What are igneous andesites
and what has the "dark melt" in the Chicxulub crater structure with them common

Norbert Brügge. Germany
Dipl.-Geol.

mailto: [email protected]
 

Investigations on now mostly lost PEMEX cores of Chicxulub andesites

Petrology of impact-melt rocks at the Chicxulub multiring basin, Yucatan, Mexico
Schuraytz, Sharpton & Marin -- GEOLOGY, v. 22, p. 868-872, 1994

Whole-rock major element compositions are similar to medium- to high-K calc-alkalic andesite to dacite. Results for Y6-N17 generally agree with those published elsewhere (Hildebrand et al., 1991). As expected from the variegated lithology of the Y6-NI9 breccia, these subsamples exhibit some compositional variability, but on average are significantly lower in SiO2 and Na2O and higher in CaO than either Y6-N17 or C1-NI0.
Pyroxene phenocryst compositions in our samples from all three core intervals are exclusively augite and, predictably, lie within the range of augite core compositions (En40-55Wo38-40Fs7-20) in andesites. The coarser grains of the C1-N10 matrix show an iron-enrichment trend, with modest, corresponding increases in Na2O, TiO2, and MnO. These variations also characterize the extent of core to rim zoning within individual phenocrysts, the increase of Fe occurring abruptly near crystal margins. Compositions within the finer-grained melt clasts in C1-N10 form a relatively tight duster with an average composition of  En49Wo42Fs9. Our analyses of augite microphenocrysts in Y6-N17 and Y6-N19 yield an average (En45Wo45Fs12) consistent with those of Kring and Boynton (1992), but contrast with the fassaitic compositions reported by Cediilo et al. (1994).
Compared to those in C1-N10, augites in Y6-N17 and Y6-N19 are generally lower in SiO 2 and molar Mg/(Mg + Fe), and higher in Na20, TiO2, and MnO. Apart from slightly higher SiO2, there are no significant compositional differences between augite microphenocrysts in the groundmass and those bordering undigested quartz clasts.
Although the feldspar mineral assemblage as a whole shows considerable chemical variability, plagioclase is the only feldspar present as a phenocryst. Consequently, those early-formed plagioclase crystals that have not suffered extensive alteration define a more restricted range of variation (andesine to labradorite) and thus are compositionally as well as texturally distinct from feldspars in the surrounding mesostasis. With decreasing An content (An59-32), coarser plagioclase phenocrysts in the C1-N10 matrix show a corresponding monotonic decrease in MgO, and an initial Fe-enrichment trend that attains a maximum at Anso, followed by a decrease in FeO. Plagioclase phenocrysts within the finer-grained melt clasts of C1- N10 tend to be more calcic, relatively constant in MgO, and higher in FeO, with an Fe-enrichment maximum at An56. Analyses of Y6-N17 and Y6-N19 are generally higher in K2O and FeO and lower in MgO than those of C1-N10 and are consistent with the average composition of groundmass plagioclase in Y6-N 17 published previously (Kring and Boynton, 1992).
The mesostasis of C1-N10 includes alkali feldspar and plagioclase ranging from oligoclase to pure albite. An example of the textural relations of these feldspars to a euhedral plagioclase phenocryst shows that albite forms at the expense of the calcic host, which in turn is surrounded by anhedral K-feldspar intergrown with quartz, epidote, minute opaque minerals, and a cryptocrystalline aluminosilicate that appears to be a devitrification product of glass. Feldspar compositions in the mesostasis of Y6-N17 are highly variable; however, with the exception of albite, our analyses indicate that they are nonstoichiometric. These anhedral, cationdeficient phases fill the interstices of the andesine and augite microphenocrysts, some of which protrude into the ubiquitous drusy cavities (Fig. 1B). Thermodynamic considerations together with textural relations between early-formed phases and the porous mesostasis suggest to us that, as in C1-N10, the albite results from secondary alteration (Schuraytz and Sharpton, 1993). Similar compositional and textural relations characterize feldspar variations in the Y6-N 19 mesostasis, although the variations in porosity are more extreme.

Mega-impact melt petrology (Chicxulub, Sudbury, and the Moon): Effects of scale and other factors on potential for fractional crystallization and development of cumulates
Warren & Claeys -- Geological Society of America, Special Paper 307, 1996

We have studied seven thin sections of "impact melt-derived rocks" from Chicxulub: one from core C1, position N9, a short distance above the C1/N10 sample of Sharpton et al. (1992) and Schuraytz et al. (1994); and three each from positions N17 and N19 of core Y6, the same as the original positions of samples described previously by Hildebrand et al. (1991), Kring and Boynton (1992), Sharpton et al. (1992), and Schuraytz et al. (1994). Our samples came from ~1,390 (C1/N9), ~1,297 (Y6/N17), and ~1,378 (Y6/N19) m below sea level.
The matrix portions of all six Y6 thin sections are aphanitic and are dominated by grains <10μm across, albeit leucocratic, texturally diverse but generally much coarser grained lithic clasts are incongruously sprinkled throughout (~20 vol%). The extraordinarily wide range of feldspar compositions in the Y6/N19 samples "tends to confirm the textural evidence for origin as a clast-rich magma". Pyroxene compositions show considerable variation between matrix portions of two nearby Y6/N19 samples. The Y6/N19-10 pyroxenes are remarkably rich in Al2O3.
Among our samples, the coarsest matrix by far is that of C1/N9, with grains mostly ~0.3 mm in maximum dimension. Texturally and mineralogically, this sample strongly resembles C1/N10 (Schuraytz et al., 1994). Only six clasts are discernible, all very fine grained (feldspar and pyroxene ~20μm), totaling ~2 vol% of the rock. A minor proportion (1%) of the matrix consists of isolated uncommonly coarse (up to 1.6 mm) grains similar to those described as phenocrysts (up to1 mm) in C1/N10 by Schuraytz et al. (1994). We retain this term for C1/N9 grains coarser than twice the prevalent 0.3 mm size, including three of the most diopside-rich pyroxenes analyzed.
C1/N9 and C1/N10 are the coarsest Chicxulub "impact" melt products yet sampled. Most other "impact" melt-like rocks (from elsewhere in the C1 and Y6 cores and from the S1 core) are described as �glass� or �microcrystalline� (Hildebrand et al., 1991; Quezada Mu�eton et al., 1992; Kring and Boynton, 1992) or as �fine- to medium-grained coherent crystalline� (Sharpton et al., 1992). Sharpton et al. (1992) described Y6/N19 as having a �medium- to coarse-grained melt rock matrix,� but a more detailed recent description by three of the same authors (Schuraytz et al., 1994) indicates that their sample from Y6/N19, like the two we studied, consists mainly of an aphanitic matrix with grains ~10 μm across.
The finer grain size of the Chicxulub samples is probably not a consequence of systematically smaller dimensions of individual Chicxulub �meltbodies,� because according to Lopez Ramos (1975) the Chicxulub cores C1, S1, and Y6 all contain layers of pure  extrusive andesite  over 200 m thick. The Y6/N17 samples came from near the middle of a 60-m-thick continuous  extrusive andesite  layer, and the Y6/N19 samples came from ~12 m below the top of a continuous layer ~220 m thick (Meyerhoff et al., 1994).

The two different melt rocks of the Chicxulub crater and where is the IR anomaly ?
Claeys et al.-- Lunar and Planetary Science, XXIX. [1361.pdf]

Yucatan melt rock samples, from the well Chicxulub-1 ( C1) and from the well Yucatan 6 (Y6), were analyzed and seem to show significant differences.
The melt rocks in C1 (C1-N9: 1390 mbsl; C1- N10: 1393 mbsl) have a very coarse-grained, aphanitic matrix containing approximately 5 vol% quartz (10 μm), 25 vol% K-feldspar (50 μm), 64 vol% plagioclase (100 μm), 5 vol% augitic pyroxene (100 μm in scale) and 1 vol% opaques minerals (< 10 μm, ilmenite, pyrite). No clasts were found.
In contrast to C1, the melt rock in well Y6 (Y6-N17: 1295.5- 1299 mbsl; Y6-N19: 1377-1379.5 mbsl) is more clast rich. Quartz, feldspar, anhydrite and carbonate clasts, range in size between 0.4 and 2 mm. All clasts except anhydrite and carbonate, are surrounded by a corona of augitic pyroxenes and K-feldspars. The clasts are usually subrounded, many are broken and their fractures are filled with matrix. Here to, many basement clasts are partly digested by the matrix. Vermicular anhydrite is also present in vein and cavities, suggesting pore filling (either from a melt or vapor phase) during the melt�s major cooling phase. The melt rock in well Y6 is more altered than its C1 counterpart and new mineral parageneses such as zeolites (stilbite, laumontite) and secondary calcite have often formed in cracks and holes.
The Y6 fine grained hypocrystalline matrix is composed of quartz (8 vol%), alkali feldspar: (13 vol%), plagioclase (66 vol%), augitic pyroxene (12 vol%) and minor constituents such as magnetite, ilmenite, sphene, zircon etc. (1 vol%). The matrix grain size is always below 10 μm, much finer than in C1 melt rock. Locally, the matrix even appears glassy or cryptocrystalline. The feldspar in the matrix consists of remnants of K-rich feldspar, mantled by a more Ca-rich feldspar phase, indicating a chemical change of the melt to a more Ca-rich composition. The abundance of Ca-rich fluids is also corroborated by the strongly augitic composition of the pyroxenes.

Thin or polish sections of Chicxulub andesites ("dark melt")
 (plagioclase-phyric pyroxene andesite)