Robert E. Zartman

The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs—A case for bi-directional transport☆

Abstract Version IV of plumbotectonics expands and refines the original model of Doe and Zartman (1979) and Zartman and Doe (1981) for explaining Pb (Sr, and Nd) isotopic systematics among major terrestrial reservoirs. A case for bi-directional transport among reservoirs is based on the observed isotopic compositions for different tectonic settings, and finds a rationale in the kinetics of plate tectonics. Chemical fractionation and radioactive decay create isotopic differences during periods of isolation of one reservoir from another, whereas dynamic processes allowing mixing between reservoirs tend to reduce these differences. Observed isotopic characteristics reflect a balance between these opposing tendencies and provide constraints on the extent and timing of fractionation and mixing processes. Plumbotectonics does not require interaction with a lower mantle or core reservoir over most of the Earths lifetime, and, in fact, achieves a material balance consistent with no such exchange of material. Important evidence of the amount and timing of crustal recycling, and of the residence times of mantle heterogeneities lies in the coupled 207 Pb 204 Pb- 206 Pb 204 Pb systematics. We believe that examination of the published data base fully supports our contention of significant bi-directional transport of material among terrestrial reservoirs. Plumbotectonics allows us to explore many aspects of reservoir interaction, and to identify parameters that provide meaningful constraints on mantle-crust differentiation. We put forth a compromise fit to many of the model variables in version IV, which can serve as a reference for future work.

Trace-element and Sr, Nd, Pb, and O isotopic composition of Pliocene and Quaternary alkali basalts of the Patagonian Plateau lavas of southernmost South America

The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The “cratonic” basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (ɛNd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the “cratonic” basalts. In contrast, the “transitional” basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, ɛNd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the “cratonic” basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The “transitional” basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the “cratonic” and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, δ18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock δ18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock δ18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The “cratonic” basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and δ18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The “transitional” basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the “transitional” basalts earlier in the Cenozoic.

Pb-, Sr- and Nd-isotopic systematics and chemical characteristics of Cenozoic basalts, eastern China

Abstract Forty-eight Paleogene, Neogene and Quaternary basaltic rocks from northeastern and east-central China have been analyzed for major-element composition, selected trace-element contents, and Pb, Sr and Nd isotopic systematics. The study area lies entirely within the marginal Pacific tectonic domain. Proceeding east to west from the continental margin to the interior, the basalts reveal an isotopic transition in mantle source material and/or degree of crustal interaction. In the east, many of the rocks are found to merge both chemically and isotopically with those previously reported from the Japanese and Taiwan island-arc terrains. In the west, clear evidence exists for component(s) of Late Archean continental lithosphere to be present in some samples. A major crustal structure, the Tan-Lu fault, marks the approximate boundary between continental margin and interior isotopic behaviors. Although the isotopic signature of the western basalts has characteristics of lower-crustal contamination, a subcrustal lithosphere, i.e. an attached mantle keel, is probably more likely to be the major contributor of their continental “flavor”. The transition from continental margin to interior is very pronounced for Pb isotopes, although Sr and Nd isotopes also combine to yield correlated patterns that deviate strikingly from the mid-ocean ridge basalt (MORB) and oceanic-island trends. The most distinctive chemical attribute of this continental lithosphere component is its diminished U/Pb as reflected in the Pb isotopic composition when compared to sources of MORB, oceanic-island and island-arc volcanic rocks. Somewhat diminished Sm/Nd and elevated Rb/Sr, especially in comparison to the depleted asthenospheric mantle, are also apparent from the Nd- and Sr-isotopic ratios.

Lead Isotope Systematics and Uranium Depletion in the Granite Mountains, Wyoming

Isotopic composition and concentration of lead in whole rock and microcline and concentration of uranium and thorium in whole-rock samples of granite from the Granite Mountains, Wyoming, have been determined. The lead isotopic composition in the whole rocks was found to be highly radiogenic with a range in Pb 206 /Pb 204 of 19.58 to 42.27; the corresponding range in microclines is 15.39 to 22.44. A Pb 206 /Pb 204 versus Pb 207 /Pb 204 plot of the whole-rock data yields an apparent isochron age of 2,790 ± 80 m.y. as the time of crystallization of the granite. Chemically determined values of U 238 /Pb 204 in the whole rocks lie between 3.3 and 18.4 and are too low to account for the amount of radiogenic lead observed. A material balance of lead, thorium, and uranium components indicates that an average of approximately 75 percent of the amount of uranium required to produce the radiogenic lead was removed from the rocks, whereas, on the average, there was no apparent loss of thorium. Loss of uranium from the granite is demonstrated to extend at least to a depth of 165 ft in a drill core. The average uranium loss from the samples analyzed represents about 20 g uranium per 1,000 kg of rock that apparently was removed during the Cenozoic and that probably constitutes the major source of uranium now in ore deposits in central Wyoming basins. The lead isotopic composition of the microclines indicates that lead was mobilized within the granite and was isolated in the feldspar during a thermal event about 1,640 + 120 m.y. ago. However, there is no evidence that the whole rocks themselves became open systems at that time. Whole-rock and microcline isochrons intersect at Pb 206 /Pb 204 and Pb 207 /Pb 204 of 13.77 and 14.86, respectively, indicating a characteristic U 238 /Pb 204 of 8.96 in the source region of the granite magma.

Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah

An area of 3,000 km 2 in and around the Grouse Creek Mountains and the Raft River Mountains exposes Precambrian, Paleozoic, and Triassic sedimentary rocks that were folded several times and displaced tens of kilometres on low-angle faults. Overturned folds and local imbrications indicate transport westward and northward during two episodes of metamorphic deformation and transport eastward after metamorphism. Metamorphic grade increases downward in the allochthonous sheets and autochthon and increases westward in the autochthon. Mineral grains are flattened into the horizontal plane, and shear strains increase upward, suggesting that the deformations were caused by gravity acting on a broadly heated dome. Rb-Sr dating of granitic plutons affected by the deformations indicates that (1) the area is underlain by adamellite, about 2.5 b.y. old, in which deformation decreased progressively downward; (2) the first metamorphic deformation probably ended before 38.2 ± 2.0 m.y. ago; and (3) the second metamorphic deformation was still underway 24.9 ± 0.6 m.y. ago. High-grade allochthonous rocks that lie on low-grade parts of the autochthon indicate as much as 30 km of eastward transport after metamorphism. Parts of the dome sagged to form broad basins 12 m.y. ago, and the coarse sediments and tuffs that accumulated in them were overrun by allochthonous sheets measuring at least 11 by 19 km. Two Rb-Sr mineral isochrons and several fission-track ages indicate that some parts of the area cooled below 400 °C only 10 m.y. ago.

Structural implications of some radiometric ages of igneous rocks in southeastern New England

U-Th-Pb zircon, Rb-Sr whole-rock, and K-Ar hornblende isotopic ages reveal two major episodes of igneous activity in southeastern New England—one in late Proterozoic Z time (600–650 m.y. ago) and the other in Devonian to Ordovician time (375–450 m.y. ago). The dated rocks are discussed in the context of several individual structural provinces that evolved more or less independently of each other prior to middle to late Paleozoic assembly. These provinces generally are bounded by major faults and(or) abrupt changes in metamorphic grade and, from east to west, include (I) the Milford-Dedham zone, (II) the Putnam-Nashoba zone, (III) the Merrimack synclinorium, and (IV) the Bronson Hill zone. From the present and other recently published studies, reasonably accurate ages representative of the two major episodes can now be assigned to many of the igneous rocks. Other units remain less well constrained but probably may be placed into one or the other of the general age groupings. Case by case examination of the geochronologic data makes us cautious in accepting some apparent fine-structure age resolution within each grouping until factors such as inherited zircon and metamorphic overprinting can be better evaluated. With the above-mentioned qualifications in mind, igneous rock ages summarized according to structural province are as follows. (I) Milford-Dedham zone: (a) late Proterozoic Z—Dedham Granite and related rocks, 630 ± 15 m.y., 595 ± 15 m.y., 612–646 m.y.; Milford Granite, 630 ± 15 m.y., 591 ± 50 m.y.; Hope Valley Alaskite Gneiss and Scituate Granite Gneiss, ages uncertain; unnamed granite near Assonet, 600–650 m.y.; (b) Devonian to Ordovician—alkalic rocks, including Quincy, Cape Ann, and Peabody Granites, Wenham Monzonite, and Nahant Gabbro. (II) Putnam-Nashoba zone: (a) late Proterozoic Z—no plutonic rocks identified but may include some metavolcanic rocks, (b) Devonian to Ordovician—Sharpners Pond Diorite, 430 ± 5 m.y.; Andover Granite, 408 ± 22 m.y., 450 ± 22 m.y.; muscovite granite near West Berlin, age uncertain; Preston Gabbro, 424 ± 5 m.y. (Ill) Merrimack synclinorium: (a) late Proterozoic Z—granitic orthogneiss unit of the Massabesic Gneiss Complex, 600–620 m.y.; metavolcanic unit of the Massabesic Gneiss Complex, 646 m.y. or older, (b) Devonian to Ordovician—Fitchburg Complex at Maiden Hill, 402 ± 11 m.y.; Fitchburg Complex at Rollstone Quarry, 390 ± 15 m.y.; Newburyport Complex, 450 ± 15 m.y.; Ayer Granite, 433 ± 5 m.y.; Chelmsford Granite, age uncertain; muscovite-biotite granite at Millstone Hill, 372 ± 7 m.y.; Canterbury Gneisses, 395 ± 10 m.y., 392 ± 9 m.y. (IV) Bronson Hill zone: (a) late Proterozoic Z—Dry Hill Gneiss of the Pelham dome, ∼600 m.y. (b) Devonian to Ordovician—Monson Gneiss, 440 ± 10 m.y. These distinctive age patterns, together with contrasts in petrography and texture of the igneous rocks, in stratigraphy of the country lock, and in metamorphic grade, accentuate the separate geologic development of each structural province. No correlation involving preassembly geology should be attempted across province boundaries without due consideration of the exotic character of each province.

The isotopic composition of lead in potassium feldspars from some 1.0-b.y. old North American igneous rocks

Abstract The isotopic composition of lead and the uranium, thorium and lead concentrations in potassium feldspars are determined for more than 30 1.0-b.y. old North American igneous rocks. Samples representing a broad spectrum in petrographic type and mode of occurrence were chosen; an effort was made to include only rocks having well-documented ages from 950 to 1140 m.y. and showing minimal evidence of subsequent metamorphism. Most samples, including those from extensive terranes of contemporaneous age, have limited lead isotope variations ( Pb 206 Pb 204 = 16.74–17.38 ; Pb 207 Pb 204 = 15.39–15.59 ; Pb 208 Pb 204 = 36.38–37.10 ), which yield model ages close to the radiometric ages. Granite, pegmatite, and rhyolite from within the Grenville province of Canada and age-equivalent rocks of New York, Virginia, Texas, and Colorado and granophyric units associated with the Duluth Gabbro Complex of Minnesota are among the materials yielding this main isotopic spectrum. Several samples were encountered which had isotopic compositions very different from the above group. Lead showing a marked deficiency in radiogenic isotopes was found in two granitic bodies associated with older Labrador Trough rocks from Quebec, in a rapakivi granite from southern Nevada, and in a small granite stock from Mellen, Wisconsin. These occurrences all involve small intrusions of granite which lie near considerably older areas of basement rock. Model ages calculated from the Pb 206 Pb 204 ratio are older than the age of the intrusions and approach the age of the host basement rock. Several possible interpretations are offered to explain the isotopic behavior encountered in this study. In particular, a “vertically differentiated crust” model is proposed which will account for both the main spectrum and the anomalous lead. The significance of lead isotopic studies in understanding crustal structure in continental regions is discussed.

Lead concentration and isotopic composition in five peridotite inclusions of probable mantle origin

Abstract The lead content of five whole-rock peridotite inclusions (four lherzolites and one harzburgite) in alkali basalt ranges from 82 to 570 ppb (parts per billion). Approximately 30–60 ppb of this amount can be accounted for by analyzed major silicate minerals (olivine ≤ 10 ppb; enstatite 5–28 ppb; chrome diopside ∼400 ppb). Through a series of acid leaching experiments, the remainder of the lead is shown to be quite labile and to reside in either glassy or microcrystalline veinlets or accessory mineral phases, such as apatite and mica. The lead isotopic composition of the peridotites ( 206 Pb/ 204 Pb= 18.01–18.90; 207 Pb/ 204 Pb= 15.52–15.61; 208 Pb/ 204 Pb= 37.80–38.86) lies within the range of values defined by many modern volcanic rocks and, in particular, is essentially coextensive with the abyssal tholeiite field. In all but one instance, isotopic differences were found between the peridotite and its host alkali basalt. Two of the peridotites clearly demonstrated internal isotopic heterogeneity between leachable and residual fractions that could not simply be due to contamination by the host basalt. However, there is no evidence that these ultramafic rocks form some layer in the mantle with isotopic characteristics fundamentally different from those of the magma sources of volcanic rocks.

U-Th-Pb and Rb-Sr geochronology of middle Proterozoic granite and augen gneiss, Salmon River Mountains, east-central Idaho

Coarsely porphyritic granite and augen gneiss in east-central Idaho have been dated by the U-Th-Pb zircon method. When plotted on concordia diagrams, the results give nonlinear data arrays that indicate the presence of at least one component of inherited radiogenic lead, as well as postcrystallization, probably recent, episodic lead-loss.Nevertheless, a systematic relationship between zircon size and convergence of the 207 Pb/ 206 Pb ages for the +100- and -100+150-mesh fractions from several samples are interpreted to provide a valid age of about 1,370 m.y. for these plutons. A single sample which is free of inheritance for all size fractions confirms this conclusion. Rb-Sr whole-rock samples are highly enriched in radiogenic strontium but do not define an isochron, probably because the system did not remain closed following crystallization of the plutons. Intrusive contacts of the granitic rocks indicate that deformation and regional metamorphism of the host Yellowjacket Formation occurred prior to 1370 Ma, rather than prior to 1500 Ma as previously suggested by others. This refinement of the geochronology (1) removes the necessity that the Yellowjacket Formation and overlying Proterozoic metasedimentary rocks of central and east-central Idaho be pre-Belt in age; (2) permits the traditional age correlation of the east-central Idaho metasedimentary sequence (Yellowjacket through Lawson Creek Formations) with the Belt Supergroup; and (3) suggests that a re-evaluation is in order for paleogeographic models of central Idaho, especially the proposed topographic high known as the Salmon River Arch.

A Permian Disturbance of K-Ar Radiometric Ages in New England: Its Occurrence and Cause

Approximately 200 K-Ar mineral and whole rock ages from New England, half of which are previously unpublished, are used to delineate an area of Permian thermal disturbance. The disturbed area, as outlined by K-Ar mica ages, forms a north-northeast-trending belt 60–80 mi wide that extends from the coast of Long Island Sound in southern Connecticut to southwestern Maine, where it terminates against rocks displaying older radiometric ages. Several possible mechanisms that may have affected the radio-metric systems of pre-existing rocks are examined: (1) contact metamorphism related to contemporaneous igneous activity, (2) alteration associated with major faulting, (3) regional metamorphism in late Paleozoic time, and (4) burial followed by uplift and erosion. Evidence is given that each of these mechanisms was operative locally, especially in the southern portion of the belt. The general lack of late Paleozoic tectonism in New Hampshire and Maine suggests that only burial is a likely cause of the disturbance there.