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USGS1983

40Ar/39Ar and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

Determinations of 40 Ar/ 39 Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give 40 Ar/ 39 Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts. 206 Pb measured in these clasts is only ∼1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the 207 Pb/ 206 Pb model ages. The Pb data indicate that the initial 207 Pb/ 206 Pb was no more than 0.08±0.07% higher than this ratio in Can˜on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The 40 Ar/ 39 Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.

Earth and Planetary Science Letters↗

USGS1983

Rb-Sr, Sm-Nd, K-Ca, O, and H isotopic study of Cretaceous-Tertiary boundary sediments, Caravaca, Spain: evidence for an oceanic impact site

Isotopic ratios and trace element abundances were measured on samples of Ir-enriched clay at the Cretaceous-Tertiary boundary, and in carbonate and marl from 5 cm below and 3 cm above the boundary. Samples were leached with acetic acid to remove carbonate, and with hydrochloric acid. Leachates and residues were measured. The Sr, Nd, O and H isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials. The data indicate that most of the clay material was derived from a terrestrial source with relatively low 87 Sr/ 86 Sr and high 143 Nd/ 144 Nd ratios. The δ 18 O data suggest that the detritus has been modified by submarine weathering. K-Ca and Rb-Sr systematics, as well as O isotope ratios of K-feldspar spherules within the boundary clay, suggest that they are predominantly authigenic and may have formed after the time of deposition. However, Sm-Nd and Rb-Sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus. The isotopic evidence for foreign terrestrial detritus in the boundary clay, the low rare earth element concentrations and high Ni concentration, support the hypothesis of a terminal Cretaceous asteroidal impact that produced a global layer of fallout. The data are most easily explained if the impact site was on oceanic crust rather than continental crust, and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere (>3 km). This would probably require a single large impactor.

Caravaca↗

USGS1983

Vapor phase exsolution as a controlling factor in hydrogen isotope variation in granitic rocks: the Notch Peak granitic stock, Utah

The Notch Peak granitic stock, western Utah, is comprised of three concentric sequentially intruded rock types, from granite at the rim, to quartz monzonite I, to quartz monzonite II at the core. The δ 18 O values of whole rocks vary about an average of 9.4 (SMOW), irrespective of the rock type and position relative to contact, suggesting that the three magmas had the same parent. The whole rock δD values in the stock range from −100 to −55. δD values increase toward the cores of both quartz monzonite I and quartz monzonite II, resulting in concentric contours. The δD contours of quartz monzonite II cross-cut those of quartz monzonite I, suggesting little isotopic interaction between these bodies and the absence of a late pervasive fluid phase. There is a positive correlation between δD values and water content of the samples, where samples from each body define a distinct field. The positive correlation is explained by isotopic fractionation attendant on vapor exsolution from the crystallizing magma. An observed increase in δD with the degree of chloritization, a trend opposite to that observed in systems where participation of meteoric water has been demonstrated, is the result of subsolidus interaction with the exsolved fluids. These results show that large variations in the hydrogen isotope ratios of a granitoid can arise by exsolution of a vapor phase from the melt on crystallization. In general, magmas with larger modal amount of primary hydrous phases will tend to have higher δD values than those with small amounts of hydrous phases. Furthermore, the relatively high δD values of chlorites at Notch Peak confirm the applicability of classical concepts of closed-system deuteric alteration to some granitoid bodies. Thus, meteoric water interaction need not be always invoked to explain hydrogen isotope variation and deuteric alteration in granitoids.

Earth and Planetary Science Letters↗

USGS1983

Geochemistry of diverse basalt types from Loihi Seamount, Hawaii: Petrogenetic implications

The wide variety of basalt types, tholeiitic to basanite, dredged from Loihi Seamount have minor and trace element abundances that are characteristic of subaerial Hawaiian basalts, thereby confirming that Loihi Seamount is a manifestation of the Hawaiian “hot spot”. Within the Loihi sample suite there are well-defined positive correlations among abundances of highly incompatible elements (P, K, Rb, Ba, Nb, light REE and Ta) and moderately incompatible elements (Sr, Ti, Zr and Hf) and between MgO, Ni and Cr. However, within the Loihi suite abundance ratios of geochemically similar elements (Zr/Hf, Nb/Ta and La/Ce) vary by factors of 1.2–1.5 and abundance ratios of highly incompatible elements such as P/Ce, P/Th, K/Rb, Ba/Th and La/Nb vary by factors of 1.2–2.5. These abundance ratios are not readily changed by different degrees of fractionation and melting. Therefore, we conclude that these samples are not genetically related by different degrees of melting of a compositionally homogeneous source.

Hawaii↗

USGS1983

Geobarometry of ultramafic xenoliths from Loihi Seamount, Hawaii, on the basis of CO2 inclusions in olivine

Abundant fluid inclusions in olivine of dunite xenoliths (∼1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO 2 (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions. Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO 2 liquid and gas, with an aggregate ϱ = ∼ 0.5–0.75 g cm −3 , and represent trapped globules of dense supercritical CO 2 (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO 2 blebs. Spherical sulfide blebs having widely variable volume ratios to CO 2 and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present. Assuming olivine growth at ∼ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO 2 P-V-T data indicates that the primary inclusions were trapped at ∼ 220–470 MPa (2200–4700 bars), or ∼ 8–17 km depth in basalt magma of ϱ = 2.7 g cm −3 . Because the temperature cannot change much during the rise to eruption, the range of CO 2 densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.

Earth and Planetary Science Letters↗

USGS1983

87Sr/86Sr ratios for basalt from Loihi Seamount, Hawaii

87 Sr/ 86 Sr ratios of 15 samples of basalt dredged from Loihi Seamount range from 0.70334 to 0.70368. The basalt types range from tholeiite to basanite in composition and can be divided into six groups on the basis of abundances of K 2 O, Na 2 O, Rb and Sr and 87 Sr/ 86 Sr ratio. The isotopic data require that the various basalt types be derived from source regions differing in Sr isotopic composition. The Loihi basalts may be produced by mixing of isotopically distinct sources, but the tholeiites and alkalic basalts from Loihi do not show a well-developed inverse trend between Rb/Sr and 87 Sr/ 86 Sr that is characteristic of the later stages of Hawaiian volcanoes such as Haleakala and Koolau.

Hawaii↗

USGS1983

Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii

Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3He 4He(RA) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the 3He/4He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower 3He 4He ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (<5 RA), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium. Crushing and melting experiments show that for modest vesicularities (<5% by volume) the Loihi glasses obey a MORB-type partitioning trend, but at higher vesicularities the data show considerably more scatter due to volatile mobilization. The high vesicularities, low extrusion pressure and generally low helium concentrations are consistent with a considerable degree of degassing. Analyses of dunites, plus a correlation between total helium concentrations with xenocryst abundances also suggest that xenocrysts are a significant carrier of contaminating (low 3He 4He) helium. 3He 4He ratios from samples of other Hawaiian volcanoes (Kilauea, Mauna Loa, Hualalai, and Mauna Kea) show a smooth decrease in 3He 4He with increasing volcano age and volume. We interpret this to be a synoptic picture of the time evolution of a hot-spot diapir: the earliest stage is characterized by primitive (> 30 RA) helium with some (variable) component of lithospheric contamination added during "breakthrough", while the later stages are characterized by a relaxation toward lithospheric 3He 4He ratios (??? 8 RA) due to isolation of the diapir from the mantle below (as the plate moves on), and subsequent mining of the inherited helium and contamination from the surrounding lithosphere. The abrupt contrast in 3He 4He ratios between Kilauea and Loihi, despite their close proximity, is indicative of the small lateral extent of the plume. ?? 1983.

Hawaii↗

USGS1983

Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount

Noble gas isotopes including 3 He/ 4 He, 40 Ar/ 36 Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount. Glass and coexisting olivine crystals have similar 3 He/ 4 He ratios (2.8–3.4) × 10 −5 , 20 to 24 times the atmospheric ratio ( R A ), but different 40 Ar/ 36 Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not. The apparent high 3 He/ 4 He ratio (3 × 10 −5 ; = 21 R A ) coupled with a relatively high 40 Ar/ 36 Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma. Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component. Excess 129 Xe has not been observed due to apparent large mass fractionation among Xe isotopes.

Earth and Planetary Science Letters↗

USGS1983

Nontronite from a low-temperature hydrothermal system on the Juan de Fuca Ridge

A deposit of Fe-rich, Al-poor, hydrothermal nontronite was recovered from the Juan de Fuca Ridge. Analyses show the deposit to be mineralogically and chemically similar to nontronite described at other oceanic localities. The deposit is located near the tip of a propagating segment of the Juan de Fuca Ridge. Rare earth elements and Sr isotopes indicate that the nontronite precipitated from seawater. A formation temperature of 57°C is suggested by oxygen isotopic composition. The low-temperature nontronite deposits apparently form from newly established hydrothermal systems associated with the propagating rift segment. More mature hydrothermal systems that deposit sulfide on the seafloor may develop from these low-temperature systems.

Earth and Planetary Science Letters↗

USGS1984

Oxygen isotope evidence for submarine hydrothermal alteration of the Del Puerto ophiolite, California

The oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the Del Puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the Great Valley sequence indicate that their alteration occurred in a submarine hydrothermal system. Whole rock δ 18 O compositions decrease progressively down section (with increasing metamorphic grade): +22.4‰ (SMOW) to +13.8 for zeolite-bearing volcaniclastic sedimentary rocks overlying the ophiolite; +19.6 to +11.6 for pumpellyite-bearing metavolcanic rocks in the upper part of the ophiolite's volcanic member; +12.3 to +8.1 for epidote-bearing metavolcanic rocks in the lower part of the volcanic member; +8.5 to +5.7 for greenschist facies rocks from the ophiolite's plutonic member; +7.6 to +5.8 for amphibolite facies or unmetamorphosed rocks from the plutonic member. Modelling of fluid-rock interaction in the Del Puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock δ 18 O can be best explained by isotopic exchange with discharging 18 O-shifted seawater at fluid/rock mass ratios near 2 and temperatures below 500°C. 18 O-depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism. Submarine weathering and later burial metamorphism at the base of the Great Valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions. The pervasive zeolite and prehnite-pumpellyite facies mineral assemblages found in the Del Puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center.

Earth and Planetary Science Letters↗

USGS1984

The isotope systematics of a juvenile intraplate volcano: Pb, Nd, and Sr isotope ratios of basalts from Loihi Seamount, Hawaii

Sr, Nd, and Pb isotope ratios for a representative suite of 15 basanites, alkali basalts, transitional basalts and tholeiites from Loihi Seamount, Hawaii, display unusually large variations for a single volcano, but lie within known ranges for Hawaiian basalts. Nd isotope ratios in alkali basalts show the largest relative variation (0.51291–0.51305), and include the nearly constant tholeiite value ( ∼ 0.51297). Pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue Tatsumoto's [31] “Loa” trend towards higher 206 Pb/ 204 Pb ratios, resulting in a substantial overlap with the “Kea” trend. 206 Pb/ 204 Pb ratios for Loihi and other volcanoes along the Loa and Kea trends [31] are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of Hawaiian tholeiites. Loihi lavas display no correlation of Nd, Sr, or Pb isotope ratios with major element compositions or eruptive age, in contrast with observations of some other Hawaiian volcanoes [38]. Isotope data for Loihi, as well as average values for Hawaiian volcanoes, are not adequately explained by previously proposed two-end-member models; new models for the origin and the development of Hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere.

Earth and Planetary Science Letters↗

USGS1984

Carbon and its isotopes in mid-oceanic basaltic glasses

Three carbon components are evident in eleven analyzed mid-oceanic basalts: carbon on sample surfaces (resembling adsorbed gases, organic matter, or other non-magmatic carbon species acquired by the glasses subsequent to their eruption), mantle carbon dioxide in vesicles, and mantle carbon dissolved in the glasses. The combustion technique employed recovered only reduced sulfur, all of which appears to be indigenous to the glasses. The dissolved carbon concentration (measured in vesicle-free glass) increases with the eruption depth of the spreading ridge, and is consistent with earlier data which show that magma carbon solubility increases with pressure. The total glass carbon content (dissolved plus vesicular carbon) may be controlled by the depth of the shallowest ridge magma chamber. Carbon isotopic fractionation accompanies magma degassing; vesicle CO 2 is about 3.8‰ enriched in 13 C, relative to dissolved carbon. Despite this fractionation, δ 13 C PDB values for all spreading ridge glasses lie within the range −5.6 and −7.5, and the δ 13 C PDB of mantle carbon likely lies between −5 and −7. The carbon abundances and δ 13 C PDB values of Kilauea East Rift glasses apparently are influenced by the differentiation and movement of magma within that Hawaiian volcano. Using 3 He and carbon data for submarine hydrothermal fluids, the present-day mid-oceanic ridge mantle carbon flux is estimated very roughly to be about 1.0 × 10 13 g C/yr. Such a flux requires 8 Gyr to accumulate the earth's present crustal carbon inventory.

Earth and Planetary Science Letters↗

USGS1984

Geochemistry of tholeiitic and alkalic lavas from the Koolau Range, Oahu, Hawaii: Implications for Hawaiian volcanism

Lavas of the post-erosional, alkalic Honolulu Volcanics have significantly lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd than the older and underlying Koolau tholeiites which form the Koolau shield of eastern Oahu, Hawaii. Despite significant compositional variation within lavas forming the Honolulu Volcanics, these lavas are isotopically (Sr, Nd, Pb) very similar which contrasts with the isotopic heterogeneity of the Koolau tholeiites. Among Hawaiian tholeiitic suites, the Koolau lavas are geochemically distinct because of their lower iron contents and Sr and Nd isotopic ratios which range to bulk earth values. These geochemical data preclude simple models such as derivation of the Honolulu Volcanics and Koolau tholeiites from a common source by different degrees of melting or by mixing of two geochemically distinct sources. There may be no genetic relationship between the origin and evolution of these two lava suites; however, the trend shown by Koolau Range lavas of increasing 143 Nd/ 144 Nd and decreasing 87 Sr/ 86 Sr with decreasing eruption age and increasing alkalinity also occurs at Haleakala, East Molokai and Kauai volcanoes. A complex mixing model proposed for Haleakala lavas can account for the variations in Sr and Nd isotopic ratios and incompatible element abundances found in lavas from the Koolau Range. This model may reflect mixing and melting processes occurring during ascent of relatively enriched mantle through relatively depleted MORB-related lithosphere. Although two isotopically distinct components may be sufficient to explain Sr and Nd isotopic variations at individual Hawaiian volcanoes, more than two isotopically distinct materials are required to explain variations of Sr, Nd and Pb isotopic ratios in all Hawaiian lavas.

Hawaii↗

USGS1984

Triple junction magmatism: a geochemical study of Neogene volcanic rocks in western California

Inception of volcanism at late Oligocene to Recent centers in the eastern Coast Ranges of California (ECR suite) regularly decreases in age northward and is correlated with the northward migration of the transform-transform-trench Mendocino triple junction (MTJ). Miocene volcanism in the southern California basin (SCB suite) is spatially and temporally associated with the transform-ridge-trench Rivera triple junction (RTJ). The tholeiitic to calc-alkaline rocks in both suites were erupted through older trench melange while arc magmatism was occurring several hundred kilometers to the east. Therefore they are not related to subduction zone magmatism, but instead to interactions of the MTJ and RTJ with the continental margin. The ECR rocks, dominantly intermediate to silicic in composition, have relatively high δ 18 O values up to 11.3, 87 Sr/ 86 Sr ratios up to 0.7055, as well as relatively high Th contents, suggesting that crustal anatexis played a dominant role in their generation. Coupled crystal fractionation and crustal assimilation by an initially basaltic magma cannot explain the high δ 18 O values and 87 Sr/ 86 Sr ratios because greater than 95% of the basalt would need to crystallize. In contrast, the SCB rocks, dominantly mafic to intermediate in composition, have relatively low δ 18 O values down to 5.2 and 87 Sr/ 86 Sr ratios down to 0.7025 suggesting that these rocks were derived dominantly from a mantle source. Whether crustal anatexis occurs is determined largely by the type of stress a triple junction imposes upon the continental margin. Both the MTJ and RTJ are associated with high heat flow and magma fluxes from the mantle. The transform-transform-trench MTJ is associated with locally variable mild extension to compression and therefore allows pooling of basaltic magma in the crust to initiate crustal melting. The high rates of continental extension associated with the transform-ridge-trench RTJ prevents such pooling of magma. The space created by decoupling of the subducted slab at a transform-transform-trench triple junction might promote passive upwelling of mantle material to fill it and induce melting to generate basalts. Mafic volcanic rocks of this origin may provide a unique view of the subcontinental mantle at the continental margin. 87 Sr/ 86 Sr ratios as low as 0.70255 for mafic volcanic rocks in the Sonoma-Tolay center associated with the MTJ contrast with high La/Sm ratios of 1.1 to 1.3 and low Zr/Nb,Hf/Ta,La/Th , and La/Ta ratios of 5.0 to 6.7, 2.6 to 3.5, 4.7 to 8.8, and 10.2 to 12.5, respectively. These data suggest that the mantle beneath parts of western California may have originally been depleted but has been enriched relatively recently. Such enrichment might have occurred by metasomatic processes associated with crustal accretion and/or juxtaposition of differing lithospheric mantle in the Mesozoic and Cenozoic.

Earth and Planetary Science Letters↗

USGS1984

A Nd, Sr and O isotopic investigation into the causes of chemical and isotopic zonation in the Bishop Tuff, California

The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial 87 Sr/ 86 Sr of 0.7060–0.7092 and δ 18 O of 5.9–10.3‰. Six constituent sanidines yield smaller ranges of initial 87 Sr/ 86 Sr of 0.7061–0.7069 and δ 18 O of 6.7–7.9. In contrast 143 Nd/ 144 Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of 0.51258 ± 1 . These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had ɛNd = −0.9, ɛSr = +23 and δ 18 O = 7‰ and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith.

Earth and Planetary Science Letters↗

USGS1984

The critical point and two-phase boundary of seawater, 200–500°C

The two-phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200–500°C. The pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel. The critical point was determined to be in the range of 403–406°C, 285–302 bar and was located by the inflection in the two-phase boundary and by the relative volume of fluid and vapor as a function of temperature. The two-phase boundary of 3.2% NaCl solution was found to coincide exactly with that of seawater over the range tested in the present study. The boundary for both is described by a single seventh-order polynomial equation. The two-phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust. Thus the boundary puts a limit of about 390°C for seawater circulating near the seafloor at active ocean ridges (2.5 km water depth), and about 465°C at the top of a magma chamber occurring at 2 km below the seafloor.

Earth and Planetary Science Letters↗

USGS1984

10Be accumulation in a soil chronosequence

We have measured the concentration of the cosmogenic isotope 10 Be in soil samples from various horizons at six sites, including three independently dated Rappahannock River terraces and a previously undated Piedmont soil to which we have assigned an age. All of the incident 10 Be can be accounted for in one of these soils and a second is within a factor of two. In three soils, whose concentrations vary widely with depth, a significant fraction of the incident 10 Be cannot be accounted for. Incomplete sampling, and enhanced Be mobility caused by organic components, are the probable reasons for the low inventory of Be from these three soils. Overall, the data from these six sites indicate that 10 Be accumulation could be used to assign ages to soils if Be is not mobilized and lost from the soil profile.

Earth and Planetary Science Letters↗

USGS1984

Uplift and submarine formation of some Melanesian porphyry copper deposits: Stable isotope evidence

Hydrogen and oxygen isotope analyses of sericites and kaolinites from four young porphyry copper deposits (Ok Tedi (1.2 Ma) and Yandera (6.5 Ma), Papua New Guinea; Koloula (1.5 Ma), Solomon Islands; and Waisoi (<5 Ma), Fiji) indicate that the fluids from which these minerals precipitated were of mixed magmatic and non-magmatic sources. The non-magmatic component of the fluid from the island arc deposits (Koloula, Waisoi) was ocean water. For Ok Tedi, the non-magmatic component was a meteoric water with an isotopic composition different from that of the present meteoric water in the region. The isotopic signature of the former meteoric water is consistent with a surface elevation of 200 m a.s.l. or less at the time of mineralization. The deposit was later exposed and supergene kaolinitization commenced at approximately 1200 m a.s.l. Uplift and erosion has continued to the present at which time the elevation of the exposed deposit is 1800 m a.s.l. This rate of uplift is consistent with that known from other geological evidence. If the rate of uplift were approximately constant during the last 1.2 Ma, the age of supergene enrichment can be dated at approximately 0.4 Ma B.P. Similarly, influx of meteoric water at Yandera occurred when the ground surface above the deposit was at an elevation of approximately 600 m a.s.l. The deposit's present elevation is 1600 m a.s.l. In this case a total uplift of approximately 2.2 km is indicated, with removal of 1.2 km of overburden by erosion.

Earth and Planetary Science Letters↗