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J.R. Hein

Publications and source records attributed to J.R. Hein.

At least 55 records · Page 3Linked to original sources
USGS1996

Ferromanganese crusts as indicators for paleoceanographic events in the NE Atlantic

Hydrogenetic ferromanganese crusts reflect the chemical conditions of the sewater from which they formed. Fine-scale geochemical analysis of crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in geochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth influenced by terrigenous input from the African continent occurred during approximately the past 12 Ma. Phosphatization of these crusts is minor. In contrast, crusts from Lion seamount, located between Madeira and the Portuguese coast, display a much more variable growth history. A pronounced increase in Ni, Cu, and Zn is observed in some intervals of the crusts, which probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic profiles indicate that Mediterranean outflow water (MOW) may play an important role in the composition of these crusts. 10 Be dating of one sample confirms that the interruption of the MOW during the Messinian salinity crisis (6.2–5 Ma ago) resulted in changes in element composition. Sr-isotope dating of the apatite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of Atlantic crusts and to determine whether crust phosphatization in the Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary data show that the old phosphatized crust generation might be as old as approximately 30–40 Ma.

Geologische Rundschau↗
USGS1996

Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts

The direct measurement of the ratio of cosmogenic 10 Be ( T 1 2 = 1.5 Ma "> T12= 1.5 Ma ) to stable terrigenously sourced 9 Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10 Be 9 Be "> 10Be9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10 Be 9 Be "> 10Be9Be ratios, time-corrected for radioactive decay of cosmogenic 10 Be using 234 U 238 U "> 234U238U , are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4–0.5 × 10 −7 ). In the Southwest Atlantic ratios increase up to 1 × 10 −7 , they vary between 0.7 and 1.0 × 10 −7 in Indian Ocean samples, and have a near constant value of 1.1 ± 0.2 × 10 −7 for all Pacific samples. If the residence time of 10 Be ( τ 10 Be ) in deep water is constant globally, then the observed variations in 10 Be 9 Be "> 10Be9Be ratios could be caused by accumulation of 10 Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both 10 Be and 9 Be reach local steady-state concentration in Pacific deep water and the global τ10 Be ≌ 600 a ">≌ τ10Be≌ 600a . An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ 10 Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10 Be ≌ 230 a ">≌ τ10Be≌ 230a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9 Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9 Be appears insufficient to provide the dissolved 9 Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater 9 Be content. If true, ocean margins would be the sites for 9 Be addition. Due to the particle-reactive nature of Be, these would also be the primary sites of Be removal. A possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10 Be 9 Be "> 10Be9Be , and establishment of a relatively constant 9 Be concentration. As τ 10 Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10 Be 9 Be "> 10Be9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10 Be 9 Be "> 10Be9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10 Be 9 Be "> 10Be9Be ratio, determined in the authigenic phase of marine sediments or hydrogenetic precipitates, should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales.

Earth and Planetary Science Letters↗
USGS1996

Distribution and sources of pre-anthropogenic lead isotopes in deep ocean water from Fe-Mn crusts

The lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead. Here the global distribution of lead isotopes in deep ocean water is presented as derived from dated (ca. 100 ka) surface layers of hydrogenetic Fe-Mn crusts. The results indicate that the radiogenic lead in North Atlantic deep water is probably supplied from the continents by river particulates, and that lead in Pacific deep water is similar to that characteristic of island and continental volcanic arcs. Despite a short residence time in deep water (80-100 a), the isotopes of lead appear to be exceedingly well mixed in the Pacific basin. There is no evidence for the import of North Atlantic deep water-derived lead into the Pacific ocean, nor into the North Indian Ocean. This implies that the short residence time of lead in deep water prohibits advection over such long distances. Consequently, any climate-induced changes in deep-water flow are not expected to result in major changes in the seawater Pb-isotope record of the Pacific Ocean.

Geochimica et Cosmochimica Acta↗
USGS1996

Hydrothermal mineralization along submarine rift zones, Hawaii

This is the first article to describe mineralization of midplate submarine rift zones and hydrothermal manganese oxide mineralization of midplate volcanic edifices. Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rocks and cement for clastic rocks; both deposit types are composed of todorokite and bimessite. Thin Fe‐Mn crusts that coat some rocks formed by a combination of hydrogenetic and hydrothermal processes and are composed of δ‐MnO 2 . The stratiform layers have high Mn contents (mean 40%) and a large fractionation between Mn and Fe (Fe/Mn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rocks leached by circulating hydrothermal fluids, clastic material (in Mn‐cemented sandstone), and seawater that mixed with the hydrothermal fluids. Mineralization on Haleakala Ridge occurred sometime during the past 200 to 400 ka, when the summit was at a water depth of more than 1,000 m. Hydrothermal circulation was probably driven by heat produced by intrusion of dikces, magma reservoirs, and flow of magma through axial and lateral conduits. The supply of seawater to ridge interiors must be extensive because of their high porosity and permeability. Precipitation of Mn oxide below the seafloor is indicated by its occurrence as cement, growth textures that show mineralizing fluids were introduced from below, and pervasive replacement of original matrix of clastic rocks.

Hawaii↗
USGS1995

238U-234U-230Th chronometry of Fe-Mn crusts: Growth processes and recovery of thorium isotopic ratios of seawater

Comparison of ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) and ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) activity ratios in oceanic Fe-Mn deposits provides a method for assessing the closed-system behaviour of 238 U- 234 U- 230 Th, as well as variations in the initial uranium and thorium isotopic ratios of the precipitated metal oxides. This approach is illustrated using a Fe-Mn crust from Lotab seamount (Marshall Islands, west equatorial Pacific). Here we report uranium and thorium isotopic compositions in five subsamples from the surface of one large 5 cm diameter botryoid of this crust, and from two depth profiles of the outermost rim of the same botryoid. The decrease of ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) and ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) activity ratio with depth in the two profiles gives mean growth rates, for the last 150 ka, of 7.8 ± 2 mm/Ma and 6.6 ± 1 mm/Ma, respectively. All data points (surface and core samples) but one, define a linear correlation in the Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U ) "> Ln ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) −Ln[ ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) ] diagram. This correlation indicates that for all points the U-Th system remained closed after the Fe-Mn layer precipitated, and that the different samples possessed the same initial Uranium and thorium isotope ratios. Furthermore, these results show that the preserved surface of this Fe-Mn crust may not be the present-day growth surface, and that the thorium and uranium isotopic ratios of seawater in west equatorial Pacific have not changed during the past 150 ka. The initial thorium activity ratio is estimated from the correlation obtained between Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U ) "> Ln ( ( 230 Th 232 Th "> 230 Th ⁄ 232 Th ) −Ln Ln ( 230 Th 232 Th − Ln ( 234 U)excess ( 238 U ) ">[ ( ( 234 U)excess ( 238 U ) "> 234 U) excess ⁄ ( 238 U) ] .

Lotab Seamount, Pacific Ocean↗
USGS1995

Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California

Compositions of lavas from seven small to medium-sized seamounts, between lat 34.0°N and 30.5°N offshore southern and Baja California, include low-K 2 O tholeiitic, transitional, and mildly to moderately alkalic basalt and their differentiates. The low-K 2 O tholeiites resemble primitive (>9% MgO) mid-oceanic-ridge basalt (MORB) with low incompatible element abundances and very depleted, concave-downward, chondrite-normalized rare-earth-element (REE) patterns and lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd ratios than typical MORB from the East Pacific Rise. The seamounts with these MORB-like lavas are inferred to have formed at or near the spreading center. Transitional and mildly to moderately alkalic basalts have higher abundances of incompatible elements and steeper slopes for chondrite-normalized REE patterns with light REE enrichment up to 150 times chondrites. The alkalic compositions indicate more variably enriched mantle sources than those of most seamounts presently located near the East Pacific Rise, but the compositions are within the mantle array defined by other ocean-island basalts. Volcanic rocks from the upper part of Rocas Alijos, a much larger and morphologically more complex edifice than the northern seamounts, located offshore central Baja California at lat ∼25°N, are all highly differentiated trachyte and trachyandesite. Based on 40 Ar/ 39 Ar laser fusion techniques, MORB-like lava from one of the northern edifices is as old as the underlying oceanic crust (>20 Ma), indicating that it originated at a spreading center. Other seamount lava ages are much younger than the oceanic crust on which they reside, ranging from 16.8 ± 0.3 to <7 Ma for some of the northern seamounts to 270 ± 16 ka for the trachyte from Rocas Alijos. Similar highly evolved lavas cap fossil spreading centers like Guadalupe and Socorro Islands, but Rocas Alijos, based on magnetic anomalies, is not an abandoned spreading center but may instead have formed on a leaky transform fault. Some of the seamounts with transitional and alkalic lavas may have formed as part of a short, age-progressive chain formed by a short-lived mantle plume. Many others, aligned along abandoned spreading centers or faults and fracture zones which are abundant in the tectonically complex region offshore southern and peninsular California, may have resulted from upwelling mantle diapirs in response to localized extension. Some of the episodes of volcanism appear to have been contemporaneous with volcanism in the continental borderland and coastal southern California, suggesting linkage between extension along the continental margin and the seamount province farther offshore. The data available for the abundant volcanic edifices of varying sizes, shapes, and orientations in this region suggest that the seamounts formed from multiple episodes of chemically diverse volcanism, tapping variably enriched, heterogeneous mantle, which occurred sporadically from early Miocene to late Pleistocene.

Baja California↗
USGS1992

Geochronology and subsurface stratigraphy of Pukapuka and Rakahanga atolls, Cook Islands: Late Quaternary reef growth and sea level history

Eustatic sea-level cycles superposed on thermal subsidence of an atoll produce layers of high sea-level reefs separated by erosional unconformities. Coral samples from these reefs from cores drilled to 50 m beneath the lagoons of Pukapuka and Rakahanga atolls, northern Cook Islands give electron spin resonance (ESR) and U-series ages ranging from the Holocene to 600,000 yr B.P. Subgroups of these ages and the stratigraphic position of their bounding unconformities define at least 5 periods of reef growth and high sea-level (0–9000 yr B.P., 125,000–180,000 yr B.P., 180,000–230,000 yr B.P., 300,000–460,000 yr B.P., 460,000–650,000 yr B.P.). Only two ages fall within error of the last interglacial high sea-level stand (∼125,000–135,000 yr B.P.). This paucity of ages may result from extensive erosion of the last intergracial reef. In addition, post-depositional isotope exchange may have altered the time ages of three coral samples to apparent ages that fall within glacial stage 6. For the record to be preserved, vertical accretion during rising sea-level must compensate for surface lowering from erosion during sea-level lowstands and subsidence of the atoll; erosion rates (6–63 cm/1000 yr) can therefore be calculated from reef accretion rates (100–400 cm/1000 yr), subsidence rates (2–6 cm/1000 yr), and the duration of island submergence (8–15% of the last 600,000 yr). The stratigraphy of coral ages indicates island subsidence rates of 4.5 ± 2.8 cm/1000 yr for both islands. A model of reef growth and erosion based on the stratigraphy of the Cook Islands atolls suggests average subsidence and erosion rates of between 3–6 and 15–20 cm/1000 yr, respectively.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1990

Age determinations and growth rates of Pacific ferromanganese deposits using strontium isotopes

87 Sr 86 Sr ratios, trace element and REE compositions, and textural characteristics were determined for three hydrogenetic Fe-Mn crusts, one hydrothermal deposit, and two mixed hydrothermalhydrogenetic crusts from the Pacific. The Sr isotope data are compared to the Sr seawater curve for the Cenozoic to determine the ages and growth rates of the crusts. The 87 Sr 86 Sr in the crusts does not increase monotonically with depth as expected if the Sr were solely derived from seawater and perfectly preserved since deposition. This indicates post-depositional exchange of Sr or heterogeneous sources for the Sr originally contained in the crusts. Textures of hydrogenetic crusts generally correlate with Sr isotopic variations. The highest porosity intervals commonly exhibit the highest 87 Sr 86 Sr ratios, indicating exchange with younger seawater. Intervals with the lowest porosity commonly have lower 87 Sr 86 Sr and may preserve the original Sr isotopic ratios. Minimum ages of crust growth inception were calculated from dense, low porosity intervals. Growth of the hydrogenetic crusts began at or after 23 Ma, although their substrates are Cretaceous. Estimated average growth rates of the three hydrogenetic crusts vary between 0.9 and 2.7 mm/Ma, consistent with published rates determined by other techniques. Within the Marshall Islands crust, growth rates for individual layers varied greatly between 1.0 and 5.4 mm/Ma. For one crust, very low 87 Sr 86 Sr ratios occurred in detrital-rich intervals. Hydrothermal Fe-Mn oxide from the active Lau Basin back-arc spreading axis (Valu Fa Ridge) has an 87 Sr 86 Sr ratio with a predominantly seawater signature ( 87 Sr 86 Sr 87 Sr 86 Sr 0.709196 "> 0.709196 ), indicating a maximum age of 0.9 Ma. One crust from an off-axis seamount west of Gorda Ridge may have begun precipitating hydrogenetically at 0.5 Ma (0.709211), and had increasing hydrothermal or volcanic input in the top half of the crust, indicated by a significantly lower 87 Sr 86 Sr ratio (0.709052).

Geochimica et Cosmochimica Acta↗
USGS1990

Insular and submarine ferromanganese mineralization of the Tonga-Lau region

Ferromanganese oxides in the Tonga-Lau region are divided into crusts and stratabound deposits. Crusts were collected from the Tonga and Lau Ridges and have Fe/Mn ratios greater than 1, and an average Co content of 0.25%. The crusts average less than 10 mm thick with a maximum of 50 mm, and growth rates of tens of millimeters per million years. The thickest crust is probably less than a million years old. Crusts formed by both hydrogenetic and hydrothermal precipitation, with the hydrothermal input averaging 76%. Stratabound deposits are divided into three types. The source rocks through which the circulating fluids passed controlled the dominant minor element compositions of the stratabound deposits from each area: Valu Fa Ridge, Mo; Tonga Ridge Ti; Tonumea, Sr and Eua, V. -from Authors

Marine Mining↗
USGS1990

Age determinations and growth rates of Pacific ferromanganese deposits using strontium isotopes

87Sr 86Sr ratios, trace element and REE compositions, and textural characteristics were determined for three hydrogenetic Fe-Mn crusts, one hydrothermal deposit, and two mixed hydrothermalhydrogenetic crusts from the Pacific. The Sr isotope data are compared to the Sr seawater curve for the Cenozoic to determine the ages and growth rates of the crusts. The 87Sr 86Sr in the crusts does not increase monotonically with depth as expected if the Sr were solely derived from seawater and perfectly preserved since deposition. This indicates post-depositional exchange of Sr or heterogeneous sources for the Sr originally contained in the crusts. Textures of hydrogenetic crusts generally correlate with Sr isotopic variations. The highest porosity intervals commonly exhibit the highest 87Sr 86Sr ratios, indicating exchange with younger seawater. Intervals with the lowest porosity commonly have lower 87Sr 86Sr and may preserve the original Sr isotopic ratios. Minimum ages of crust growth inception were calculated from dense, low porosity intervals. Growth of the hydrogenetic crusts began at or after 23 Ma, although their substrates are Cretaceous. Estimated average growth rates of the three hydrogenetic crusts vary between 0.9 and 2.7 mm/Ma, consistent with published rates determined by other techniques. Within the Marshall Islands crust, growth rates for individual layers varied greatly between 1.0 and 5.4 mm/Ma. For one crust, very low 87Sr 86Sr ratios occurred in detrital-rich intervals. Hydrothermal Fe-Mn oxide from the active Lau Basin back-arc spreading axis (Valu Fa Ridge) has an 87Sr 86Sr ratio with a predominantly seawater signature ( 87Sr 86Sr 0.709196), indicating a maximum age of 0.9 Ma. One crust from an off-axis seamount west of Gorda Ridge may have begun precipitating hydrogenetically at 0.5 Ma (0.709211), and had increasing hydrothermal or volcanic input in the top half of the crust, indicated by a significantly lower 87Sr 86Sr ratio (0.709052). ?? 1990.

Geochimica et Cosmochimica Acta↗
USGS1989

Morphology of sea-floor landslides on Horizon Guyot: application of steady-state geotechnical analysis

Mass movement and erosion have been identified on the pelagic sediment cap of Horizon Guyot, a seamount in the Mid-Pacific Mountains. Trends in the size, shape and preservation of bedforms and sediment textural trends on the pelagic cap indicate that bottom-current-generated sediment transport direction is upslope. Slumping of the sediment cap occurred on and that the net bedload transport direction is upslope. Slumping of the sediment cap occurred on the northwest side of the guyot on a 1.6?? to 2.0?? slope in the zone of enhanced bottom-current activity. Submersible investigations of these slump blocks show them to be discrete and to have a relief of 6-15 m, with nodular chert beds cropping out along the headwall of individual rotated blocks. An evaluation of the stability of the sediment cap suggests that the combination of the current-induced beveling of the sea floor and infrequent earthquake loading accompanied by cyclic strength reduction is responsible for the initiation of slumps. The sediment in the area of slumping moved short distances in relatively coherent masses, whereas sediment that has moved beyond the summit cap perimeter has fully mobilized into sediment gravity flows and traveled large distances. A steady-state geotechnical analysis of Horizon Guyot sediment indicates the predisposition of deeply buried sediment towards disintegrative flow failure on appropriately steep slopes. Thus, slope failure in this deeper zone would include large amounts of internal deformation. However, gravitational stress in the near-surface sediment of the summit cap (sub-bottom depth < 14 m) is insufficient to maintain downslope movement after initial failure occurs. The predicted morphology of coherent slump blocks displaced and rafted upon a weakened zone at depth corresponds well with seismic-reflection data and submersible observations. ?? 1990.

Deep Sea Research Part A, Oceanographic Research P↗
USGS1988

Maps showing the Seabeam bathymetry and sedimentologic and biologic sample locations on Horizon Guyot, Mid-Pacific Mountains and a summary of existing data

Horizon Guyot (Fig. 1) is a 300-km-long, 75-km-wide volcanic ridge with a relatively flat summit that is diagnostic of guyots (Hess, 1946). The U. S. Geological Survey (USGS) began a study of Horizon Guyot in 1983 as part of a program on the origin, distribution, and composition of ferromanganese-oxide precipitates that encrust the hard substrate of sea floor edifices, such as seamounts and volcanic ridges (Hein and others, 1985a). Mass movement and bedload transport of sediment appears to influence the thickness of these crusts on seamount flanks (Hein and others, 1985b). Because Horizon Guyot has been studied more extensively than any other volcanic edifice in the Mid-Pacific Mountains (Heezen, Fischer, and others, 1971; Lonsdale and others, 1972; Winterer, Ewing, and others, 1973), it was chosen as the principal site for a USGS study of sediment transport processes and the geotechnical behavior of sediment on seamounts (Cacchione and others, 1988; Schwab and others, 1988). In March, 1987, Horizon Guyot was again investigated using the R/V ATLANTIS II and the D.S.R.V. ALVIN (cruise 118-12); sponsored by the National Science Foundation. Although primarily a biologic investigation, observations from 10 submersible dives, bottom samples collected at depth using ALVIN and from the surface using the ATLANTIS II, and Seabeam swath-bathymetry (sponsored by the USGS and the Office of Naval Research) add to the overall Horizon Guyot data set. In this report, we summarize the existing data base, present a Seabeam bathymetric map of the study area, ALVIN dive tracklines, the sample locations, and a brief description of the samples collected or other station activities on the ATLANTIS II cruise 118-12. The detailed bathymetric map of the study area (Plate 1) was constructed by merging data obtained by a Deep-Tow study (Lonsdale and others, 1972) (Fig. 1) with data obtained from the swath-bathymetry mapping system onboard the ATLANTIS II. Detailed information on the Seabeam bathymetric system is given by Renard and Allenou (1979).

Open-File Report↗
USGS1984

Paleogene geology and chronology of southwestern Umnak Island, Aleutian Islands, Alaska

The rocks of southwestern Umnak Island (Nikolski area) consist mainly of a slightly deformed marine sedimentary sequence intruded by hypabyssal quartz diorite sills and small plutons. Laminated and thin-bedded argillaceous and tuffaceous mudstone and siltstone reflect volcanic arc sedimentation from late Eocene to early Oligocene time. Zeolite facies minerals formed during diagenesis include heulandite, analcime, quartz, chlorite, and smectite. Radiometric dates from sill rock and plutons yield apparent ages of about 30 Ma, i.e, middle Oligocene. Chemical data from igneous rocks exhibit calc-alkaline and tholeiitic volcanic arc differentiation trends. Dikes of pyroxene basalt yield K/Ar apparent ages as old at 17 Ma, and a columnar-jointed plug of pyroxene basalt is as young as 1 Ma. The fossil ages and radiometric dates from southwestern Umnak Island are similar to those reported from other central and eastern Aleutian islands, and indicate uniformity in the chronology and tectonic development of the archipelago during the Paleogene. Paleomagnetic data suggest possible northward movement but remain equivocal and more work is indicated.

Alaska↗
USGS1982

Uranium and thorium enrichment in rocks from the base of DSDP Hole 465A, Hess Rise, central North Pacific

Uranium and thorium are concentrated in Cretaceous limestone, chert, ash, basalt, and other rock types at Deep Sea Drilling Project Site 465 located on the southern Hess Rise in the central North Pacific. U concentrations, up to 194 ppm on a carbonate-free basis, are among the highest recorded for any deep-sea deposits. U was initially derived from seawater and concentrated by absorption on terrigenous (humic) organic matter in limestone in a shallow marine environment. U and Th were probably concentrated further by low-temperature hydrothermal fluids emanating from the basaltic basement. Mainly montmorillonite, an alteration product of basalt and ash, and organic matter in sedimentary rocks acted as hosts for U and Th. The unique combination of sediments rich in humic organic matter, abundant smectite in altered ash and basalt, and warm hydrothermal solutions provided the necessary conditions for migration and concentration of U and Th. To better understand the conditions limiting the migrating and concentration of U and Th, other rocks deposited during the ocean-wide Cretaceous anoxic events should be analyzed for these elements. ?? 1982.

Chemical Geology↗