An evaluation of the behavior of the rare earth elements during the weathering of sea-floor basalt

We present rare earth element (REE) data for fresh and altered tholeiitic basalts sampled during a dredging transect at 23°N in the Atlantic Ocean and covering a time span of 0 to 57 million years. These data have been used to evaluate the behavior of the REE during low-temperature weathering processes. Compositional trends from altered basalt interiors to palagonitized rinds in individual pillow samples indicate significant mobility of the light REE: some elements are enriched by four orders of magnitude in rinds relative to interiors. The heavy REE show no selective mobilization and can be used in a normalization procedure which indicates that the light REE are enriched in altered interiors relative to fresh interiors of the basalts. Cerium behaves anomalously and accords with either its abundance in seawater or its fractionation from seawater during the formation of ferromanganese deposits.These results indicate that REE data from fresh glassy or crystalline basalt samples only may be used with confidence in petrological models.     

Radioactive disequilibrium in altered mid-oceanic basalts

A young (<1 m.y.) tholeiitic basalt dredged from the Mid-Atlantic Ridge displays a²³⁴U excess and a²³⁰Th deficiency that have resulted from the addition of seawater uranium during weathering at seafloor temperatures. Two older samples, though they acquired substantial amounts of uranium from seawater, are depleted in²³⁴U, indicating preferential leaching of this isotope. Hydrothermally altered samples suggest that some uranium loss may have occurred. Possible isotopic effects of preferential²³⁴U leaching, however, are obscured by secondary addition of seawater uranium at lower temperatures.     

Geochemistry of late Cenozoic lavas on Kunashir Island, Kurile Arc

Middle Miocene to Quaternary lavas on Kunashir Island in the southern zone of the Kurile Arc were examined for major, trace, and Sr–Nd–Pb isotope compositions. The lavas range from basalt through to rhyolite and the mafic lavas show typical oceanic island arc signatures without significant crustal or sub-continental lithosphere contamination. The lavas exhibit across-arc variation, with increasingly greater fluid-immobile incompatible element contents from the volcanic front to the rear-arc; this pattern, however, does not apply to some other incompatible elements such as B, Sb, and halogens. All Sr–Nd–Pb isotope compositions reflect a depleted source with Indian Ocean mantle domain characteristics. The Nd and Pb isotope ratios are radiogenic in the volcanic front, whereas Sr isotope ratios are less radiogenic. These Nd isotope ratios covary with incompatible element ratios such as Th/Nd and Nb/Zr, indicating involvement of a slab-derived sediment component by addition of melt or supercritical fluid capable of mobilizing these high field-strength elements and rare earth elements from the slab. Fluid mobile elements, such as Ba, are also elevated in all basalt suites, suggesting involvement of slab fluid derived from altered oceanic crust. The Kurile Arc lavas are thus affected both by slab sediment and altered basaltic crust components. This magma plumbing system has been continuously active from the Middle Miocene to the present.     

Metallogenesis along a fossil oceanic fracture zone: Arakapas fault belt,Troodos Massif,Cyprus

The Arakapas fault belt, southern Troodos Massif, which is interpreted as a fossil oceanic fracture zone, contains a sedimentary and volcanic “fill” including basaltic lava breccias, volcaniclastic sandstones, intercalated Fe-rich mudstones, and minor Cu sulphide mineralisation. The volcaniclastic sedimentary rocks were produced by submarine weathering and gravity redeposition of basalt within the transform zone. In contrast, the Fe-mudstones predominantly originated as chemical precipitates from hydrothermal solutions generated by hydrothermal leaching of basalt by seawater. The rare earth elements, and some trace metals, including Co, Ni, Pb, V and Zr, are thought to have been incorporated from seawater into the ferruginous precipitates. Unusually high Fe/Mn ratios of the Fe-mudstones relative to most modern ocean-ridge sediments may reflect deposition in relatively oxygen-depleted waters on the floor of a major bathymetric depression created by the transform faulting; release of acidic thermal waters would have further suppressed Mn precipitation. Contrary to recent suggestions of metallogenesis along oceanic fracture zones, none of the major Cyprus cupriferous sulphide bodies lie along the fossil Arakapas transform zone.     

Mineral classification from quantitative X-ray maps using neural network: Application to volcanic rocks

We developed a mineral classification technique of electron probe microanalyzer (EPMA) maps in order to reveal the mineral textures and compositions of volcanic rocks. In the case of lithologies such as basalt that include several kinds of minerals, X-ray intensities of several elements derived from EPMA must be considered simultaneously to determine the mineral map. In this research, we used a Kohonen self-organizing map (SOM) to classify minerals in the thin-sections from several X-ray intensity maps. The SOM is a type of artificial neural network that is trained using unsupervised training to produce a two-dimensional representation of multi-dimensional input data. The classified mineral maps of in situ oceanic basalts of the Juan de Fuca Plate allowed us to quantify mineralogical and textural differences among the marginal and central parts of the pillow basalts and the massive flow basalt. One advantage of mineral classification using a SOM is that relatively many minerals can be estimated from limited input elements. By applying our method to altered basalt which contains multiple minerals, we successfully classify eight minerals in thin-section.     

The Permian seamount stratigraphic sequence in Chiang Mai,North Thailand and its tectogeographic significance

The widespread Permian carbonate strata outcropped in northwestern Thailand are considered as the evidence for the Late Paleozoic shallow Tethys. Our investigation, however, shows that basalt can be discovered usually under the Permian carbonate sequence in Chiang Mai-Fang area, northwestern Thailand. The basalt belongs to subalkalic basalt and potassic trachybasalt. They are characterized by high P and Ti in major elements, by high content, enriching LREE, lacking δEu anomaly in rare earth elements, and by enrichment of the large ion lithophile element (LILE) (K, Rb, Ba) and high field strength element (HFSE) (Nb, Ta, Zr) in trace elements, which can be compared with the characters of the oceanic island basalt in Three Rivers (Jinsha River, Lancang River, and Nujiang River) area, southwestern China. Therefore, the Permian carbonate in the studied area was deposited on a sea-mount, rather than on a stable carbonate platform. The oceanic basin is correlated to the Late Paleozoic ocean represented by the Changning-Menglian Belt in southwestern China and they are a major basin of the Paleo-Tethyan Archipelagoes Ocean. The result indicates nonexistence of a Shan-Thai Block in the Late Paleozoic.     

Causes of spatial compositional variations in Mariana arc lavas: Trace element evidence

New inductively coupled plasma mass spectrometry (ICP-MS) trace element data are presented on a suite of arc lavas from the northern Mariana and southern Bonin island arcs. The samples were dredged from seamounts in the Central Island Province (CIP), the Northern Seamount Province (NSP) and the Volcano Arc (VA), and they range in composition from low-K tholeiites to shoshonites. Previous studies on these samples concluded that the primary compositional control was two-component mixing between a fluid-metasomatized mid-ocean ridge basalt (MORB) source and an enriched, ocean island basalt (OIB)-like, mantle component, with subducted sediment material playing a secondary role. However, the new trace element data suggest that the compositional variations along the Mariana arc can be better explained by the addition of spatially varying subduction components to a spatially varying mantle source. The data suggest that the subduction component in the CIP and VA is dominated by aqueous fluids derived from altered oceanic crust and a pelagic sediment component, while the subduction component in the NSP is dominated by more silicic fluids derived from volcanogenic sediments as well as from pelagic sediment and altered oceanic crust. The mantle wedge in the CIP and VA is depleted relative to a normal mid-ocean ridge basalt source by loss of a small melt fraction, while the mantle wedge in the NSP is enriched either by possible gain of a small melt fraction or addition of a sediment-derived melt. Because the subduction of seamounts controls the arc and back-arc geometries, so the concomitant variation between subducted material and mantle composition may be no coincidence. The high field strength element (HFSE) data indicate a high degree of melting (∼ 25–30%) throughout the arc, ∼ 10% of which may be attributed to decompression and ∼ 20% to fluid addition.     

Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks

Five minor and trace elements, known to be chemically stable during alteration and metamorphism, have been combined in a set of binary diagrams that distinguish fresh tholeiites from alkali basalts. Of the five elements: Ti, P, Zr, Y, Nb, only P shows slight mobility during metamorphism, which is not sufficient to alter greatly the point distribution on the binary diagrams. Using these stable elements altered basaltic rocks: greenstones, spilites and amphibolites may be distinguished in the same way as fresh basalts, and their original magma may be identified as tholeiitic or alkaline basalt. All five elements are readily and rapidly determined, using XRF, thus this method may be applied as a rapid, easy way of discriminating the magma types of altered basaltic rocks. Using this method it can be demonstrated that alkali basalt magma was produced in minor quantities in the Precambrian.     

Trace elements in ocean ridge basalts

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.     

Geochemistry of the oldest MORB and OIB in the Isua Supracrustal Belt, southern West Greenland: Implications for the composition and temperature of early Archean upper mantle

Abstract Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO₂ and Na₂O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al₂O₃, and depleted in TiO₂, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg? and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.     

An iron-rich deposit from the Northeast Pacific

An iron-rich deposit dredged from the upper flank of Dellwood Seamount in the Northeast Pacific has been analyzed for major and trace elements, rare-earth contents and uranium isotopic composition. In terms of mineralogy and overall chemical composition, the deposit resembles other iron-rich deposits variously attributed to volcanic hydrothermal activity. Both the relative concentrations of the rare-earth elements and the isotopic composition of uranium rule out seawater as the sole source of elements in this deposit. The rare-earth element pattern indicates that these elements were derived from the underlying basalt. The²³⁴U/²³⁸U ratio is significantly higher than in seawater and can best be explained by preferential leaching of²³⁴U generated by decay from its parent²³⁸U in the underlying rock and subsequent redeposition of the excess²³⁴U together with the Fe and minor metals. These data are consistent with a model for the origin of submarine metal-rich solutions involving mobilization of elements from the interior of slowly cooling basalts by circulating seawater.     

He and Ne isotopes in oceanic crust: implications for noble gas recycling in the mantle

In an attempt to determine the helium and neon isotopic composition of the lower oceanic crust, we report new noble gas measurements on 11 million year old gabbros from Ocean Drilling Program site 735B in the Indian Ocean. The nine whole rock samples analyzed came from 20 to 500 m depth below the seafloor. Helium contents vary from 3.3×10⁻¹⁰ to 2.5×10⁻⁷ ccSTP/g by crushing and from 5.4×10⁻⁸ to 2.4×10⁻⁷ ccSTP/g by melting. ³He/⁴He ratios vary between 2.2 and 8.6 Ra by crushing and between 2.9 and 8.2 by melting. The highest R/Ra ratios are similar to the mean mid-ocean ridge basalt (MORB) ratio of 8±1. The lower values are attributed to radiogenic helium from in situ α-particle production during uranium and thorium decay. Neon isotopic ratios are similar to atmospheric ratios, reflecting a significant seawater circulation in the upper 500 m of exposed crust at this site. MORB-like neon, with elevated ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios, was found in some high temperature steps of heating experiments, but with very small anomalies compared to air. These first results from the lower oceanic crust indicate that subducted lower oceanic crust has an atmospheric ²⁰Ne/²²Ne ratio. Most of this neon must be removed during the subduction process, if the ocean crust is to be recirculated in the upper mantle, otherwise this atmospheric neon will overwhelm the upper mantle neon budget. Similarly, the high (U+Th)/³He ratio of these crustal gabbros will generate very radiogenic ⁴He/³He ratios on a 100 Ma time scale, so lower oceanic crust cannot be recycled into either MORB or oceanic island basalt without some form of processing.     

Transport and exchange of U-series nuclides between suspended material,dissolved load and colloids in rivers draining basaltic terrains

This study presents uranium and thorium concentrations and activity ratios for all riverine phases (bedload, suspended load, dissolved load and colloids) from basaltic terrains in Iceland and the Azores. Small basaltic islands, such as these, are thought to account for ~ 25% of CO₂ consumed by global silicate weathering, and for ~ 45% of the flux of suspended material to the oceans. These data indicate that [U] and [Th] in the dissolved and colloidal fractions are strongly controlled by pH, and to a much lesser extent by levels of dissolved organic carbon (which are low in these environments). At high pH, basalt glass dissolution is enhanced, and secondary mineral formation (e.g. Fe-oxyhydroxides and allophane) is suppressed, resulting in high dissolved [U], and low colloidal [U] and [Th], indicating a direct chemical weathering control on elemental abundances. When the dissolved (²³⁴U/²³⁸U) activity ratio is >~1.3 (i.e. when physical weathering, groundwater contribution or soil formation are high), there is little isotope exchange between dissolved and colloidal fractions. At lower activity ratios, the dissolved load and colloids have indistinguishable activity ratios, suggesting that when chemical weathering rates are high, secondary clay formation is also high, and colloids rapidly adsorb dissolved U. Many of the suspended sediment samples have (²³⁴U/²³⁸U) activity ratios of > 1, which suggests that uptake of U onto the suspended load is important. Identical (²³⁰Th/²³²Th) in suspended, dissolved and colloidal samples suggests that Th, like U, is exchanged or sorbed rapidly between all riverine phases. This particle-reactivity, combined with poorly constrained contributions from groundwater and hydrothermal water, and short-term variations in input to soils (volcanic and glacial), suggests that U-series nuclides in riverine material from such basaltic terrains are unlikely to reflect steady state erosion processes.     

Production de26Al dans Fe et Si par protons de 0.6 et 24 GeV

A total of 139 breccia and crystalline rock fragments in the size range 2–4 mm from four Apollo 15 soil samples have been examined. Two of the sample stations are on the mare surface (4 and 9A) and two are on the Apennine Front (2 and 6). Approximately 90% of the fragments from the Apennine Front are brown-glass “soil” breccias, but those from the mare surface are 60%–70% basalt. Several textural varieties of mare basalt have been recognized, but within experimental error there is no difference in their⁴⁰Ar-³⁹Ar ages. The major non-mare (Pre-Imbrian) crystalline rock types in the Apennine Front regolith are KREEP basalt, anorthositic rocks, recrystallized norite (including anorthositic norite) and recrystallized polymict breccias; however, such crystalline rocks are rare in the samples examined. Apparently, the near surface Imbrium ejecta below the regolith has not been thermally recrystallized, and probably there are no outcrops of crystalline rocks upslope from the sample stations.     

Effect of sea water interaction on strontium isotope composition of deep-sea basalts

Analyses of rim-to-interior samples of fresh tholeiitic pillow basalts, deuterically altered holocrystalline basalts, and older, weathered tholeiitic basalts from the deep sea indicate that ⁸⁷Sr/⁸⁶Sr ratios of the older basalts are raised by low temperature interaction with strontium dissolved in sea water. ⁸⁷Sr/⁸⁶Sr correlates positively with H₂O in these basalts; however, there is little detectable modification of the strontium isotope composition in rocks with H₂O contents less than 1%. The isotope changes appear to be a function of relatively long-term, low-temperature weathering, rather than high-temperature or deuteric alteration. Strontium abundance and isotopic data for these rocks suggest that strontium content is only slightly modified by interaction with sea water, and it is a relatively insensitive indicator of marine alteration. Average Rb-Sr parameters for samples of apparently unaltered basalt are: Rb= 1.11ppm; Sr= 132ppm; ⁸⁷Sr/⁸⁶Sr= 0.70247.     

Isotopic evidence for uranium exchange during low-temperature alteration of oceanic basalt

Measurements of uranium concentration and the²³⁴U/²³⁸U activity ratio in oceanic basalts which have undergone low-temperature seafloor alteration indicate that uranium uptake is a pervasive occurrence but that the various phases involved behave differently with respect to this process. Palagonite exhibits uranium contents 8–20 times higher than unaltered glass coupled with low²³⁴U/²³⁸U, suggesting ongoing preferential leaching of²³⁴U. Altered crystalline interiors of several old basalts have²³⁴U/²³⁸U > 1, indicative of recent uranium exchange with seawater. The data also provide evidence for uranium sources with²³⁴U/²³⁸U higher than the seawater value of 1.14. Manganese crusts on basalts of a variety of ages have isotopic ratios indicating that they either are recent deposits or also have experienced continuing uranium exchange with seawater.     

Petrology and geochemistry of basaltic rocks of the Lau Basin

The Lau Basin is a marginal sea, located between the Tonga and Lau Ridges, in the southwestern Pacific. The basin is on the “inner” or concave side of the Tonga Trench-Arc system and is situated above the deep seismic zone dipping westward from the Tonga Trench. The Tonga Trench-Arc system is undoubtedly located above a zone of crustal shortening as evidenced by the deep seismicity and vulcanism. However, the geological and geophysical data give strong support to the contention that the Lau Basin has formed by crustal dilation.Rocks dredged from ridges and seamounts in the basin are sub-alkaline basalt. The average major element composition of least altered samples is: SiO₂ 48.8%, TiO₂ 1.2%, K₂O 0.18%, P₂O₅ 0.08%, H₂O+ 0.30%, Fe^III/Fe^II = 0.26,CaO/Al₂O₃ = 0.77. The data for Lau Basin basalt (LBB) show close similarity to data of typical oceanic ridge basalt (ORB). Trace element abundances (ppm): Ni 160, Cr 390, Sr 100, Ba < 31, Rb < 1 also resemble ORB values. K/Rb in a least altered and unfractionated sample is 860, Ba/Sr is 0.1, Ba/Rb is 8. Strontium isotope data show the only marked variance from ORB chemistry with LBB values ranging from ⁸⁷Sr/⁸⁶Sr=0.7020 to 0.7051. The low Sr abundances in the samples suggest the possibility of crustal Sr contamination to explain the radiogenic Sr enrichment. An alternate possibility is that the mantle source rocks were enriched in ⁸⁷Sr. Variation within dredge hauls and between dredge sites may be explained by low-pressure fractional crystallization of magmas separated from the mantle at about 50 km depth.The basin probably began to open in middle to late Miocene time either by the disruption of a single andesitic island arc by splitting along its axis or by dilation of the area between two closely spaced concentric arcs. Mantle counterflow in the asthenosphere above the downgoing oceanic lithosphere slab is the probable driving force for dilation and has provided a continuous supply of parent material for the basalt of the basin floor.     

A direct test of the Vine-Matthews hypothesis

Paleomagnetic studies of the basalt samples of Mid-Atlantic Ridge recovered during DSDP Leg 45 and the FAMOUS Project have led to a revision of our view of the oceanic igneous crust as a recorder of geomagnetic field reversals. The discovery of several magnetic polarity reversals with depth in the crust has indicated that oceanic igneous basement should not necessarily be considered magnetized uniformly in direction, or even polarity, in a given vertical cross section. Statistical arguments, based on the ratio of the average time of crustal formation to the average length of a magnetic polarity interval, indicate that magnetic reversals with depth are to be expected in typical ocean crust, but also that this does not conflict with current theories of plate tectonics or exclude the upper layers of the crust from making a major contribution to the overlying linear magnetic anomalies. Certain ratios of average crustal formation time to average polarity interval do, however, result in an effective zero magnetization for the oceanic crust and these conditions may be responsible for the reduced amplitude of magnetic anomalies in some areas.     

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth''s core

We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earth's core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earth's primitive mantle.

Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle ( 0.036) is too low to represent the primitive value ( 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.     

Gamma ray spectrometric analysis of hydrothermally altered dacite samples from the PACMANUS hydrothermal field: Implications for the interpretation of gamma ray wireline measurements

Spectral gamma ray wireline logging derives the concentrations of potassium, thorium, and uranium of the formation by measuring the gamma ray spectrum under the assumption of secular equilibrium. These measurements were carried out in a number of boreholes drilled by Ocean Drilling Program Leg 193 at the PACMANUS hydrothermal field. Spectral gamma ray logs from several of these holes show intervals of increased radioactivity, associated with high uranium values of up to 60 ppm (Snowcap hydrothermal field) and 25 ppm (Roman Ruins hydrothermal field). Nine samples of hydrothermally altered dacite were analyzed using gamma spectrometry to examine the origin of elevated radioactivity and to test for distortion of secular equilibrium. Core spectrometry indicates that secular equilibrium is distorted. Distortion can be explained by either an uptake of nuclides of the lower part of the ²³⁸U decay series, or by removal of ²³⁸U or ²³⁴Th from the rocks. In all cases wireline uranium logs and core spectrometry do not reflect true uranium concentrations of the formation but uranium is overestimated by a factor of 3–4. At Roman Ruins, uranium values from wireline logging show higher uranium concentrations over large intervals than uranium values from core spectrometry and from published core geochemistry, even when a possible leaching of uranium is taken into account. Wireline logs indicate that depth intervals of increased radioactivity are related to the occurrence of stockwork mineralization. In these depth intervals, core recovery was extremely low. In addition to the distortion of secular equilibrium, low core recovery and preferential loss of softer and more altered material cause a sampling bias between core samples and wireline data and may explain the observed difference in uranium concentrations between core and wireline logs.