m. Most of the veins in Hole U1350A are calcite veins, consisting of either crystalline blocky calcite or crossfiber calcite. Alteration of Hole U1350A samples is comparable to the alteration mineralogy encountered in Hole U1347A on Tamu Massif.

The chemical effects of the alteration are comparatively modest at Site U1350. The shipboard chemical data reveal that the basement section is composed of variably evolved tholeiitic basalts (Fig. F11). Broad similarities with the Site U1347 and Site 1213 basalts are present, but the Site U1350 lavas also differ in important respects: for example, they show systematic downhole variation in several key chemical parameters, exhibit a wider range of TiO₂, Zr, and Mg numbers, extend to higher Mg# values (as high as 68.5), and have higher Sr concentrations. They also show a wider spread of Zr/Ti ratios, implying that variations in amount of partial melting and/or source composition were important.

The overall structure of the basement can be divided by the hyaloclastite of Core 324-U1350A-24R (Unit III) into two kinds of extrusive structures: (1) sheet flow lavas with intercalated pillows in the upper part and (2) piled-up pillows in the lower part. Both parts are characterized by synmagmatic structures, including inter- and intrapillow structures. Interpillow structures are structures observed in the rocks surrounding the pillows. In the upper part of the hole, the surrounding rocks are characterized by hyaloclastite breccias or calcite fillings. In the lower part of the hole, interpillow structures are characterized by limestone, which shows only weak bedding. Intrapillow structures are similar in texture throughout the hole, but differ in size from ~5 to ~30 cm depending on the size of the pillows. They are characterized by thin glassy margins, radially aligned vesicles, concentric vesicular zones, and spheroidal shapes. In addition, structural features can be divided by syn- and postmagmatic structures. Synmagmatic structural features are represented by amygdaloidal structure, irregular vein networks or curved veins, and breccias. Postmagmatic structures are conjugate veins and joints. Dip angles of both veins and joints vary irregularly with depth. A larger population of veins and/or joints is found in the lower part of the hole (Unit IV).

The physical property data separate the basement into three distinct units or divisions based on magnetic susceptibility: (1) Cores 324-U1350A-7R through 11R (~150–190 mbsf), (2) Cores 12R through 21R (~190–270 mbsf), and (3) Cores 22R through 26R (~270–316 mbsf), and into three distinct, but different, units based on NGR: (1) Cores 324-U1350A-7R through 10R (~150–175 mbsf), (2) Cores 11R through 24R (~175–295 mbsf), and (3) Cores 24R through 26R (~295–316 mbsf). Interestingly, neither set of divisions corresponds with any of the stratigraphic units, except for the lowermost NGR division. Magnetic susceptibility shows a decreasing downhole trend from >2000 x 10^–5 SI in the upper portion of Subunit IIa to <1000 x 10^–5 SI in the lowermost Unit IV. A sharp decrease in magnetic susceptibility is found between Cores 324-U1350A-21R and 22R within the pillow basalts of Subunit IIc. This decrease corresponds to the disappearance of sediment interbeds and the onset of a continuous stack of pillows.

Data from the Natural Gamma Ray Logger yields generally low counts (<5 cps), with two notable exceptions. The exceptions occur in the upper portion of Subunit IIa and in the lower part of Unit IV. Examination of spectra from both of these intervals shows that counts are dominated by products of ⁴⁰K decay. This is in agreement with the increased K-rich alteration (clays) seen in the lowermost part of Unit IV. The GRA bulk density is consistently around 2.5 g/cm³ throughout Units II–IV. Bulk density measurements of discrete samples agree fairly well with the maximum values of the whole-round data, having uniform values with an average of 2.61 ± 0.17 g/cm³ (2σ). Porosity measurements on discrete samples are from 3.43% to 28.45% and display a good negative correlation with P-wave velocity, as expected. P-wave velocity shows no appreciable anisotropy and averages 4.793 ± 1.249 km/s, which is typical for basaltic material and of a similar range to measured velocities in igneous rocks in all sites. Overall, discrete physical property measurements do not change significantly with different stratigraphic units.

A total of 42 samples were analyzed for paleomagnetism (18 AF and 24 thermal demagnetizations). AF demagnetizations were characteristic of low-coercivity magnetic minerals, except for the samples from Sections 324-U1350A-25R-2 to 26R-1. All the AF-demagnetized samples have a stable direction pointing toward the origin, but the inclinations derived from principal component analysis show a large spread, between –12° and 27°. Thermal demagnetizations indicate a large distribution of Ti content in the samples. Principal component analysis carried out on the thermal demagnetization specimens defined stable magnetization components but also led to a scatter of inclination values. Overall, inclinations are shallow, both positive and negative, but with large variations even within the same stratigraphic unit.

Because the logging tools were unable to leave the drill pipe and enter the open hole, only gamma ray measurements could be recorded from inside the BHA. The gamma ray data in the shallow sediments show an anomaly from the seafloor to ~25 m wireline log matched depth below seafloor. The contributions to this anomaly are mainly an increase in Th and a smaller contribution from U.

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