IGNEOUS
PETROLOGY
Introduction
Holes 1157A and 1157B were
cored into igneous basement from 200.0 to 216.4 and 130.6 to 171.0 mbsf,
respectively. Hole 1157A (Sections 187-1157A-2R-1 through 4R-1) was drilled
16.40 m into basement, resulting in 2.92 m (17.8%) recovery. All recovered
material from this hole has been assigned to one lithologic unit: basaltic
rubble with intervals of basalt-carbonate breccia. The unit is composed of
poorly sorted lithic clasts, derived from aphyric basalt and moderately
plagioclase-olivine phyric basalt. The material is interpreted as a talus pile;
no primary magmatic stratigraphic significance can be associated with the
position of individual pieces in the hole.
Hole 1157B (Sections
187-1157B-2R-1 through 9R-1) was drilled 40.4 m into basement, resulting in
11.70 m (28.96%) recovery. Lavas from this hole were assigned to a single
lithologic unit of moderately plagioclase-olivine phyric basalt. The unit is
interpreted as basalt pillow lava, based on the abundance of glassy chilled
margins recovered (i.e., 22% of all pieces), the presence of V-shaped
pieces, and pieces with curved glassy rinds.
Hole
1157A
Unit
1
Unit 1 of Hole 1157A
consists of basaltic rubble with two intervals of basalt-carbonate breccia
(Section 187-1157A-2R-1 [Pieces 3-7] and Section 3R-1 [Pieces 12-19]). There are
two different igneous lithologies among the rubble and breccia clasts: a dark
gray aphyric basalt and a medium gray plagioclase-olivine phyric basalt. The two
lithologies are present throughout the hole in roughly equal proportions. There
is no apparent variation in relative proportions with depth in the hole, and
both lithologies can be observed in the same piece of breccia in Section
187-1157A-3R-1 (Piece 6) (Fig. F1).
Both basalt types are characterized by weathered outer surfaces and are slightly
to moderately altered throughout the unit. Alteration is concentrated in halos
that have formed adjacent to veins, fractures, and outer weathered surfaces and
consists predominantly of replacement of olivine and groundmass by a mixture of
Fe oxyhydroxides and clay (see "Alteration").
Although the alteration halos for most breccia clasts lie adjacent to the
carbonate sediment matrix, not all clasts have alteration halos, indicating that
the weathering and/or alteration responsible for the halos occurred before
formation of the lithified rubble deposit.
Petrography
of Basaltic Rubble Clasts
Moderately
Plagioclase-Olivine Phyric Basalt. The moderately plagioclase-olivine
phyric basalt contains 1%-2% equant to skeletal olivine and 1%-6% tabular to
prismatic plagioclase phenocrysts. Plagioclase is typically seriate and,
although crystals range in size to 5 mm, most are <1-2 mm long. Larger
crystals tend to have equant (subhedral) or rounded (anhedral) shapes.
Sieve-textured plagioclase is common, although variable in character, ranging
from crystals with only embayed cores to crystals with only embayed rims. Zoning
is commonly discontinuous in sieve-textured plagioclase, but most crystals are
unzoned. All plagioclase phenocrysts are twinned. In more altered areas,
plagioclase crystals are stained along microcracks by Fe oxyhydroxides (see "Hole
1157A" in "Alteration"); elsewhere plagioclase is
unaltered. Olivine phenocrysts (0.5-3 mm) are present throughout and range from
equant (subhedral to euhedral) to skeletal; they are present both in
glomerocrysts in association with plagioclase and as discrete crystals. In
alteration halos, olivine phenocrysts are partially to totally replaced by Fe
oxyhydroxides and clay; elsewhere, they are usually unaltered (see "Hole
1157A" in "Alteration").
Groundmass textures are
microcrystalline and range from intersertal to immature sheaf quench
morphologies. Acicular plagioclase (aspect ratios up to 40:1) forms as much as
30% of the groundmass; small (<100 µm) equant olivines form ~2% of the
groundmass. In most cases, clinopyroxene is restricted to quench crystal
morphologies intergrown with plagioclase sheafs. However, in some areas the
crystallization of clinopyroxene is unusually enhanced, leading to formation of
granular, anhedral clinopyroxene as large as 20 µm. In Sample 187-1157B-3R-1,
97-101 cm, small crystals of clinopyroxene have euhedral terminations on one
end, indicating growth into a miarolitic cavity (Fig. F2)
that is now filled with secondary calcite (see "Hole
1157A" in "Alteration"). These cavities represent
areas of volatile concentration during the final stage of crystallization, which
would have facilitated the growth of clinopyroxene. Minute (<2 µm) equant
opaque minerals make up ~1%-2% of the groundmass. Dark brown mesostasis—which
here includes glass plus quench crystals of plagioclase, clinopyroxene, and
olivine that are not readily distinguishable—constitutes >50% of the rock.
Spherical vesicles are present in abundances of <1% and are typically very
small (<50-100 µm in diameter); they are usually unfilled; but, in some
altered areas, they are filled with calcite (see "Hole
1157A" in "Alteration").
Aphyric Basalt.
The aphyric basalt consists of <1% olivine microphenocrysts (0.3-0.8 mm) in a
microcrystalline groundmass dominated by sheaf quench textures (Fig. F3).
The olivine microphenocrysts are equant, and most are skeletal (Fig. F3).
The groundmass consists of ~35% acicular plagioclase (aspect ratio up to 50:1),
1%-2% small (<100 µm) equant olivine (Fig. F3),
and 1%-2% small (<20 µm) equant opaque minerals. Similar to the moderately
plagioclase-olivine phyric basalts described above, the groundmass contains some
areas in which clinopyroxene is present as larger, granular crystals rather than
the more typical quench plumose morphologies. Although miarolitic cavities are
not observed, the local enhancement of crystal growth suggests similar
conditions for the evolution and liberation of volatiles during crystallization.
As with the phyric basalts, these crystal clots are associated with secondary
calcite. Dark brown, quench-textured mesostasis makes up >50% of the rock.
Vesicles are usually small (<0.5 mm), uniformly distributed, and make up
<1% of the rock; some are filled with calcite, but most are unfilled.
Chilled Margins.
Chilled margins were recovered on nine of the rubble clasts (i.e., 18% of the
pieces). Six of these are aphyric and three are phyric basalts. Few of the
pieces retain a significant thickness of clear glass, and most consist of glass
+ spherulitic quench crystals. The zone of coalesced spherulites is typically
thin (3-4 mm), and the spherulites are relatively small (~100 µm diameter). One
exception is interval 187-1157A-3R-1, 124-132 cm, which is notable for its
coarser spherulitic texture (individual spherulites as large as 2 mm) and wider
chilled margin (>2.5 cm) (Fig. F4).
Petrography
of the Breccia
There are two breccia
intervals in Unit 1 of Hole 1157A (Section 187-1157A-2R-1 [Pieces 3-7] and
Section 3R-1 [Pieces 12-19]). The breccias are poorly sorted and are cemented by
a range of carbonate, carbonate + clay, and clay sediments and/or cements. The
clasts are angular and range from sand-sized particles of palagonite and altered
olivine to pieces of aphyric and moderately plagioclase-olivine phyric basalt
that are larger than the diameter of the core (>8 cm). The petrography of the
basalts is identical to the aphyric and phyric basalts described in detail
above.
The matrix varies from a
lithic-poor calcarenite (e.g., Section 187-1157A-2R-1; Fig. F5)
to a lithic-rich (~80% lithic fragments) calcareous sediment (e.g., Section
187-1157A-3R-1; Fig. F6).
In most places, the calcarenite is a mottled, pinkish gray. In thin section, it
is seen to consist of >95% round to oval grains, each of which is a single
calcite crystal (0.1-0.3 mm). The crystals are poorly cemented by interstitial
clay (Fig. F7).
The proportion of clay increases locally, and small (<2 mm) lithic fragments
are concentrated in these clay-rich areas. Included among the lithic fragments
are glass shards (unaltered and retaining quench crystals; Fig. F8)
and palagonite. The latter varies in appearance from laminated (Fig. F9)
to vermiform (Fig. F10).
Also present are angular fragments of unaltered feldspar (<0.3 mm maximum
dimension), but these constitute <1% of the lithic clasts. Mn oxide
concretions as large as 0.3 mm are common throughout, but most are <<0.1
mm (Fig. F7A).
The lithic-rich calcareous
sediment in Section 187-1157A-3R-1 (Fig. F6)
is similar in appearance to the lithic-rich areas of the calcarenite described
above but includes larger (e.g., 0.5-3 cm) clasts of slightly to moderately
altered basalt (Fig. F6),
as well as smaller fragments of palagonite and glass. Material associated with
basaltic chilled margins (e.g., palagonite and altered spherulitic basalt)
predominates. The fine-grained matrix consists of a mixture of pale pink to buff
clay and micritic calcite crosscut by an anastomosing network of sparry calcite
that encircles clasts, forms thin veins, and replaces both matrix and clasts.
Sediment was not recovered
in significant volumes in Sections 187-1157A-3R-2 through 4R-1, but small
remnant patches (a few millimeters to several centimeters across and <1 mm
thick) of micritic calcite and/or clay are observed adhering to the outer
surfaces of individual pieces. These patches and the ease with which the
sediment matrix parts from the larger basaltic clasts suggest that the breccia
sensu stricto may be underrepresented in the material recovered.
Hole
1157B
Unit
1
Petrography
of Basalts
Unit 1 of Hole 1157B is a
moderately plagioclase-olivine phyric basalt (Fig. F11),
the petrography of which is very similar to the phyric basalts recovered in Hole
1157A. It consists of 1%-2% equant to skeletal olivine and 1%-6% tabular to
prismatic plagioclase phenocrysts. The plagioclase phenocryst content varies
unsystematically throughout the unit, with some pieces appearing nearly aphyric.
Large plagioclase phenocrysts (up to 7 mm) are present, but most (>90%)
average 2-3 mm in length. The larger crystals tend to have blocky (subhedral) to
rounded (anhedral) shapes and are more likely to display discontinuous zoning as
well as sieve textures (both embayed cores and embayed rims were observed).
Smaller plagioclase phenocrysts are typically prismatic and unzoned, although
there are sieve textures in these crystals as well. Many plagioclase phenocrysts
are crosscut by microcracks that are stained with Fe oxyhydroxides (see "Hole
1157B" in "Alteration"), but otherwise plagioclase
is unaltered. Olivine phenocrysts are present throughout as equant (subhedral)
to skeletal crystals, most of which are <2 mm in size, although crystals as
large as 5 mm are observed. Outside of alteration halos, they are usually
unaltered. Cr spinel is present in trace amounts, both as small (<50 µm)
anhedral to subhedral inclusions in olivine and plagioclase and as larger (0.6
mm) euhedral microphenocrysts.
Between 10% and 30% of the
phenocrysts are included in glomero-crysts of plagioclase, olivine, and
plagioclase + olivine (± clinopyroxene). Two types are observed. One consists
of a relatively loose collection of prismatic plagioclase ± small equant to
skeletal olivine (Fig. F12)
that probably forms by aggregation and/or equilibrium growth of phenocrysts
during crystallization of the magma. The second glomerocryst type consists of
plagioclase partially to totally enclosed in olivine; this type is usually made
up of fewer, but generally larger, anhedral crystals. The textural relationship
requires that plagioclase crystallized before or together with olivine and that
the relationship of the crystals to each other is more typical of cumulate
textures (Fig. F13).
Glomerocrysts of this type may be xenocrysts incorporated into the magma from a
magma chamber. Clinopyroxene phenocrysts were only observed in hand specimen in
a single plagioclase + olivine + clinopyroxene glomerocryst in Section
187-1157B-4R-1 (Piece 12). Since clinopyroxene is not observed as a discrete
phenocryst phase anywhere in Hole 1157B, it is inferred that the clinopyroxene
is part of a xenocrystic glomerocryst.
Groundmass textures range
from intersertal to immature sheaf quench morphologies. Mineralogically, the
groundmass consists of as much as 50% acicular plagioclase (aspect ratios of up
to 30:1; see Fig. F14),
~2% small (<100 µm) equant olivine, and 1%-2% minute (<5 µm) equant
opaque minerals. As with the phyric basalts of Hole 1157A, clinopyroxene is
usually restricted to quench crystal morphologies intergrown with plagioclase
sheaves. However, miarolitic cavities are common and in some cases relatively
large—up to 0.7 mm across. Anhedral to subhedral crystals of clinopyroxene and
Fe-Ti oxide minerals line or partially fill these cavities, and the crystals are
significantly larger than those in the adjacent groundmass (Fig. F15).
Alteration tends to be higher in these areas as well (Fig. F16;
also see "Hole 1157B"
in "Alteration"). The miarolitic cavities may explain some of the
irregularly shaped pits observed on the cut surfaces of core pieces. Dark brown,
quench-textured mesostasis makes up >50% of the rock. Spherical vesicles
constitute <1% of the rock and are typically <100 µm in diameter; they
are usually unfilled but, in more altered areas, may be filled with calcite (see
"Hole 1157B"
in "Alteration").
Chilled margins are
present on 22% of the phyric basalt pieces recovered, and more than half of
these margins retain a significant thickness of clear glass (1-6 mm). Similar to
the phyric basalts of Hole 1157A, the zone of coalesced spherulites is typically
thin (3-4 mm) and composed of relatively small spherulites (~100 µm in
diameter). In Sample 187-1157B-2R-1, 48-51 cm, the spherulitic overgrowths on
plagioclase are <25 µm wide; overgrowths on olivine crystals are rare to
absent (Fig. F17),
suggesting very rapid cooling. The abundance of glassy chilled margins recovered
and the presence of V-shaped
pieces and pieces with curved glassy rinds (Fig. F18)
suggest that these basalts are pillow lavas.
Petrography
of Sediments
Sediment was not recovered
in significant volumes in Hole 1157B. However, small patches a few millimeters
to several centimeters across and 
1
mm thick are common throughout; larger accumulations adhere to the outer
surfaces of some pieces or serve as the matrix cementing fragments of glass +
palagonite (Fig. F19).
In addition, there are strong spatial and compositional relationships between
the sediments and some thick veins that crosscut the basalts, suggesting that
these represent a continuum of processes ranging from physical infilling to
chemical precipitation. For example, interval 187-1157B-3R-2, 109-130 cm (Fig. F20),
has a pocket of sediment at one end that is continuous with a composite vein (~3
mm wide) that extends >20 cm along the length of the piece. The vein is
typical of those observed throughout Hole 1157B and consists of three materials:
(1) pale buff micrite ± clay, (2) darker grayish brown micritic to very finely
crystalline sparry calcite, and (3) more coarsely crystalline sparry calcite
that crosscuts the first two. These composite veins do not appear to have formed
solely by precipitation of calcite and/or clay, since they commonly contain up
to 40% small lithic fragments (Fig. F21),
similar to those in the breccia matrix in Hole 1157A. The origin of the clasts
(i.e., whether they are derived from the adjacent wall rock or are transported
from elsewhere) is difficult to assess because most are in an advanced state of
alteration and disaggregation (Fig. F22).
Many are clearly disaggregated from the adjacent wall rock, but the high
abundance of palagonite fragments suggests that many are transported into the
pillow interior from its rim. Thus, the veins appear to have a complex origin
involving
- Infill of micritic calcareous sediment ± lithic fragments ±
fluids (e.g., Fig. F20),
- Reaction and dissolution of the wall rock, which replaces primary
phases with clays, Fe oxyhydroxides, fibrous amphibole/chlorite?, and/or calcite
(Figs. F22, F23),
- Recrystallization of micritic calcite to very finely crystalline
sparry calcite (Fig. F24),
and, finally,
- Generation of thin sparry calcite veins and vug fillings.
The veins and their effect
on the alteration of the basalts are described in more detail in "Hole
1157B" in "Alteration."
