PLATINUM-GROUP ELEMENTS IN THE CORES OF POTASSIUM FELDSPAR SPHERULES FROM THE CRETACEOUS-TERTIARY BOUNDARY AT CARAVACA ( SPAIN )

The abundant spherules present in the Cretaceous-Tertiary boundary layer at Caravaca are diagenetically transformed to potassium feldspar. Before our study no possible relicts of the precursor material had been reported. but in this paper we describe the presence of cores in these spherules that could represent a relict of the «unknown precursor». These cores are made up of C mixed with Si. Mg. AL Cr. Ca among other elements. Laser Ablation System analysis also reveals an enrichment in pe; E could suggest an extraterrestrial origin for this material. PI. Pd and Ir do not show a chondritic ratio: however. asevere modification of their concentration could be expected during the early diagenetic processes.

Palabras clave: ElemClllos del f!,rupo del PI. /éldespil/o pOlásico.es/erula.l.limile Crelácico-Terciario. lntroduction One of the most outstanding characteristics of the Cretaceous-Tertiary (KIT) houndary layer at Caravaca (Betic Cordilleras, SE Spain) is the presence of K-fcldspar spherules.Smit & Klaver (19R 1) were the first to descrihe these spherules, suggesting that the potassium feldspar formed at high temperature as a consequence of an impact event.The quench-crystal textures presented amI the morphology similar to microtektites led these authors to link the origin of the spherules to an extraterrestrial holide.Suhsequent research has shown that the feldspar is of diagenetic origin and formed at low temperature (e.

1994).
The textures and morphology of the spherules really support an extraterrestrial origin although their characteristics differ from those of microtektites.Applying the new term proposed hy Glass & Burns (19R7) and used later hy Smit el al. (1992).the di agenetically altered spherules from the KIT houndary should he considered as microkrystites.Among ot-her differences, the microkrystites show evidence of a crystalline phase, whereas the microtektites are of glassy nature, and microkrystites are associated with iridium anomalies.
As in Caravaca, in most of the known KIT sections the spherules are diagenetically altered, with total replacement of the original material.In this case the K-feldspar replaced the precursor and no relicts have been reported before now.
In this paper we describe the presence of a possible relict of the «unknown precursor».The discovery of cores in the K-feldspar spherules, whose composition differs from that of the K-feldspar, suggests the existence of such relicts.In addition, the high content of platinum group elements (PGE) in the cores also supports the possible extraterrestrial origin of this material.

Methods
The spherules were isolated from > 63 [tm fraction and then separated by hand-picking using a stereoscopic microscope.For the study of the K-feldspar spherules and their cores, the following techniques were used: X-ray diffraction using a Philips diffractometer with automatic slit (Department of Mineralogy and Petrology of the University of Granada); Scanning Electron Microscopy (SEM) (DSM 950 equipped with Link microanalysis QX 2000) and Electron Microprobe (Cameca Camebax SX 50) (Centro de Instrumentación Científica of the University of Granada); SEM (Cambridge S360 coupled with a Link ANlOOOO ED detector, University of Bari).In particular, the C in the core of 35 % of the spherules was detected and quantified using the Link detector in windowless position with acceleration voltage set at 9 kV.Traceelement microanalyses on sorne cores were carried out using a Perkin Elmer 302 Laser Ablation System coupled to a PE Scier ICP-MS Elan 5000 Spectrometer (Perkin Elmer, Uberlingen, Germany).Calibration was done in two ways: externally, with NBS-612 glass; internally, using SiOl (previously determined by microprobe in the same sections) as standard.The detection limits is 0.1-0.15ppm.

Potassium feldspar spherules and composition of the cores
The bulk mineralogy of the KIT boundary layer at Caravaca is essentially characterized by 80 % of smectites (Si 3 .63 AIIV O .37 ) 0 10 (AIVl uo Mg O .34 Feo46 Ti o .04 ) Ko.Z3 (OH)z, 10 % of calcite and 5 % of guartz, as well as small amounts of kaolinite, illite, palygorskite, chamosite (Fe = 2.21 a.f.u. and Mg = 0.98 a.f.u), celestite, barite, gypsum, rutile and zircon.Transmission Electron Microscopy has been used to study both smectites and minor minerals.The total carbon content (TOC) in the KIT boundary layer is 0.27 % and 0.23 % at the beginning of the Danian (1 cm aboye KIT), with much lower values in later Tertiary levels (0.08 %, sample at 10 cm aboye KIT).
K-feldspar spherules are very abundant at Caravaca.Their size usually ranges from 100 !-lm to 500 !-lm,They present a porous structure in which the crystals of K-feldspar are distributed in fibroradial and dentritic textures (Martínez Ruiz et al., 1992;Martínez Ruiz, 1994).The EPMA analyses of the K-feldspar reveal a composition (Table 1) close to the KA1Si 3 0 s end member.Its structural characterisation corresponds to a monoclinic Kfs (ó = O) with a moderately ordered Si:Al distribution (T¡o and T¡m = 0.41, 2T z = 0.19) similar to that of orthoclase or adularia.
However, one of the most interesting results obtained during our study was the discovery of cores made up of C and a mixture of Si, Ti, Al, Mn, Mg and Na (Table 1) whose sizes usually range from 10 to 20 !-lm (fig.la).The SEM backscattered image of the spherule cores reveals a composition with an average atomic number clearly lower than that of K-feldspar (fig.la).This is due to the systematic presence of C, with most freguent percentages of around 50 %, although proportions of 90 % have been punctually detected by SEM.EPMA analyses reveal that there is scarcely any chemical variation in any one core, which indicates that the carbonaceous material is mixed with the other chemical elements on an extremely fine scale.The Ablation Laser System analyses also reveal strong enrichment in Ir (0.59 ppm), Pd (15.70 ppm), Pt (2.89 ppm), Ru (6.33 ppm), Rh (4.31 ppm) and Ni (3000 ppm).

Discussion
Cores found in the impact-related K-feldspar spherules from the Cretaceous/Tertiary boundary layer at Caravaca are possibly a relict of the precursor mate- rial of the spherules, but it is diffieult to affirm their exacl nature.The eores could be similar to earOOn rich veins described in ureilites by Tomeoka & Takeda (1990), whose origin is linked to carbonaceous ehondrites.In Caravaea an organic associalion of Ir has been proposed; Schmitz (1988) considered thal 50 % of the bulk sample Ir is associated with organic eompounds.On the other hand, the presence of relict carOOn (microdiamonds) has been reponed in the Crelaceousffertiary OOundary layer at Red Deer Va-Iley (Alberta, Canada) by Carlisle & Braman (1991).
Recently an exlraterrestrial origin for these mierodiamonds was confirmed by Carlisle (1992).
The values oblained for the PGE are highly significant and show a c1ear enrichmenl tha! could suggest an extralerrestrial source.Howevcr it is diffieult to prove extralerreslrial derivalion of Ihe PGE-rich cores, which do not present chondritic ratios.In facI, the spherule cores presenl a ratio ¿(Ir, Pd, Pt)/Ni ,.. 0.006, which is much higher than the chondrilic ratio (0.002).Indeed, even the ralios bclween the differenl PGE show enrichmcnt in Pd and Pt as regards Ir, and of Pd as regards Pt (PdJIr '" 26.61, Ptllr ~4.90, PdlPt ~5.40).
To explain the high concentration of PGE in the spherule cores, it is necessary to take in account that PGE can be remobilized in most geological environmenls.Although the processes contribuling lO remobilization of PGE in sedimentary environmenls are not well understood, among other rcasons beeause they are presenl in concentrations that are someti• mes very diffieult to detect. it is c1car thal remobili• zatioo is possible in this environmenl.Dyer el al. (t989) carried out different experimcnts to show that certain types of cyanobacteria can play an imponant role in Ir concentration.During processes of early diagenesis of the scdimenls.microbial activilY could have therefore considerably modified Ihe Ir concen• tralion.The expcrimcnt by De Lange el al. (1991) showed Ihat Ir anomalies may be explained by remobilizalion and redox controlled precipitalion and Mountain & Wood (1988) also showed Ihat mobility of PGE is possible at temperalures characleriSlic of sedimentary environmenls.Other authors as Bowles (1986), Wallace el al. (1990( ), Evans el al. (1993) ) or Sawlowicz (1993) have reponed low temperature mo• bilization of PGE.
Isotopic composition of Ihe OOundary c1ay and K•feldspar spherules (DePaolo el al., 1983) reveals tha! the Krr at Caravaca underwent severe modifi• cation during diagenetic proeesses, with important neoformalion of mineral phases.There is evidence whieh proves Ihat the Kfs is c1early of authigenic origin and formed at low temperalure, such as the fact that ils composilion corresponds to an end-member of the alkaline series, and, moreover, is of high chemical purity (Table 1), both of which are characteristics of this type of genesis (Kastner & Siever, 1979); also the ()lll() value is 27 %o (Epstein, 1982).In addition, the presence of pollen grains replaced by Kfs (6g.lb) is also significant.This indicales Ihe Kfs could therefore have replaced the spherules and possibly other particles.
The PGE may have been remobilized during dia-genetic alteration, and therefore chondritic ratios of these elements could not be expected.During the early diagenetic processes the presence of organic matter, complex-forming elements, and the pH and Eh conditions could have considerably affected the concentrations of the PGE.
Other impact signatures such as Ni-rich spinels of cosmic origin (e. g.Bohor, 1990; Robin el al., 1992)  and shocked minerals (e. g.Bohor, 1990) support the hypothesis that the Ir is due to an extraterrestrial origin.However, the high concentration in Ir, Pd or Pt could be the result of concentration by terrestrial processes rather than a consequence of exclusively extraterrestrial contamination.

Conclusions
Carbon and PGE-rich cores in the K-feldspar spherules from Caravaca are possibly a relict of the precursor material of the spherules.The high PGE content could suggest an extraterrestrial origin, although these elements do not show a chondritic ratio.Other impact signatures in the boundary layer may support an extraterrestrial origin of the PGE, but the considerable alteration that took place in the KIT layer during the early diagenetic processes could led to a severe modification of their original patterns.Therefore the enrichment is not only the consequence of extraterrestrial source but also the result of terrestrial processes, in particular diagenesis and carbon acting as a geochemical trap for the PGE.

Table l .
-Microprobe data (wt %) oC the KCs spherules with sizes outside this range less freguent.Morphologies are globulous, either spherical or drop-shaped.