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C. E. Hedge

Publications and source records attributed to C. E. Hedge.

30 records · Page 2Linked to original sources
USGS1970

Distribution of rubidium between sodic sanidine and natural silicic liquid

Phenocrysts of sodic sanidine from twelve upper Cenozoic units of silicic ash-flow tuff and lava from the Western United States contain from 0.25 to 0.45 the Rb present in the associated groundmass materials. The ratios of potassium to rubidium in the sanidines are, on the average, about four times greater than those of the groundmass. Separation of phenocrystic sanidine from salic melts provides an efficient method for raising the Rb content and lowering the K/Rb ratio of the melts, although the amount of differentiation probably is limited by continuous reequilibration of the alkalis between crystal and liquid phases through ion exchange. Syenites of cumulate origin will have appreciably lower Rb contents and higher K/Rb ratios than the melts from which they precipitated. Available data on the distribution of Rb between synthetic biotite and K-sanidine demonstrate that the separation of biotite probably will not deplete salic melts in Rb relative to K.

Contributions to Mineralogy and Petrology↗
USGS1970

Strontium isotopes in some Cenozoic lavas from Oregon and Washington

The 87 Sr/ 86 Sr ratios have been determined in 22 volcanic rocks from Oregon and Washington. The emphasis of the study was on andesites and dacites, but several basalts were included. Most of the samples have a very limited range in strontium isotopic composition and all fall within variations previously defined for oceanic basalts. There are no systematic differences between lavas erupted through the pre-Cenozoic basement and those erupted in the Columbia River Embayment. It is concluded that the role played by crustal rocks in the generation of these andesites and dacites is minor or nonexistent, but assimiliation of crustal material may have been a factor for some of the basalts of the Columbia River Group.

Oregon, Washington↗
USGS1970

Isotopic composition of strontium in sea water throughout Phanerozoic time

Isotopic analyses of strontium in primary fossil carbonate reveal significant variations in Sr 87 Sr 86 "> Sr87Sr86 of sea water during the Phanerozoic. The strontium isotopic composition may have been uniform from the Ordovician through the Mississippian, with an average Sr 87 Sr 86 "> Sr87Sr86 of 0.7078. A subsequent decrease in this value into the Mesozoic is interrupted by two provisionally documented positive pulses in Sr 87 Sr 86 "> Sr87Sr86 —one in the Early Pennsylvanian and one in the Early Triassic. The lowest observed value (0.7068) occurred in Late Jurassic time, and this was followed by a gradual increase to 0.7075 in the Late Cretaceous and a more rapid increase through the Tertiary to 0.7090 for modern sea water. These variations are thought to be the result of a complex interplay of periods of intense volcanism and epeirogenic movements of the continents on a worldwide scale.

Geochimica et Cosmochimica Acta↗
USGS1970

Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota

Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.

Minnesota↗
USGS1969

Primitive and contaminated basalts from the Southern Rocky Mountains, U.S.A

Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K 2 O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K 2 O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K 2 O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts. Contamination causes lead isotope ratios, 206 Pb/ 204 Pb and 208 Pb/ 204 Pb, to become less radiogenic, but it has little or no effect on 87 Sr/ 86 Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks. Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for 206 Pb/ 204 Pb and 208 Pb/ 204 Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios. The primitive basalts have: 206 Pb/ 204 Pb ∼ 18.09–18.34, 207 Pb/ 204 Pb ∼ 15.5, 208 Pb/ 204 Pb ∼ 37.6–37.9, 87 Sr/ 86 Sr ∼ 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of 206 Pb/ 204 Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas 208 Pb/ 204 Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. 87 Sr/ 86 Sr appears to be equal to or a little greater than those of the oceanic localities. These 206 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios are distinctly less radiogenic and 87 Sr/ 86 Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.

Colorado↗
USGS1968

Lead and strontium isotope studies of the Boulder Batholith, Southwestern Montana

The isotopic composition of lead in feldspar varies widely from pluton to pluton of the Late Cretaceous Boulder batholith, encompassing the following ranges in isotopic values: 16.9-18.1 for Pb 206 /Pb 204 ; 15.4-15.7 for Pb 207 /Pb 204 ; and 37.7-38.5 for Pb 205 /Pb 204 . Although each pluton has a characteristic isotopic composition, the fact that Pb 206 /Pb 204 for the Butte Quartz Monzonite varies by 1.3 percent, compared to a range of about 8.6 percent for the batholith as a whole, suggests that perfect isotopic mixing of the magma was not always attained. Whole rock initial Sr 87 /Sr 88 for the entire batholith ranges from 0.705 to 0.710, comparable to ranges for the Sierra Nevada and British Columbia batholiths. Ore leads of the Butte district are isotopically very similar to feldspar leads of the host Butte Quartz Monzonite. This isotopic similarity was interpreted by Murthy and Patterson (1961a) to indicate complete mixing of independently derived feldspar lead and ore lead rather than close genetic relationship. However, detailed study of the only sample in the present investigation, from the Donald pluton, that clearly has mixed leads shows significant differences in lead isotopic composition between megacryst K-feldspar and both the groundmass K-feldspar and plagioclase, indicating that isotopic mixing of leads is not complete, even within a single hand specimen. Spatial relations of isotopic differences among the various plutons strongly suggest that complete mixing of leads of different isotopic composition from the magma did not occur in the Butte Quartz Monzonite nor in any other pluton of the batholith, and that the isotopic similarity between Butte ore and feldspar lead of the host rock indeed stems from a genetic association. Lead and strontium isotope ratios generally behave in a similar way; that is, the more radiogenic the lead in a rock, the more radiogenic the strontium. The source of the lead which could produce the isotope variations observed for the batholith is calculated to have a mean age of about 2,200 m.y., which is compatible with ages of Precambrian crystalline rocks in the region, and an isotopic makeup with U 238 /pb 204 <9, Th/U > 4, and Rb/Sr of 0.03-0.09. However, no large volumes of prebatholith rocks exposed in the Boulder batholith region are of the required isotopic composition, which is typical of basaltic-gabbroic or quartz dioritic chemical composition. Models involving complete melting of possible source materials to account for the isotope variations are considered and rejected. Mechanisms involving par.tial melting of lower crustal or upper mantle source material appear to be the most viable in explaining the observed isotopic compositions. Assimilation of upper crustal material (i.e., the Precambrian Belt and pre-Belt rocks, in which the lead and strontium are more radiogenic than in the batholith rocks) may have accompanied partial melting of the lower crust or upper mantle and indeed must have played a major role if the source material had a composition isotopically comparable to that observed for oceanic tholelites (i.e., low Th/U, U 238 /Pb 204 and Rb/Sr), but would be relatively unimportant if the source material were sufficiently radiogenic to begin with as might be expected from basalt, gabbro, or quartz diorite compositions 2,200 m.y. old. © 1968 Society of Economic Geologists, Inc.

Montana↗
USGS1967

87Sr/86Sr ratios in some eugeosynclinal sedimentary rocks and their bearing on the origin of granitic magma in orogenic belts

Rb and Sr contents and 87 Sr/ 86 Sr values were determined for samples of eugeosynclinal sedimentary rocks, mostly graywackes, from Oregon and California. These data are compatible with the theory of anataxis of eugeosynclinal sedimentary rocks in orogenic belts to produce granitic magmas provided that the melting occurs within several hundreds of m.y. after sedimentation. The low ( 87 Sr/ 86 Sr) 0 values of the eugeosynclinal sedimentary rocks are related to the significant amounts of volcanogenic detritus present which probably were originally derived from the mantle.

Earth and Planetary Science Letters↗
USGS1966

Preliminary investigation of the source of lead and strontium in deep geothermal brines underlying the Salton Sea geothermal area

A radiogenic tracer study has been made of lead and strontium in deep geothermal brines , salt from brine , and the upper Quaternary rhyolite domes near the Salton Sea in the Imperial Valley, California. The data on the brines and rhyolite glasses are compared to those from Quaternary Colorado River and Tertiary sediments of the area . The brines sampled contain '-'100 ppm Pb, sufficient for consideration as a possible ore-forming fluid. The isotopic data indicate that 80-100 percent of the strontium and 50-100 percent of the lead in these brines were acquired from the associated sediments. The isotopic compositions of lead and strontium in the Quaternary rhyolite glasses are distinctly different from those of the sediments and brines . The strontium data suggest that the rhyolite glasses were derived from basaltic or gabbroic material but may have attained as much as 40 percent of their strontium from upper crustal rocks such as the sediments analyzed.

California↗
USGS1966

Geochronology of the St. Kevin granite and neighboring precambrian rocks, northern Sawatch Range, Colorado

Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths ( St . Kevin Granite and granite of Cross Creek), and various minor intrusive rocks . A whole- rock Rb-Sr isochron age on the St . Kevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the St . Kevin and numerous other rocks are either about the same as the St . Kevin whole- rock age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor Precambrian intrusive masses that are probably younger than St . Kevin Granite yield mica ages within analytical error of the St . Kevin age, indicating that these rocks can be younger than the granite by only a few tens of millions of years. The mica ages, both Rb-Sr and K-Ar, are thought to be minimal, but a K-Ar age of 2020 m.y. on horn-blende probably reflects excess argon. Mica ages from all rocks known geologically to be older than St . Kevin Granite are low and are interpreted as heating ages reflecting intrusion of the granite , in some cases modified further by heating during Laramide time. In this area, Precambrian intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the St . Kevin Granite and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from Colorado although weak regional metamorphism may also have been a factor.

Coloradao↗
USGS1965

Potassium, rubidium, strontium, thorium, uranium, and the ratio of strontium-87 to strontium-86 in oceanic tholeiitic basalt

The average concentrations of potassium, rubidium, strontium, thorium, and uranium in oceanic tholeiitic basalt are (in parts per million) K, 1400; Rb, 1.2; Sr, 120; Th, 0.2; and U, 0.1. The ratio Sr 87 to Sr 86 is about 0.702, that of K to U is 1.4 x 10 4 , and of Th to U is 1.8. These amounts of K, Th, U, and radiogenic Sr 87 are less than in other common igneous rocks. The ratios of Th to U and Sr 87 to Sr 86 suggest that the source region of the oceanic tholeiites was differentiated from the original mantle material some time in the geologic past.

Science↗
USGS1963

Radiogenic strontium-87 as an index of geologic processes

The abundance of radiogenic Sr 87 relative to Sr 86 at the time of crystallization has been determined for 45 rocks. The total range in the ratio Sr 87 /Sr 86 is less than 2 percent. Ratios for recent lavas range from 0.702 to 0.711. Oceanic basalts are closely grouped at 0.703, whereas ratios for continental volcanic rocks spread from 0.702 to 0.711. Among the volcanic rocks, ranging from basalt to rhyolite, no correlation was found between original ratio and rock type. Older mafic and felsic rocks that include both plutonic and extrusive types also cover this same range in original Sr 87 /Sr 86 ratios; however, there is a definite trend with geologic time. Precambrian rocks give values as low as 0.700. The data indicate that Sr 87 /Sr 86 of the weathering crust has changed 1.1 percent in 3000 million years, while the ratio in the mantle has changed no more than 0.5 percent.

Science↗