Further published Results
Site M0077: Upper Peak Ring
"Upper Peak Ring interval lithologies are divided
into Units 2 and 3. Unit 2 is polymict breccia with impact melt
rock fragments and is thus suevite. Unit 3 is impact melt rock
with occasional (<25%) clasts. Units 2 and 3 are divided into
Subunits 2A–2C, 3A, and 3B based on sedimentary features and
matrix or groundmass characteristics.
S. Gulick, J. Morgan and others --
IODP Publications., Vol. 364 (2017);
The upper surface of Unit 2 in the lithologic sequence is defined
by the sharp stylolitized contact at the top of the cross-bedded
suevite in Section 40R-1, 109.4 cm (617.33 mbsf). Unit 2 is
an approximately 104 m thick suevite (interval 40R-1, 109.4
cm, to 87R-2, 90 cm; 617.33–721.62 mbsf) and is predominantly
composed of various types of green to black impact melt rock
clasts, as well as to a lesser degree lithic fragments, including
Mesozoic sedimentary rocks and crystalline basement rocks. The
maximum clast size, defined here as the long axis length of
the largest clast in each section, varies from 0.2 cm to more
than 25 cm. Overall, the unit has a fining-upward trend, although
upward coarsening or grain size oscillations in maximum clast
size are locally observable, especially in the upper part of
The second principal lithology of the Upper Peak Ring interval
consists of a ~25 m thick succession of impact melt rocks. It
is dominantly clast-poor impact melt rock, but clast-rich intervals
Subunit 3A is ~16 m thick (interval 87R-2, 90 cm, to 92R-3,
17 cm; 721.62–737.56 mbsf) and is characterized by a mixture
of dark green and black clast-poor impact melt rocks. The first
occurrence of the dark green melt rock marks the top of the
subunit. The green melt rock often forms a matrix with angular
fragments of black melt rock. These black melt rock fragments
commonly entrain various mineral fragments and <1 mm to ~1 cm
lithic fragments and also can contain vesicles and flow lines.
The green melt rock reacts to HCl and thus may be altered through
hydrothermal processes. The green melt rock was also found to
be intimately intermixed with black melt rock, forming multiple
millimeter to centimeter thin schlieren of green material within
the black melt rock.
Black impact melt rock also occurs in larger regions up to 0.5
m in size. These regions include centimeter- to decimeter-sized
clasts of target material. The smaller clasts are partially
resorbed and in some cases heavily shocked, making lithologic
identification difficult. Larger clasts in the impact melt rock
are composed of metamorphic and igneous target lithologies,
including decimeter-sized clasts of granodiorite (Sample 91R-1,
44–68 cm) and gneiss (Sample 91R-1, 94–110 cm) and rare clasts
of red sandstone (e.g., Sample 90R-3, 8–15 cm).
One thin section was available for this subunit (Sample 89R-3,
39–43 cm). The impact melt rock in this sample is variably altered,
with some parts consisting of microcrystalline melt rock with
tiny phenocryst laths in an aphanitic groundmass (with partly
digested clasts and blebs of opaque minerals), green domains
consisting of smectite (likely a secondary alteration product
from a formerly glassy melt), zeolite, silica, and chloritoid/chlorite.
Up to ~0.5 cm angular to subrounded clasts in the melt rock
are variably assimilated, some with reaction coronas. A few
silica clasts with ballen quartz occur, and one clast of quartz
with decorated planar deformation features was noted.
Subunit 3B occurs below the green schlieren-bearing black impact
melt rock and is a ~9 m thick, black, coherent, clast-poor impact
melt rock (interval 92R-3, 17 cm, to 95R-3, 117 cm; 737.56–747.02
mbsf). The contact occurs at the lowest occurrence of green
schlieren. Although the green schlieren are absent, the black
melt rock still contains observable flow banding and vesicles.
It also entrains a variety of mineral and lithic clasts. In
some parts, the clasts are evenly distributed. Other areas contain
clusters and subtle alignments of clasts. As in Subunit 3A,
clasts are composed of metamorphic and igneous target lithologies,
but Subunit 3B lacks visible clasts of sedimentary lithologies.
Granitoid clasts dominate and increase in abundance toward the
base of the subunit, reaching 42 cm in size before the base.
Subunit 3B contains clasts of previously solidified impact melt
rock that seem to have separated, and the gaps are filled with
melt rock. The lower surface of this subunit is the top of the
shocked granitoid target in Section 95R-3 at 747.02 mbsf.
One thin section was available for this subunit (Sample 93R-3),
revealing that this impact melt rock is composed of a fine-grained
matrix made of tiny plagioclase laths and opaque minerals (magnetite)
in a melt (now altered to clay minerals). Mineral clasts including
quartz, alkali-feldspar, and plagioclase occur, as well as partially
assimilated mineral and rock (mainly granitoids) clasts and
some melt rock fragments. Quartz grains display planar deformation
features and are in some cases toasted. Plagioclase grains with
possible planar deformation features were also noted."
About the composition of the "impact melt" in Unit 3
only this is in the article (no analysis of the "dark
"The lowermost portion, which is dominated
by impact melt rock, is chemically distinct with higher concentrations
of SiO2, TiO2, Al2O3, FeO, and, notably, oxide totals that are
on average 20 wt% higher than those of the suevite above."
"In Core 87R through Section 95R-2 (719.54–744.88 mbsf), the
mineralogical assemblage is mainly composed of albite and anorthite
(average = 30%), as well as quartz (average = 10%). Here, calcite
is almost completely absent in the samples. The clay fraction
is dominated by smectite (average = 20% of saponite) and traces
of mica (phlogopite and glauconite), chlorite (clinochlore),
vermiculite, clinoptilolite, and serpentine (lizardite and antigorite).
Accessory minerals are again highly variable, including feldspar
(sanidine, orthoclase, bassanite, and nepheline), zeolite (microcline
and andesine), silicates (talc, forsterite, sodalite, diopside,
and ferrosilite), evaporite (gypsum), and oxides (corundum and
magnetite). Of particular note is the occurrence of coesite,
a high-pressure polymorph of quartz, in Core 87R at 719.99 mbsf,
near the top of Unit 3."
Variety of mineral and lithic
clasts in "impact melt" rock from Subunit 3A (93R-3)
"Impact melt" rock sample from Subunit 3A.
A. Intermix of green and black impact melt rocks with
angular to subrounded clasts (89R-3).
B. Microcrystalline black melt rock and green domains
C. Melted silica clast in black melt rock.
Chicxulub Impact Structure - Study of Large Impact
Formation and Effects (conference talk and tutorial video)
Sean Gulick -- Scientific Computing with
Python, Austin, Texas, July 10-16, 2017
Some pictures with my short comment
no statement about the composition of this "dark melt"
SPECIAL SESSION: IODP-ICDP EXPEDITION 364 TO
THE CHICXULUB IMPACT CRATER; March 21, 2017:
Macro- and microscopic
Evidence of impact Metamorphism in Rocks from the Chicxulub
L. Ferričre et al. -- Lunar and Planetary Science XLVIII
The M0077A core was subdivided into three
main lithological units: a (post-impact) section (from 505.7
to 617.3 mbsf), an upper (peak ring) section of "suevite" and
melt rocks (from 617.3 to 747.0 mbsf), and a lower (peak ring)
section mainly consisting of granitoid rocks (with aplite and
pegmatite dikes) intruded by different types of subvolcanic
dikes, and intercalations of millimeter to decameter thick "suevite"
and melt rock units (from 747.0 to 1334.7 mbsf).
The upper (peak ring) section consists of 104 m of "suevite"
(polymict lithic breccia with mm to over 25 cm in size melt
clasts and lithic mineral and rock fragments) on top of ~25
m melt rock (dominantly clast-poor but with clast-rich
intervals). The matrix of the "suevite" is calcitic (ranging
from micritic to sparitic). Clasts include a variety of more
or less shocked mineral and rock fragments (sedimentary [including
isolated fossils], metamorphic, and igneous lithologies, with
carbonate and granitoid being the most abundant rock types)
and melt fragments withaltered (green to brown in color in plane-polarized-light;
clay minerals) glassy to microcrystalline textures. Many of
the melt fragments have flow textures and are occasionally vesicular.
They often contain relic mineral clasts (dominated by feldspars
and quartz) and shocked lithic rock fragments, or are, themselves,
coated with an additional layer of melt. Quartz grains show
planar fractures (PFs) and/or (decorated) planar deformation
features (PDFs), with up to 3 sets. A few toasted quartz grains
were noted. Silica glass, generally recrystallized, with a chert-like
appearance (and with ballen silica also occur. Other minerals
also exhibit shock features, especially plagioclase and alkali-feldspar
(with PFs and PDFs). Possible coesite (??) was observed in a
large silica-rich melt fragment. The melt rock from the lower
part of the upper (peak ring) section is green to black in color,
with flow banding, and in some cases vesicular. The green and
black melts are locally intermixed, forming schlieren of green
material (altered melt?) in a black-colored melt. A large variety
of clasts are present (as in "suevite", with the exception of
sedimentary rock clasts that were not found), in some cases
so heavily shocked and/or hydrothermally altered that it was
difficult to identify them. Shock features similar to those
in "suevite" were observed in the melt rocks, such as PFs and
PDFs in quartz, toasted quartz, and a variety of shock features
in other minerals.
The lower (peak ring) section consists mainly of pervasively
deformed granitoid basement rocks (granite to syenite), overall
coarse-grained, with locally cm to dm thick aplitic and pegmatitic
sections. All main rock-forming minerals, i.e., alkali-feldspar,
plagioclase, quartz, and biotite, show signs of shock deformation.
In the case of quartz, locally PFs were even visible as a result
of preferential hydrothermal alteration. Almost all quartz grains
are shocked, with PFs, feather features (FFs), and/or (decorated)
PDFs ; up to 4 sets of PDFs are seen. Kinkbanding was also observed
for some quartz grains. Similar shock features were observed
in alkali-feldspar and plagioclase. Biotites and chlorites are
often kinked. Based on qualitative evaluation of the thin sections
no noticeable shock attenuation with depth was observed. Preliminary
quantitative results seem to indicate little or no shock attenuation
For the first time shatter cones were
found at Chicxulub. They are well-developed in some of the subvolcanic
dikes intruding the granitoid basement rocks, such as
in aplite at 777.2 and 777.4 mbsf and in phonotephrite dikes
at 1125.1, 1131.7, 1137.5, and 1138.3 mbsf. A possible poorly-developed
shatter cone was also noted in a coarse-grained granitoid sample
at 909.6 mbsf. Finally, "suevites"and melt rocks also occur
in the lower (peak ring) section in the form of small dikes
and large bodies, such as the ~100 m thick occurrence between
1215 and 1316 mbsf. Similar shock features as in "suevites"
and melt rocks from the upper (peak ring) section were observed.
Emplacing impact melt in the Chicxulub peak ring
David A. Kring et al. -- Lunar and Planetary
Science XLVIII (2017); 1213.pdf
Distribution of Melt in the Core: After
penetrating (post-impact) sediments (Unit
1), the top of the (peak ring) was encountered at 617.33
mbsf, beginning with a 104-m-thick polymict, melt-bearing breccia
(Unit 2) with a calcitic matrix
that may represent a plume of carbonate ash. The most abundant
clasts in the breccia are melt fragments. That unit has been
sub-divided (2A, 2B, 2C) based on sedimentary and matrix features.
A melt rock, Unit 3, extends ~26
m to a depth of 747 mbsf. It is dominantly a clast-poor melt
rock, but clast-rich intervals occur at ~722, ~732-734, and
~744 mbsf. It has been subdivided to reflect a change from green
schlieren-bearing black melt (3A) to a basal ~9.5 m-thick coherent
black melt unit (3B). Those units cover granite and related
basement lithologies. Thin, <1 m-thick melt horizons were logged
within the granite. A thicker (~4 m) series of melt and melt-bearing
breccia horizons were logged at ~1000 mbsf and ~58 m of melt
and melt-bearing breccias dominate the lower 100 m of core.
The total thickness of the basement interval sampled by the
borehole is 588 m.
Clast Content: There is a diverse array of sedimentary,
metamorphic, and igneous clasts within those units. Sedimentary
lithologies are carbonate, chert that in many cases is visibly
associated with and derived from carbonate, shale, sandstone,
and red silt-stone. Metamorphic lithologies are gneiss, mylonite,
schist, amphibolite, and quartzite. Marble was also logged,
but thin-section studies are needed to determine if it is a
target unit or shock-modified carbonate. Igneous lithologies
include granite, granodiorite, diorite, dacite, felsite, and
mafic clasts that were variously logged as gabbro, diabase,
and dolerite. Carbonate and granite are the most abundant lithologies.
Sedimentary, metamorphic, and igneous lithologies are found
in the uppermost units, but carbonate was not logged in the
bottom eight cores (from ~722 to ~747 mbsf) of the Unit 3 melt
rock, and granite dominates the clast assemblage at the base
of that interval. Thin melt horizons within the granite only
have clasts of (locally derived) granite. However, the horizon
at ~1000 mbsf contains clasts of melt, granite, granodiorite,
dolerite, and gneiss. The basal interval of melt and melt-bearing
breccias also has a diverse array of metamorphic and igneous
clasts, but no sedimentary clasts.
Discussion: For the melt-bearing horizons within the
granite, we consider four hypotheses. (1) The "impact" melts
in the granite may have been injected into the walls of the
transient crater and transported with the bounding granite during
uplift and collapse into a peak ring. This seems unlikely, however,
because the granite is heavily sheared while the impact melt
rocks and melt-bearing breccias are not. The melts were introduced
after most (albeit not all) of the deformation of the peak ring
The "impact" melts in the granite may have instead been (2)
produced by melting along shear planes and faults in the basement.
That is a likely source of the thin, <1 m melt horizons, which
only contain clasts of granite. However, there is a wide variety
of clasts in the thicker melt-bearing units at ~1000 mbsf and
at the base of the borehole that would require transport of
multiple lithologies (granodiorite, gneiss, dolerite) an unknown
distance along an intrusive conduit and emplacement in granite.
We cannot discount, however, (3) the infusion of melt from adjacent
melt pools along open fractures in the peak ring. The melts
at the base of the borehole lie at a greater depth than "impact"
melt in the adjacent crater trough, although that source may
require transport through fractures over a distanceof at least
2 km, without quenching, while mixing with clasts from multiple
An alternative (ludicrously) hypothesis
(4) is prompted by the lack of sedimentary clasts in the deeper
"impact" melts. If the dynamic collapse and Displaced Structure
Uplift models are correct, then as the peak ring collapsed and
deformed outward, it may have overrun and flowed over surficial
melt-bearing components before the fallback breccia with its
sedimentary clast components landed. The repetition of "impact"
melts within the granitic sequence implies there was internal
shearing within the granite as it displaced outward, allowing
it to cover breccias at least twice.
Preliminary chemical Data for drill cores of the Chicxulub
impact structure's peak ring
A. Wittmann et al. -- Lunar and Planetary Science XLVIII
Upper "Peak Ring"
The 50 samples from the 130 m thick section of "suevite" and
(impact) melt rocks between 618.22‒744.07 mbsf show pronounced
vertical variation in their major and minor element concentrations.
Based on these chemical variations, the upper (peak ring) section
can be subdivded into an upper sorted "suevite" section from
617.34‒684 mbsf that is grossly chemically homogenous. A lower
sorted "suevite"section from 687‒718 mbsf shows more chemical
variation compared to the subsection above, while average concentrations
are grossly similar.
The lowermost section between 720‒744 mbsf is dominated by
melt rocks and is chemically distinct with higher concentrations
of SiO2, TiO2, Al2O3, FeO compared to the "suevite" subsections
and notably, oxide totals that are typically 20 wt% higher than
those of the "suevite" subunits above.
Lower "Peak Ring":
We analyzed 194 samples from the 586 m thick section of
granite, subvolcanic dikes, "suevite"
and melt rocks between 747.89‒1332.75 mbsf. The granite
lithologies in this section are remarkably similar in composition,
suggesting compositions of granites and syenites in the TAS
diagram and metasedimentary protoliths. Subtile differences
are present, though, for example Na2O concentrations from 748.89‒948.39
mbsf are lower than in the section between 948.39 and 1332.75
m. Very low incompatible trace element compositions suggest
a volcanic arc setting for the emplacement of these granites,
which is also supported by their magnesian character.
Compared to the granitic rocks, the most common intrusive rock,
a dark, aphanitic, subvolcanic lithology, has much lower SiO2
and K2O contents, while MgO, FeO, CaO, MnO, TiO2 are significantly
enriched. A less common, brown, fine-grained subvolcanic lithology
that exhibits shatter cones displays a chemical affinity to
the dark subvolcanic dike lithology in that it shows similar
depletion and enrichment trends for the major, minor and trace
element concentrations compared to their granite host rocks.
In the TAS diagram it plots in the field
for phonotephrite, while the dark subvolcanic dikes plot in
the fields for foidite and basanite.
Intercalcations of "suevite" and melt rocks show variable compositions
but tend to be relatively depleted in K2O and enriched in TiO2,
CaO, FeO, MgO, and MnO compared to the granitic rocks.
Ages and Geochemistry of the Basement Granites of the
Chicxulub impact crater
L. Xiao et al. -- Lunar and Planetary Science XLVIII
The granitic rocks are intruded by three or
more types of subvolcanic dikes or dike
swarms including felsite/phonotephrite, dacite/trachyte and
diabase/dolerite. Major rock-forming minerals of the
granitoid are alkali feldspar, plagioclase, quartz and biotite.
Accessory minerals observed in thin section include zircon,
apatite, sphene, and opaques. Shock metamorphism is extensive
throughout the core; planar deformation features developed in
quartz and plagioclase at mineral scale.
Five samples taken from depths of 829mbsf, 927mbsf, 979mbsf,
1076mbsf and 1200 mbsf were dated. Their 206Pb/238U ages
are 304+10Ma,321+7.7Ma, 313+14Ma, 325.9+7.5Ma and 340.8+9.9
Ma. A total of 147 analyses yielded major element oxide contents
for SiO2 of 64wt% to 79wt%, Al2O3 of 10 to 17wt%, Na2O of 3.5wt%
to 7wt%, and K2O of 2wt% to 8wt%. In the TAS diagram, most samples
plot within the granite field, while some are in the quartz
monzonite and syenite field.
The zircon dating results suggest that
the granites were formed mostly around 300-340 Ma ago.
This large age variation of the five samples may indicate that
this was not a single granitic intrusion
Some published pictures