Basement material from Hole 1243B was first recovered in Core 203-1243B-2R at 108.6 mbsf (curated depth). Basement was drilled and cored to a total depth of 195.3 mbsf, which represents 87.1 m of basement section. Recovery averaged 25%, ranging from 1.6% in Core 203-1243B-16R to 63.7% in Core 7R (this recovery statistic does not include 5.3 m of drilling breccia/cuttings recovered in the deepest core) (Fig. F11). On the basis of hand specimens, thin section descriptions, and shipboard geochemical analyses (see also "Core Descriptions" in the "Explanatory Notes" chapter), eight basement units were defined (Fig. F11; Table T2). Units 1, 3, 4, 5, 6, 7, and 8 are volcanic basaltic units. Units 1, 3, and 7 are aphyric basalt. Units 4, 5, and 6 are sparsely plagioclase and olivine phyric basalt. Unit 8 consists of moderately plagioclase and olivine phyric basalt. Unit 2 is formed by a piece of limestone (see also "Sedimentology"). All basement basaltic units are interpreted as pillow lavas based on the presence of glassy margins and associated vesicular zones. No evidence of thicker massive lava flows was found in the cores. This interpretation of the environment of eruption is further confirmed by downhole measurements in Hole 1243B (see "Downhole Measurements"). ICP-AES analyses conducted on board indicate that all units are tholeiitic except Unit 4, which consists of alkali basalt (see also "Geochemistry"). The basement units range in thickness from 0.065 m (Unit 2) to 11.175 m (Unit 3) (Table T2). At the bottom of Hole 1243B, 5.3 m of drilling breccia was recovered. This consists of broken angular fragments of pillow basalt (Core 203-1243B-19B).
This section describes the different lithologies and petrographic primary features of the basement sequence in Hole 1243B as observed in hand specimens and thin sections. Detailed descriptions of each core section can be found in "Site 1243 Visual Core Descriptions." Detailed description of the thin section can be found in "Site 1243 Thin Sections." The main features of thin sections are reported in Table T3. Further information on alteration assemblages, veins, and vesicle fillings can be found in "Alteration."
Basalt recovered from Hole 1243B is aphyric to sparsely plagioclase and olivine phyric (Fig. F11). Textures in the groundmass are generally aphanitic to fine grained. The degree of alteration, overall, is low (see Fig. F11) (see "Alteration"). Glassy rinds are present throughout the recovered basement section, and most of the glass is slightly altered, though some samples of fresh glass were found. Textural gradations were observed along pillow margins, with coarser-grained material present further from the glassy rind. Vesicularity ranges from nonvesicular in Units 7 and 8 up to sparsely vesicular with some moderately vesicular pieces in Units 3, 4, and 6 (Fig. F11). Rounded to elongate vesicles are present throughout the units, ranging in size from <1-mm- to 2-cm-long pipe vesicles. Twenty-three shipboard samples were taken for thin section observations from both pillow rims and pillow interiors, including some samples selected for alteration features. Microphenocrysts of plagioclase and olivine are mostly fresh and euhedral to subhedral. Quenched plagioclase crystals are evidenced in thin sections by skeletal and swallow-tail structures. Opaque minerals in the groundmass include euhedral magnetite and needle-shaped ilmenite.
Basement Unit 1 is composed of aphyric basalt (Fig. F12A). This unit consists of pillow lavas, some showing glassy margins. The groundmass is holocrystalline and composed of euhedral lath-shaped plagioclase (An48-50), subhedral, prismatic augite clinopyroxene, interstitial glass, and opaque minerals (Fig. F12B, F12C). Opaque minerals are euhedral to subhedral cubic magnetite crystals and rare elongate needles of ilmenite. Blotches of primary sulfides are also present. Unit 1 is sparsely vesicular, with randomly distributed empty (60%) and filled (40%) round vesicles.
Unit 2 is a palagonite- and peloid-bearing foraminiferal limestone with manganese oxide granules up to 5 mm across and randomly distributed throughout the rock. The texture is sparse foraminiferal pelbiomicrite, formerly wackestone (see "Sedimentology").
Unit 3 comprises aphyric pillow basalt very similar to that of Unit 1 (Fig. F13A). The groundmass is hypocrystalline and consists of plagioclase, clinopyroxene, glass, and opaque minerals (Fig. F13B, F13C). The plagioclase crystals are euhedral to subhedral with tabular, lath, swallow-tail, needlelike, and radial shapes/forms. The clinopyroxene crystals (augite) are generally subhedral to anhedral, ranging in size from 0.1 to 0.5 mm. Magnetite is the dominant opaque mineral, identified by its euhedral cubic form. Unit 3 is sparsely to moderately vesicular with randomly distributed filled and empty vesicles.
Unit 4 comprises pillowed sparsely plagioclase and olivine phyric basalt (Fig. F14A). The boundary between Units 3 and 4 was not clearly observed but was inferred from the appearance of microphenocrysts in Unit 4. Plagioclase microphenocrysts (An48-50) are euhedral and tabular to lath shaped, range in size from 0.4 to 2 mm, and are associated with minor amounts of euhedral olivine microphenocrysts (Fig. F14B, F14C). The groundmass is aphanitic to fine grained with a hypocrystalline texture and consists of plagioclase, clinopyroxene, olivine, glass, and opaque minerals. The presence of olivine in the groundmass denotes the alkaline affinity of the basalt (see also "Geochemistry"). Two different types of clinopyroxene were identified in thin section: augite, clinopyroxene, and scarce, but easily distinguishable, brown Ti-enriched augite (Fig. F14D). Some minor amounts of spinel (<1%) were identified in thin section. Unit 4 is sparsely to moderately vesicular with rounded to elongate, randomly distributed vesicles ranging up to 2 mm in length.
Unit 5 comprises pillowed sparsely plagioclase and olivine phyric basalt (Fig. F15A). The boundary between Units 4 and 5 was inferred from the absence of olivine in the groundmass, brown clinopyroxene, and geochemical differences (see "Geochemistry"). Microphenocrysts of plagioclase and olivine are euhedral to subhedral (Fig. F15B, F15C). The groundmass in Unit 5 is fine-grained hypocrystalline comprising plagioclase, augite clinopyroxene, glass, and opaque minerals (Fig. F15D). Unit 5 is sparsely vesicular, having rounded randomly distributed filled and empty vesicles (Fig. F15E).
Unit 6 comprises pillowed sparsely plagioclase and olivine phyric basalt (Fig. F16A). The boundary between Units 5 and 6 was inferred from the presence in groundmass of brown clinopyroxene (likely Ti-enriched augite) (Fig. F16B, F16C). Tabular to lath-shaped plagioclase and olivine microphenocrysts are euhedral to subhedral, varying in size from 0.6 to 2 mm and from 0.5 to 1 mm, respectively (Fig. F16D). The groundmass is aphanitic to fine-grained hypocrystalline and consists of plagioclase, clinopyroxene, glass, magnetite, and ilmenite (Fig. F16A). Vesicularity ranges from sparse to moderate. Rounded empty and filled vesicles are randomly distributed through the unit (Fig. F16A).
Unit 7 comprises pillowed aphyric basalt (Fig. F17A). The boundary between Units 6 and 7 was inferred from the disappearance of microphenocrysts in Unit 7. The groundmass is holocrystalline and composed of plagioclase, augite clinopyroxene, glass, and opaque minerals (mainly magnetite) (Fig. F17B, F17C). Several plagioclase aggregates were observed through the unit. Vesicularity ranges from nonvesicular to sparsely vesicular; filled and empty vesicles are <0.5 mm, equant, and randomly distributed (Fig. F17A).
Unit 8 comprises pillowed moderately plagioclase and olivine phyric basalt (Fig. F18A). Skeletal plagioclase microphenocrysts up to 0.7 mm in size are present, sometimes aggregated. Olivine microphenocrysts are euhedral, fresh, and have an average size of 0.3 mm (Fig. F18B, F18C). The groundmass texture is aphanitic, hypohyalline, and variolitic (Fig. F18D). The groundmass is composed of plagioclase, augite clinopyroxene, glass, and opaque minerals. Textural gradation is well observed in Section 203-1243B-18R-1 [Pieces 4, 5, and 7] from a glassy rim to a vesicular interior with vesicles ranging up to 1 mm in diameter. Vesicularity ranges from nonvesicular to sparsely vesicular with rounded empty or filled vesicles.
The pillow lavas of the Hole 1243B basement sequence show the following characteristic zones, from their upper outer part to their interior:
The lower part of the pillow displays identical features, but the thickness of each zone is approximately one-half that of the upper part. Glassy margins may be observed from the tops of pillows (upper glass zone [UGZ]), bottoms (lower glass zone [LGZ]), or both (upper and lower glass zone [ULGZ]). In Hole 1243B, basement sequence glassy margins were observed in 46 rock pieces (12.8% of basement recovered). Among those 46 samples, only 11 pieces are oriented. Nine pieces (19.6%) and two pieces (4.3%) are from UGZ and LGZ, respectively. Rock pieces with ULGZ were not recovered in Hole 1243B nor was interpillow material recovered. More glassy pillow tops than pillow bottoms were recovered, which can be explained by:
The relationship between core recovery and pillow texture is discussed further in the "Appendix."
Major and trace element abundances were determined by ICP-AES (see "Core Descriptions" in the "Explanatory Notes" chapter) for nine samples from Hole 1243B. The selected samples were taken from pillow interiors, and the results are reported in Table T4. At least one sample from each basement unit was analyzed, except Unit 6, which was not sampled for ICP-AES.
Basement Units 1, 3, 5, 7, and 8 are tholeiitic basalt, and Unit 4 is composed of alkali basalt, following chemical (alkali silica diagram) and petrographic classifications (Fig. F19). Al2O3, Ti/Zr, and Ba/Sr are reported vs. depth in Figure F20. No systematic variations were observed downhole except for the distinctive characteristics of alkali basalt in Unit 4. Units 1, 3, 5, and 8 display very similar geochemical characteristics, whereas Unit 7 shows a slightly lower content of Al2O3 and lower Ti/Zr ratios.
Within the tholeiitic units, the samples are almost unaltered, as evidenced by their low loss on ignition (LOI) values (LOI = <0.90 wt%) (see Table T4) (see "Alteration"), and this confirms the petrographic observations. The samples exhibit moderate compositional variation within narrow ranges of SiO2 (49.99-51.64 wt%), MgO (5.74-7.29 wt%), or TiO2 (1.43-2.12 wt%) (Table T4). Mg# (Mg# = Mg/[Mg+Fe2+] x 100, with Fe2+ estimated as 85% of total iron) for this group of units, ranges from 50.2 to 65.7 (Fig. F21; Table T4). The samples from Units 1 and 3 display the most primitive compositions (Mg# = 59.4 - 65.6). Variation diagrams of Al2O3 vs. Mg# show a trend consistent with slight variations in the proportions of fractionated phases (i.e, olivine, plagioclase, and clinopyroxene) between the different units (Fig. F21). Units 1, 3, and 5 display very homogeneous characteristics; Units 7 and 8 are slightly more differentiated (Mg# = 55.6-50.1). Massive fractionation of plagioclase, and subsequently magnetite, is consistent with the lower Al2O3 content of Unit 7. The trace element ratio Ti/Zr is plotted against Zr/Y and Ba/Sr ratios in Figure F22. Ti/Zr ratios are slightly higher in Units 7 and 8 as are Zr/Y ratios, which is consistent with their more fractionated characteristics. The relatively unaltered nature of these samples allows us to consider Ba and Sr concentrations as representing the original magmatic elemental composition. We observe a constant Ba/Sr ratio for these units (Fig. F22). In a primitive mantle-normalized trace element diagram (Fig. F23), samples from Units 1, 3, 5, 7, and 8 have generally subparallel patterns, with (Zr/Ti)N and (Zr/Y)N ranging from 1.2 to 1.9 and from 1.1 to 2.2, respectively.
The alkali basalt from Unit 4 shows distinctive geochemical features. Because the LOI contents of these samples are higher (up to 2.7 wt%), we expected their alkali abundances to be affected by postmagmatic alteration. However, abundances in Y, Nb, Ba, and Sr are very similar in the altered (LOI = 2.7 wt%) and least-altered (LOI = 1.4 wt%) samples from Unit 4, indicating that secondary processes have not affected these elements (Figs. F20, F21, F22; Table T4). On this basis, we tentatively conclude that the high TiO2 (2.11-2.12 wt%), along with Ba, Zr, and Nb contents, are original magmatic features of Unit 4 basalt. Compared to the tholeiitic basalt of Units 1, 3, 5, 7, and 8, basalt from Unit 4 has higher Ti/Zr, Ba/Sr, and Zr/Y ratios (Fig. F22). In a primitive mantle-normalized incompatible element diagram, they show an enriched pattern with (Zr/Ti)N ranging from 1.8 to 1.9 and (Zr/Y)N from 2.2 to 2.3, similar (at least for the shipboard incompatible trace element data) to those of alkali basalt worldwide (Fig. F23). The alkaline signature of Unit 4 is consistent with the fractionation of Ti-enriched clinopyroxenes, which appear to join olivine rather than plagioclase as a fractionation phase.
The shipboard petrographic and geochemical data led to the following preliminary conclusions, which will require careful testing as additional geochemical and isotopic data become available. The petrographic and geochemical characteristics of Units 1, 3, 5, 7, and 8 indicate relatively unevolved lavas with minimal differentiation at the crustal level. These tholeiitic units likely derived from a single mid-ocean-ridge basalt (MORB)-type mantle source. The partial melting event that led to the formation of these magmas probably took place at a shallow level with moderate partial-melting degree (>5%). The distinctive geochemically enriched signature of Unit 4 does not match with the identical moderate-high degree of partial melting of a depleted MORB-type mantle source (the terms "enriched" and "depleted" are used to indicate relative enrichment/depletion in the highly incompatible element abundances). Such enrichment in trace elements can be related to (1) the low partial-melting degree of a depleted MORB-type mantle source—a source similar to the one from which the tholeiitic basalt originated or (2) the partial melting of a distinctive (enriched) mantle source such as metasomatized upper mantle. No systematic spatial and temporal relationships were determined for the Hole 1243B basement section, and the petrogenetic conditions in which these tholeiitic and alkaline magmas formed remain tentative, pending further shore-based studies.
Eight basaltic basement units were identified in Hole 1243B. Most basaltic rocks have undergone slight secondary alteration. Alteration mineralogy was defined in rocks from Hole 1243B by color, habit, and hardness in hand specimens, by optical properties in thin sections, and by analogy with well-studied minerals identified during previous legs. Because of the low degree of alteration, no XRD measurements were conducted on samples from Hole 1243B, and the identification of secondary minerals in Hole 1243B, therefore, remains tentative pending further shore-based studies.
The effects of alteration in rocks from Hole 1243B are defined within the basaltic units in terms of alteration assemblages, vein and vesicle filling when present, and alteration chemistry.
In this section, we describe the alteration paragenesis for each basaltic unit, as recorded during core description and following thin section examination. We also include the vein and vesicle filling as recorded during hand-specimen observation. Additional data are available in the thin section descriptions and the alteration and vein logs (see the "Core Descriptions" contents list).
All the aphyric basalt recovered in Unit 1 is generally fresh, but the extent of alteration slightly increases toward vesicles and fine-grained margins. Units 3 and 7 are slightly altered. The alteration assemblages and vein and vesicle fillings are identical for these three units, and typical assemblages are presented in Figure F24. Color throughout the units varies from medium gray to medium light gray to light gray (N5-N6-N7) in the interior of lobes to light brown (5YR 5/6), dark yellowish orange (10YR 6/6), brownish brown (10YR 5/4), moderate brown (5YR 4/4), and moderate reddish brown (10YR 4/6) at altered fine-grained lobe margins, around veins, and next to vesicles. In very few sections, a greenish black (5G 2/1) was recorded. As the colors show, Fe oxyhydroxide formation is pervasive through the unit and is most prominent at the margins. Most of the groundmass is fresh or slightly altered to brown clay mineral (nontronite) and Fe oxyhydroxide. Glass is unaltered to moderately replaced by palagonite. In Unit 1, vesicles are sparse, and some are filled and/or lined with Ca carbonate (calcite) and zeolite. One vein was observed to be filled by small amounts of greenish clay minerals and Ca carbonate (calcite). Unit 3 is sparsely veined. Veins are 1-2 mm wide and are mostly filled with Ca carbonate (calcite), brown clay (nontronite), zeolite (analcite), and Fe oxyhydroxide (Fig. F24). Unit 3 is sparsely to moderately vesicular. Some vesicles are filled with and/or lined by Ca carbonate (calcite), brown clay (nontronite), and zeolite (analcite). In one section, vesicles are filled with green clay (saponite) (Fig. F24). Unit 7 is sparsely vesicular to nonvesicular, some vesicles are filled with and/or lined by Ca carbonates (calcite), brown clay (nontronite), and Fe oxyhydroxide.
Units 4, 5, and 6 consist of sparsely plagioclase and olivine phyric basalt. The degree of alteration is slight to moderate through Units 4 and 6, although it should be noted that the extent of alteration rarely exceeds 10% replacement, with a maximum of 15%. Unit 5 is slightly altered. The alteration assemblages and vein and vesicle fillings are identical for these three units, and typical features are presented in Figure F25. Color varies from medium gray to medium light gray to light gray (N5-N6-N7) to dark yellowish orange (10YR 6/6), moderate yellow brown (10YR 5/4), and pale yellowish brown (10YR 6/2). The maximum degree of alteration is found in Section 203-1243B-7R-2 [Pieces 1A-1C], which shows a light olive-gray color (5Y 5/2). Olivine microphenocrysts are partly replaced by reddish brown Fe oxyhydroxide (iddingsite). Plagioclase microphenocrysts are slightly altered to sericite. Part of the groundmass is replaced by brown clay (nontronite) and Fe oxyhydroxide. Glass is replaced by palagonite in the margins (Fig. F25). Veins are sparse (0.1-1 mm wide) and filled with Ca carbonate (calcite), brown clay (nontronite), and Fe oxyhydroxide (Fig. F25). One wider vein (4 mm) is found in Section 203-1243B-7R-1 [Piece 11] and is filled with the same assemblage. Some vesicles are filled with and/or lined by Ca carbonate (calcite), zeolite (analcite and phillipsite), brown clay (nontronite), and Fe oxyhydroxide.
Unit 8 consists of slightly altered moderately plagioclase and olivine phyric basalt. Color varies within the unit from medium light gray (N6) to dark gray (N3). The altered part is dark yellowish orange in color (10YR 6/6) because of the Fe oxyhydroxide formation. Olivine is partly replaced by reddish brown Fe oxyhydroxide (iddingsite). Some plagioclase microphenocrysts are slightly altered to sericite. Groundmass is altered to brown clay and Fe oxyhydroxide. Glass is replaced by palagonite. Unit 8 is sparsely vesicular to nonvesicular. Some vesicles are filled with and/or lined by zeolite.
All shipboard samples analyzed during Leg 203, except one (Sample 203-1243B-9R-2, 135-138 cm), have LOI values <2 wt%. LOI is a proxy for the degree of alteration and is reported vs. depth on Figure F26. LOI values vary downhole from 0.1 to 2.7 wt%, with an increase in Unit 4. This trend confirms macroscopic and petrographic observations of the samples, with a very low degree of alteration for the all basement section recovered. Mobile elements such as K, Sr, or Ba show the same geochemical characteristics as immobile elements such as Ti, Zr, or Y, indicating very low to no chemical mobility related to alteration (see Fig. F20) (see "Geochemistry").
Overall, the basement sampled in Hole 1243B is fresh to slightly altered. The alteration paragenesis is clearly dominated by Fe oxyhydroxide, brown clay (nontronite), Ca carbonate, and zeolite. These mineral associations are indicative of low-temperature interactions between rocks and seawater-derived fluids. This paragenesis is typical of the lowest-temperature stage of oceanic crust alteration (also known as "seafloor weathering" stage) (Alt, 1995). Temperature estimates for smectite (nontronite) formation are within the range of 30° to <150°C. The presence of Fe oxyhydroxide and smectite (nontronite and saponite) suggests circulation of large volumes of oxidizing fluids within the lava pile with a coeval precipitation of Ca carbonate (calcite) and zeolite formation.