Comments (here below) to the Reply by Gerta Keller, Thierry Adatte, Gerald Baum, Zsolt Berner, a reply to Comment by Schulte et al., Earth and Planetary Science Letters (2008) in press, doi: 10.1016/j.epsl.2007.12.025

4. Yellow clay - original Chicxulub ejecta layer

 Schulte et al. claim that without "true Chicxulub ejecta spherules, i.e. round- or drop-shaped spherules with internal cavities and vesicles ... a volcanic origin for the yellow clay layer is more plausible." However, restricting Chicxulub impact ejecta to well-preserved spherules and relegating altered glass to volcanic origin makes little sense, especially since they argue that the K-T clay, iron and glauconite spherules from the Tethyan realm represent Chicxulub spherules. It is well known that glass alters to clay. Therefore, it should not be surprising{1} that the original spherule layer is completely weathered into cheto smectite clay minerals under the humid climatic conditions of West Texas, as also observed throughout Mexico, the Chicxulub crater core Yaxcopoil-1, Guatemala and Belize [11, 32-33]. The presence of ghost spherules transformed into clay minerals in the yellow clay excludes a volcanic (bentonite) origin, as also evident by the absence of characteristic volcanic minerals (e.g. plagioclase, biotite, apatite, amphibole etc). Shocked quartz and Ir have never been observed even in the well-preserved spherule layers, as tacitly acknowledged by Schulte et al. in their Figure 1. Moreover, the so-called Balcones volcanic province noted by Schulte et al. as possible origin for the yellow clay is much older (70-87 Ma) with peak activity during the Campanian [34-35] and deposition as alkali basalt to phonolite lava flows, sills, dykes and rare thin bentonite ash and tuffs [36-37].   In contrast, the yellow clay layer at Brazos (CMA-B) is exclusively composed of cheto smectite and mineralogically similar to the clay in the overlying reworked spherules layers, as well as cheto smectite from altered impact glass in the Chicxulub crater breccia and spherule layers from Haiti, Belize and Guatemala [32-33]. In the yellow clay, as well as all of these altered impact glass layers, ESEM and EDX analyses of well-crystallized smectite reveal a webby morphology and show that the major element is a typical Mg-smectite (Si, Al, Mg with minor Fe) characterized by excellent crystallinity and very high intensity of the 001 reflection (Fig. 3). After heating, the 9.6å reflection is very reduced compared with ethylen-glycol solvated preparation implying a particular cationic configuration of the interlayer as observed in bentonite [32-33, 38].   Schulte et al. claim that high potassium in the yellow clay is incompatible with pure smectite mineralogy and suggests the presence of a large amount of illite smectite mixed layers. However, XRD analysis shows that all the typical reflections characterizing pure smectite [39] are recognized (001 at 17Å, 002 at 8.46Å, 003 at 5.64Å, 004 at 4.23Å etc) and no illite mixed-layer peaks are detected. The significant K content is due to the fact that geochemical analysis was not performed on the clay fraction, but on the rare glass relicts found in the yellow clay and spherule layers of the event deposit. Indeed, the same method was used by Schulte et al. [6] who reported similar high K20 values (5%- 8%) from spherules from NE-Mexico [3, 6]. The use of FeO+MgO, K2O+Na2O and CaO ternary diagrams, similar to Schulte et al.'s binary diagrams [6, Fig.9, p. 130], is therefore appropriate to characterize and correlate spherules, even though some diagenetic overprint occurred{3}. Their critique that we did not correlate our spherule geochemistry with ejecta spherules from the Tethyan realm directly contradicts their own criteria for characteristic Chicxulub impact spherules (see above). Spherules found in the K-T boundary clay in the Tethys largely consist of iron, iron hydroxyde or glauconite and are clearly not of Chicxulub origin {4}.

1) No, the alteration to smectite is no surprise, since, including Beloc in Haiti, all (or partially at Beloc) spherules are altered to smectite (so, by their own admission, the frequent use of glassy by Keller 2007 is erroneous). What is surprising is that the spherules in the event beds have perfect shapes, while in the yellow layers all forms aredestroyed. The humid climate counts for both!
2) That's the point of our critique. These ghost spherules don't exist! The one shown by Keller is a cross-section of a plant root, the other is ultra vague.
3) This whole section is pulling wool over our eyes. The Balcones province provenance was just one suggestion, but the frequent bentonites in the Maastrichtian from Alberta to the Mimbral area show abundant volcanism all along the nascent Rocky Mountain and Sierra Madre ranges (Laramide orogeny), so well within range of Brazos. The info about the clay minerals is irrelavant. All glass fragment in the area, irrespective of volcanic of impact origin, will be transformed into smectite, cheto-smectite if you will, so that does not make the a difference
4) Misrepresentation of these spherules. The basis mineralogy is smectite, from altered glass, and secondary alterations are towards k-spar and minor iron oxides, pyrite, arsenopyrite. Ni-rich Spinels (not mentioned here) are an important component of these crystalline spherules, as well as pseudomorphs after clinopyroxene. olivine and plagioclase. The spinel-rich spherules are enriched in iridium, and the overall chemistry is fully compatible with the mixture of vaporized target rocks at Chicxulub, like dolomite, anhydrite, and pan-african basement. So where is the "clearly not" based on??
Here below follow some comments on the reply by Gerta Keller, Thierry Adatte, Gerald Baum, Zsolt Berner (in Press) 2008 to <<Chicxulub impact predates K-T boundary: New evidence from Brazos, Texas’, a comment by Schulte et al.>> , which is a critique on:
G. Keller, T. Adatte, Z. Berner, M. Harting, G. Baum, M. Prauss, A. Tantawy, D. Stueben, Chicxulub impact predates K-T boundary: New evidence from Brazos, Texas, Earth and Planetary Science Letters 255 (2007) 339-356