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2 Chemical and Isotopic Composition of Rocks Many important geochemical problems can probably be solved only with samples obtained from deep drill holes in the continen- tal crust. Much more can still be done, of course, with surface materials. However, major advances in knowledge are likely to come about only through geochemical studies of new samples from heretofore inaccessible portions of the earth's crust. Enormous advances have taken place in our ability to obtain important geo- chemical information from very tiny samples, mainly as a result of the study of lunar rocks. Any information obtained from lu- nar samples was extremely valuable, even for samples that could not be placed into accurate geologic context. The lunar sampling program and the continental drilling program are very dissimilar, and have vastly different geochemical sampling requirements. To make major new advances in our understanding of the geochem- ical evolution of the earth, it is essential that studies be made of samples whose relationships to each other and to other known geologic features and structures are accurately known. Although it is certainly true that important geochemical in- formation could be obtained from cuttings from very deep holes, simply because such samples are not obtainable in any other way, the information would be of limited value. The data thus acquired 10
11 would pale into insignificance compared to the increase in our un- derstanding of the evolution of the earth that would come about through studies of sections of continuous core from a series of deep holes in the continental crust. GENERAL SAMPLING REQUIREMENTS FOR GEOCHEMISTRY More often than not, the most valuable geochemical samples taken from surface outcrops are not those from the relatively abun- dant rock types, but those rare samples at important geologic con- tacts, or those present as veins, mineralized fractures, alteration envelopes, or fault zones. Such anomalous materials generally are more readily observed in core samples than in surface outcrops. Given the choice in sampling near-surface rocks, even where out- crops are excellent, geochemists and petrologists typically choose drill-core material over outcrop samples because of the freshness and continuity of drillcore. Thus, a good case can be made that continuous core is better for most geochemical sampling than the best surface outcrops. For many petrological purposes, it is important to make ac- curate major and trace element analyses of whole-rock samples. This is particularly useful in igneous petrology, but it has in- creasing significance in metamorphic and hydrothermal studies as well. A key problem is collection and preparation of the sample to ensure that it is: (1) large enough to be representative of the rock mass being sampled; (2) collected and handled in such a way that it is free of contamination, at least for the elements being analyzed; and (3) fresh and unweathered. In recovered drilling samples, item (3) will not be a problem, and with high quality core, requirements (1) and (2) also can be met. Special care will have to be taken to ensure that samples are free of contamination from drilling mud or tools. Because of the low permeability antic- ipated for core samples, surface contamination and contamination from fluid penetration can be eliminated by sawing or grinding off the outer part of the core. However, if only cuttings are available, then most whole-rock studies cannot be done, because items (1) and (2) simply cannot be satisfied. For example, in trace-element studies, a few grains of galena (PbS) from a vein that was inad- vertently mixed in the cuttings would invalidate the lead isotopic composition of a whole-rock sample.
12 The problem of obtaining a representative rock sample that is homogeneous on the scale of interest for any type of whole-rock chemical analysis is related to the size of crystals that make up the rock. A standard procedure is to crush a large sample and take a small split of crushed rock for final preparation of powder suitable for analysis. For fine- and medium-grained rocks, about 0.5 kg of crushed rock is sufficient. This is equivalent to about 40 cm of (uncontaminated) 2.5 cm (1 inch) diameter core or 4 cm of 7.6 cm (3 inch) diameter core (Table 2). Coarse grained rocks, such as porphyritic granites or augen gneisses that might be encountered, could require crushing on the order of 10 kg of material (800 cm or 80 cm, respectively, of the above cores) in order to obtain a representative split of perhaps 0.1 kg for powdering. Concern for contamination by crushing equipment becomes significant when working with such large samples. Splits of less than 1 kg could be taken from rock crushed for mineral separations for geochronology so that very large segments of core would not be destroyed solely for chemical analysis. Clearly, larger core diameter is desirable to minimize the proportion of contaminated material that will need to be removed before crushing. MAJOR ELEMENT GEOCHEMISTRY Characterization of igneous rocks recovered from continental drilling requires accurate knowledge of major element composi- tions. Such information is vital to correlations of metamorphic rocks beneath the drill site with those exposed elsewhere at the surface. Moreover, knowledge of rock composition is necessary for many aspects of petrologic studies of metamorphic rocks, espe- cially those involving mass transfer. Problems of contamination are less severe than for complementary trace element studies, but samples must be large enough to be representative of the rock volume of interest. Core samples are essential for meaningful whole-rock analyses. Representative samples of all igneous rocks returned from the hole, and sufficient metamorphic rocks to ad- equately characterize geologic units, should be analyzed. Rock powders prepared for major element analysis will be suitable for many other geochemical determinations.
13 TRACE ELEMENT GEOCHEMISTRY Although most of the elements occurring in rocks are present in only low concentrations, accurate knowledge of their abun- dances is important in interpreting the origin of all types of rocks. It is likely that virtually every trace element will be analyzed in certain important samples obtained from the deep continental drilling program. For many samples, only the more geochemically useful trace elements will be analyzed, but these include a wide variety: several of the rare earths, as well as Ba, Sr, Rb, Zr, Ilf, Nb, Ta, U, Th, and most of the valuable ore metals (Ni, Cu, Zn, Pb, Hg, Mn, Cr, Sn, Ag, Au, etc.). Mineral separates also may be analyzed. Trace element studies will be critical in establishing correla- tions between geologic units in the southern Appalachian hole and surface outcrops. Carbonate rocks beneath the sole of the thrust brought up as lenses in the Brevard fault zone, for example, may be correlative with rocks exposed to the west in the Valley and Ridge province. Continental or oceanic crustal signatures of ig- neous rocks and some indication of the tectonic environment of their formation will be apparent from trace element data. Vertical variations in trace element abundances will help to establish the extent of elemental mobility during metamorphism and deforma- tion. RADIOGENIC ISOTOPE GEOCHEMISTRY Radiogenic isotope geochemical techniques are among the most definitive of all geochemical studies, and for that reason their sampling requirements constrain those of the entire geochemical program. The most important radiogenic-isotope systems are the U-Pb, Th-Pb, Sm-Nd, Rb-Sr, and K-Ar parent-daughter pairs. From each we obtain: (1) the time of crystallization of the rock and(or) age information about the geologic events that affected the rock since its formation; and (2) the geologic processes that created the rock or later affected it, including the type of parent material and the nature of any later metamorphism or hydrother- mal alteration. In addition, the distribution of these elements in the crust will provide important information for a large number of geochemical and geophysical studies, including heat-flow, fluid- flow, and elemental migration, because the radioactive parents
14 listed above are the major heat-producing materials in the earth's interior. Except for K, all the radiogenic isotope systems listed above occur as trace elements in most minerals and rocks of the earth's crust. They are thus extremely susceptible to contamina- tion. The most useful type of drilling sample would be a pristine section of core large enough (preferably at least 4 cm in diameter) to obtain interior samples. In addition to its utility as a geochronological tool, radiogenic isotope geochemistry has the power to characterize deep crustal rocks on the basis of the isotopic compositions of Sr, Nd, and Pb in igneous rocks emplaced at shallower levels or erupted at the surface (e.g., Farmer and DePaolo, 1984). Because metamor- phism affects the mobility of these elements and their parents in different ways, components of magmas that were derived in part from metamorphic rocks can be detected and the general age and degree of metamorphism determined (e.g., amphibolite vs. granulite). Radiogenic isotopes also yield information on mantle or oceanic crustal contributions to igneous rocks. For example, isotopic studies of igneous rocks within a tectonostratigraphic ter- rane, now present as a thin thrust sheet, and very likely to indicate whether that terrane originally formed on oceanic or continental crust; if continental, the age of the crust might also be obtained. Isotopic studies should be carried out on igneous rocks from all terranes encountered in the southern Appalachian drill hole in or- der to test hypotheses of terrane origins and indicate the nature of underlying crustal types. The mobility of radiogenic daughter products makes them ideal tracers for studying elemental mi- gration during metamorphism and deformation. Vertical profiles through terranes and their contacts would contribute greatly to multidisciplinary investigations of elemental mobility. STABLE ISOTOPE GEOCHEMISTRY Stable isotope studies have become extremely important in understanding the origin and history of sedimentary, igneous, and metamorphic rocks. Major categories of investigation are: (1) isotopic geothermometry (O'Neil and Clayton, 1964), which re- quires the separation and analysis of individual minerals from cogenetic mineral assemblages; (2) fluid-rock interaction (Taylor, 1974), which also requires analysis of mineral separates, vein and
15 fracture material, and fluid inclusions in minerals; and (3) iso- topic tracers of geologic processes, utilized in a somewhat similar way and often in conjunction with radiogenic isotope systematics (James, 1982). The geochemically most important stable isotopes are the el- ements of low atomic number or those that have a complex chem- istry. Particularly important are those with a variety of oxida- tion states, namely, hydrogen (D/H), carbon (13C/12C), nitrogen (15N/14N), oxygen (18O/16O), and sulfur (34S/32S). All these ele- ments except oxygen are present in small amounts in most rocks. However, some of them may be present as major constituents of certain minerals (sulfides, carbonates), and in such cases it is es- sential to obtain mineral separates. Others are present in most minerals in trace amounts or as fluid inclusions (hydrogen, nitro- gen, carbon). Contamination problems are least severe for oxygen, but even for 18O/16O, at least 0.1 kg samples are needed because mineral separates (typically about 100 mg) are usually essential for geochemical studies. Thus the same considerations of contam- ination, accurate representation of geologic units, and knowledge of interrelationships between samples that have been stated for other geochemical studies apply to the sample requirements for almost all stable isotope analyses. Identification and recovery of small veins enriched in sulfides, carbonates, or hydroxyl-bearing minerals will generally be necessary to make useful S, C, and H isotope interpretations of continental drilling samples. This can only be done with drill-core material. Results of stable isotope studies will define the extent of fluid mobility within terranes and across their contacts. Fluids can be characterized as to their origin, and this information can be added to interpretations based on metamorphic petrology. The character of protoliths of metasedimentary rocks, such as those below the master sole thrust, may be elucidated. Stable isotope data will also contribute to identifying crustal or mantle components of igneous rocks encountered in the hole. If any mineralized areas (e.g., sulfide bodies) are penetrated, stable isotope investigations will be essential in determining the origin of ore-forming fluids and the extent of rock-fluid interaction.
