Average Igneous Rock Research Articles

Chlorine as a Discriminant Element to Establish the Provenance of Central Mediterranean Obsidians

Abstract Chlorine is a minor element present in obsidians in quantities greater than in average igneous rocks. The chlorine concentration in obsidians is generally low, of the order of tenths of wt %, but it exhibits an appreciable differentiation among geological sources. Despite these characteristics, chlorine has rarely been taken into consideration as a possible indicator of obsidian provenance and it does not appear in the chemical analytical tables accompanying the geochemical characterisation of obsidian samples. In this work, after an overview of chlorine geochemistry and cycle, we present thirty-one new electron microprobe (EPMA) analyses, including Cl, of geologic obsidians sampled from the four sources of the Central Mediterranean, exploited in prehistoric times (Monte Arci, Palmarola, Lipari and Pantelleria). The results are compared with 175 new EPMA analyses, including Cl, of archaeological obsidians already characterised in previous work and of known provenance. As such it was possible to ascertain that each source has a characteristic chlorine concentration, showing the utility of its use in the studies of obsidian provenance. Furthermore, given that the solubility of chlorine in silicate melts is correlated to its alkali content, in particular sodium, we assessed the efficacy of simple binary graphs Cl vs Na2O to better constrain the provenance of the obsidian samples.

Iron isotope constraints on diagenetic iron cycling in the Taixinan seepage area, South China Sea

Authigenic pyrite is a common mineral in seepage areas, where it is mainly derived from sulfate-driven anaerobic oxidation of methane (SO4-AOM). However, reports on the iron isotopic composition of SO4-AOM generated pyrite are limited, and the mechanisms controlling the fractionation of iron isotopes are insufficiently understood. Here, we present new iron isotope data for pyrite and iron oxides from sediments of the Taixinan seepage and gas hydrate-bearing area (sites HD109 and GMGS2-16) and revisit published data for pyrite from the Shenhu area to unravel the impact of SO4-AOM on the iron isotope composition of pyrite. The δ56Fe values (relative to average igneous rock) of pyrite were found to vary from −0.86 to −0.56‰ for site HD109, and from −1.57 to −0.25‰ for site GMGS2-16. In contrast, the iron isotopic compositions of iron oxides from site GMGS2-16 reveal positive values throughout the core, spanning from +0.52‰ to +0.74‰. In addition, the extent of pyritization (Fepy/FeHR) of the sediment was investigated, revealing a variability between 0.01 and 0.29 for HD109, and between 0.00 and 0.36 for GMGS2-16. A previous study on the Shenhu seepage area recognized an enrichment of 56Fe in pyrite at (paleo-)sulfate-methane transition zones (SMTZ), which was regarded as a consequence of pronounced SO4-AOM. However, peaks in δ56Fepy values are not observed at (paleo-)SMTZs at sites HD109 and GMGS2-16 in this study. The negative δ56Fepy values are likely to reflect the dominance of dissimilatory iron reduction (DIR) during diagenesis in this area. In addition, for all samples of the Taixinan and Shenhu areas, their δ56Fepy values, and the extent of pyritization, a clear positive correlation becomes apparent, where both δ56Fepy values and Fepy/FeHR ratios from the Taixinan area are lower. Hence, it can be inferred that the availability of sulfide is among the key factors that affect the iron isotopic composition of pyrite. With respect to the Taixinan area, the low degree of pyritization was apparently insufficient to alter the iron isotopic composition of the iron sources (i.e., iron oxides) to a significant degree.

The enrichment of heavy iron isotopes in authigenic pyrite as a possible indicator of sulfate-driven anaerobic oxidation of methane: Insights from the South China Sea

Sulfate-driven anaerobic oxidation of methane (SO4-AOM) and organiclastic sulfate reduction (OSR) in marine sediments commonly lead to the precipitation of authigenic pyrite with characteristic sulfur isotopic compositions. Yet, no reports on the iron isotopic composition of SO4-AOM generated pyrite exist to the best of our knowledge and the processes controlling the distribution of iron isotopes during this biogeochemical process are not understood. To investigate the mechanisms involved in iron isotope fractionation during SO4-AOM, we analyzed the isotopic composition of authigenic pyrite from two seafloor sites (HS148 and HS217) in the Shenhu gas hydrate area, South China Sea. The δ56Fe values of pyrite were found to vary from −0.349‰ to +0.267‰ in core HS148, and from −0.793‰ to +0.178‰ in core HS217 (relative to average igneous rock) with distinct trends towards higher values with increasing burial depth in both cores. The δ56Fe values correlate positively with the extent of pyritization (Fepy/FeHR) derived from a sequential iron extraction procedure. OSR was found to drive early diagenetic pyrite formation at shallow depth, resulting in relatively 56Fe depleted pyrite. Peaks in the extent of pyritization of 0.45 and 0.47 at ca. 6 and 7m below the sediment surface, respectively, correlate with high δ56Fe and high δ34S values, and are interpreted to reflect periods of enhanced SO4-AOM in paleo-sulfate methane transition zones (SMTZs). The great variability in iron isotopic composition and the observed trends are best explained as a function of the availability of dissolved sulfide, reflecting sulfide formation by OSR and SO4-AOM. The successive conversion of reactive iron to pyrite via sulfidization apparently resulted in the enrichment of 56Fe in the iron reservoir upon the preferential consumption of isotopically light iron, leading to the formation of successively 56Fe-enriched pyrite especially at or in close proximity to the SMTZ. Therefore, we hypothesize that the combined enrichment of heavy iron isotopes and 34S in authigenic pyrite is a possible tracer of SO4-AOM in modern and ancient sedimentary sequences.

Isotopic evidence for Fe cycling and repartitioning in ancient oxygen-deficient settings: Examples from black shales of the mid-to-late Devonian Appalachian basin

Abstract We have measured iron (Fe) isotope compositions of bulk samples and chemically extracted pyrite in two black shale units: the Oatka Creek Formation (OCF) and the younger Geneseo Formation (GF) deposited in the Appalachian basin during the mid-to-late Devonian. The depositional redox conditions for these units are well established through multiple paleoproxies, including degree of pyritization (DOP) and ratios of total Fe to Al (Fe T /Al), suggesting that both deposits reflect oxygen-deficient environments, but that euxinia (anoxia with hydrogen sulfide in the bottom waters) was more frequent and persistent during deposition of the OCF. Iron isotopes show systematic variations that are consistent with the inferred water column redox conditions. Samples from the OCF yield low and variable bulk Fe isotope compositions (− 0.44‰ to 0.03‰ in δ 56 Fe relative to average igneous rocks) that are inversely correlated with Fe T /Al, whereas bulk δ 56 Fe values ( δ 56 Fe T ) of the GF fall in a narrower range (− 0.09‰ to 0.12‰). δ 56 Fe values of pyrite ( δ 56 Fe Py ) display good correlation with δ 56 Fe T in the OCF, but no such correlation is observed in the GF. The Fe isotope data and other paleo-redox indicators, when viewed collectively, point to a benthic Fe source on the shelf and shelf-to-basin transfer that operated during deposition of the OCF, similar to what has been observed in the modern Black Sea. For the first time this study confirms the strength of Fe isotopes in delineating this Fe enrichment mechanism in the ancient geological record and emphasizes the utility of the Fe isotope proxy for fingerprinting and quantifying ancient biogeochemical cycling of Fe.

Pore-fluid Fe isotopes reflect the extent of benthic Fe redox recycling: Evidence from continental shelf and deep-sea sediments

Pore-fluid Fe isotopes may be a unique tracer of sediment respiration by dissimilatory Fe-reducing bacteria, but to date, pore-fluid Fe isotope measurements have been restricted to continental shelf settings. Here, we present δ56Fe values of pore fluids from two distinct sedimentary settings: (1) a riverine-dominated site on the northern California margin (Eel River shelf; 120 m water depth) and (2) biogenic opal-rich volcaniclastic deep-sea sediments from the Southern Ocean (north and south of the Crozet Plateau; 3000–4000 m water depth). The Fe isotope compositions of Crozet region pore fluids are significantly less fractionated (δ56Fe = +0.12‰ to −0.01‰) than the Eel River shelf (δ56Fe = −0.65‰ to −3.40‰) and previous studies of pore-fluid Fe isotopes, relative to average igneous rocks. Our data represent the first measurements of Fe isotope compositions in pore fluids from deep-sea sediments. A comparison of pore-fluid δ56Fe with the relative abundance of highly labile Fe in the reactive sedimentary Fe pool demonstrates that the composition of Fe isotopes in the pore fluids reflects the different extent of sedimentary Fe redox recycling between these sites.

A Geochemical View of Weathering and the Origin of Sedimentary Rocks and Natural Waters

The geochemistry of weathering plays a major role in determining the general composition of sedimentary rocks and natural waters. Chemical weathering converts igneous rocks to the chemical and mineralogical constituents of both clastic and chemical sedimentary rocks, as shown in a newly developed diagram that additionally illustrates the chemical ancestry of carbonate rocks in mafic igneous rocks. New variation diagrams illustrate the chemical differentiation of various sedimentary rocks from average igneous rocks, in which the degree of differentiation depends on previous reactions in chemical weathering. Chemical weathering also modifies surface waters by donating the dissolved solids, especially bicarbonate, dictated by weathering reactions. This interplay of weathering and freshwater chemistry, combined with subsequent chemical and biological deposition from the oceans, illustrates the linkage of the rock and water cycles from rainfall to weathering to transport of dissolved solids to deposition to ev...

K-Rb relations in the alkaline suites of the Eastern Ghats precambrian belt, India

K-Rb relations in the major alkaline suites from the Eastern Ghats Precambrian belt of India have been studied. All the suites show progressive enrichment of Rb with differentiation, but the Sivamalai suite with a dominance of feldspathoidal members has extremely low Rb concentrations. The K/Rb ratios, except those for Kunavaram, lie above the MT for average igneous rocks. Possible cause for fluctuation in K/Rb is discussed. The high K/Rb ratio is a possible inherent character of the magmas corresponding to their derivation from deeper parts of the upper mantle.

Chemical balance of the Earth's crust

The chemical composition and lithologic proportions of average sedimentary and igneous rocks are analyzed by a linear least squares regression. The best set of internally consistent data for sediments in geosynclines, continental platforms, deep ocean basin and the average igneous and sedimentary rock are shown below. These results are based on published estimates of the chemical composition of igneous and sedimentary rocks and the relative volume of sediments on the continents and in the deep ocean basins. SiO 2 A1 2O 3 Fe(total) MgO CaO Na 2O K 2O CO 2 Geosynclines 55.2 12.0 3.6 3.2 8.3 1.0 2.4 8.6 Platforms 49.5 10.7 3.2 3.3 12.3 0.9 2.2 10.8 Av. sed. 55.9 13.8 4.4 3.6 5.8 1.2 2.6 5.2 Av. ig. 59.0 15.0 5.0 3.6 7.0 3.4 3.2 The lithologic proportions are: geosyncline—24% sandstone, 58% shale, and 16% carbonate; platform—23% sandstone, 49% shale and 26% carbonate; average sedimentary rock—13% sandstone, 33% shale, 12% carbonate, and 41% pelagic; average igneous rock is approximately 2/3 granodiorite and 1/3 tholeiite. These data indicate a chemical balance of major elements in the Earth's upper crust during the last 1.5 b.y.

PETROS—a data bank of major-element chemical analyses of igneous rocks for research and teaching

PETROS is a fixed-format magnetic tape data bank of major-element chemical analyses of igneous rocks divided into groups representing selected geographic areas and petrologic provinces. The 20,000 analyses and additional calculated average igneous rock compositions may be used for a variety of computer-based research and teaching applications. Interactive programs greatly expand the accessibility and usefulness of PETROS.

Early Evolution of Oceans--A Weathering Model: ABSTRACT

The long-term chemical composition of seawater is controlled by the generalized reaction: primary igneous rocks + water + acid volatiles = sediments + oceans + atmosphere. Unstable crustal minerals are weathered by water and acid volatiles, and local equilibrium between the products of the reaction--oceanic sediments, seawater and the atmosphere--is attained. To obtain a better picture of the evolution of the oceans as this reaction proceeds (minerals formed, mass transfers involved, changes in seawater composition), we simulated with a model calculation on a high speed computer the irreversible attack of average igneous rock--represented by an idealized mineral assemblage--by water and acid volatiles. We assumed a single-stage degassing process under reducing conditions at 25°C and 1 atm. The predicted final solid products at equilibrium, ranked according to decreasing mass, are amorphous silica (chert), clay minerals, carbonates, and K-feldspar. The predicted composition of the early ocean resembles that of present seawater except that (1) the dissolved sulfur is in reduced form, (2) the solution is saturated with amorphous silica and (3) the salinity is about twice that of today because of nonremoval of NaCl as evaporites. Extension of these results to more realistic systems can at best be semiquantitative because of lack of sufficient thermochemical data. Furthermore, the recycling of sediments makes it very difficult to estimate early environmental conditions from present remnants of Precambrian sediments. Some generalizations can nevertheless be made with confidence. A more basic initial crustal material such as oceanic basalt would lead to larger amounts of clays and carbonates in the sediments at the expense of chert, and to a large concentration of dissolved ferrous iron in the ocean. Degassing of water preferentially to other volatiles would not affect the outcome of the weathering process unless the escape rates of the volatiles differed by several orders of magnitude. Although our model clearly represents one extreme, rapid degassing, the available geologic evidence does not preclude its having taken place. It is encouraging that the results of the calculation are in general accord with what has been reported previously. End_of_Article - Last_Page 348------------

Sedimentary Volumes and their Significance

Sedimentary volumes are of prime interest in many fields of geology: as measures of erosional rates, of geochemical balance, and recently, with the virtual demonstration of continental drift, as measures of movement of the continental and oceanic plates. The Basement Map of the United States, published by the U.S. Geological Survey in 1968, provides a partial basis for an improved estimate of the volume of Phanerozoic rock in the center, minous United States. The map requires correction for this purpose, because all metamorphic rocks of whatever age have been classed as basement. We have, therefore, attempted to allow for the metamorphic rocks of Phanerozoic age. We have made estimates of volumes for areas not controlled by contours on this map and have used such offshore data as we have been able to assemble from the literature in order to extend our estimates to include offshore sediments reasonably attributable to erosion from the area of the contiguous United States. Our results are as follows: We consider this estimate to be within 10 percent of the true volume. Of it, we estimate about 3.2 ×10 6 km 3 to be volcanic rock, not representing erosion of pre-existing rock. The remaining 56.8 × 10 6 km 3 , rounded to 57 × 10 6 km 3 , we consider products of continental denudation. This volume is so large, representing, as it does, only 5.3 percent of the continental surface of the earth and only a sixth of recognizable geologic time, that it appears to invalidate schemes of geochemical balance such as those of Clarke, Goldschmidt, and others. These students assume that the salt in the sea is a measure of the amount of some “average igneous rock” that has been eroded during the whole of geologic time to produce some “average sedimentary rock.” Instead, our result points strongly toward the hypothesis of Livingstone, Gregor, Earth, and others that the oceanic salt is merely the cyclic salt not yet returned to the continents in a continuing cycle. Assuming that this volume was derived from erosion of the contiguous United States—an assumption that we recognize as invalid in detail, though not seriously in error—we obtain an ostensible average rate of Phanerozoic erosion of about 10 m/ m.y., about a sixth of the present rate. But inasmuch as present erosion is attacking a surface that exposes about 76 percent sedimentary rocks and only 24 percent igneous, most of its product is recycled rather than first-cycle sediment. An analysis of the broad features of the paleo-geography of the country indicates that a similar disproportion between first-cycle and recycled sediment has been characteristic of nearly all the Phanerozoic. The ostensible erosion rate is therefore spurious, and it is likely that the average erosion rate durin g the Phanerozoic was more than half that of the present, and perhaps was nearly or quite equal to it. The great disparity in volumes of sediment offshore in the Atlantic and Pacific—in a ratio of more than 5 to 1—is consonant with expectations if the continent has been moving westward and over-riding the Pacific Basin on a Benioff fault system activated at the beginning of the Mesozoic, though now dormant.

The average igneous rock and the geochemical balance

The composition of the average igneous rock has been calculated utilizing new analytical data and paying special attention to the role of andesites. The resulting composition, waterfree and in weight per cent, is: SiO 2 62.6; TiO 2 0.75; Al 2O 3 15.7; Fe 2O 3 2.85; FeO 3.29; MnO 0.14; MgO 2.87; CaO 4.80; Na 2O 3.30; K 2O 3.23 and P 2O 5 0.27. This is very close to earlier estimates based on geochemical balance equations. The calculated ratio of limestone to other sedimentary rocks differs markedly from current estimates. It is suggested that this discrepancy arises from the fact that metamorphic terrains are left out of consideration in such estimates.

Hydrothermal Melting of Shales

AbstractPT curves for the beginning of melting of five analysed shales in the presence of water vapour under pressure are 20° C. to 40° C. higher than the corresponding curve for granite. About 150° C. above the beginning of melting, the shales are half-melted; this is higher than the liquidus curve of most granites. Refractive indices of the quenched liquids (1·495–1·505) indicate a granitic or granodioritic composition. Quartz, cordierite, mullite, hypersthene, anorthite, etc., are developed in the partially fused shales. Partial fusion of shales by a granitic magma, even if superheated, would produce a liquid no more basic than granodiorite. The chemical characteristics of the shales are compared with average igneous rocks, and there appears to be no possibility that fusion of shales could produce a basaltic magma. Complete fusion would produce a melt with composition distinct from normal igneous magmas.

Helium, argon, and carbon in some natural gases

Thirty-nine samples of natural gases representing varied chemical compositions and geological occurrences were analyzed for their helium, radiogenic argon, and atmospheric argon contents. The total range in the (He/A)_(rad) ratio was found to be 1.6 to 130 with most samples having values between 6 and 25. This range of values is essentially equal to the production ratio from the uranium, thorium, and potassium in average igneous rocks and a wide variety of sediments. This indicates that all of these natural gases have obtained their radiogenic gases from rather average rock types. This is true in spite of the fact that the gases range in helium content from 37 to 62,000 ppm. A theoretical discussion of the origin of helium and argon in natural gases is given. It can be shown from the ratio of nitrogen to atmospheric argon that most of the nitrogen in these gases cannot come from the entrapment of air. From a consideration of the concentration of atmospheric argon in natural gases it is possible to estimate the proportion of gaseous and aqueous phases assuming diffusive equilibrium. The isotopic composition of the carbon in the methane of these gases was found to be very light. It was shown that for coexisting CH_4-CO_2 pairs the carbon dioxide was always isotopically heavier.

The effect of metamorphism on metal distribution near base metal deposits

Sedimentary rocks near 3 base metal deposits were analyzed to determine possible changes in Zn, Cu, and Fe content caused by alteration or metamorphism. Of the sediments in the Hanover, New Mexico district of contact metasomatic Zn-Cu deposits, only the Devonian (Percha) fissile shale contains sufficient Zn (70 p. p. m. Zn, 48 p. p. m. Cu, and 4% Fe) to contribute significantly to the metal content of the deposits. However, this shale has the same metal content (within + or -20 p. p. m. Zn and Cu and within + or - 1% Fe) when unaltered, as in the recrystallized and silicified zone (excluding the skarn). Extraction of 20 p. p. m. Zn from the altered area of the shale is quantitatively insufficient to account for the 600,000 metric tons Zn in the skarn and ore deposits of the zone. In the northern Mississippi valley district, neither the Maquoketa shale (35 p.p.m. Zn, 65 p.p.m. Cu, and 3% Fe), nor the oil rock (20 p. p. m. Zn, 60 p. p. m. Cu, and 7% Fe) has a sufficient metal content in comparison with the background level given by average igneous rock (100 p. p. m. Zn, 70 p. p. m. Cu, and 5% Fe) to be the source for the metals in the deposits. Near the margins of the San Francisco del Oro district, Chihuahua, Mexico, the gradient of Zn content from the veins into the silicified wall rocks passes through a minimum below that of unaltered shale (123 p. p. m. Zn, 75 p. p. m. Cu, and 5% Fe). However, the quantity of Zn shown by this gradient to have been extracted from the shale is only a negligible fraction of the metal content of the entire deposit. It is quantitatively impossible for the metal content of these 3 districts to have been derived from the surrounding Paleozoic or younger sediments by lateral secretion.

Thorium, uranium and potassium in some sandstones

Thorium, uranium, and potassium concentrations have been determined in nineteen sands and sandstones by γ-ray spectrometry, fluorometric uranium analysis, and α-counting. The samples were selected so that both common and extreme thorium and uranium ratios would be represented. The average and nearly uniform values found in orthoquartzitic, clay-free sands were potassium (as metal) 0.64 ± 0.04 per cent; thorium 1.7 ± 0.1 p. p. m.; and uranium 0.45 ± 0.05 p. p. m., giving a Th/U ratio of 3.8 ± 0.8. The Th/U ratio found is very near to that of average igneous rocks and may represent an independent determination of the average crustal Th/U ratio. Heavy mineral separations on some of the ordinary sands indicate that very little of the thorium, uranium, and potassium in a common sand is associated with the heavy detrital grains. Neglecting a very minor feldspar content, the radioactivity of the common sandstones studied is contained almost entirely in the quartz, where it appears to be largely related to microscopic and submicroscopic inclusions. The heavy detrital minerals do not begin to affect the Th/U ratio or concentration ratios until they occur in large quantities as in placer sands. Within experimental error, the modern beach sands studied were in radioactive equilibrium. These preliminary data provide some independent and experimental comparisons for the average thorium and uranium content of sandstones calculated from geochemical balances. The data are also helpful in constructing models of the changes in thorium and uranium ratios that might be observed in moving from a beach line through shallow water sediments to deep-water sediments.

GEOCHEMISTRY OF PELITIC ROCKS. PART III: MAJOR ELEMENTS AND GENERAL GEOCHEMISTRY

Research Article| July 01, 1956 GEOCHEMISTRY OF PELITIC ROCKS. PART III: MAJOR ELEMENTS AND GENERAL GEOCHEMISTRY DENIS M SHAW DENIS M SHAW MCMASTER UNIVERSITY, HAMILTON, ONTARIO Search for other works by this author on: GSW Google Scholar Author and Article Information DENIS M SHAW MCMASTER UNIVERSITY, HAMILTON, ONTARIO Publisher: Geological Society of America Received: 17 Jun 1955 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1956, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1956) 67 (7): 919–934. https://doi.org/10.1130/0016-7606(1956)67[919:GOPRPI]2.0.CO;2 Article history Received: 17 Jun 1955 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DENIS M SHAW; GEOCHEMISTRY OF PELITIC ROCKS. PART III: MAJOR ELEMENTS AND GENERAL GEOCHEMISTRY. GSA Bulletin 1956;; 67 (7): 919–934. doi: https://doi.org/10.1130/0016-7606(1956)67[919:GOPRPI]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Compilation of 155 analyses gives the following average composition of all pelitic rocks in per cent, from low to high grade (standard deviations in parentheses): SiO2, 61.54 (4.68); TiO2, 0.82 (0.61); A12O3, 16.95 (4.21); Fe2O3, 2.36 (1.97); FeO, 3.90 (2.25); MgO, 2.52 (1.91); CaO, 1.76 (2.03); Na2O, 1.84 (1.18); K2O, 3.45 (1.32); H20, 3.47 (2.25); CO2, 1.67 (2.37).After classifying the analyses into two groups (clays, shales, and slates; phyllites, schists, and gneisses) statistical discrimination tests showed no change in composition during regional metamorphism, beyond loss of H20 and CO2.The Littleton formation of New Hampshire is a series of pelitic rocks, but is not completely representative of the group, since it forms a sedimentary petrographic province. Thus, the formation is somewhat deficient in CaO, Na2O and CO2, contains more Al2O2 and possibly TiO2, and is in a more reduced state at the low-grade level than the average pelitic rock. It is also more restricted in composition. Moreover this formation underwent minor metasomatism during metamorphism, resulting in the addition of about half a per cent of CaO and Na2O and the loss of H2O, as well as minor element changes.Comparison of pelitic rocks with the average igneous rocks shows that the alkalies and alkaline earths are relatively concentrated in shales as follows: Li > K > Na; Sr > Ba > Mg > Ca. These relations cannot be explained solely on the basis of relative ionic potentials. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Igneous intrusions and metamorphism in some phosphatic rocks of southwestern Montana

Exploration work to locate commercial grade phosphate rock and phosphatic shale deposits has been going on in southwest Montana for several years. Geologic mapping has resulted in finding four locations where sills, dikes, and stocks intrude or cut off the phosphate-bearing beds of the Phosphoria formation (Permian). The metamorphic effects, or lack of them, produced in the phosphatic sedimentary rocks by intrusions are of commercial and scientific interest.In three of the areas, there have been no detectable changes in the phosphate minerals resulting from the intrusions, although the sedimentary rocks have been indurated. At the remaining area, Fleecer Mountain, the phosphatic rocks have been indurated, bleached from dark gray and black to light gray, and collophane and francolite minerals have been converted to colorless, very fine-grained apatite. Some phosphatic material has migrated. Megascopic veinlets filled with francolite developed in shale, and microscopic quartz veinlets containing some apatite grains formed in phosphatic shaleChemical analyses made on igneous rocks collected from the dikes, sills, and stock do not show an excess of P 2 O 5 over that present in average igneous rocks of similar types.

The “total” amount of sediments and the composition of the “average igneous rock”

Assuming sediments to derive exclusively from granite-granodiorite and basalt the chemical composition of the “average igneous rock” is found to be almost identical with that of the Appalachian and Cordilleran igneous rocks as determined by A. Knopf (1916). The total amount of weathered igneous rocks is found to be 194 ± 21, the amount of shale 172 ± 28, sandstone 17.3 ± 21, limestone 12.8 ± 4.3, and dolomite 6.8 ± 3.0, all in kg per cm 2 earth surface. The total amount of sediments formed by complete weathering is thus 209 ± 35 in the same unit. The results and their degree of accuracy are discussed and compared with the results of previous calculations.

TRACE ELEMENTS IN PELITIC ROCKS

The range in composition of pelitic shales, schists, and gneisses has been investigated for a number of trace elements and has been compared with previous work on similar materials. An interesting distribution law has appeared (logarithmic), namely that the range in composition extends from zero to about twice the mean value for almost every element. The probability that a rock will fall outside this range is low. Fairly good positive correlation has been found between chromium, vanadium, and nickel, with cobalt participating to some extent. All these elements tend to be high or low together, but the most regular correlation is between nickel and chromium. In general, the content of the trace elements studied is what would be expected from erosion and transfer from the average igneous rock, with no loss or gain on the way. Copper is an exception with a clear loss, whereas lithium and strontium show an increase. Comparison with suites of silicic and basic igneous rocks emphasizes some differences between these and pelitic rocks, but also shows that in many cases the composition fields overlap extensively. Certain revisions in crustal abundance (average igneous rock) figures are suggested.