IGNEOUS
PETROLOGY
Holes 1161A and 1161B were
cored into igneous basement from 116.0 to 145.3 and 158.5 to 167.0 mbsf,
respectively. Hole 1161A (Sections 1W-CC through 5R-1) was drilled 29.3 m into
basement, resulting in 4.39 m of recovered core (15.0% recovery). All recovered
material from this hole has been assigned to one lithologic unit: basaltic
rubble with intervals of basaltic breccia. The unit is composed of intermingled
lithic clasts, derived from aphyric to moderately plagioclase-olivine phyric
basalt.
Hole 1161B (Sections
187-1161B-1W-1 through 3R-1) was drilled 8.5 m into basement, resulting in 0.86
m of recovered core (10.12% recovery). This hole also yielded basaltic rubble
with intervals of basaltic breccia, similar to the material recovered from Hole
1161A, and all of this material was assigned to a single unit. The material from
both holes is interpreted as talus; no primary magmatic stratigraphic
significance can be associated with the position of individual pieces in either
hole.
Unit
1
Unit 1 of Hole 1161A
consists of basaltic rubble with several intervals of basaltic breccia (i.e.,
Sections 3R-2 [Pieces 7, 8, and 11], 4R-1 [Pieces 9-12], and 5R-1 [Pieces 11-14,
17]). At least three different igneous lithologies occur among the rubble clasts
(Table T2): (1)
light gray, microcrystalline, sparsely to moderately plagioclase-olivine phyric
basalt (Fig. F1);
(2) medium gray, microcrystalline, aphyric to sparsely plagioclase (± olivine)
phyric basalt (Fig. F2);
and (3) mottled buff to gray-brown, fine-grained aphyric basalt (Fig. F3).
Only the first two lithologies are observed within the breccias. All three
lithologies are intermixed throughout the hole, but Type 1 predominates,
constituting 61% of the recovered rubble clasts. Type 2 and Type 3 basalts occur
in similar proportions (i.e., 17% and 22%, respectively). All basalt types are
moderately to highly altered throughout the unit. In highly altered pieces,
alteration is characterized by pervasive replacement of olivine phenocrysts and
groundmass phases by a mixture of Fe oxyhydroxides and clay; in moderately
altered pieces, a similar type of oxidative alteration is concentrated in halos
that have formed adjacent to veins, fractures, and outer weathered surfaces (see
"Alteration").
Petrography
of Basaltic Rubble Clasts
Sparsely to Moderately
Plagioclase-Olivine Phyric Basalt (Type 1). Type 1 basalt consists of 1%-2%
equant olivine and 1%-3% tabular to prismatic plagioclase phenocrysts. Between
20% and 70% of the phenocrysts occur as glomerocrysts. These generally consist
of a relatively loose collection of prismatic plagioclase (<3 mm in length)
± small (<1 mm) and equant to skeletal olivine; more rarely these consist of
clusters of olivine phenocrysts or microphenocrysts. They probably form by
aggregation and/or equilibrium growth of phenocrysts during crystallization of
the magma. In contrast, larger plagioclase phenocrysts (3-4 mm) tend to have
equant (subhedral) or rounded (anhedral) shapes and occur as solitary crystals.
These typically display discontinuous zoning and are sieve-textured, suggesting
disequilibrium with the host magma. Some smaller prismatic phenocrysts also show
sieve textures, but these crystals tend to have fewer embayments, the embayments
tend to parallel cleavage or twin planes, and the crystals are less likely to
show discontinuous zoning. This suggests that the sieve textures in these
crystals developed as a growth feature rather than by partial resorption. All
plagioclase phenocrysts show albite twinning and are generally unaltered. Equant
olivine phenocrysts (0.5-3 mm) are present throughout and range from subhedral
to euhedral. However, fresh olivine is rarely observed, being totally replaced
by Fe oxyhydroxides ± clay in most pieces (see "Alteration").
Two subhedral spinel microphenocrysts containing melt inclusions up to 40 µm
were observed in Sample 2R-1, 10-13 cm (Fig. F4).
Groundmass textures are
microcrystalline to cryptocrystalline, ranging from intersertal (Fig. F5)
to immature sheaf quench morphologies to spherulitic with decreasing distance
from the chilled margins. Acicular to skeletal plagioclase (aspect ratios up to
30:1) forms 20%-40% of the groundmass, and small (<100 µm) equant olivines
form 2%-3% of the groundmass. In most cases clinopyroxene is restricted to
plumose quench crystal morphologies, but the size of clinopyroxene and Fe-Ti
oxide crystals is enhanced adjacent to and within miarolitic cavities (Fig. F5),
similar to some basalts from Site 1157. In these cavities clinopyroxene forms
anhedral, granular to elongate crystals up to 200 µm, and equant Fe-Ti oxides
as large as 50 µm are observed. Elsewhere, Fe-Ti oxides occur as minute (<2
µm) equant crystals and make up ~1%-2% of the groundmass. Dark brown mesostasis—which
here includes glass plus indistinguishable quench crystals of plagioclase,
clinopyroxene, and olivine—makes up >50% of the rock. Vesicles are rare.
Where present, they tend to be small (<0.3 mm in diameter), spherical, and
lined or filled with clay minerals (see "Alteration").
Aphyric to Sparsely
Plagioclase (± Olivine) Phyric Basalt (Type 2). Type 2 basalts are similar
to those of Type 1 but are distinguished by containing fewer plagioclase
phenocrysts (1%-2% tabular to prismatic crystals) and rare olivine
microphenocrysts (
1%).
Plagioclase phenocrysts also have a tendency to be small (<1-2 mm long),
although large solitary crystals up to 7 mm are also present. The latter tend to
be similar to the large plagioclase phenocrysts observed in Type 1 basalts,
i.e., they have equant (subhedral) or rounded (anhedral) shapes, display
discontinuous zoning, and are sieve textured. Approximately 25% of the
pheno-crysts occur as glomerocrysts, which in this basalt type are dominated by
small prismatic plagioclase crystals. All plagioclase phenocrysts show albite
twinning and are generally unaltered. Although not abundant (1%),
equant olivine microphenocrysts (<1 mm) are usually present and range from
subhedral to euhedral. Fresh olivine is observed in the less altered interiors
of some pieces, but, in most cases, it is totally replaced by Fe oxyhydroxides +
clay (see "Alteration").
Groundmass textures are
microcrystalline, ranging from intersertal to various quench morphologies (e.g.,
sheaf and plumose) close to chilled margins. Acicular to skeletal plagioclase
forms ~20% of the groundmass. However, in contrast to Type 1 basalts, groundmass
olivine is not present in the one available thin section (Sample 3R-1, 107-108).
Fe-Ti oxides occur as minute (<10 µm) equant crystals and make up ~1% of the
groundmass. Dark brown, quench-textured mesostasis constitutes 50%-75% of the
rock. Small spherical vesicles (<200 µm in diameter) are filled with clay ±
Fe oxyhydroxides in alteration halos but can be unfilled in less altered
portions of the rock (see "Alteration").
Aphyric Basalt (Type
3). Type 3 basalts are distinguished from Type 2 basalts by being fine
grained as opposed to being microcrystalline (Table T2).
They consist of <1% prismatic to tabular plagioclase microphenocrysts (1-2
mm) (Fig. F3),
and rare olivine microphenocrysts (<1 mm) in a subophitic to intergranular
groundmass (Fig. F6).
The groundmass consists of ~45% subhedral to anhedral plagioclase and ~35%
anhedral clinopyroxene. Most plagioclase crystals are normally zoned, but some
microphenocrysts and larger groundmass crystals show discontinuous zoning.
Alteration to a birefringent clay mineral commonly occurs along discontinuous,
subparallel microcracks (Fig. F7).
Although now totally replaced by Fe oxyhydroxides + clay (iddingsite), equant
olivine crystals (0.1-0.3 mm) originally composed ~15% of the rock (Fig. F6).
Fe-Ti oxide minerals range from equant to elongate shapes that fill interstitial
areas between silicate minerals and are as large as 300 µm.
Chilled Margins.
Chilled margins were recovered on 14 of the rubble clasts (i.e., 17% of the
pieces). Only one chilled margin was recovered on a Type 2 basalt (Sample 3R-2
[Piece 18]); the remainder are on Type 1 basalts. Few of the pieces retain a
significant thickness of clear glass, and most consist of glass plus spherulitic
quench crystals heavily altered to palagonite. In one case the chilled margin
appears to be partially replaced by blue cryptocrystalline silica and/or clay?
(see "Alteration").
Sample 3R-1 (Piece 12) is an exception, consisting of <1 mm of palagonite and
4-5 mm of clear glass + phenocrysts (Fig. F8).
The zone of coalesced spherulites is relatively thin (3-4 mm), and the
spherulites are small (~100-200 µm in diameter). The entire sequence of chilled
margin textures (i.e., spherulitic to quench plumose and sheaf morphologies)
extends for >3 cm into the interior of the piece. This sequence of quench
textures is typical of those observed in the other, albeit more altered, samples
with chilled margins from this hole.
Petrography
of the Breccia
There are several breccia
intervals in Unit 1 of Hole 1161A: Section 3R-2 (Pieces 7, 8, and 11), Section
4R-1 (Pieces 9-12), and Section 5R-1 (Pieces 11-14 and 17). The breccias are
poorly sorted (Fig. F9);
sorting by size or density is not apparent. Clast sizes range from small pebbles
(<4 cm) to coarse sand (<1 mm). Only clasts of basaltic derivation are
visible in the breccia; included are basalt, palagonite ± glass ± spherulites,
olivine (rare), and plagioclase. Whether individual basaltic clasts are derived
from aphyric or phyric basalt is difficult to determine because of their small
size (basalt clasts as small as 100 µm are readily identifiable) and, in some
cases, their high degree of alteration (see "Alteration").
However, larger aphyric and phyric basalt clasts are clearly identical to the
microcrystalline Type 1 and Type 2 basalts described in detail above.
Fine-grained aphyric basalt (Type 3), however, has not been identified as clasts
in the breccia.
Approximately 15% of the
sand-sized clasts are angular plagioclase crystals (Fig. F10).
Most of these are unaltered, but some are altered to a birefringent material
along microcracks (Fig. F11).
This alteration style is similar to that of the plagioclase crystals in the
fine-grained aphyric basalts (see Fig. F7),
suggesting that some feldspars may be derived from Type 3 basalts.
Basaltic clasts are
typically angular, whereas palagonite ± glass clasts are angular to subrounded;
the latter commonly show concentric layers of different colors that parallel the
shape of the piece (Fig. F9).
This suggests that at least some of the alteration occurred within the breccia
rather than before deposition (see "Alteration").
Basalt dominates among the clasts larger than ~5 mm, but palagonite and basalt
are present in roughly equal abundance in the very coarse sand to granule size
range (i.e., 1-4 mm).
The matrix material
between the breccia clasts is a cream to white clayey silt, most of which comes
from the breakdown of highly altered material derived from basaltic chilled
margins. Clasts that were originally composed of glass ± spherulites are now
composed of pale yellow to buff-colored clay; "ghosts" of spherulites
and plagioclase microlites can still be observed (Fig. F10).
Although some of these clay pseudomorphs retain original clast outlines, others
are partially disaggregated, and these appear to be the main source of
interstitial clay matrix for the breccia. Similarly, a single crystal of
unaltered olivine was observed in the breccia matrix in Sample 187-1161A-4R-1,
65-68 cm, adjacent to a moderately plagioclase-olivine phyric basalt clast (Fig.
F10). The fact
that olivine crystals are relatively rare in the breccia matrix, by comparison
with plagioclase, suggests that in most cases the olivine has been quickly
broken down into clay minerals and incorporated into the finer grained matrix.
The matrix is loosely
cemented by thin (~10 µm) selvages of crypto-crystalline silica and/or clay
that surround the clasts (Figs. F11,
F12A); locally it is
cemented by crystalline quartz (e.g., Section 187-1161A-4R-1). Mn oxide lines
the edges of some pore spaces between clasts, adjacent to the silica cement
(Fig. F12B). Patches of Mn-Fe
oxyhydroxides and Mn oxide concretions also are present throughout the matrix
(Fig. F10).
Minor amounts of calcite were detected by the reaction of the core pieces with
dilute HCl (~5%-10%) in a few places in Section 187-1161A-5R-1. Whether this was
due to the presence of a calcareous sediment or to local precipitation of a
calcite cement was impossible to determine because of the fine-grained nature of
the matrix.
Unit
1
Petrography
of Basalts
Similar to Hole 1161A,
Unit 1 of Hole 1161B consists of basaltic rubble with several intervals of
basaltic breccia: Section 187-1161B-1W-1 (Pieces 3-6) and Section 187-1161B-2R-1
(Pieces 2-4, 9). Four different igneous lithologies occur among the rubble
clasts. Three of these are identical to the basalts in Hole 1161A, i.e., Types
1, 2, and 3 (Table T2).
The fourth lithology in Hole 1161B (Type 4) is aphyric basalt, distinguished
from Type 3 aphyric basalt by its medium gray color and microcrystalline
groundmass texture (Table T2;
Fig. F13).
Types 1, 2, and 4 basalts are observed as clasts within the breccias in Hole
1161B; the fine-grained aphyric basalt (Type 3) is not.
The four types of basalt
clast are intermixed throughout the hole, but Type 4 aphyric basalt
predominates, constituting 47% of the recovered rubble clasts. Types 1 and 2
basalts occur in equal proportions (i.e., 23%), whereas Type 3 fine-grained
aphyric basalt makes up only 7% of the clasts. Types 1 and 2 basalts range from
moderately to highly altered. In highly altered clasts, the alteration is
characterized by pervasive replacement of olivine and groundmass by a mixture of
Fe oxyhydroxides and clay; in moderately altered pieces, a similar type of
oxidative alteration is concentrated in halos that have formed adjacent to
veins, fractures, and outer weathered surfaces. Type 4 aphyric basalts are
slightly to moderately altered, with alteration restricted to relatively narrow
alteration halos along edges of pieces and around fractures (see "Alteration").
No thin sections were made of basalts from Hole 1161B, so only macroscopic
observations are provided below. However, at this level of examination, Types 1,
2, and 3 basalts are virtually indistinguishable between the two holes. Only a
few differences were noted:
- One Type 1 basalt in Hole 1161B is highly phyric (Sample
187-1161B-3R-1 [Piece 3]; Fig. F14)
rather than moderately phyric. It contains ~4% equant olivine and ~8%
tabular to prismatic plagioclase phenocrysts. The phenocrysts are slightly
larger, on average, than in other Type 1 basalts, with plagioclase crystals
up to 8 mm long and equant olivines as large as 4 mm.
- Small (<0.2 mm) spherical vesicles occur in the Type 2
basalts in Hole 1161B and constitute <1% of the rock. These are filled
with a yellowish-green clay ± Mn oxide in altered areas but elsewhere are
unfilled (see "Alteration").
Miarolitic cavities up to 1.5 mm across were also identified in one piece,
Sample 187-1161B-2R-1 (Piece 10).
- Rare (<<1%) spherical vesicles as large as 1 mm in
diameter were observed in one Type 3 basalt (Sample 187-1161B-1W-1 [Piece
11]); these are concentrated on one side of the piece and filled with yellow
to green smectite.
Microcrystalline
Aphyric Basalt (Type 4). The microcrystalline aphyric basalt is medium
gray. It is devoid of phenocrysts, although small, sparse, olivine
microphenocrysts are present throughout. Spherical vesicles form ~1% of the rock
and range from unfilled to lined or filled with calcite or green to white clay
or blue cryptocrystalline silica.
Chilled Margins.
Chilled margins were recovered on three rubble clasts (10% of the pieces); one
is Type 2 phyric basalt and two are Type 4 aphyric basalts. None of the pieces
retain a significant thickness of clear glass—they consist of partially
palagonitized glass plus spherulitic quench crystals (see "Alteration").
The spherulites are small (<0.2 mm) in all cases.
Petrography
of the Breccia
Basaltic breccia occurs in
several intervals in Unit 1 of Hole 1161B: Section 1W-1 (Pieces 3-6) and Section
2R-1 (Pieces 2-4, 9). The breccias are similar to those recovered in Hole 1161A
(Figs. F15, F16,
F17, F18),
with only a few differences:
- Type 4 aphyric basalt clasts are common in the breccias in Hole
1161B, in addition to Type 1 and Type 2 phyric basalts (Fig. F15).
Chilled margins occur on clasts of all basalt types among the breccia clasts
(Figs. F16, F17).
- The maximum clast size is slightly smaller (<2.5 cm in Hole
1161B, ~4 cm in Hole 1161A).
- Basalt clasts in the breccias of Hole 1161B show a wider range
of alteration characteristics, pervasively altered basalt, basalt with clay,
and/or palagonite alteration halos, and/or clay pseudomorphs after basalt
clasts (Figs. F15, F17)
(see "Alteration").
As in Hole 1161A, spherulitic textures are still visible in the clay
pseudomorphs of basalt chilled margin clasts (Fig. F18).
- As in Hole 1161A, basalt dominates among the pebble-sized clasts,
and palagonite and basalt are present in roughly equal proportions in the
very coarse sand to granule-size range. However, in Section 187-1161B-1W-1,
palagonite ± glass dominates the 1- to 2-mm-size clast range. Sample
187-1161B-1W-1, 16-21 cm, for example, is made up entirely of palagonite +
glass fragments (Fig. F19).
However, this is not a hyaloclastite, since a contact between this breccia
type and one containing predominantly basalt clasts is seen in interval
187-1161B-1W-1, 25-31 cm (Fig. F16).
In addition, clasts showing a range of alteration characteristics are found
in the same piece (Fig. F15).
The latter suggests a multistage alteration history for some of the basalt
clasts; some of that alteration must have occurred prior to breccia
formation (see "Alteration").
- The matrix material between breccia clasts is similar in Holes
1161A and 1161B, being a cream to white clayey silt composed of highly
altered material derived from basaltic chilled margins. However, Hole 1161B
matrix material is cemented by a greater range of silica types (i.e., white
to blue-gray cryptocrystalline material to drusy or euhedral clear quartz).
Botryoidal to euhedral quartz is also observed filling vugs and pore spaces
(e.g., Section 187-1161B-2R-1).
