Search

Davidson, Guide, Pioneer, Gumdrop, and Rodriguez Seamounts, located at the continental margin offshore California between latitudes 37.5°N and 34.0°N, may represent a previously unrecognized type of intraplate oceanic volcanism. Morphologically unlike typical oceanic-island volcanoes or near-ridge seamounts, they are complex, northeast-trending ridges that reflect the ridge-parallel structure of the underlying oceanic crust. 40Ar/39Ar laser fusion ages of mineral separates indicate at least two episodes of volcanism at ca. 16 and ca. 12 Ma, younger by 7–11 m.y. than the underlying ocean crust. Volcanic rocks are predominantly differentiated alkalic basalt, hawaiite, and mugearite. The lack of coherent liquid- lines-of-descent is consistent with small batches of magma forming, fractionally crystallizing, and erupting in isolation from the previous and subsequent batches. The presence of mantle xenoliths suggests that magmas originated in the upper mantle. Xenoliths of alkalic cumulates and xenocrysts of feldspar, amphibole, and titanomagnetite are consistent with fractionation in the upper mantle as well. Sr, Nd, and Pb isotope ratios of some samples are from depleted-mantle sources like those of MORB (mid-oceanic-ridge basalt); others indicate more variably enriched mantle sources. Magmas formed by small- percentage partial melting of variably enriched MORB-source-type mantle, according to the samples' high abundances of incompatible elements such as Nb, Ta, and Th. Coeval middle Miocene basalts in onshore coastal California have a similar isotopic range, but their trace elements show a subduction-related signature. If the coeval volcanic rocks offshore originated in a slab window, as proposed for those onshore, they were not influenced by a slab component. Although a slab-window origin is plausible for the middle Miocene volcanism, later episodes of small, sporadic eruptions on- and offshore probably resulted from decompression melting of mantle rising along existing zones of weakness undergoing extension related to continued movement along transform-fault systems.

Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yield 207Pb/206Pb vs. 238U/206Pb concordia ages ranging from 1.13±0.04 Ma to 1.25±0.04 (1σ) Ma. The weighted mean of the U/Pb model ages is 1.18±0.03 Ma. The U–Pb ages coincide closely with 40Ar/39Ar age spectrum plateau and `terminal' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350°C by ∼0.9–1.0 Ma. Interpretation of the feldspar 40Ar/39Ar age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350° to 300°C between 1.0 and 0.4 Ma or transitory reheating to 300–350°C at about 0.4–0.6 Ma. Subsequent rapid cooling to below 260°C between 0.4 and 0.2 Ma is in agreement with previous proposals that vapor-dominated conditions were initiated within the hydrothermal system at this time. Heat flow calculations constrained with K-feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2 to 0.6 Ma.

After two centuries of scientific endeavor, scientists have concluded that the age of the Earth is 4.54 billion years. This value, which is based on the relationships between lead isotopes in meteorites and in the Earth, has an uncertainty of less than 1 percent and is consistent with numerous radiometric age measurements on ancient rocks found on the Earth and Moon as well as on meteorites. In addition, the antiquity of the Earth is consistent with evidence indicating that the Milky Way Galaxy and the universe are of the order of 14-16 billion years (abbreviated Ga) in age.

We have obtained high-resolution (21-63 steps) Ar-40/Ar-39 age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from Apollo 17 samples collected from the pre-Imbrian highlands in the easternmost part of the Serenitatis basin. The samples include poikilitic melt rocks inferred to have been formed in the Serenitatis basin-forming impact, aphanitic melt rock whose compositions vary and whose provenance is uncertain, and granulite, gabbro, and melt clasts. Three of the poikilitic melts have similar age spectrum plateau ages (72395,96, 3893 +/- 16 Ma (2sigma); 72535,7, 3887 +/- 16 Ma; 76315,150, 3900 +/- 16 Ma) with a weighted mean age of 3893 +/- 9 Ma, which we interpret as the best age for the Serenitatis basin- forming impact. Published Ar-40/Ar-39 age spectrum ages of Apollo 17 poikilitic melts are consistent with our new age but are much less precise. Two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of greater than or equal to 3869 Ma (72558,7) and greater than or equal to 3743 Ma (77135,178). Plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 +/- 16 Ma to 3937 +/- 16 Ma and have probably been affected by the ubiquitous (older?) clasts and by post- formation heating (impact) events. Plateau ages from two of the aphanitic melt 'blobs' and two granulites in sample 72255 fall in the narrow range of 3850 q 16 Ma to 3869 q 16 Ma with a weighted mean of 3862 +/- 8 Ma. Two of the aphanitic melt blobs from 72255 have ages of 3883 +/- 16 Ma and greater than or equal to 3894 Ma, whereas a poikilitic melt clast (of different composition from the 'Serenitatis' melts) has an age of 3835 +/- 16 Ma, which is the upper limit for the accretion of 72255. These data suggest that either the aphanitic melts vary in age, as is also suggested by their varying chemical compositions, or they formed in the 72255 accretionary event about 3.84-3.85 Ga and older relict material is responsible for the dispersion of ages. In any case the aphanitic melts do not appear to be Serenitatis products. Our age for the Serenitatis impact shows, on the basis of the isotopic age evidence alone, that Serenitatis is greater than 20-25 Ma and probably greatr than 55-60 Ma older than Imbrium (less than or equal to 3870 Ma and probably less than or equal to 3836 Ma (Dalrymple and Ryder, 19931). Noritic granulite sample 78527 has a plateau age of 4146 +/- 17 Ma, representing a minimum age for cooling of this sample in the early lunar crust. So far there is no convincing evidence in the lunar melt rock record for basin-forming impacts significantly older than 3.9 Ga.

A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 microns-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basaits. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the Ar-40/Ar-39 laser incremental heating technique, is 1647 +/- 11 Ma (2 sigma); it is expressed as an age spectrum of seventeen steps over 96% of the Ar-38 released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; Ar-38 cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50 C/s is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and the young age are perhaps all most consistent with an impact origin, but nonetheless not absolutely definitive.

The coarse-grained lunar meteorites, Yamato-793169 and Asuka-881757, represent a new type of low-Ti mare basalt. This paper reports the results of a UThPb, SmNd, and ArAr isotopic study of Yamato-793169 performed as part of a consortium studies of lunar basaltic meteorites. The isotopic study was carried out on a small sample (100 mg) so that only three density fractions could be separated. These fractions were leached with dilute acid in order to eliminate terrestrial Pb contamination. However, the leaching procedure did not completely remove this contamination in some fractions and also apparently caused a fractionation of elements (U, Th, Pb) due to preferential leaching effects, producing a secondary disturbance of the systematics. Furthermore, the ArAr analyses indicate that the isotopic systematics in this meteorite might have been disturbed sometime later than 750 Ma. For these reasons, the ages obtained using different isotopic systems disagree with each other and a precise formation age could not be obtained for this meteorite. However, using what results there are, two reasonable interpretations can be made: (1) the SmNd system yielded an age of 3.4 Ga that could be interpreted as the formation age, assuming that this system is possibly the least disturbed during the metamorphic event(s) that this meteorite experienced at least once, and that all other isotopic systems and their corresponding ages were disturbed, and (2) the U-Pb system yielded a nearly concordant age of 3.8 Ga that could be interpreted as the formation age and the SmNd isotopic systematics were somehow disturbed. We prefer the second interpretation for the reasons discussed below. On a concordia diagram, the CDT (Canon Diablo troilite)-corrected UPb isotopic data yield a discordia line similar to the lunar catastrophic array, indicating that the source of the meteorite formed during early lunar differentiation (∼4.4 Ga) and that the basalt was generated near 3.9 Ga. Total ArAr age on plagioclase is 3.26 Ga, which seems to be too old if the formation age is 3.4 Ga, because low temperature fractions lost large amounts of radiogenic 40 Ar during the late thermal event. If we assume a formation age of 3.8 Ga, the estimated source 238 U/ 204 Pb (μ) is 21.6 ± 3.5 and ϵ Nd is 3.9 ± 0.3. These results indicate that the source of Yamato-793169 is more depleted than Apollo 12 and 15 LT basalts, but less depleted than Asuka-881757. Therefore, Yamato-793169 may represent a new type of LT- or VLT-like mare basalt that is different from Asuka-881757. The wide variety of lead and neodymium isotopic characteristics among LT and VLT mare basalts indicate that the lunar mantle was very heterogeneous with respect to trace element abundances.

40 Ar 39 Ar age spectra of biotite from a mineralized vein in the ore-bearing, Noril'sk I intrusion and from picritic-like gabbrodolerite from the weakly mineralized, Lower Talnakh intrusion show that these bodies were emplaced at 249 ± 2 Ma, which is not significantly different from the age of the Permian-Triassic boundary. The ore-bearing intrusions postdate the lower third of the flood-basalt sequence in the Noril'sk area and, on the basis of geochemistry, can best be correlated with lavas slightly younger than those which they cut. Thus, flood basalt was erupted at the time of the Permian-Triassic mass extinction event, although its role in this event is, as yet, ill defined. Additional new 40 Ar 39 Ar age data for a group of intrusive and extrusive rocks on the western margin of the Siberian craton indicate that mafic magmatism extended over a period of several tens of million years, whereas paleomagnetic data suggest that the bulk of the Siberian flood-basalt sequence near Noril'sk has been erupted in only a million years or so. 40 Ar 39 Ar ages of plagioclase from early flood-basalt flows are about 2% younger than those obtained for biotite from the crosscutting, Noril'sk I intrusion, probably because of slight alteration and Argon loss from the plagioclase.

The dominant ore host at Camflo is a small monzonite pluton cut by main-stage gold-bearing quartz veins containing hydrothermal titanite and K feldspar. Main-stage veins are spatially and temporally related to brittle-ductile faults. Emplaced into most faults are younger transitional-stage quartz-pyrite lodes with sericite-rutile alteration halos. Titanite in main-stage gold quartz veins is replaced by rutile-calcite-quartz where overprinted by transitional-stage sericitic alteration.A U-Pb zircon age from the Camflo monzonite indicates that the stock was emplaced at 2685 + or - 10 Ma. A U-Pb age of 2621 + or - 4 Ma for hydrothermal titanite and a Pb-Pb isochron age of 2621 + or - 7 Ma defined by seven titanite and two K feldspar samples indicate that the gold quartz veins formed approximately 60 m.y. later which precludes a direct genetic link between these two events. Rutile analyses plot below the titanite-feldspar Pb-Pb isochron and so support field and petrographic evidence suggesting rutile did not precipitate from the same fluid that formed the titanite and K feldspar.Incremental heating and total fusion 40 Ar- 39 Ar ages of hydrothermal micas related to gold quartz veins and quartz-pyrite lodes are 70 to 170 m.y. younger than the hydrothermal U-Pb ages. In addition, 40 Ar- 39 Ar ages of micas on the 1650 level are approximately 10 to 15 m.y. older than like micas on the 3475 level. The younger 40 Ar- 39 Ar ages are interpreted as cooling, and not recrystallization or reset, ages. 40 Ar- 39 Ar ages indicate cooling rates of approximately 1 degrees C/m.y. for 2600 to 2550 Ma (rutile-vein muscovite), approximately 0.6 degrees C/m.y. for 2550 to 2511 Ma (vein muscovite-wall-rock muscovite), and approximately 0.3 degrees C/m.y. for 2507 to 2452 Ma (large biotite-small biotite). An apparent uplift rate of approximately 0.04 km/m.y. for 2520 to 2478 Ma is calculated by comparing biotite and white mica ages from the 1650 and 3475 levels.The U-Pb and 40 Ar- 39 Ar thermochronology is consistent with an accretionary tectonic setting for Camflo during the terminal stages of tectonic construction of the Superior province. A critical feature of this setting is large volumes of mantle-derived basalt underplating the lower crust during peak granulite formation at approximately 2648 to 2640 Ma. CO 2 -rich fluids liberated by solidification of ponded mafic magma after peak granulite formation interacted with other fluids, magmas, and rock either at their source or during migration in shear zones to upper crustal levels where they deposited significant quantities of gold at Camflo at approximately 2621 Ma. Slow cooling and slow apparent uplift at Camflo ensued for >150 m.y. after magmatic under-plating, resulting in the young 40 Ar- 39 Ar cooling ages recorded by hydrothermal minerals. This model is consistent with ages for granulite facies metamorphism in the Kapuskasing structural zone, postpeak metamorphic ages for CO 2 -H 2 O fluid inclusions in regional granulite terranes, and geochronological data from other gold deposits and may have widespread implications for the formation of gold deposits in the southern Superior province.

We have undertaken UThPb, SmNd, RbSr, and 40 Ar 39 Ar isotopic studies on Asuka 881757, a coarse-grained basaltic lunar meteorite whose chemical composition is close to low-Ti and very low-Ti (VLT) mare basalts. The PbPb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 ± 28 Ma, which is similar to the U-Pb (3850 ± 150 Ma) and Th-Pb (3820 ± 290 Ma) internal isochron ages. The Sm-Nd data for the mineral separates yield an internal isochron age of 3871 ± 57 Ma and an initial 143 Nd 144 Nd value of 0.50797 ± 10. The Rb-Sr data yield an internal isochron age of 3840 ± 32 Ma ( λ( 87 Rb) = 1.42 × 10 −11 yr −1 ) and a low initial 87 Sr 86 Sr ratio of 0.69910 ± 2. The 40 Ar 39 Ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 ± 12 Ma. We interpret these ages to indicate that the basalt crystallized from a melt 3.87 Ga ago (the Sm-Nd age) and an impact event disturbed the Rb-Sr system and completely reset the K-Ar system at 3.80 Ga. The slightly higher Pb-Pb age compared to the Sm-Nd age could be due to the secondary Pb (from terrestrial and/or lunar surface Pb contamination) that remained in the residues after acid leaching. Alternatively, the following interpretation is also possible; the meteorite crystallized at 3.94 Ga (the Pb-Pb age) and the Sm-Nd, Rb-Sr, and K-Ar systems were disturbed by an impact event at 3.80 Ga. The crystallization age obtained here is older than those reported for low-Ti basalts (3.2–3.5 Ga) and for VLT basalts (3.4 Ga), but similar to ages of some mare basalts, indicating that the basalt may have formed from a magma related to a basin-forming event (Imbrium?). The age span for VLT basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least ~500 million years. The impact event that thermally reset the K-Ar system of Asuka 881757 must have been post-Imbrium (perhaps Orientale) in age. The lead isotopic composition of Asuka 881757 is nonradiogenic compared with typical Apollo mare basalts and the estimated 238 U 204 Pb (μ) value for the basalt source is 10 ± 3. This source-μ value is the lowest so far measured for lunar rocks. A large positive ϵNd value (7.4 ± 0.5) and the time averaged 147 Sm 144 Nd ratio for the basalt source are similar to those for some Apollo 12, 15, and 17 basalts, suggesting a LREE-depleted mantle, which is consistent with the global magma ocean hypothesis. The U-Th-Pb, Sm-Nd, and Rb-Sr data on Asuka 881757 suggest that the basalt was derived from a low U Pb , low Rb Sr , and high Sm Nd source region, mainly composed of olivine and orthopyroxene with minor amounts of plagioclase (or clinopyroxene) and with sulfides enriched in volatile chalcophile elements. The basalt source may be deep in origin and different in chemistry from those previously estimated from studies of Apollo and Luna mare basalts, indicating heterogeneous sources for mare basalts.

We have obtained 15 high resolution (21–51 step) 40Ar/39 age spectra on six Apollo 15 impact melt rocks of different compositions using a continuous laser system on submilligram subsamples and on single crystal plagioclase clasts. Four of the six samples gave reproducible age spectra with well-defined intermediate temperature plateaus over 48% or more of the 39Ar released; the plateaus are interpreted as crystallization ages. Samples 15304,7,69, 15294,6,21, and 15314,26,156 gave virtually identical plateau ages whose weighted mean is 3,870±6 Ma. These three melt rocks differ in composition and likely formed in three separate impact events. Sample 15356,9 gave replicate plateau ages that average 3,836±12 Ma and date a fourth and younger impact event. The age spectra for samples 15308,9 and 15414,3,36 increase with increasing increment temperature and may have been formed in or affected by impacts at about 2,700 Ma and 3,870 Ma, respectively. So far there continues to be no convincing evidence in the lunar record for impact melts older than about 3.9 Ga.

My subject is “small” science—not small in physical dimensions or significance, but science done by capable individuals or small teams of scientists who are motivated solely by curiosity about how nature works. I mean science unencumbered by the goals, direction, or guidance of administrators, funding agencies, or politicians. I mean science done without the slightest consideration of its practical worth to society or humankind. How important is “small” science? I think it is the single most important thing that has made possible the technological revolution of the 20th century. This is by no means a new idea; it was expressed 45 years ago by Vannevar Bush, who wrote, “New products and processes are founded on new principles and conceptions which, in turn, are developed by research in the purest realms of science.” I also think, however, that the contributions of small science to the wealth and welfare of humankind are not well recognized today, either among the population at large or among those who fund science. It is fairly obvious to anyone who has looked at research budgets over the past decade or two that small science is being increasingly neglected in favor of big science and of science directed toward perceived national and global problems. In the long run this neglect will be detrimental to society, and the trend must be reversed.

Lava flows of the Ninole Basalt, the oldest rocks exposed on the south side of the island of Hawaii, provide age and compositional constraints on the evolution of Mauna Loa volcano and the southeastward age progression of Hawaiian volcanism. Although the tholeiitic Ninole Basalt differs from historic lavas of Mauna Loa volcano in most major-element contents (e.g., variably lower K, Na, Si; higher Al, Fe, Ti, Ca), REE and other relatively immobile minor elements are similar to historic and prehistoric Mauna Loa lavas, and the present major-element differences are mainly due to incipient weathering in the tropical environment. New K-Ar whole-rock ages, from relatively fresh roadcut samples, suggest that the age of the Ninole Basalt is approximately 0.1–0.2 Ma, although resolution is poor because of low contents of K and radiogenic Ar. Originally considered the remnants of a separate volcano, the Ninole Hills are here interpreted as faulted remnants of the old south flank of Mauna Loa. Deep canyons in the Ninole Hills, eroded after massive landslide failure of flanks of the southwest rift zone, have been preserved from burial by younger lava due to westward migration of the rift zone. Landslide-induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread Basaltic ash that overlies the Ninole Basalt. Subaerial presence of the Ninole Basalt documents that the southern part of Hawaii Island had grown to much of its present size above sea level by 0.1–0.2 Ma, and places significant limits on subsequent enlargement of the south flank of Mauna Loa.

The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent 40Ar/39Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic 40Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a 14C age of 400 ± 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria.

The accuracy of the ages of primary mineral standards used as fluence monitors is the limiting factor in calculating accurate 4 0 Ar/ 3 9 Ar ages. A primary mineral standard can be dated only by measuring its K and radiogenic-Ar contents, using calibrated 3 8 Ar tracers. We describe the analytical procedure for first-principles calibrations of 3 8 Ar tracers in the U.S. Geological Survey (USGS) laboratory in Menlo Park, Calif. Two laboratories in Japan that have made first-principles calibrations of 3 8 Ar tracers report good agreement with the values obtained in the USGS laboratory for the radiogenic- 4 0 Ar content of the SORI biotite standard. The ages of fluence monitors determined in the USGS laboratory and those reported in the literature differ by nearly 2 percent. However, the radiogenic- 4 0 Ar content of the primary mineral standard on which these ages are based has been measured by first-principles Ar calibration in only one laboratory, and that value has never been verified by first-principles measurement in other laboratories.

At least 13 times during the Pleistocene Epoch lava flowed into the inner gorge of the Grand Canyon and formed lava dams, as high as 600 m, that temporarily blocked the flow of the Colorado River. K-Ar ages on these lava dams indicate that the seven youngest formed within a short period of time between about 0.6 and 0.4 mega-annum (Ma). The physiography of the lava dam remnants within the canyon shows that each dam was destroyed by erosion, the Colorado River rapidly reaching its pre-existing grade level, before the next dam was emplaced by new eruptions. The total time for emplacement and destruction for an individual lava dam was probably as little as 0.01–0.02 million years. The K-Ar ages of the two oldest dams, the Lava Butte dam (0.577 ± 0.054 Ma) and the Prospect dam (0.684 ± 0.051 Ma) are somewhat younger than the physiography of their remnants suggest.

Results are reported on 26 high-resolution (16-51 steps) Ar-40/Ar-39 age spectra obtained on 12 Apollo-15 melt rocks of different composition using a continuous laser system on submg fragments of recrystallized melt and single-crystal plagioclase clasts from impact melt rocks collected at the Apennine Front where the Imbrium and Serenitatis basins intersect. A table is presented with the summary of the Ar-40/Ar-39 spectrum data, which represent 891 individual temperature step analyses. Also presented are 20 of the 26 age spectra along with their respective K/Ca plots. Melt rock fragments and plagioclase clasts from seven of the 12 samples analyzed yielded reproducible, intermediate-T Ar-40/Ar-39 age spectrum plateaus, which were interpreted as crystallization ages that represent the times of impact of bolides onto the lunar surface.

K-Ar conventional and 40Ar/39Ar ages on plagioclase from altered basalt samples show that Khatchaturian and Rachmaninoff Seamounts in the Musicians Seamounts, north-central Pacific, are 65.2 ± 2.6 and 86.6 ± 5.2 m.y. old, respectively. The minimum age of Wentworth Seamount, located on the Hawaiian Ridge near Midway, is 71 ± 5 m.y. Wentworth appears to be a Cretaceous volcano that was incorporated into the Hawaiian volcanic chain. A single boulder of rhyolite dredged from the northern slope of the seamount underlying Necker Island has an age of 77.6 ± 1.7 m.y. Apparently Necker is a composite seamount constructed of both Cretaceous and late Tertiary volcanoes. With one possible exception, the ages of these and other Cretaceous seamounts in the north-central Pacific are less than or equal to the age of the adjacent sea floor indicating that the seamounts formed at or near the crest of the East Pacific Rise.

K-Ar and paleomagnetic data from cores through a sequence of basalt flows in the eastern Snake River Plain provide evidence for a brief (0.005 to 0.01 m.y.) reversal of the geomagnetic field 0.46 ± 0.05 m.y. ago. This reversed polarity event has also been found in sea-floor magnetic anomalies and in sediment cores and is probably the Emperor event of Ryan [1972].

Long Valley caldera, a 17- by 32-km elliptical depression on the east front of the Sierra Nevada, was formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68-0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende-biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende-biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self-sealing of near-surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated northwest-trending Sierra Nevada frontal faults that tap hot water at depth.

At the top of the Waimea Canyon Basalt on the island of Kauai, rare flows of alkalic postshield-stage hawaiite and mugearite overlie tholeiitic flows of the shield stage. These postshield-stage flows are 3.92 Ma and provide a younger limit for the age of the tholeiitic shield stage. The younger Koloa Volcanics consist of widespread alkalic rejuvenated-stage flows and vents of alkalic basalt, basanite, nephelinite, and nepheline melilitite that erupted between 3.65 and 0.52 Ma. All the flows older than 1.7 Ma occur in the west-northwestern half of the island and all the flows younger than 1.5 Ma occur in the east-southeastern half. The lithologies have no spatial or chronological pattern. The flows of the Koloa Volcanics are near-primary magmas generated by variable small degrees of partial melting of a compositionally heterogeneous garnet-bearing source that has about two-thirds the concentration of P2O5, rare-earth elements, and Sr of the source of the Honolulu Volcanics on the island of Oahu. The same lithology in the Koloa and Honolulu Volcanics is generated by similar degrees of partial melting of distinct source compositions. The lavas of the Koloa Volcanics can be generated by as little as 3 percent to as much as 17 percent partial melting for nepheline melilitite through alkalic basalt, respectively. Phases that remain in the residue of the Honolulu Volcanics, such as rutile and phlogopite, are exhausted during formation of the Koloa Volcanics at all but the smallest degrees of partial melting. The mantle source for Kauai lava becomes systematically more depleted in 87Sr/86Sr as the volcano evolves from the tholeiitic shield stage to the alkalic postshield stage to the alkalic rejuvenated stage: at the same time, the lavas become systematically more enriched in incompatible trace elements. On a shorter timescale, the lavas of the Koloa Volcanics display the same compositional trends, but at a lower rate of change. The source characteristics of the Koloa Volcanics, considered along with those of the Honolulu Volcanics, support a mixing model in which the source of rejuvenated-stage lava represents large-percent melts of a plume source mixed with small amounts of small-percent melts of a heterogeneous mid-ocean-ridge source.

40 Ar/ 39 Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess 40 Ar indicate that saddle-shaped age spectra are diagnostic of excess 40 Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar isochron diagram reliably reveal the crystallization age in such samples.

40 Ar/ 39 Ar age data on alkalic and tholeiitic basalts from Diakakuji and Kinmei Seamounts in the vicinity of the Hawaiian-Emperor bend indicate that these volcanoes are about 41 and 39 m.y. old, respectively. Combined with previously published age data on Yuryaku and Ko¯ko Seamounts, the new data indicate that the best age for the bend is 42.0 ± 1.4 m.y. Petrochemical data indicate that the volcanic rocks recovered from bend seamounts are indistinguishable from Hawaiian volcanic rocks, strengthening the hypothesis that the Hawaiian-Emperor bend is part of the Hawaiian volcanic chain. 40 Ar/ 39 Ar total fusion ages on altered whole-rock basalt samples are consistent with feldspar ages and with 40 Ar/ 39 Ar incremental heating data and appear to reflect the crystallization ages of the samples even though conventional K-Ar ages are significantly younger. The cause of this effect is not known but it may be due to low-temperature loss of 39 Ar from nonretentive montmorillonite clays that have also lost 40 Ar.

40 Ar/ 39 Ar age spectra and 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochrons were determined by incremental heating for 11 terrestrial rocks and minerals whose geology indicates that they represent essentially undisturbed systems. The samples include muscovite, biotite, hornblende, sanidine, plagioclase, dacite, diabase and basalt and range in age from 40 to 1700 m.y. For each sample, the 40 Ar/ 39 Ar ratios, corrected for atmospheric and neutron-generated argon isotopes, are the same for most of the gas fractions released and the age spectra, which show pronounced plateaus, thus are consistent with models previously proposed for undisturbed samples. Plateau ages and isochron ages calculated using plateau age fractions are concordant and appear to be meaningful estimates of the crystallization and cooling ages of these samples. Seemingly anomalous age spectrum points can be attributed entirely to small amounts of previously unrecognized argon loss and to gas fractions that contain too small (less than 2 per cent) a proportion of the 39 Ar released to be geologically significant. The use of a quantitative abscissa for age spectrum diagrams is recommended so that the size of each gas fraction is readily apparent. Increments containing less than about 4–5 per cent of the total 39 Ar released should be interpreted cautiously. Both the age spectrum and isochron methods of data reduction for incremental heating experiments are worthwhile, as each gives slightly different but complementary information about the sample from the same basic data. Use of a least-squares fit that allows for correlated errors is recommended for 40 Ar/ 36 Ar vs 39 Ar/ 36 Ar isochrons. The results indicate that the 40 Ar/ 39 Ar incremental heating technique can be used to distinguish disturbed from undisturbed rock and mineral systems and will be a valuable geochronological tool in geologically complex terranes.

New conventional K-Ar, 40 Ar/ 39 Ar, and petrochemical data on alkalic basalt pebbles from the basalt conglomerate overlying tholeiitic flows in the Midway drill hole show that Midway evolved past the tholeiitic shield-building stage and erupted lavas of the alkalic suite 27.0 ± 0.6m.y. ago. The data also show that previously published conventional K-Ar ages on altered samples of tholeiite are too young by about 9 m.y. These results remove a significant anomaly in the age-distance relationships of the Hawaiian chain and obviate the need for large changes in either the rate of rotation of the Pacific plate about the Hawaiian pole or the motion of the plate relative to the Hawaiian hot spot since the time of formation of the Hawaiian-Emperor bend. All of the age data along the Hawaiian chain are now reasonably consistent with an average rate of volcanic propagation of 8.0 cm/yr and with 0.83°/m.y. of angular rotation about the Hawaiian pole.

Thomas G. Barnes, Emeritus Professor of Physics at the University of Texas, El Paso, and a creationist, argues that the geomagnetic field was created by unknown processes at the time of creation of the earth and has been decaying irreversibly and exponentially, with a half-life of about 1400 years, since then. He calculates that the field would have been impossibly large in 8000 B.C. and concludes that the earth, therefore, is less than 10,000 years old. He denies that the earth's field has reversed polarity in the past, and attempts to refute the dynamo theory of the origin of the field. Barnes' arguments and conclusions are commonly cited in creationist literature as definitive proof that the earth is very young. Barnes is wrong, and has ignored or misrepresented much of the data on the earth's magnetic field. Barnes calculations are based on observatory measurements showing that the earth's dipole field strength has decreased since 1835. But these same measurements also show a corresponding increase in...

Twenty-two KAr ages were determined for sanidine samples from 10 rhyolite domes of the Mono and Inyo Craters to test the applicability of KAr dating to volcanic rocks of Recent age. Comparison of the results with ‘blank’ and dosed analyses shows that radiogenic 40Ar was detected and was measured to within a factor of two or better. The estimated standard deviation of precision is 12% for analyses containing 5% or more radiogenic 40Ar. A statistical analysis suggests that real differences in apparent age were detected between three of the analyzed domes. The ages, which range from 6400 to 10,200 years for experiments with 5% or more radiogenic 40Ar, are, in general, consistent with ionium ages on 5 of the same samples and with glacial and 14C evidence of age. They suggest that most of the Mono Craters volcanos are on the order of 10,000 years old or less. The results also suggest that the problem of excess 40Ar may not be severe for KAr dating of volcanic rocks.

Four basalt samples for whole-rock K-Ar dating were analyzed with an electron microprobe to locate potassium concentrations. Highest concentrations of potassium were found in those mineral phases which were the last to crystallize. The two reliable samples had potassium concentrated in fine-grained interstitial feldspar and along grain boundaries of earlier formed plagioclase crystals. The two unreliable samples had potassium concentrated in the glassy matrix, demonstrating the ineffectiveness of basaltic glass as a retainer of radiogenic argon. In selecting basalt samples for whole-rock K-Ar dating, particular emphasis should be placed on determining the nature and condition of the fine-grained interstitial phases.

New paleomagnetic and potassium-argon dating measurements have been made of basalt flows from Nunivak Island, Alaska, with the following results. (1) The best estimate of the age of the Brunhes/Matuyama polarity epoch boundary is found to be 0.694 m.y. (2) The best estimate of the age of the Gauss/Gilbert boundary is 3.32 m.y. (3) Three normally magnetized flows with ages from 0.93 to 0.88 m.y. are in accord with previous estimates of the age and duration of the Jaramillo normal event. (4) One normally magnetized flow with an age of 1.65 ± 0.09 m.y. supplies additional evidence for the Gilsa normal event. (5) Two new normal events are identified within the Gilbert reversed epoch, the “Cochiti normal event” with an age of 3.7 m.y. and the “Nunivak normal event” with an age of 4.1 m.y.

Thermoluminescence glow curve and decay characteristics of Apollo 12 fines and soil samples, suggesting lower mean daytime surface temperature at site

A time-scale for polarity reversals of the geomagnetic field has been established for the last 4.1 m.y., using the potassium-argon method of dating rocks. The data for this time-scale consist of polarity- and age-determinations for about 240 rocks made by a number of workers; these are summarized in tabular form. At least 17 geomagnetic reversals occurred during the period covered, and intervals of constant polarity ranged in length from 700 000 years to about 50 000 years or possibly less. Two possible correlations of this time-scale with the Pliocene-Pleistocene boundary are discussed. In several parts of the world glacial deposits have been shown to be as old as 3 m.y., in some cases by application of the reversal time-scale either on land or in deep-sea sediment cores.

Submarine pillow basalts from Kilauea Volcano contain excess radiogenic argon-40 and give anomalously high potassium-argon ages. Glassy rims of pillows show a systematic increase in radiogenic argon-40 with depth, and a pillow from a depth of 2590 meters shows a decrease in radiogenic argon40 inward from the pillow rim. The data indicate that the amount of excess radiogenic argon-40 is a direct function of both hydrostatic pressure and rate of cooling, and that many submarine basalts are not suitable for potassium-argon dating.

Repetitive K-Ar age determinations on whole-rock samples from a late Cenozoic basalt give a standard deviation of precision of about 2% for both the within-sample and the within-flow studies. K analyses of 10 chips cut from one hand specimen give a total spread of 3.11% and σ = 0.93%; 12 Ar analyses give a total spread of 6.76% and σ = 1.84%. The calculated ages give a mean value of 3.42×106 years with a total spread of 7.60% and σ = 1.90%. Calculated ages of seven samples taken from the same basalt at points up to nearly 3 km apart is 3.32×106 years with a total spread of 6.33% and σ = 2.11%. The total spread in the K and the Ar values from these samples, however, is over 20%.

Geological evidence obtained from samples of rocks from Cocos Island, Costa Rica, seems to agree with the idea that this island could have been a stepping stone for organisms migrating to the Galapagos Islands.

Potassium-argon analyses of P-207 in twenty-four laboratories and rubidium-strontium analyses in ten laboratories indicate that for material of this typo the average interlaboratory precision for KAr ages is 2.6% and for RbSr ages is 3.4%, and the average intralaboratory precision for KAr ages is 1.6% and for RbSr ages is 2.9%.