Albert M. Kudo

An igneous plagioclase thermometer

An empirical approach has been taken to develop a geothermometer based on plagioclase-magmatic liquid equilibrium. Compositions of coexisting plagioclase and liquid (glass) obtained by electron microprobe analysis of quenched samples from equilibrium melting experiments of natural granitic rocks at water pressures of 0.5 and 1.0 kilobars have been used along with data from the equilibrium experiments of Bowen (1913, 1915), Prince (1943) and Yoder et al. (1957) to calibrate this geothermometer. Applications of this geothermometer to natural occurrences demonstrate that it can provide useful information on temperature of equilibration of coexisting plagioclase and liquid in rocks ranging in composition from basalt to rhyolite. The plagioclase geothermometer is in good general agreement with other geothermometers wherever these are applicable. Where temperatures are known from other sources it can be used to predict the equilibrium compositions of plagioclase in magmas as well as to provide a rough estimate of water pressure.

Chemical and strontium isotopic investigations of ultramafic inclusions and basalt, Bandera Crater, New Mexico

Abstract A new locality for lherzolite inclusions in basalt has been found in a cinder eruption of Bandera Crater, New Mexico. Red-spinel lherzolites had 87 Sr 88 Sr ratios of 0.7045–0.7055, green-spinel lherzolites 0.7023–0.7040 and basalt 0.7028–0.7034. The red spinel lherzolites apparently have no genetic relationship to the basalt.

Sr isotopic disequilibrium in lherzolites from the Puerco necks, New Mexico

Abstract Sr isotopic ratios in coexisting minerals in a lherzolite inclusion from the Puerco necks, 60 miles west of Albuquerque, New Mexico, are significantly different. Petrographic and chemical data indicate that these inclusions are typical of other localities except that the composition of each mineral in the ultramafic inclusion appears to be independent of the mode and is homogeneous among dunites, lherzolites and websterites. The lherzolites have 87 Sr/ 86 Sr ratios higher than their host Pliocene basalts. It seems likely that the lherzolites are fragments of a part of the mantle which was not involved in any significant thermal event which could have restored isotopic equilibrium during Mesozoic time. Carters partial fusion-partial crystallization model for the origin of ultramafic inclusions is unacceptable for the Puerco inclusions.

Xenoliths in recent basaltic andesite flows from Arenal Volcano, Costa Rica: inference on the composition of the lower crust

Basaltic andesite flows erupted between 1973 and 1980 from Arenal Volcano contain abundant inclusions of anorthosite, olivine gabbro, and pyroxenites, and megacrysts of olivine and anorthite. The anorthosites with large (20 mm) anorthite grains (An96-92) exhibit deformation twinning and granulation between grain boundaries. Some olivine gabbros have angular clasts of anorthite with bent twins, pyroxene, and olivine in a finer-grained matrix which is distinctly foliated. These textural features suggest that these inclusions were deformed. An exotic (xenolithic) origin is supported in part by the mineral compositions and the estimated temperatures of equilibration: a temperature of about 975° C is obtained by two-pyroxene and Fe-Ti oxide geothermometers for the gabbros, but two-pyroxene temperatures are higher (1064 to 1120° C) for the basaltic andesite host. The olivine gabbro is thought to have crystallized at a pressure between 8.5 and 9.5 kb; whereas the lava phenocrysts crystallized at a much lower pressure of less than 5 kb. These xenoliths probably represent fragments of the lower crust below Arenal volcano. The lava flows show evidence for some contamination especially from fragments of anorthite broken apart from the larger megacrysts and xenoliths. A few phenocrysts of plagioclase in the lava samples have deformation twins. The unusually high Al2O3 content (19.4 to 23.2 wt%) of the lava samples can be attributed directly to the addition of anorthite; in fact the observed chemical variation in the lava flows (the increasing alumina and lime contents with decreasing silica) can be explained by this contamination.

Assimilation-fractional crystallization of Polvadera Group rocks in the northwestern Jemez Volcanic Field, New Mexico

Pliocene Polvadera Group rocks in the northwestern Rio Grande rift-marginal portion of the Jemez Volcanic Field record the rapid transition from weakly alkaline Lobato Basalt magmatism (48–52% SiO2; 7.9 Ma) through calc-alkaline Lobato andesite and dacite (53–64% SiO2) and Tschicoma dacite-rhyodacite magmatism (63–69% SiO2; 7.4 Ma). Petrologically, Lobato andesite and dacite and Tschicoma dacite-rhyodacite represent a cogenetic suite of differentiates (the La Grulla Plateau or LGP suite) distinctive from the bulk of Polvadera Group rocks including Tschicoma andesite. Increasing (87Sr/86Sr)O ratios with differentiation within the LGP suite from 0.7051 (54% SiO2) to 0.7064 (68% SiO2), trace element variations, and disequilibrium mineral assemblages suggest open system differentiation involving 87Sr-enriched upper crust. A likely parental magma is the voluminous Lobato Basalt ((87Sr/86)O= 0.7043–0.7050) which was erupted predominantly earlier and to the east toward the rift axis. The best model for petrogenesis involves bulk assimilation of locally wide-spread Proterozoic (1.4–1.6 Ga) upper crustal granite by fractionally crystallizing Lobato Basalt. Assimilation-fractional crystallization (AFC) modeling of Sr-isotope and trace element variation (DePaolo 1981) indicates that ∼40% crystallization of Lobato Basalt accompanied by ∼10% addition of granite reproduces the observed geochemical and Sr-isotopic diversity. Neither magma mixing, nor mafic recharge have complicated the magmatic evolution of the LGP suite. Crustal thickness and/or retarded tectonism could have facilitated conditions necessary for evolution by AFC to occur within the upper crust.

The petrology of Poás volcano lavas: basalt-andesite relationship and their petrogenesis within the magmatic arc of Costa Rica

Abstract Basalt, andesite and dacite from Poas volcano exhibit a distinct petrogenetic evolution. The basalts (Al 2 O 3 = 16–17.4%, K 2 O = 1.43–1.61%) are hypersthene-normative tholeiites with phenocrysts of labradorite, olivine, and two pyroxenes. Orthopyroxene (En 67-56 ) is generally rimmed by clinopyroxene (Fs 10–18 Wo 39–48 ). In all basalts and in a few andesites, pyroxene phenocrysts have Al- and Ti-rich salite rims that surround cores of orthopyroxene or augite having lower values. The Al z values (% of tetrahedral site occupied by Al) for these augites are above 6%, thus defining on the Al z vs. TiO 2 diagram an intermediate trend between arc-related cumulates and rift-related tholeiites and ophiolite-layered cumulates. Moreover, the presence of olivines with coronae of wormy intergrowths of orthopyroxene and Ti-magnetite (similar to those occurring in metagabbros of the granulite facies and characteristic of Arenal gabbroic xenoliths) together with anorthite megacrysts (An 94-91 ) suggest a possible contamination origin for these basalts. Sr-isotope ratios for the basalts are between 0.703816 and 0.703849 whereas andesites and dacites have lower ratios (0.703676–0.703715): these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust. Pearce element-ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase.

The origin of Pliocene-Holocene basalts of New Mexico in the light of strontium-isotopic and major-element abundances

Strontium isotope ratios of nepheline-normative basalts of New Mexico are similar to those of oceanic basalts and these undersaturated basalts have probably originated deep in the mantle under all structural provinces in the state. The hypersthene-normative basalts have highly variable Sr-isotope ratios (0.7029–0.7078) indicating either an origin in mantle enriched in radiogenic Sr or crustal contamination which is likely for one flow with variable87Sr/86Sr ratios. The most acceptable hypothesis for the origin of the hypersthene-normative basalts requires that they be formed primitively in the mantle at a shallow depth with a Sr content low enough to reflect a change in the Sr ratios by small additions of crustal material of higher Sr-isotope ratios.

Igneous origin of the orbicular rocks of the Sandia Mountains, New Mexico: Summary

This paper presents new data obtained from the field, 34 whole-rock silicate analyses, and age dates, in order to provide evidence for a late-stage, water-enriched, igneous origin for the orbicular rocks of the Sandia Mountains in New Mexico. A recent paper by Thompson and Giles (1974) proposed a metasomatic origin for these orbicules which resulted from reaction of the Sandia granite with a large biotite monzonite-diorite xenolith (see p. 912). Our data, collected since 1972 without knowledge of the work of Thompson and Giles, indicate that the biotite monzonite is not a xenolith but rather a product of a magmatic event during the late stages of the Sandia granite formation. In 197.1 (abs.), Daugherty and Asquith suggested an igneous origin in which the orbicules were formed by alternating precipitation of biotite and plagioclase in a convecting magma, one phase being precipitated in a hotter region and the other in a cooler part of the magma. Phase relationships can be used to argue against this theory since, two different phases could not be precipitating in different parts of a magma simultaneously at different temperatures. During the circulation of an orbicule from a cooler to a hotter (or vice versa) region of a magma, a cotectic boundary must be crossed, which is impossible to do. Kelley and Northrop (1975) in their memoir on the Sandia Mountains reviewed briefly the occurrence of the orbicular rocks and, without new data, concluded that a metasomatic origin as proposed by Thompson, and Giles (1974) was unacceptable and that an early magmatic origin was most likely. Using zircon distribution patterns, Held and Harris (1978, abs.) interpreted an igneous origin from one specimen of the Sandia orbicular rock. We believe that our new data will help resolve this controversy.

Petrology and geochemistry of the Paliza Canyon Formation and the Bearhead Rhyolite, Keres Group, Jemez Mountains, New Mexico

Basaltic andesite, andesite, dacite, and rhyodacite of the Paliza Canyon Formation (9.1 to 8.5 m.y.) and the Bearhead Rhyolite (7.1 to 6.5 m.y.) belong to a high-K, calc-alkaline association in the southern Jemez Mountains, New Mexico. The majority of the rocks of the Paliza Canyon Formation contain augite, hypersthene, and plagioclase phenocrysts. Olivine is an important phenocryst in the basaltic andesite, but it is absent in the other rock types. In the rhyodacite, hornblende is a major phenocryst at the expense of the pyroxenes. Characteristically, a very restricted range in the Fe/Mg composition is found for the pyroxenes (augite from 0.33–0.29) and hornblende (0.41–0.47) throughout the whole range of rock types, from basaltic andesite (∼56% SiO 2 ) to rhyodacite (67% SiO 2 ). On the other hand, the Bearhead Rhyolite lacks pyroxene, and it has two feldspars (sanidine and plagioclase) and bipyramidal quartz and biotite. Its SiO 2 content ranges between 76.5% and 77.3%. The Bearhead Rhyolite lies on the two-feldspar surface, or below it, in the sanidine field; the Paliza Canyon rocks plot within the plagioclase volume of the granite system. Both major- and trace-element trends in the Paliza Canyon Formation do not project to the Bearhead Rhyolite composition. Major- and trace-element modeling suggests that the sequence from basaltic andesite to rhyodacite in the Paliza Canyon Formation can be formed by simple fractional crystallization. The Bearhead Rhyolite cannot be formed from the Paliza Canyon rhyodacite by this process, however. Removal of the observed phenocrysts found in the rhyodacite would form a rhyolite enriched in Ba, Hf, Th, U, and light rare earth elements (LREE) and depleted in V and Sc; the Bearhead Rhyolite, however, is depleted in the former group and enriched in the latter. These conclusions are supported by initial 87 Sr/ 86 Sr ratios: three samples of Bearhead Rhyolite have initial ratios of 0.7056, 0.7073, and 0.7076; two samples of Paliza Canyon basaltic andesite, on the other hand, have ratios of 0.7041 and 0.7044. The most likely source for the Bearhead Rhyolite is lower crustal material which may have been fused partially from the heat provided by the andesitic magmas of the Paliza Canyon Formation.

Dacites of Bunsen Peak, the Birch Hills, and the Washakie Needles, northwestern Wyoming, and their relationship to the Absaroka volcanic field, Wyoming and Montana

Three porphyritic dacite plugs from the western Absaroka volcanic belt in northwestern Wyoming have been studied, and their petrography, chemistry, and ages are treated in terms of the regional igneous geology of the Absaroka volcanic field. The dacite from the northernmost plug studied, Bunsen Peak, is 47.6 ± 1.9 m.y. old, as dated by the fission-track method on apatite; the Birch Hills dacite is 40.5 ± 2.6 m.y. old, as dated by the fission-track method on apatite; and the dacite from the southernmost plug, Washakie Needles, is 38.8 ± 1.6 m.y. old, as dated by the fission-track method on sphene. It appears from the dates available that the oldest igneous activity in the Absaroka volcanic field occurred at the northwestern end about 53.5 m.y. ago. The activity migrated to the southeast, ending about 38.8 m.y. ago at the Washakie Needles. The Absaroka volcanic field has been subdivided into two belts. The western belt is composed of normal calc-alkalic igneous rocks, and the eastern belt is composed of potassium-rich rocks. When the available analyses of the province are treated in terms of the system quartz-plagioclase-orthoclase, it becomes apparent that the rocks of the two belts lie on two distinct differentiation trends. The trend for rocks of the western belt is best explained by fractional crystallization of plagioclase from an intermediate magma. The trend for rocks of the eastern belt is best explained by crystallization of both plagioclase and potassium feldspars. The mafic members of the eastern belt rocks are similar to shoshonitic rocks.