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RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 14, ES1001, doi:10.2205/2014ES000535, 2014
Morphology and geochemistry of zircon in bentonite ash at the Paleocene-Eocene boundary in the Anthering Formation, Eastern Alps, Austria
A. F. Grachev1, S. E. Borisovsky2, and T. F. Zinger3
Received 28 January 2014; accepted 28 January 2014; published 11 February 2014.
The morphology and concentrations of trace elements and rare earth elements (REE) of zircon from a transitional layer of bentonite ash at the Paleocene-Eocene boundary at a locality north of Salzburg in the Eastern Alps suggest a magmatic genesis of the zircon, and data on the REE distribution in the mineral testify to an oceanic nature of the magmas. A more reliable criterion of the nature of the magmatic source is data on the Li concentrations; the latter in some of the zircon grains are much lower than the minimum values typical of the continental crust. It is hypothesized that a zircon type exists that is related to plume magmatism. KEYWORDS: Zircon; Paleocene and Eocene bentonite; geochemistry; Eastern Alps.
Citation: Grachev, A. F., S. E. Borisovsky, and T. F. Zinger (2014), Morphology and geochemistry of zircon in bentonite ash at the Paleocene-Eocene boundary in the Anthering Formation, Eastern Alps, Austria, Russ. J. Earth. Sci., 14, ES1001,
doi:10.2205/2014ES000535.
Introduction
Data on the mineralogy and geochemistry of transitional layers produced during the critical epochs in the Phanerozoic that were related to mantle plumes and the mass extinction of the biota is of paramount importance for a broad circle of scientists. One of such events, which is referred to as the Paleocene-Eocene thermal maximum (PETM) was related to the activity of the Iceland plume at 55 Ma, which resulted in the ejection of 1200 km3 of ash material of predominantly basalt and trachyte composition into the atmosphere, with this ash preserved in several stratigraphic units at the Paleocene-Eocene boundary in West Europe [Egger and Bruckl, 2006; Egger et al., 2005; Larsen et al., 2003]. As a result, the temperature of the oceanic water rapidly increased to 8°C, and this led to the passage of approximately 2000 x 109 tons of methane to the atmosphere and the extinction of close to 50% of benthic foraminifers [Kennet and Stott, 1991; Zachos et al., 2005].
Volcanic events analogous to those related to the Iceland
1Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia
2 Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Moscow, Russia
3Institute of Precambrian Geology and Geochronology, Rus-
sian Academy of Sciences, St. Petersburg, Russia
Copyright 2014 by the Geophysical Center RAS.
http://elpub.wdcb.ru/journals/rjes/doi/2014ES000535.html
plume were multiply repeated during the Earth’s history and resulted in transitional layers or beds at critical boundaries (Cambrian explosion, Ordovician-Silurian, Permian-Triassic, Cretaceous-Paleogene), with these beds and layers containing bentonite clay that replaced the ash [Christidis and Huff, 2009; Clayton et al., 1996; Compston et al., 1992, and others]. The chemical composition of ashes of various age and their sedimentation and redeposition conditions are known well enough, whereas data on the geochemistry of their heavy-fraction minerals, first of all, zircon are absent, except only for the U and Th concentrations, which usually reported when the zircons are dated by the U-Pb method.
This publication reports our recently obtained data on zircon from bed X1 of bentonite ash at the Paleocene-Eocene boundary in the bottom portion of the Anthering Formation exposed north of Salzburg, Austria. The formation contains 22 bentonite beds < 1.5 cm thick, including bed X1, whose thickness is 3 cm [Huber et al., 2003]. The bentonites contain similar SiO2, Al2O3, and MgO concentrations but remarkably different TiO2 ones. In contrast to the other beds, bed X1 is of trachyte composition and is poor in TiO2 and P2O5 but relatively rich in Zr, Nb, Hf, Ta, Th, and light rare earth elements (LREE) [Huber et al., 2003].
Sample Preparation Techniques
The contents of clasyogenic minerals contained in bed X1 are low, and the predominant grain sizes of these minerals
range from 0.05 to 0.10 mm. The minerals were separated from their host rocks using a sample treatment procedure that was adjusted for the purposes of this research.
The key feature of this procedure was the selective fractionation of the material of the sample in water and gathering the clay fraction after certain time intervals until the water suspension became clear. This was done to accurately enough segregate the clay (< 5 ^m) and grain (> 5 ^m) size fractions of the sample without significant losses of the latter.
The clay constituent of the sample was differentiated into a number of size classes via the successive separation of discrete portions of the suspension at certain time intervals, and each of these fractions was then further separated in water into two density fractions: the heavy fraction consisted of minerals whose density was grater than 3 g cm-3, and the light one of minerals < 3 g cm-3 in density.
The grainy constituent usually consisted of minerals of various density and, hence, required further separation in heavy liquids. For this purpose we used bromo-form (2.89 g cm-3) and obtained two fractions: heavy
(> 2.89 g cm-3) and light (< 2.89 g cm-3). An ultraheavy fraction of ore minerals was separated from the heavy one in bromoform. These minerals were the most interesting in the context of our research. The fractions were then examined under an optical microscope, and 62 zircon grains were hand-picked from these fractions for our further studies.
Methods of Zircon Analysis
Zircon grains were analyzed at the Laboratory for the Analysis of Mineral Materials at the Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, on an Leol JXA-8200 electron microprobe equipped with five wave-dispersive and one energy-dispersive spectrometers, at a beam current of 300 nA and beam diameter of 3 ^m. Table 1 reports the analyzed elements, their analytical lines, analyzer crystals, differential (dif) or integral (int) modes of counter operation, exposure times, standards, and detection limits. Phosphorus was analyzed with regard for overlap of the ZrLa and PKa lines. The corrections were calculated with the ZAF routine, using the respective Jeol software.
The contents trace elements in zircon were determined by secondary ion mass spectrometry (SIMS) using a Cameca IMS-4f ion microprobe at the Yaroslavl branch of Institute of Physics and Technology, Russian Academy of Sciences.
Our samples were prepared for analysis in the form of cylindrical epoxy pellets with zircon grains. In the polished surfaces of the pellets, the polished sections of all of the grains were coplanar. The working surface of the pellets was sputter coated with an Au film 0.03 ^m thick on a magnetron deposition setup.
The basics of the measurement technique corresponded to those reported in [Fedotova et al., 2008; Nosova et al., 2002; Smirnov et al., 1995]. Primary O- ions were accelerated to about 14.5 keV and focussed at the sample surface
to a 30 ^m spot. The ion current was 4.5 nA. Positive secondary ions were collected from an area 25 ^m in diameter, limited by a field aperture. Molecular and ion clusters were energy filtered using an offset voltage of -100 V, with an energy window of 50 eV. Five counting cycles were carried out with a discrete transition between mass peaks within a given set. The counting time was varied depending on signal intensity and was determined automatically by statistical control. The maximum counting time for any species in each cycle was 30 s.
The absolute concentrations of each element were calculated from the measured intensities of positive single-atom secondary ions, which were normalized to the intensity of secondary 30Si+ ions, using the relative sensitivity factors C = Ii/I30Si x K*. Calibration curves were based on the measurements of the set of well characterized standard samples [Jochum et al., 2000]. The signals of 153Eu+, 174 Yb+, 158Gd+, and 167Er+ were corrected to the interfering oxides of Ba and lighter REE in accordance to the scheme reported by Bottazzi et al. [1994]. The intensity of 174Hf+ calculated from measured 178Hf+, using value of 170.5 known for 178Hf+/174Hf + ratio of hafnium natural isotopes, was also subtracted from 174Yb+. Additional stripping procedure was used to correct the signals of 138Ba+, 139La+, 140Ce+, 141Pr+, for interfering isobaric ions ZrSiO+ specific to zircon [Hinton and Upton, 1991]. Besides, intensities of 88Sr+ and 89 Y+ were stripped from the interfering doubly charged hafnium ions 176Hf++ and 178Hf++. The ratio of singly (177Hf+) to doubly (177Hf++) charged hafnium positive secondary ions determined experimentally was 581.
The intensity of 92 Zr1 H+ secondary ions was determined via measuring the intensity of 94Zr1H+ at 95 a.m.u. and known ratio 92Zr/94Zr= 0.9868 of natural abundances of these zirconium isotopes and then subtracted from the measured intensity of mass peak at 93 a.m.u. to get the signal of 93Nb+:
I (95 a.m.u) = 94Zr1H+
92Zr1H+ = 94Zr1H+ x 0.9868
93Nb+ = I (93 a.m.u.) - 92Zr1H+
Detection limit of niobium in zircon nevertheless limited by the value ~ 10 wt ppm because of high intensity of neighboring 92Zr+.
The phosphorous content was estimated using stripping procedure. The contribution of 30Si1H+ to the measured intensity of mass peak at 31 a.m.u. was determined from intensity 29Si+ and known abundances of natural silicon isotopes:
28Si1H+/30Si+ = I(29 a.m.u.)/30Si+ - 1.517
30Si1H+/30Si+ =28 Si1H+/30Si+/29.875
31P+/30Si+ = I(31 a.m.u.)/30Si+ -30 Si1H+/30Si+
Table 1. Conditions of determination the elemental compositon of zircons on Leol JXA-8200 electron microprobe
Element Analytical line Analyzer crystals Modes of counter operation Exposure time, s Standards Detection limit, ppm
Si Ka TAP dif 10 Zircon -
P Ka TAP dif 90 Apatite 105
Fe Ka LIF int 100 Andradite 90
Zr La PET int 10 Zircon -
Hf Ma TAP dif 100 HfO2 85
Y La TAP dif 90 Y2O3 80
U Ma PET int 120 UO2 75
Th Ma PET dif 120 ThO2 70
age of 1.95 ± 0.64), with a lognormal distribution, which indicates that the grains were disintegrated in the course of sedimentation, when angular fragments were produced. At the same time, it is pertinent to mention that acicular zircon grains are absent, whereas variably rounded grains are, conversely, abundant (Figure 1). The zircons are pink transparent and/or semitransparent or, more rarely, opaque.
The great morphological diversity of zircon in bed X1 makes it possible to distinguish a number of groups: (1) cat-aclased, (2) oval, (3) euhedral and subhedral, and (4) corroded. It is worth mentioning that several zircons are cat-aclased; these were detected among the grains mounted on a double-sided adhesive tape and in the polystyrene pellet (Figure 2, Figure 3). The morphologies of the cataclased zircon grains are analogous to those of zircons in cataclased rocks [Zinger et al., 2010]. This does not rule out that the angular geometries were produced by breaking previously cataclased during sedimentation.
The zircon grains are often oval, corroded, or sub- to euhe-dral (Figure 4). We have also found unusual spongy zircons whose growth zones are riddled with round pores (Figure 5). Zircons of this type are not rare, and this led us to distinguish them as an individual group, which crystallized at different circumstances, simultaneously with the liberation of the gas phase from the rapidly decompressed melt. Originally the pores were mistaken for SiO2 inclusions, but the EDS spectra showed lines of both Si and Zr. Several zircons bear inclusions, usually quartz, xenotime, plagioclase, occasional apatite, and even more rare pyrite (Figure 6). The
Figure 2. Cataclased zircon grains mounted on a double-sided adhesive tape.
Figure 1. Panoramic micrograph of the polished polystyrene pellet with impregnated mineral grains. Pale grains are zircon, pale gray is rutile, and dark gray is silicates.
Morphology and Inner Structure of the Zircons
Bed X1 contains morphologically diverse zircon grains and fragments (Figure 1) ranging from 30 to 100 ^m. The aspect ratios of the grains vary from 1.2 to 4 (at an aver-
Figure 3. Cataclased zircon grains in a polystyrene pellet.
Figure 4. Zircon grains of various morphology: (a) oval, (b) corroded, (c) sub- and euhedral.
corroded zircons host hollows left after the dissolution of mineral inclusions.
The morphology of 62 zircon grains was examined under an electron microscope, and their inner structure was studied in cathodoluminescence (CL) and back-scattered electrons (BSE). The most representative images are displayed in Figure 7.
The surface of most of the grains shows traces of catacla-sis and destruction (grain 26, Figure 7a), numerous caverns (grains 6 and 19 in Figure 7a; 55 and 37 in Figure 7b; and 3, 12, 40, and 44 in Figure 7c). Some of the grains have completely porous spongy surface (grains 56, 25, and 23 in
Figure 5. Zircon crystal with small round cavities on growth planes in a polished polystyrene pellet.
Figure 7a; 28 in Figure 7b; and 45 in Figure 7c). The prism faces of some grains are intersected by minute cracks (grains 32 and 34 in Figure 7b; 9, 13, 33, and 35 in Figure 7d). At the same time, some of the grains display evidence of partial dissolution and healing of prism faces (grains 9 and 59 in Figure 7d) and the growth of new faces of prisms and pyramids (grains 46, 33, and 35 in Figure 7d).
As can be seen in CL and BSE in sections of the same grains, they are highly heterogeneous: display thin 55, 37, 34, 45) or coarser (56, 19, 3) zoning or its fragments (6, 25, 23, 28, 3, 40, 9, 13, 59), can be partly recrystallized and unzoned (12, 44), or show evidence of the destruction of grain cores (26, 25, 55, 37, 32, 34, 46, 33). The character of the older “primary magmatic” zoning of the zircon grains suggests that they crystallized under different conditions and likely from differentiating melt. It is thereby important to emphasize that the surfaces of most of the zircons are cavernous and porous, in spite of the fact that these grains can have different inner structures.
A discrete group comprises zircons with evidence of postsedimentary transformations. Zircons of this group typically display traces of partial dissolution and redeposition of the material in minute healed cracks and spongy surface (9, 13, 59, 46, 33, 35) and the growth of newly formed crystal faces (33, 46, 35). This process can be discerned in the inner structure of the crystals as the development of minute veinlets (9, 46), banding (13, 40), and patchy patterns that can be seen in CL and BSE images. Thereby the destructed cores of the grains can be partly healed (32, 33, 34). Oc-
Figure 6. Inclusions in zircon: (a) quartz (dark) and xenotime (pale rim around quartz increase) in detrital zircon; (b) rounded quartz inclusion in corroded zircon; (c) pyrite inclusion in zoned zircon grain.
casional crystals (13, 59, 33, 35) show the development of outer rims of various thickness, from a few to 15 ^m.
The formation and transformation sequences of the zircon crystals led us to think that the youngest of these processes were related to postsedimentary and diagenetic processes of hydrothermal nature. Note that these processes affected already compositionally heterogeneous and variably modified zircon grains.
Bulk Composition of the Zircons
the crystal structure of the mineral. The ZrO2/HfO2 ratio varies from 80 to 30. Analyses of individual grains led us to conclude that lower ZrO2 /HfO2 ratios can occur in both the rims of the crystals and their domain with disturbed crystal structure and developed in relation with transformations of the grains.
Analysis of the zircon grains in their various parts indicates that the zircons have a heterogeneous major-component composition, which can be identified because of the small beam diameter of the microprobe (3 ^m), and this should be taken into account when data on the bulk, trace-element, and REE compositions of zircons are compared.
Table 2 reports 186 microprobe analyses of 62 zircon grains. Each of the grains was analyzed in its various parts, from the core to peripheral portions, to reveal possible compositional heterogeneities. To identify possible patterns and trends of the compositional variations, the analytical data were processed using factor analysis (method of principal components), which indicates that the compositional variations are controlled by variations in the concentrations of UO2, ThO2, and Y2O3 (factor 1) and ZrO2 and SiO2 (factor 2) (Table 2).
The factor diagram (Figure 8) clearly demonstrates that most of the zircon bulk compositions define a clearly individualized data point swarm, with certain zircon compositions differing in having elevated Th, U, Y, and P concentrations, which usually increase from grain cores to their peripheries. This tendency is, in fact, described by factor 1 (Table 3). This trend is pronounced in binary diagrams for both the whole set of the zircon analyses (Figure 9, Figure 10) and individual grains (Figure 11, grains 33, 35, 46). The HfO2-Y2O3 diagram (Figure 9) demonstrates the pronounced bimodal character of the distribution of the HfO2 concentrations: 85% of the grains contain 1.2% HfO2 on average, whereas a small fraction of the grains contain higher HfO2 concentrations, up to 3.5% in grain 9 (Figure 12), as is typical of hydrothermally recycled zircon [Hoskin, 2005].
As can be seen in the ZrO2-ZrO2 /HfO2 diagram for all of the analyses (Figure 13), the ZrO2 concentrations vary within a narrow range of 65-66%. The only exception is a single analysis (grain 55-1) reflecting the destruction of
Trace Element Geochemistry
Zircon is known to contain, along with Zr, Si, and O, also Hf, Y, Th, U, P, Ca, Na, Fe, Al, Mn, Ti, Sr, Ba, and REE [Hoskin and Schaltegger, 2003], and hence, zircon analyses for trace elements and REE are widely utilized to reproduce (usually with the application of binary diagrams; [Grimes et al., 2007, 2009] and several others) the provenance of zircons found in sedimentary rocks.
Table 4 presents the trace element and REE composition of 41 zircon grains, which were analyzed within their central parts (cores) and in the margins (the diameter of the electron beam was 25 ^m). The table also reports the crystallization temperatures of the zircons calculated by the Ti thermometer [Watson et al., 2006].
In order to identify possible correlations between the behavior of trace elements (without REE), we applied factor analysis, which demonstrates that all of our analyses can be classified into two groups according to the correlated behaviors of U, Y, and Th (factor 1, weight 32%) and Li and Hf (factor 2, weight 18%) (Table 5, Figure 14).
Similar to the classification by major components, most of our analyses define a fairly compact group. At the same time, some of the analyses are anomalous and principally differ from those combined in the group. It is worth mentioning grains 19, 25, and 33, whose major component and
6 of 17
25-1 25-2
23-3 23-2
25-1 25-2
23-3, .23-2
c
\
!
25-5 iMPP ■
25-4
25-3 Ü
Clear
Figure 7. Micrographs of zircon grains from bed XI of bentonite ash. Zircon morphologies (left), cathodoluminescence (CL) images (center), and back-scattered electron (BSE) images (right). Arrows indicate spots analyzed for bulk composition (major components) (Table 2), and circles are spots analyzed for trace elements, including REE (Table 4). Numerals are grain numbers. 26 - Fragment of a prismatic zircon grain with traces of cat aclasis and destruction of the crystal structure of the crystal core; 6 - oval grain with corroded pyramid faces, CL and BSE images showing a sharp boundary between the grain core (with fragments of coarse zoning) and heterogeneous rim (pale in CL domain is younger); 56 - short-prismatic crystal with cavernous surface and (b) zoning; 19 -fragment of a prismatic crystal with corroded pyramid and prism faces and with a zoned-sectorial inner structure; 25, 23 - long-prismatic zircon crystals with completely spongy surface sharp boundaries between the zoned cores and rims; 55, 37 - fragments of prismatic zircon grains with cavernous surface, thin zoning, and with the partial destruction of the crystal structure manifested in the form of small specks (perhaps, ”cavities”) seen in BSE-pale layers; 28 - fragment of a long-prismatic crystal cavernous surface and fragments of coarse zoning; 32, 34 - prismatic crystals with poorly pronounced cavernous surfaces of prisms and pyramids, prism faces are cut by cracks. In grains 32 and 34, the cores and rims are separated by a sharp boundary, seen in CL and BSE, to which inclusions (?) or cavities are restricted (seen in BSE as dark specks). To zoom grain image click on the corresponding number in red square.
ES1001 GRACHEV ET AL.: MORPHOLOGY AND GEOCHEMISTRY OF ZIRCON ES1001
7 of 17
ACL 9
An- f~ A n A Afc O Af>
35-4 35-3 35-2
A K_9
A r* r~ A r* A A r* O
35-4 35-3 35-2
;u 12.00 hV 163.71 uni « 130.-Î7 D
( M X Î
-X'si ri
Clear
Figure 7. Continued. Micrographs of zircon grains from bed XI of bentonite ash. Zircon morphologies (left), cathodoluminescence (CL) images (center), and back-scattered electron (BSE) images (right). Arrows indicate spots analyzed for bulk composition (major components) (Table 2), and circles are spots analyzed for trace elements, including REE (Table 4). Numerals are grain numbers.
3, 12 - oval grains with a cavernous surface and with fragments of zonal-sectorial structure and a thin rim seen in CL; 40 - grain fragment with cavernous surface and (b) coarse zoning; 44 - long-prismatic zircon with (a) cavernous surface and unzoned core; 45 - prismatic crystal with cavernous surface and zoned core and a rim; 9 - prismatic crystal with a veinlet cutting across the magmatic zoning (dark in CL and pale in BSE); 13 - Fragment of a prismatic crystal with elements of zoning and thin outer rim. The boundary with the rim is lined by an object dark in CL and pale in BSE; 59 - Oval grain with extensively healed cavernous surface. CL and BSE images show a core with fragmentary primary magmatic zoning and a gradual transition to the younger rim; 46, 33 - prismatic crystals with healed minute fractures at prism faces and with the growth of newly formed pyramid faces. The CL and BSE images show intense decomposition of the crystal cores and a heterogeneous zoned structure of the rims, whose boundaries are lined with zones with inclusions or cavities, which are seen in BSE as dark specks; 35 - euhedral crystal and small fragments of a spongy structure and a crack. The CL and BSE show a core with ”shadow” zoning in the newly formed coarsely zoned rim. To zoom image click on the corresponding number in red square.
ES1001 GRACHEV ET AL.: MORPHOLOGY AND GEOCHEMISTRY OF ZIRCON ES1001
Table 2. Whole rock composition (wt.%) of zircons
SiO2 P 2 O5 FeO ZrO2 HfO2 Y2O3 UO2 ThO2 Total ZrO2 /HfO2 N grain
1 31.63 0.61 0.01 64.92 1.27 0.45 0.03 0.00 98.93 51.32 2-1
2 32.31 0.70 0.01 61.24 1.04 0.77 0.08 0.04 96.19 59.05 2-2
3 32.06 0.00 0.02 66.05 1.02 0.05 0.00 0.00 99.18 64.94 3-1
4 32.09 0.00 0.02 66.06 1.01 0.12 0.00 0.00 99.30 65.67 3-2
5 32.07 0.00 0.02 66.01 1.22 0.10 0.02 0.01 99.43 54.28 4-1
6 31.95 0.05 0.01 65.48 1.17 0.30 0.08 0.04 99.09 55.77 4-2
7 32.24 0.00 0.01 65.70 1.30 0.14 0.02 0.00 99.41 50.61 5-1
8 32.12 0.09 0.01 65.52 1.30 0.26 0.11 0.06 99.47 50.25 5-2
9 32.11 0.00 0.01 66.01 1.08 0.14 0.00 0.02 99.37 61.01 6-1
10 32.12 0.00 0.01 66.29 1.11 0.05 0.01 0.00 99.58 59.88 6-2
11 32.09 0.06 0.01 65.85 1.44 0.09 0.04 0.00 99.58 45.73 6-3
12 32.73 0.00 0.01 65.26 1.30 0.07 0.02 0.02 99.41 50.08 6-4
13 32.33 0.00 0.02 65.97 1.06 0.11 0.00 0.01 99.51 62.01 7-1
14 32.07 0.21 0.01 64.95 1.52 0.31 0.11 0.01 99.19 42.73 7-2
15 32.18 0.18 0.01 66.03 1.02 0.22 0.01 0.00 99.65 64.93 8-1
16 32.26 0.00 0.02 65.52 1.49 0.23 0.09 0.03 99.65 43.86 9-1
17 32.49 0.00 0.02 66.29 1.12 0.04 0.00 0.00 99.95 59.29 9-2
18 32.40 0.00 0.01 65.52 2.05 0.04 0.28 0.00 100.30 31.93 9-3
19 32.29 0.00 0.01 66.36 1.03 0.04 0.00 0.00 99.74 64.49 9-4
20 32.25 0.00 0.42 62.91 3.20 0.07 0.27 0.00 99.11 19.65 9-5
21 32.12 0.06 0.02 65.66 1.02 0.34 0.02 0.03 99.26 64.63 10-1
22 32.34 0.00 0.01 66.10 1.08 0.10 0.00 0.01 99.64 61.26 10-2
23 32.16 0.17 0.01 65.17 1.46 0.24 0.02 0.00 99.23 44.73 10-3
24 32.21 0.15 0.01 66.02 1.17 0.23 0.02 0.02 99.82 56.52 12-1
25 32.29 0.10 0.02 65.33 1.17 0.31 0.03 0.05 99.30 55.88 12-2
26 32.34 0.00 0.04 65.74 1.41 0.12 0.02 0.02 99.67 46.79 12-3
27 32.17 0.10 0.01 65.39 1.78 0.13 0.16 0.01 99.75 36.70 13-1
28 32.16 0.03 0.01 66.29 0.94 0.08 0.00 0.01 99.51 70.44 13-2
29 32.24 0.02 0.01 66.48 0.96 0.06 0.00 0.01 99.78 69.25 13-3
30 32.21 0.01 0.02 66.30 0.99 0.10 0.01 0.01 99.65 67.18 13-4
31 32.24 0.00 0.01 66.16 1.38 0.06 0.06 0.02 99.92 48.08 13-5
32 32.33 0.00 0.01 66.18 1.13 0.07 0.00 0.01 99.72 58.77 14-1
33 32.14 0.04 0.03 65.03 1.18 0.47 0.13 0.18 99.20 55.02 14-2
34 32.32 0.00 0.01 66.35 1.08 0.05 0.00 0.00 99.81 61.49 14-3
35 32.08 0.15 0.01 65.51 1.11 0.17 0.02 0.01 99.07 58.86 15-1
36 32.04 0.13 0.02 64.48 0.98 0.61 0.08 0.05 98.38 65.79 15-2
37 32.30 0.06 0.02 65.69 1.08 0.11 0.03 0.00 99.30 61.00 15-3
38 32.19 0.08 0.02 65.43 1.07 0.25 0.03 0.01 99.06 61.32 15-4
39 32.07 0.18 0.01 65.71 1.43 0.20 0.03 0.00 99.62 46.08 16-1
40 31.60 0.53 0.01 64.36 1.09 0.71 0.30 0.17 98.78 58.99 16-2
41 31.86 0.41 0.02 65.09 1.47 0.38 0.06 0.01 99.29 44.16 16-3
42 32.01 0.17 0.01 65.47 1.29 0.18 0.00 0.00 99.13 50.87 17-1
43 31.70 0.47 0.01 64.98 1.10 0.47 0.03 0.01 98.77 59.18 17-2
44 32.28 0.00 0.06 65.52 1.35 0.09 0.15 0.06 99.50 48.49 18-1
45 32.28 0.00 0.06 65.52 1.35 0.09 0.15 0.06 99.50 48.49 18-2
46 32.07 0.14 0.04 64.96 1.00 0.57 0.19 0.20 99.17 64.76 18-3
47 31.16 1.17 0.01 63.62 0.99 1.24 0.06 0.04 98.29 64.00 19-1
48 32.23 0.20 0.01 65.81 1.24 0.15 0.01 0.01 99.66 52.99 19-2
49 32.12 0.27 0.02 65.72 1.28 0.23 0.01 0.01 99.65 51.46 19-3
50 32.16 0.06 0.03 66.11 1.28 0.14 0.11 0.07 99.95 51.85 20-1
51 32.08 0.07 0.09 65.91 1.33 0.18 0.16 0.12 99.95 49.48 20-2
52 32.24 0.07 0.04 66.53 1.17 0.11 0.04 0.02 100.21 57.06 20-3
53 32.22 0.07 0.01 66.76 1.11 0.12 0.01 0.00 100.30 60.20 20-4
54 31.99 0.17 0.05 65.46 1.33 0.29 0.11 0.05 99.45 49.11 21-1
55 31.70 0.39 0.10 63.12 0.93 1.15 0.67 1.01 99.06 67.58 21-2
56 31.81 0.31 0.06 64.71 0.88 0.88 0.30 0.23 99.18 73.53 21-3
Table 2. Whole rock composition (wt.%) of zircons (continuation)
SiO2 P2O5 FeO ZrO2 HfO2 Y2O3 UO2 ThO2 Total ZrO2/HfO2 N grain
57 31.83 0.36 0.12 63.64 0.85 0.88 0.81 1.49 99.98 74.87 21-4
58 32.10 0.35 0.01 65.77 1.22 0.33 0.01 0.01 99.79 54.09 23-1
59 31.99 0.46 0.01 65.34 1.34 0.43 0.02 0.00 99.57 48.94 23-2
60 31.98 0.38 0.02 65.54 1.33 0.36 0.01 0.00 99.61 49.13 23-3
61 32.19 0.17 0.02 65.58 1.84 0.09 0.35 0.00 100.23 35.64 23-4
62 32.28 0.05 0.01 66.45 1.05 0.10 0.02 0.02 99.97 63.46 24-1
63 32.19 0.15 0.01 66.11 1.02 0.21 0.04 0.05 99.78 65.00 24-2
64 32.35 0.05 0.01 66.61 1.13 0.05 0.00 0.00 100.20 58.89 24-3
65 32.21 0.10 0.01 65.89 1.09 0.28 0.07 0.09 99.74 60.45 24-4
66 32.38 0.00 0.01 66.63 1.07 0.00 0.00 0.00 100.10 62.27 24-5
67 31.68 0.52 0.01 65.40 1.33 0.49 0.05 0.00 99.48 49.32 25-1
68 31.77 0.41 0.11 64.91 1.16 0.64 0.32 0.12 99.42 56.20 25-2
69 32.10 0.18 0.03 65.77 1.04 0.30 0.06 0.02 99.50 63.06 25-3
70 31.74 0.49 0.01 65.01 1.35 0.59 0.24 0.06 99.49 48.08 25-4
71 31.86 0.45 0.02 65.28 1.38 0.45 0.05 0.00 99.49 47.27 25-5
72 32.21 0.13 0.02 65.95 1.31 0.18 0.12 0.04 99.94 50.54 26-1
73 32.24 0.10 0.01 66.22 1.31 0.11 0.05 0.01 100.06 50.43 26-2
74 32.14 0.25 0.02 65.51 1.48 0.25 0.03 0.00 99.68 44.32 27-1
75 32.13 0.25 0.01 65.80 1.35 0.23 0.01 0.00 99.78 48.92 27-2
76 31.64 0.71 0.02 65.06 1.11 0.71 0.05 0.02 99.31 58.51 27-3
77 32.12 0.32 0.01 65.66 1.21 0.31 0.01 0.00 99.64 54.22 27-4
78 32.09 0.29 0.01 65.62 1.37 0.28 0.02 0.00 99.69 47.79 28-1
79 31.82 0.48 0.01 65.48 1.30 0.51 0.04 0.01 99.64 50.44 28-2
80 31.81 0.54 0.02 65.42 0.99 0.72 0.05 0.04 99.58 65.95 29-1
81 32.08 0.33 0.01 65.72 1.16 0.31 0.00 0.01 99.63 56.46 29-2
82 32.27 0.03 0.01 66.07 1.20 0.15 0.07 0.02 99.81 54.97 30-1
83 32.35 0.00 0.01 66.18 1.29 0.04 0.02 0.01 99.89 51.30 30-2
84 31.79 0.53 0.01 65.48 1.17 0.55 0.03 0.01 99.57 56.16 31-1
85 31.74 0.35 0.03 64.18 0.95 1.18 0.17 0.19 98.77 67.70 31-2
86 32.06 0.30 0.02 65.55 1.26 0.26 0.01 0.01 99.48 51.86 31-3
87 32.31 0.08 0.02 65.57 1.60 0.18 0.30 0.16 100.22 40.98 32-1
88 32.34 0.04 0.01 66.16 1.22 0.14 0.06 0.05 100.03 54.19 32-2
89 32.36 0.05 0.03 66.23 1.32 0.11 0.08 0.05 100.22 50.21 32-3
90 32.45 0.02 0.41 64.32 1.30 0.01 0.02 0.02 98.53 49.63 32-4
91 32.30 0.07 0.02 65.74 1.53 0.13 0.06 0.02 99.87 42.88 33-1
92 31.89 0.23 0.12 63.39 1.68 0.90 0.89 0.37 99.46 37.76 33-2
93 32.01 0.25 0.06 64.95 1.54 0.50 0.23 0.07 99.60 42.18 33-3
94 31.70 0.25 0.11 63.05 1.68 0.90 0.83 0.35 98.87 37.44 33-4
95 32.35 0.04 0.01 65.97 1.51 0.06 0.02 0.01 99.97 43.57 34-1
96 31.91 0.21 0.04 64.53 0.99 0.92 0.20 0.15 98.95 65.19 34-2
97 32.44 0.04 0.02 65.94 1.47 0.12 0.04 0.01 100.06 44.98 34-3
98 31.78 0.71 0.01 65.16 1.23 0.67 0.03 0.01 99.59 53.15 35-1
99 32.48 0.00 0.02 65.67 1.79 0.05 0.13 0.00 100.13 36.77 35-2
100 32.45 0.00 0.01 65.76 1.87 0.01 0.05 0.00 100.15 35.18 35-3
101 32.37 0.00 0.01 65.72 2.02 0.00 0.08 0.00 100.20 32.57 35-4
102 32.44 0.05 0.01 66.83 0.95 0.18 0.02 0.02 100.51 70.43 36-1
103 32.39 0.08 0.02 66.91 0.93 0.19 0.02 0.03 100.57 72.34 36-2
104 32.33 0.08 0.01 66.35 1.25 0.11 0.07 0.03 100.23 52.91 37-1
105 32.05 0.16 0.13 64.54 1.16 0.47 0.56 0.70 99.77 55.45 37-2
106 32.54 0.00 0.02 65.59 1.90 0.04 0.33 0.00 100.42 34.61 38-1
107 32.39 0.13 0.01 66.30 1.32 0.19 0.02 0.01 100.38 50.31 38-2
108 32.13 0.35 0.02 65.82 1.00 0.44 0.15 0.07 99.97 65.75 38-3
109 32.36 0.12 0.02 66.37 1.30 0.21 0.03 0.01 100.41 50.94 38-4
110 32.43 0.07 0.01 66.07 1.35 0.18 0.22 0.09 100.42 48.91 39-1
111 32.42 0.02 0.02 66.41 1.27 0.17 0.11 0.04 100.45 52.41 39-2
112 32.37 0.04 0.01 67.00 0.89 0.07 0.00 0.01 100.38 75.62 40-1
Table 2. Whole rock composition (wt.%) of zircons (continuation)
SiO2 P2O5 FeO ZrO2 HfO2 Y2O3 UO2 ThO2 Total ZrO2/HfO2 N grain
113 32.45 0.00 0.03 66.64 1.20 0.07 0.00 0.01 100.39 55.62 40-2
114 32.45 0.01 0.05 66.24 1.28 0.18 0.04 0.05 100.30 51.92 40-3
115 32.43 0.05 0.02 66.76 1.12 0.07 0.00 0.02 100.46 59.77 41-1
116 32.47 0.06 0.01 66.65 1.11 0.10 0.01 0.01 100.42 60.21 41-2
117 32.07 0.47 0.02 64.70 1.79 0.36 0.19 0.01 99.59 36.16 42-1
118 32.44 0.07 0.01 66.59 1.07 0.12 0.02 0.02 100.33 62.35 42-2
119 32.14 0.33 0.01 66.06 1.26 0.31 0.01 0.00 100.12 52.30 42-3
120 32.39 0.10 0.01 66.39 1.05 0.33 0.02 0.07 100.37 63.05 43-1
121 32.39 0.21 0.01 66.06 1.35 0.21 0.02 0.00 100.26 48.83 43-2
122 32.37 0.20 0.02 66.11 1.37 0.21 0.01 0.00 100.29 48.11 43-3
123 31.97 0.15 0.08 65.21 0.91 0.81 0.18 0.16 99.46 71.34 44-1
124 32.44 0.00 0.02 66.97 0.91 0.12 0.00 0.00 100.46 73.35 44-2
125 32.22 0.04 0.03 65.82 0.85 0.71 0.03 0.02 99.70 77.71 44-3
126 32.32 0.05 0.03 66.04 1.08 0.41 0.05 0.04 100.03 61.20 45-1
127 32.32 0.11 0.02 66.06 1.29 0.29 0.08 0.03 100.19 51.25 45-2
128 32.27 0.00 0.01 66.64 1.24 0.00 0.03 0.00 100.20 53.61 46-1
129 31.83 0.19 0.06 65.44 1.02 0.59 0.28 0.14 99.54 64.48 46-2
130 32.07 0.09 0.02 66.50 1.01 0.11 0.09 0.01 99.91 65.84 46-3
131 31.64 0.24 0.05 65.21 0.92 0.83 0.34 0.19 99.41 71.19 46-4
132 32.03 0.04 0.02 66.41 1.24 0.13 0.04 0.02 99.93 53.51 46-5
133 32.18 0.02 0.01 66.57 1.20 0.05 0.02 0.01 100.06 55.34 47-1
134 32.22 0.03 0.01 66.64 1.22 0.05 0.02 0.01 100.20 54.66 47-2
135 31.98 0.10 0.02 66.38 1.05 0.23 0.08 0.09 99.93 63.34 47-3
136 31.94 0.23 0.01 66.54 1.18 0.20 0.00 0.01 100.11 56.63 48-1
137 31.94 0.28 0.01 66.19 1.52 0.25 0.05 0.00 100.22 43.57 48-2
138 32.08 0.17 0.01 65.83 1.35 0.37 0.12 0.05 99.98 48.94 49-1
139 31.98 0.22 0.01 65.62 1.36 0.38 0.15 0.05 99.77 48.18 49-2
140 32.06 0.14 0.01 66.26 0.98 0.28 0.07 0.04 99.82 67.68 50-1
141 32.10 0.15 0.01 66.71 0.95 0.14 0.03 0.01 100.10 70.07 50-2
142 32.09 0.14 0.01 65.98 0.99 0.31 0.09 0.04 99.64 66.51 50-3
143 32.05 0.12 0.01 66.07 0.95 0.33 0.08 0.04 99.65 69.77 50-4
144 32.16 0.12 0.01 66.60 0.95 0.11 0.02 0.01 99.97 70.47 50-5
145 32.09 0.10 0.01 66.39 1.23 0.11 0.03 0.00 99.96 53.89 52-1
146 32.10 0.08 0.01 66.41 1.26 0.10 0.03 0.01 100.00 52.75 52-2
147 32.17 0.00 0.02 66.44 1.41 0.00 0.07 0.00 100.12 47.26 53-1
148 32.16 0.04 0.01 66.69 1.26 0.04 0.02 0.00 100.23 52.81 53-2
149 32.05 0.01 0.02 65.87 1.82 0.05 0.25 0.07 100.15 36.13 54-1
150 32.25 0.02 0.01 66.70 1.28 0.01 0.04 0.02 100.33 52.27 54-2
151 32.22 0.01 0.01 66.84 1.25 0.00 0.00 0.01 100.34 53.38 54-3
152 31.97 0.13 0.04 65.94 1.03 0.51 0.10 0.12 99.84 64.15 54-4
153 32.12 0.07 0.01 66.46 1.26 0.09 0.09 0.03 100.13 52.74 55-2
154 31.90 0.11 0.01 66.12 1.20 0.28 0.24 0.14 100.01 55.19 55-3
155 32.16 0.07 0.02 66.90 1.01 0.11 0.01 0.00 100.28 66.11 56-1
156 31.93 0.04 0.01 66.49 0.88 0.34 0.02 0.02 99.74 75.30 56-2
157 32.03 0.03 0.02 66.76 0.95 0.12 0.01 0.01 99.92 70.05 56-3
158 31.87 0.19 0.01 66.17 1.34 0.18 0.02 0.00 99.78 49.31 57-1
159 31.85 0.20 0.01 66.06 1.52 0.18 0.01 0.00 99.85 43.46 57-2
160 31.80 0.30 0.02 66.05 1.28 0.26 0.01 0.00 99.71 51.64 57-3
161 31.96 0.10 0.01 66.26 1.12 0.21 0.01 0.01 99.69 59.22 58-1
162 32.19 0.00 0.01 66.20 1.34 0.03 0.01 0.01 99.77 49.47 58-2
163 32.02 0.14 0.01 66.62 1.11 0.14 0.01 0.01 100.05 60.18 58-3
164 32.12 0.04 0.02 66.46 1.20 0.05 0.02 0.01 99.91 55.57 59-1
165 31.66 0.57 0.01 65.03 1.51 0.46 0.14 0.01 99.38 43.12 59-2
166 30.97 0.72 0.02 64.55 1.60 0.55 0.34 0.01 98.76 40.27 59-3
167 32.08 0.05 0.01 66.85 1.24 0.05 0.01 0.01 100.30 53.74 60-1
168 31.97 0.14 0.03 65.46 1.17 0.48 0.10 0.09 99.42 56.09 60-2
Table 2. Whole rock composition (wt.%) of zircons (end)
SiO2 P2O5 FeO ZrO2 HfO2 Y2O3 UO2 ThO2 Total ZrO2/HfO2 N grain
169 32.02 0.02 0.01 66.37 1.38 0.01 0.04 0.01 99.86 48.20 60-3
170 32.09 0.06 0.01 66.24 1.20 0.06 0.03 0.02 99.70 55.29 61-1
171 32.07 0.05 0.01 66.35 1.29 0.09 0.10 0.06 100.01 51.64 61-2
172 32.02 0.05 0.02 66.58 1.30 0.07 0.04 0.03 100.10 51.41 61-3
173 31.38 0.64 0.01 64.93 1.10 0.84 0.13 0.07 99.10 59.08 62-1
174 31.72 0.30 0.01 65.68 1.40 0.28 0.04 0.00 99.43 46.95 62-2
175 31.43 0.52 0.02 65.04 1.10 0.78 0.11 0.08 99.08 59.29 62-3
176 32.08 0.01 0.01 66.50 1.17 0.03 0.01 0.01 99.82 56.99 63-1
177 32.06 0.03 0.01 66.75 1.20 0.07 0.02 0.02 100.17 55.44 63-2
178 32.05 0.08 0.01 65.96 1.37 0.09 0.04 0.04 99.64 48.25 63-3
179 31.69 0.40 0.01 65.78 1.03 0.48 0.02 0.03 99.44 63.74 64-1
180 31.95 0.14 0.01 66.56 1.13 0.12 0.00 0.00 99.92 58.70 64-2
181 32.18 0.01 0.01 66.66 1.38 0.00 0.01 0.00 100.24 48.48 65-1
182 32.19 0.00 0.01 66.41 1.44 0.00 0.01 0.00 100.07 46.25 65-2
183 32.14 0.03 0.02 66.21 1.10 0.15 0.04 0.03 99.72 59.97 65-3
184 31.87 0.07 0.02 65.92 1.96 0.06 0.30 0.00 100.20 33.62 66-1
185 31.59 0.61 0.01 65.46 1.17 0.55 0.05 0.01 99.45 55.85 66-2
186 31.61 0.55 0.16 64.50 1.59 0.61 0.45 0.06 99.51 40.70 66-3
trace element compositions are anomalous (Figure 8, Figure 14) due to similarities between U, Y, and Th, which are combined within factor 1 in both instances. Another anomalous grain (grain 26) is extremely rich in Ti (115 ppm). We cannot rule out that a Ti-bearing inclusion occurred within the excitation spot of the ion probe beam and thus affected the analysis. The analyses of this grain were rejected from the temperature calculations.
The geochemical meaning of factor 1 is self-evident, because the variations in the U, Y, and Th concentrations are significant (Table 4), and correlations between these elements are strong (rTh,u = 0.86 and ru,Hf = 0.39).
Factor 2 involves two elements (Li and Hf), whose correlated coefficient is significant (r = 0.53). The interpretation of factor 2 is determined by the bimodal distribution of the Li and Hf concentrations.
Analysis of the variations in the Li concentration indicates
Table 3. Factor loadings matrix of major elements
Oxide Factor 1 Factor 2
SiO2 -0.15 0.81
FeO 0.57 -0.24
P2O5 0.48 0.31
ZrO2 -0.35 -0.84
HfO2 0.15 -0.48
Y2O3 0.74 0.44
UO2 0.89 -0.01
ThO2 0.78 0.08
Input to total
variability 33% 24%
Note:marked loadings are > 0.70.
that a few of our zircon grains contain < 1 ppm Li (grains 3, 12, 13, 40, 44, 45, and 59), whereas all other grains (i.e., most grains in our selection) contain > 10 ppm Li. Data on Li concentration in zircon of various genesis are still sparse. It is known that magmatic zircon that crystallized from magma of primitive composition contains < 8 ppm Li, while zircon from differentiated melts in the continental crust contains 10-70 ppm Li [Ushikubo et al., 2008]. At the same time, Li concentrations in zircons in gabbro in the continental crust and kimberlites are lower than 0.01 ppm, and these values vary from 0.1 to > 100 ppm for zircon in the continental
Figure 8. Factor diagram for major components of zircon (N = 176).
i 3.0
2.0
►
1.5 1.0 0.5 H 0.0
♦
Figure 9. Hf2Ü3-Y2O3 diagram for all grains (N = 176).
Figure 10. P2O5-Y2O3 diagram for all grains (N = 176).
■
0 °* X 0
018 ■ 9 ▲ 48 □ 6 0 59 X66
Figure 13. ZrO2-ZrO2/HfO2 diagram for all grains (N 176).
Figure 11. P2O5-Y2O3 diagram for grains 33, 35, and 46.
Figure 12. Hf2O3-Y2O3 diagram for selected grains.
Figure 14. Factor diagram for trace elements in the zircons (N = 41). Solid circles correspond to zircons containing < 1 ppm Li.
crust. According to [Spicuzza et al., 2012] the lower limit of Li concentration in zircons from the continental crust is close to 0.5 ppm.
If the aforementioned bimodality in the Li concentration of zircon is generally typical of magmatic rocks, which is indirectly corroborated by data on lunar zircon [Spicuzza et al., 2012], this fact is of paramount importance for zircon identification. It is important that data on the Li isotopic composition testify that the rate of Li selfdiffusion in zircons is low [Ushikubo et al., 2008].
This led us to conclude that the 62 zircon grains from bed Xi comprise two zircon types, of course, with regard for the fact that the Li detection limit in this situation was 0.1 ppm.
The Hf concentration also shows a bimodal distribution. For example, anomalous grains 35 and 37 contain 11,415 and 13,833 ppm Hf, respectively, i.e., much more than the other grains do. A bimodal distribution can also be seen in the
13 of 17
Table 4. Trace and rare-earth elements concentrations (ppm) in individual zircon grains
No. Ba Sr Li Nb Hf Ti Y Th u La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Th/U T, °C
2 8 3 19.7 - 9418 14 4372 31.98 301 0.36 1.94 0.15 1.96 6.86 0.09 56.67 333 769 1324 203 0.11 769
3 9 2 0.45 - 7434 12 1290 31.22 78 0.29 11.76 0.23 2.60 3.98 1.30 21.15 91 223 495 89 0.40 756
4 11 4 3.13 - 8626 15 3003 311 861 0.95 31.6 1.26 11.25 13.33 0.57 60.73 250 520 867 130 0.36 771
5 10 3 18.77 - 9602 6 2238 112 503 0.34 6.84 0.15 1.72 5.79 0.21 37.0 176 394 671 105 0.22 697
6 12 2 1.74 - 7296 23 777 61 93 0.38 18.33 0.15 1.63 3.44 0.60 17.24 64 131 235 37 0.66 814
9 12 3 22.16 - 8992 8 488 20 770 0.36 5.13 0.09 0.52 0.96 0.07 5.53 31 95 285 59 0.03 713
10 11 3 8.30 - 7972 13 1749 41 178 0.38 3.84 0.13 1.42 3.66 0.09 24.53 129 319 575 90 0.23 760
12 1 2 0.80 59.3 8388 8 3300 357 416 0.06 10.00 0.28 3.34 7.71 0.77 50.50 254 561 915 134 0.86 722
13 0 1 0.76 20.1 6977 21 1189 59 147 0.20 10.15 0.19 3.27 5.95 1.60 32.00 106 185 275 44 0.40 805
16 1 2 38.11 17.7 9703 15 3962 296 1316 0.20 5.72 0.14 2.30 7.79 0.09 56.80 310 700 1233 194 0.22 772
17 2 2 20.30 12.8 8639 20 4199 83 340 0.25 2.51 0.22 4.04 10.80 0.14 72.58 345 751 1208 187 0.24 801
19 8 3 22.35 16.4 7970 31 9725 251 566 0.46 6.29 1.63 26.81 48.00 0.72 253 888 1543 2210 342 0.44 848
20 2 2 3.26 15.8 8144 19 1884 182 486 1.85 22.11 0.38 3.05 5.14 0.69 28.75 130 325 690 118 0.37 797
24 1 1 9.00 2.8 7278 12 1571 244 321 0.17 32.12 0.24 3.52 6.48 1.98 32.90 115 251 524 96 0.76 753
25 2 2 22.07 15.9 7992 26 5150 383 1556 0.17 10.93 0.27 4.78 12.12 0.38 82.34 399 865 1445 223 0.25 828
26 7 3 68.69 16.5 9401 115 2114 198 1077 0.47 12.16 0.93 9.25 12.78 8.77 46.70 170 349 686 110 0.18 -
27 0 2 19.84 12.1 9383 16 3786 52 284 0.32 1.73 0.21 2.85 7.59 0.11 58.27 286 605 983 157 0.18 782
30 1 1 5.99 12.9 8449 6 1489 222 715 3.73 28.00 1.25 7.90 4.89 0.26 24.35 110 256 464 76 0.31 694
32 1 1 12.87 10.9 9311 22 2449 385 695 0.13 35.35 0.62 10.94 25.87 10.61 76.63 184 268 450 76 0.55 811
33 2 4 6.74 130 11415 13 6140 1038 4344 0.18 26.00 0.16 2.21 8.21 0.09 68.00 441 1070 1873 59 0.24 761
35 1 1 41.77 16.8 13833 2 271 4.88 790 0.02 0.98 0.01 0.08 0.27 0.10 3.21 19.14 35.39 54.41 90 0.006 604
37 1 2 21.18 10.0 8986 4 2201 427 1168 0.75 59.38 0.41 4.69 8.15 1.86 45.61 170 365 688 134 0.37 667
38 1 2 20.11 18.0 8497 11 2826 299 1089 0.17 20.32 0.17 2.80 7.62 0.29 52.30 218 529 904 44 0.27 741
39 0 2 17.69 14.1 8691 13 1810 352 1349 0.09 25.41 0.09 1.75 4.28 0.27 28.28 132 308 586 194 0.26 763
40 - 2 0.36 14.0 8212 12 1711 283 415 0.08 60.17 0.25 2.23 4.06 0.88 22.00 101 287 660 119 0.68 755
41 1 1 4.36 6.3 7395 27 757 114 127 0.08 20.87 0.08 1.08 2.16 0.62 11.86 49 123 281 51 0.90 833
42 1 2 15.66 8.9 8161 13 2738 52 294 0.17 12.00 0.10 1.77 4.99 0.35 38.73 195 444 773 125 0.18 759
44 - 2 0.13 10.0 5786 15 6216 123 318 0.52 11.07 0.67 11.72 25.32 5.14 151 538 994 1476 227 0.39 775
45 1 1 0.85 15.6 8264 11 2414 168 563 0.06 14.14 0.15 3.05 7.08 0.24 47.30 199 413 655 101 0.30 746
46 8 5 3.20 9.0 7247 18 4954 694 1799 0.60 42.65 0.49 6.11 12.44 1.11 77.71 380 812 1404 213 0.39 792
47 1 2 8.32 6.0 8505 17 697 130 292 0.99 35.82 0.48 3.75 3.51 0.65 17.00 55 123 248 41 0.44 786
48 - 2 23.77 4.7 9953 10 3894 31 429 0.14 0.71 0.09 1.23 3.98 0.06 39.25 269 649 1128 178 0.07 735
50 - 2 2.11 9.2 6803 13 2668 217 689 0.20 17.80 0.18 2.99 6.30 1.50 42.70 195 430 753 131 0.31 759
52 - 1 22.50 5.6 8144 10 1050 95 431 0.06 2.67 0.05 1.14 2.93 0.06 18.50 78 174 311 51 0.22 735
53 2 1 5.89 2.6 7009 10 1428 151 363 0.10 25.50 0.48 8.00 10.30 2.87 40.40 116 212 381 65 0.42 734
54 - 2 4.31 6.1 8073 38 2293 422 462 0.08 30.20 0.31 4.54 7.61 1.69 45.20 173 380 695 122 0.91 867
55 5 3 10.30 17.6 8522 19 4551 1087 2552 0.72 68.10 0.84 9.93 21.99 3.00 107 368 731 1187 178 0.43 798
58 - 1 2.34 10.4 7954 12 1614 77 212 0.10 15.80 0.13 2.14 5.07 0.66 30 131 274 488 78 0.36 752
59 - 2 0.54 8.4 1025 18 4490 70 958 0.27 1.58 0.14 1.90 6.10 0.13 51 318 781 1491 233 0.07 793
61 - 1 33.95 8.5 9000 12 988 215 628 0.06 25.50 0.14 2.08 4.26 1.07 19.70 73 165 343 58 0.34 753
64 - 1 3.00 5.7 7741 14 4416 131 251 0.19 3.84 0.68 11.78 21.59 0.41 110 384 723 1077 172 0.52 856
ES1001 GRACHEV ET AL.: MORPHOLOGY AND GEOCHEMISTRY OF ZIRCON ES1001
Figure 15. Th-U diagram for zircons from bed Xi in comparison with this mineral from gabbro from the Mid-Atlantic Ridge [Zinger et al., 2010], dacite from the Juan de Fuca Ridge [Schmitt et al., 2011], and acid tiff from Iceland [Car-ley, 2010].
HfO2-Y2 O3 diagram, which also displays two zircon populations. High Hf concentrations are known to be indicative of hydrothermal alteration of zircon [Hoskin, 2005]. Miner-alogical evidence confirming the effect of hydrothermal processes is quartz, xenotime, plagioclase, occasional, apatite, and even more rare pyrite inclusions in some zircons (Figure 6).
Figure 17. Ti-Y diagram for zircons: solid circles - this publication, trend for MORB is according to [Grimes et al.,
2009].
We can now discuss diagrams most commonly used to distinguish between zircon types [Grimes et al., 2007; and others]. In the Th-U diagram (Figure 15), most of the data points of zircons from bed Xi plot within the field of zircons from the products of historical volcanic eruptions in Iceland and partly in the field of zircons in basalts from the Juan de Fuca and Mid-Atlantic ridges. The situation with the data points of the zircons in the U/Yb-Hf diagram is somewhat different: zircons from bed Xi and zircons from Iceland lavas fall within the continental crust field (Figure 16). Other diagrams of this type (which are not presented here) rather demonstrate that it is not possible to use any two parameters, or their ratios, to identify the provenance of detrital zircon in sedimentary rocks. For example, the Ti-Y diagram (Figure 17) displays, unlike the Th-U diagram (Figure 15), that the trend for zircons of the MORB source divides our zircons into two roughly equal populations.
Table 5. Factor loadings matrix of trace elements
Figure 16. U/Yb-Hf diagram for zircons: circles - this publication, red rectangle - compositional field of zircons in tuffs from Iceland [Carley, 2010]. The boundary line between zircons in continental and oceanic crust is according to [Grimes et al., 2007].
Element Factor 1 Factor 2
Ba 0.21 0.32
Sr 0.04 0.30
Li 0.32 0.78
Hf 0.16 0.71
Ti 0.08 0.63
Y 0.75 -0.05
Th 0.74 -0.14
U 0.81 0.10
Input to total
Variability 31.8 18%
Note:marked loadings are > 0.70.
10
La Ce Pr Nd SmEu GdTb Dy Ho Er TmYb Lu
Figure 18. Chondrite-normalized [McDounough and Sun, 1995] REE patterns for zircon: circles - this publication, squares - zircon from gabbro [Zinger et al., 2010].
Rare Earth Elements
The chondrite-normalized REE patterns of zircon make it possible to distinguish three its types: magmatic, meta-morphic, and hydrothermal [Hoskin and Schaltegger, 2003; Li et al., 2000; and others], which differ from one another in enrichment in LREE and Ce and Eu anomalies.
Our zircons are obviously enriched in LREE, show positive Ce and negative Eu anomalies (Table 3, Figure 18), and can be classed with the magmatic type, as can be seen by the overlap of their trends with those of zircons from oceanic gabbroids.
The REE patterns exhibit no evidence of hydrothermal alterations, because zircons from bed Xi do not meet the main geochemical criterion for distinguishing hydrothermal zircons: LREE differentiation and a gradual decrease in the Ce anomaly.
Discussion and Conclusions
An interpretation of the data obtained in the course of our study requires knowledge of the paleogeographic environment in which the bentonite was formed. It is known that bentonite can be of two types: purely volcanic one and that containing a few percent pelagic and terrigenous components and broadly variable contents of pyroclastic grains and nonvolcanic material [Thurow and von Rad, 1992].
Our data on the heavy fraction of the material of bed Xi indicate that this material contains several minerals: garnet, rutile, leucoxene, xenotime, feldspars, Ni and Fe sulfides, magnetite, and titanomagnetite. Previously the most detailed data were obtained on the rutile, which was classified into six types, including this mineral from metamorphic rocks. This mineral association suggests that the material
Table 6. Estimated zircon crystallization temperatures
Grain number of zircons Ti content, ppm Temperature, °C
2 14.31 769
3 12.39 756
4 14.65 771
5 6.24 697
6 22.70 814
9 7.54 713
10 12.91 760
12 8.37 722
13 20.87 805
16 14.77 772
17 19.88 801
19 31.46 848
20 19.13 797
24 11.94 753
25 26.03 828
26 115.39 1007
27 16.44 782
30 5.98 694
32 22.02 811
33 13.12 761
35 1.72 604
37 4.25 667
38 10.50 741
39 13.45 763
40 12.25 755
41 27.26 833
was supplied from more than one (at least two) sources: a distant volcanic one, which was related to the activity of the Iceland plume, and a proximal Alpine source of clastic material. At a conservative estimate of the sedimentation rate of about 2 cm per 103 years [Egger et al., 2005], bed X1 should have been produced fairly quickly (during approximately 1500 years), but the supply of terrigenous material to the sedimentation basin at a low sea level in the latest Paleocene-earliest Eocene should have been fairly significant [Kennett and Stott, 1991]. The bentonite of bed X1 should thus be classed with a mixed type, and this put forth the question as to which criteria can be applied to elucidate the provenance of the zircons.
In analyzing binary diagrams commonly utilized to identify the nature of zircons, it was demonstrated that it is not possible to use any two trace elements or their ratios to identify the source of detrital zircons in sedimentary rocks.
The crystallization temperatures of the zircons calculated with the Ti thermometer [ Watson et al., 2006] broadly vary from 604 to 867°C at an average of 766±53°C (grain 26 containing 115 ppm Ti was rejected) (Table 6). Currently available estimates of zircon crystallization temperatures vary within the range of 653 ± 124° C for acid magmatic rocks and 758 ± 111°C for mafic rocks [Fu et al., 2008].
Based on this criterion and with regard for the results of factor analysis of major and trace elements, it can be
hypothesized that our zircons mostly affiliate with the magmatic (but not the hydrothermal or metamorphic) type. The data on the REE distribution testify to an oceanic nature of the magma. A more reliable criterion of an oceanic nature of the source is the Li concentration, with the concentrations of this element in some of the grains being much lower than the minimum values typical of the continental crust.
Our data on the morphology and inner structure of the zircons also suggest a magmatic genesis of most of these grains. It is pertinent to mention their distinctive features: their structure, which suggests that the zircons crystallized simultaneously with the dramatic decompression of the melt. Certain morphological features of the zircons, for instance, the occurrence of cataclased grains, make the zircons similar to this mineral from the Mid-Atlantic Ridge [Zinger et al.,
2010].
At the same time, it is worth mentioning that several grains show obvious evidence of hydrothermal alteration, which affected both the morphology of the zircon grains and their inner structure, as can be seen in the CL images, and resulted in the high Hf concentrations. The currently widely used classification of zircons into continental and oceanic types [Grimes et al., 2007; and several others] does not cover all diverse types of magmatic zircons. The data presented above on Li concentrations in the zircons provide grounds to suggest that a zircon type can exist that is related to plume magmatism, which evolves regardless of the type of the crust and is controlled by processes at the Earth’s core/lower mantle interface. An analogous conclusion was drawn in [Vasquez et al., 2010] based on data on zircon in gabbro and diorite xenoliths in the Holocene basalt of Hualali volcano in Hawaii; these researchers suggest to distinguish a zircon type related to hotspots (hot spot zircons).
This problem can be solved only via further studies of Li concentrations in magmatic zircons in lavas form oceanic islands and intraplate continental basalts (flood basalts in Siberia, Deccan, Ethiopia, etc.).
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S. E. Borisovsky, Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Moscow, Russia
A. F. Grachev, Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia (afgrachev@gmail.com)
T. F. Zinger, Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia
