Mediterranean

Frédéric Mélières, Université Pierre et Marie Curie, Paris, France Hervé Chamley, Université de Luminy, Marseille, France Francis Coumes, Elfaquitaine, Pau, France and

Pierre Rouge, Compagnie Française des Pétroles, Bordeaux, France

INTRODUCTION

This report synthesizes X-ray mineralogy data from Sites 371, 372, 373, 374, 375, 376, 377, and 378 on Leg 42A in the Mediterranean Sea. Before the cruise, a very careful statement on the routine sampling policy was established by the scientific party, making available one sample per section of core for routine X-ray mineralogy analysis. In this way, around 450 samples were taken, the study of which was shared among different laboratories:

  • bulk X-ray mineralogy: all sites (F. Meiieres)
  • clay mineralogy: Sites 374, 375, and 376 (H. Chamley)
  • clay mineralogy: Site 372 (F. Coumes)
  • clay mineralogy: Sites 371, 373, 377, and 378 (P. Rouge)

The upper Miocene evaporites, encountered at six sites, were not sampled for routine X-ray mineralogy analysis because of the highly diversified lithologies found. It was decided that the study of the evaporitic facies (mainly gypsum, anhydrite and halite) would be carried out with an essentially petrographic approach (Garrison et ai, this volume).

Here we present information on the methods used by the different laboratories, a site-hy-sitc discussion of the results, and comparisons between sites in chronologic sequence.

METHODS Bulk X-ray Mineralogy: All Sites

The method used, basically operating on non-oriented powder with the use of an internal standard for quantitative determinations, is described elsewhere (Meiieres, 1974). A resume of the basic steps follows.

The samples are manually roughly ground in an agate mortar, without any pretreatment (such as removing salts by washing in distilled water1). After being oven-dried at 40°C, 50 mg of internal standard (sodium fluoride) are mixed with 250 mg of sediment by grinding in the mortar. The mixture is then

1 Consequently. halite reported on analytical data doe;, not exist as a mineral in the sediment; it results from evaporation of interstitial water when the samples arc dried.

mounted in a rotating sample holder (Mdlieres, 1973), the speed of which (10 rev/sec) insures a good reproducibility of the diffracted peak intensities. Two identical Siemens diffractometers, equipped with fine focus copper tubes, are used to run the X-ray diffraction scans. CuK« radiation is isolated by a nickel filter. The geometry of the X-ray beam is determined by a 0.5" entering slit, a 0.2 mm reeeiving slit, and a frontal knife. The X-ray source intensity is set in order to obtain all the peaks of the constituents (including the internal standard, but omitting the clay minerals), in the best recording conditions. The first scan is then recorded (15° to 45° 20) at 1° 20/mm. A second scan is run for clay minerals (2° to 17° 26) under the same conditions but with appropriate gain, depending on the abundance of these minerals in the sample. The third is a slow scan (0.5° 20/min or less) resolving the doublet kaolinite(002)/ehlorite(004) (24° to 26° 26). After the sample, in its holder, is exposed to a current of ethylene-glycol aerosol for five minutes, to expand the smectite minerals, a fourth scan (with the same conditions as the second one) is run.

The identification of minerals is carried out by reference to standard diffractograms made in the laboratory on pure natural constituents, and, if necessary, with ASTM data and classical handbooks (Brown, 1963; Borgand Smith, 1969).

The quantitative analysis is carried out using calibrated curves giving the mass ratio (given mineral versus internal standard) from the peak intensities ratio. All these curves were prepared in the laboratory using the classical procedure of artificial mixtures (Klug and Alexander, 1967). Depending on minerals, three kinds of measurement of peak intensities are used: (a) for minerals well crystallized and known to be constant in crystallinity (detrital quartz and feldspar, heavy minerals, internal standard, etc...), only the peak height above the background is used; (b) for minerals well crystallized, but offering various degrees of crystallinity such as carbonates, both peak height and width at half-height are used (two entries in the calibrated curve); (c) for minerals more or less well crystallized (clay minerals, opal-CT), the weighted peak area above the background is exclusively used. The amounts of clay minerals are estimated on the glycolated diffractogram. The 10A peak is used for illite; the 17A peak is used for smectite (this includes pan of the expandable irregular mixed layer). The 7Â peak yields the total amount of chlorite + kaolinite, the balance between these minerais being made using the peaks height ratio, at, respectively, 3.54Â and 3.58À, according to the Biscayc ( 1964) method. Amounts of irregular mixed-layer (mainly illite-semec-tite) arc estimated from the area above the background between the 10Â illite peak and that of smectite at I7Â. This procedure in fact underestimates the true amount of the mixed-layer, because the expandable part of these minerals is more or less incorporated in the 17Â peak area through the ethylene-glycol treatment. The precision of the quantitative measurement ranges from 5% to 15% relative, and depends mainly on the nature of the minerals.

Data concerning the amorphous material content are not reported here because of their insignificance. The amorphous material content can actually be estimated subtracting the total of all crystallized constituents from 100. In fact, in the hemipelagic sediments of the Mediterranean Sea, mainly consisting of detrital silicates and/or biogenic carbonates, the amorphous material content is usually low (below 10%). In such a case, the amorphous material content value has a range of error which is often greater than the value itself; for instance, if the total of crystallized constituents is 93% +15%, the resulting amorphous material content will be 7% ± 15%, which lacks significance. For this reason, the percentages of total crystallized constituents are summed to 100%.

Clay Mineralogy: Sites 371, 373, 377, and 378

The samples are mixed with distilled water for 10 hours, and then are decarbonated in 10/JV hydrochloric acid. The excess acid is removed by successive centrifugations. The less than 2 urn fraction is collected by décantation employing Stockes' law to calculate settling times, and then oriented aggregates are made on glass slides. A Philips X-ray diffractometer with copper Ni-filtered radiation is used to run three scans as follows: (a) from 2° to 22° 26 (8°/min) on natural sample; (b) from 2° to 22° 20 (8°/min) on glycolated sample; and (c) from 24° to 26° 29 (2Vmin) on natural sample. Quantitative determinations are carried out using peak heights above the background; the amount of each mineral is considered to be directly proportional to the peak height. The I0Â peak (natural sample) is used for illite and the l4Âpeak (natural sample) for smectite, eventually corrected for chlorite influence, deduced from the 14Â peak on glycolated sample. The amount of irregular mixed-layer (illite-smectite) is estimated by the difference of diagram traces between "natural" and "glycol" samples, between 11Â and 13A. The 7Â peak gives the total kaolinite + chlorite. The ratio of these two minerals being deduced from the ratio of the peak heights at 3.58À and 3.54Â, respectively. The total of percentages is summed to 100%.

Clay Mineralogy: Site 372

The method and diffractometer used are very similar to that employed at Sites 371, 373, 377, and 378.

Minor differences are: (a) clay mineral analysis is performed on the less than 5 ^m fraction (oriented aggregates); (b) the speed of the X-ray diffraction scans is 2° 20/min; and (c) the peak heights used for quantitative estimates, are corrected by classical proportional factors (Johns et al„ 1954),

Clay Mineralogy: Sites 374, 375, and 376

The sample preparation is similar to that above. Both oriented aggregates and oriented pastes are made from the less than 2i*m fraction. A CGR Theta 60 diffractometer (copper radiation focused by a quartz curved-crystal monochromator) is used to run the X-ray diffraction scans at 1° 20/min. A receiving slit of 1.25 mm is used for a better determination of mixed-layer minerals. Beside the ethylene-glycol treatment (expansion of smectites), a hydrazine-hydrate treatment is used to characterize the kaolin minerals. The heat treatment consists of two hours at 490°C, Semiquantitative evaluations are based on the peak heights and areas; smectite and attapulgite are corrected in addition to peak height, and well-crystallized kaolinite is corrected in diminution with regard to middle crystalized illite or chlorite. The balance between chlorite and kaoiinite is made from peak heights ratio (3.54A and 3.58A. respectively). When this ratio is 1, the amount of chlorite is assumed to be two times that of kaolinite.

Slight, but unavoidable, discrepancies are to be expected in the quantitative data, since this analytical work was carried out by a team working at several laboratories.

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