Macro- and microscopic Evidence
of impact Metamorphism in Rocks from the Chicxulub peak ring
L. Ferričre et al. -- Lunar and Planetary Science XLVIII (2017);
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
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 with depth.
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
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
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 had occurred.
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 peak-ring lithologies.
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 (2017);
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"
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
L. Xiao et al. -- Lunar and Planetary Science XLVIII (2017); 1311.pdf
The granitic rocks are intruded by three or more types
of subvolcanic dikes or dike swarms including felsite/phonotephrite,
dacite/trachyite 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
during the Carboniferous
Note: There remain at least 2300 m of basement rocks underneath,
which were not drilled and which can be much older.