Strontium Isotope Study Research Articles

Evidence for mantle metasomatism: an oxygen and strontium isotope study of the Vulsinian District, Central Italy

This paper presents new O and Sr isotope data for lavas from the northern part of the Roman perpotassic province. The samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the Vulsinian District, the olivine leucite melilitite of San Venanzo, and the kalsilite diopside melilitite of Cupaello. Previous oxygen isotope work on the lavas of the Vulsinian District suggested crustal contamination of “normal” mantle-derived magmas. The new data cover the ranges previously found. O and Sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca. 10% of continental crustal material. The saturated lavas probably assimilated large amounts (up to ca. 50%) of crust. Lavas chemically identified as corresponding to little modified mantle-derived liquids are high in both 87Sr/ 86Sr andδ 18O: 0.7103−0.7107, +7.8 to +9.4 (Vulsini), 0.7104, +12.3 (San Venanzo) and 0.7112, +14.4 (Cupaello). These high values are interpreted to have been inherited from a metasomatized parental mantle. Hydrous fluids enriched in large-ion lithophile elements and high inδ 18O and 87Sr/ 86Sr are thought to have mixed with mantle of “normal”δ 18O and 87Sr/ 86Sr. The fluids probably origi dehydration of continent-derived sediments, which were subducted beneath a mantle wedge in the continent-continent collision of the Corsica-Sardinia block and the Adriatic (Italian) plate. This hypothesis is supported by Pb and Nd isotopic evidence and is probably valid for the entire Roman Province.

Geochemical and Strontium-Isotope Studies of Reactions between Basic Magma, Chalk, and Flint, and the Role of Groundwater, in the Carneal Plug, Co. Antrim, Northern Ireland

Geochemical and Strontium-Isotope Studies of Reactions between Basic Magma, Chalk, and Flint, and the Role of Groundwater, in the Carneal Plug, Co. Antrim, Northern Ireland

The rainbow range, British Columbia: A miocene peralkaline shield volcano

The Rainbow Range is a Late Miocene shield volcano (30 km diameter, 370 km 3) whose stratiform flanks surround a complex central zone. Over a 2-m.y. interval, extrusion of low viscosity, silicic peralkaline lavas and minor basaltic lavas built up the gently sloping (5–8°) flanks, forming a shield volcano rather than a composite cone. Comenditic trachytes are the lowest flows exposed on the north flank of the volcano. Thin mugearite flows rest unconformably on the comenditic trachytes. Comendites unconformably overlie the mugearites and account for at least 75% of the volume of flows within the flank zone. These lavas are distinguished from the comenditic trachytes by lower Al 2O 3 (13%), higher total iron as Fe 2O 3, (7%), and extremely depleted Sr (1–10 ppm) and Ba (10–100 ppm). Strontium isotopic studies combined with petrologic data suggest that Rainbow Range lavas originated from alkali basalt magma trapped in an intracrustal magma chamber and tapped at several intervals after it underwent crystal fractionation. A best-fit mathematical model for the origin of the suite involves step-wise derivation of the lavas in the order hawaiite → mugeartite → comenditic trachyte → comendite, with the major phases precipitating in the order: olivine, clinopyroxene, plagioclase, iron-titanium oxides, and alkali feldspar.

A neodymium and strontium isotopic study of the Mesozoic calc‐alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California

Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by εNd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have εNd = +8.0 to −6.4. In both batholiths, εNd correlates strongly with initial 87Sr/86Sr. Decreasing εNd values are accompanied by increasing 87Sr/86Sr and increasing δ18O; the correlation with δ18O being more pronounced for the Peninsular Ranges. The εNd values show regular geographic variations, as was found previously for initial 87Sr/86Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low εNd values (−11 to −16) and Precambrian model Sm‐Nd ages (1.5 to 1.9 Æ). The country rock εNd values, and those of primitive oceanic island arcs (εNd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O‐Sr isotopic relations and the variation of 147Sm/144Nd with εNd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal‐ and mantle‐derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8‐Æ crust, yielding an average crust formation age of nearly 1 Æ for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with εNd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5‐Æ Sm‐Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm‐Nd isotopic studies for aiding in construction of tectonic models.

Neodymium and strontium isotope study of ophiolite and orogenic lherzolite petrogenesis

Neodymium isotopic analyses have been measured on nine ophiolites and four orogenic lherzolites. ε Nd varies from +12 to +3 in the ophiolites and from +18 to +2 in the orogenic lherzolites. The majority of the analyses plot on a ε Nd-ε Sr correlation line as defined by Nd and Sr isotopic analyses of oceanic basalts. However, certain ophiolitic and lherzolitic samples exhibit high 87Sr/ 86Sr ratios and as such lie to the right of the correlation line towards seawater compositions. From these data one can postulate several origins for ophiolites including that of mid-ocean ridges and ocean islands. If the orogenic lherzolites are interpreted as representative of the mantle occurring below active ridges a more complex model is required involving mantle heterogeneity and multi-episodic chemical fractionation starting prior to 2 Ga ago.

A neodymium, strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust

In the Samail ophiolite, 147Sm- 143Nd, 87Rb- 87Sr, and 18O/ 16O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and 18O/ 16O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the 147Sm- 143Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial 143Nd/ 144Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ε Nd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The 87Sr/ 86Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (ε Sr = −19.7 to +30.5) and the δ 18O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the 18O/ 16O water-rock fractionation.

Isotopic composition of strontium in Indian kimberlites

Strontium isotopic studies on twenty three whole rock kimberlites from two petrographic provinces in India show variation of initial 87Sr 86Sr ratios from 0.7027 to 0.7102. The variation is unrelated to the degree of alteration. Between the micaceous and basaltic varieties there is some overlap in the Sr isotopic ratios. Leaching experiments on whole rock samples showed more highly radiogenic Sr in leaches compared to the bulk samples. In two diatremes, the initial 87Sr 86Sr ratios show a positive correlation with Rb Sr which is believed to reflect a source event earlier than the formation of the kimberlites. The observed Sr isotopic data can be caused by (i) melting of a heterogeneous source or (ii) disequilibrium partial melting in the source region. In the former case, variable isotopic composition would be a necessary consequence of melting in small subsystems.

Neodymium and Strontium Isotope Evidence for Crustal Contamination of Continental Volcanics

Combined neodymium and strontium isotope studies on Tertiary volcanics from northwest Scotland indicate that their parental mantle isotopic compositions have been substantially modified in many instances by contamination with the Precambrian continental crust through which they were erupted. The occurrence of samarium-neodymium and rubidium-strontium "pseudoisochrons" of different ages in these contaminated continental volcanics indicates that they are artifacts of the contamination processes and have no temporal significance with respect to mantle fractionation events.

A study of strontium isotopes in lakes and surficial deposits of the ice-free valleys, southern Victoria Land, Antarctica

The ice-free valleys of southern Victoria Land, Antarctica, contain saline lakes including Lake Vanda in Wright Valley and Lake Bonney in Taylor Valley. The source of the salts dissolved in the brines has not yet been identified. Strontium in water-soluble salts of soil samples in Wright Valley has 87 Sr 86 Sr ratios ranging from 0.7119 to 0.7157 with an average of 0.7144 ± 0.0008 (1 σ). This value is very similar to the 87 Sr 86 Sr ratio of brines in Lake Vanda ( 87 Sr 86 Sr = 0.7149 ± 0.00017 ), but differs significantly from the 87 Sr 86 Sr ratios of seawater (0.7094 ± 0.00012) and basaltic rocks of the McMurdo Volcanics (0.7044 ± 0.00046). The Sr in Lake Vanda therefore could not have originated from seawater or from volcanic rocks of the area, but may have been derived by chemical weathering of the igneous and metamorphic rocks exposed in Wright Valley. The 87 Sr 86 Sr ratios of Lake Bonney are invariant with depth and average 0.7130 ± 0.00014 (1 σ), whereas the Sr concentrations increase from 0.7345 ppm near the surface to a maximum of 39.92 ppm at a depth of 25 m. The Sr concentrations at greater depth decrease slightly to 35.88 ppm at 30 m. The Sr in water-soluble soil salts near Lake Bonney and of the Taylor Red Cone is similar isotopically to the Sr in the lake. The 87 Sr 86 Sr ratios of soil salts and of meltwater in Taylor Valley decrease systematically from Lake Bonney toward the coast. The water in Lake Fryxell, which is closest to the coast, has an 87 Sr 86 Sr ratio of 0.7090 and is identical to that of seawater. The evidence from this and similar studies in other parts of the world indicates that the 87 Sr 86 Sr ratios of saline brines in closed continental basins are representative of the Sr in the rocks underlying the basin. The Sr in carbonate and sulfate minerals precipitated from such brines preserves the 87 Sr 86 Sr ratios that existed at the time of deposition. Because of these relationships we suggest that stratigraphic variations of the 87 Sr 86 Sr ratios of non-marine carbonate or sulfate rocks reflect changes in the geology of the drainage basin. The isotopic composition of Sr of such rocks may therefore provide useful information about the geologic histories of continental basins.

Lead and Strontium Isotopic Study of Igneous Rocks and Ores, Gold Hill District, Utah

A multistage model is presented to account for the different lead isotopic compositons of the northern and southern parts of the Gold Hill stock. The stock was found actually to be two bodies of different ages - Oligocene in the north (38 m. y.) and Jurassic in the south (152 m.y.). Initial 87 Sri 86 Sr ratios computed from measurements made on the intrusive rocks are 0.715 ± 0.001 for the north and 0.709 ± 0.001 for the south. Lead in the northern intrusion at Gold Hill and 100 km eastwards in the Oquirrh Mountains region is shown by the model to have originated in an Archean upper crustal environment (determined 11 =238U1204Pb ;::;::: 12) from 2.7 b.y. to 1.65 b.y. ago. Subsequently, this lead was incorporated in 1.65 b.y. lower crust and subjected to granulite or higher rank metamorphism (determined 11 ;::;::: 4 to 7), and remained there until being remobilized in the Oligocene by plutonic activity. The ore and feldspar lead in the southern intrusion at Gold Hill, however, seems to have remained in upper crustal environments (11 ;::;::: 12) continuously since 2.7 b.y. ago (e.g., there was no high rank metamorphism of this material in the Precambrian) until being remobilized in Jurassic time. By implication, this must apply to much of the miogeosynclinal material of similar lead isotopic composition found westwards in Nevada.

A strontium isotopic study of the volcanic rocks of the Myoko volcano group, central Japan

Thirty-one selected volcanic rocks from the Myoko volcano group which comprises a volcanic chain of four independent volcanoes of Quaternary to Recent age are analyzed for 87Sr/86Sr ratios. The rocks of the lizuna volcano, the oldest among the Myoko volcano group, have higher 87Sr/86Sr ratios and show a larger scatter ranging from 0.70437 to 0.70556 than those of other volcanoes. The Kurohime volcanic rocks have a restricted range of 87Sr/86Sr ratios (0.70403∼0.70435). 87Sr/86Sr ratios of the Myoko volcanic rocks are almost the same in average to those of the Kurohime volcanic rocks, although somewhat varied ranging from 0.70378 to 0.70461. A single analysis of the Yakeyama volcanic rock yielded a 87Sr/86Sr ratio of 0.70427. A characteristic pattern in 87Sr/86Sr ratios is observed through the volcanic activity of the Myoko volcano group; 87Sr/86Sr ratios are high in the early stage of the volcanic activity and then decrease to low values, the late eruptives being characterized by constant 87Sr/86Sr ratios. The negative correlation between 87Sr/86Sr and Rb/Sr, and positive correlation between 87Sr/86Sr and Sr found in the rocks of the Iizuna volcano are interpreted to show the occurrence of contamination by materials with high 87Sr/86Sr ratios (>0.7056), low Rb/Sr ratios ( 300 ppm). Sialic crustal contamination may have played only a minor role.

Strontium isotope study of the composite batholith between Princeton and Okanagan Lake

Twenty-seven whole-rock samples of granitic rocks and associated mafic inclusions define a Rb–Sr ispchron of 156 ± 6 m.y. with an initial ratio of 0.70415 ± 0.00017 (95% confidence limits). K–Ar and Rb–Sr data for granodiorite, quartz monzonite, and granite are concordant with this Upper Jurassic age. K–Ar dates suggest that diorite and quartz diorite are distinctly older, 165 to 185 m.y. old. The Rb–Sr data for these more mafic rocks alone (166 ± 53 m.y.) is not accurate enough to confirm this, but is not in conflict. Country rock samples are generally similar to the batholith in Sr-isotopic composition, and nearly all mafic inclusions are equilibrated with their enclosing granitic host. The Cordilleran transition zone between relatively low initial Sr isotope ratios (0.704 and less) and relatively high ratios (0.706 and greater) appears to lie just east of the batholith studied.

Age and origin of the Stone Mountain Granite, Lithonia district, Georgia

Research Article| July 01, 1976 Age and origin of the Stone Mountain Granite, Lithonia district, Georgia JAMES A. WHITNEY; JAMES A. WHITNEY 1Department of Geology, University of Georgia, Athens, Georgia 30602 Search for other works by this author on: GSW Google Scholar LOIS M. JONES; LOIS M. JONES 1Department of Geology, University of Georgia, Athens, Georgia 30602 Search for other works by this author on: GSW Google Scholar RAYMOND L. WALKER RAYMOND L. WALKER 2Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Search for other works by this author on: GSW Google Scholar Author and Article Information JAMES A. WHITNEY 1Department of Geology, University of Georgia, Athens, Georgia 30602 LOIS M. JONES 1Department of Geology, University of Georgia, Athens, Georgia 30602 RAYMOND L. WALKER 2Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1976) 87 (7): 1067–1077. https://doi.org/10.1130/0016-7606(1976)87<1067:AAOOTS>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES A. WHITNEY, LOIS M. JONES, RAYMOND L. WALKER; Age and origin of the Stone Mountain Granite, Lithonia district, Georgia. GSA Bulletin 1976;; 87 (7): 1067–1077. doi: https://doi.org/10.1130/0016-7606(1976)87<1067:AAOOTS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Petrographic, geochemical, and strontium isotopic studies have been conducted on samples of the Stone Mountain Granite (leucocratic quartz monzonite) from the Lithonia district, within the Inner Piedmont province of Georgia. A Rb-Sr isochron obtained from 10 whole-rock and 3 mineral samples yielded an age of 291 ± 7 m.y. and an initial Sr87Sr86 ratio of 0.7250 ± 0.0005. Petrologic and geochemical data, combined with the high initial strontium isotopic ratio, suggest that the origin of this peraluminous quartz monzonite can best be explained by the anatexis of an older peraluminous, granitic crustal material during Late Pennsylvanian time. The depth of intrusion was probably around 12 km, with initial magma temperatures of 700°C or less. Magma generation could have occurred at depths of 22 to 28 km, depending on the regional geothermal gradient at the time. The most likely source material is the Lithonia Gneiss, which has a peraluminous, granitic composition and underlies the area.The Stone Mountain pluton extends the 300-m.y.-old intrusive belt in the southern Appalachians across the Piedmont province to just south of the Brevard lineament. Its implied depth of intrusion fits well with certain models for the development of the Inner Piedmont and Blue Ridge provinces in the Carolinas because it requires rapid removal of considerable overburden. The origin and magmatic history of this intrusive unit may find use as a model for other peraluminous granitic plutons having similar petrologic, geochemical, and isotopic characteristics. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Strontium isotopic studies of the volcanic rocks of the Saunda arc, Indonesia, and their petrogenetic implications

Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low 87Sr 86Sr ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an 87Sr 86Sr ratio of 0.7139. The relationship between 87Sr 86Sr and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in 87Sr 86Sr despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in 87Sr 86Sr from West Java to Bali and there is some evidence for increasing 87Sr 86Sr with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher 87Sr 86Sr ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand). A model of 87Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high 87Sr 86Sr ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high 87Sr 86Sr ratio in the Lake Toba rhyolite implies a crustal origin.

K–Ar and Rb–Sr isotopic age studies on the Wolf Rock nosean phonolite, Cornwall

SummaryPotassium–argon and rubidium–strontium isotopic age studies of the Wolf Rock phonolite of SW England confirm its age as 131 Ma, with an initial87Sr/88Sr ratio of 0.703. The existence of a Mesozoic volcanic province of this age on the margin of the Labrador–Biscay rift is discussed with special reference to its implications for the early evolution of the N.Atlantic Ocean, and the general relationship of alkaline igneous activity to major periods of plate reorientation.

Lead and Strontium Isotopic Studies in the Cascade Mountains: Bearing on Andesite Genesis

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. New isotopic analyses of lead are reported for 40 volcanic rocks from the Cascade Mountains of Washington, Oregon, and California. Strontium isotopic compositions were also determined in 33 volcanic rocks from the same area. In addition, lead and strontium isotopic data are given for feldspar and whole-rock samples of prominent varieties of crystalline basement rocks from northern Washington. The Sr87/Sr86 values of the volcanic rocks average 0.7037, with no significant difference between andesite and high-alumina basalt. The ratios exhibit no measurable correlation with strontium concentrations over a range of 200 to 1,500 ppm. Strontium in the Cascade rocks is slightly more radiogenic than that in oceanic ridge basalts, but less radiogenic than in most continental basalt, including the Columbia River basalt. Comparisons with published data for strontium in Pacific Ocean sediments and coastal graywackes in Oregon indicate much higher Sr87/Sr86 values in the sedimentary rocks when compared to the Cascade volcanic rocks, thus ruling out anatectic models involving large sediment contributions. Lead isotopic compositions from the volcanic rocks are variable but tend to be rather constant at a single volcanic center. A noteworthy feature of the lead data are the higher Pb207/Pb204 values (or larger µ values in a model age diagram) for the Cascade volcanic rocks compared to published Pb207/Pb204 values and µ values for Pacific Ocean tholeiitic basalt. The Pb206/Pb204 values from the Cascade volcanic rocks are higher than in oceanic sediments from the northeast Pacific Ocean but are lower than the average ratio for three samples of Oregon coastal sedimentary rocks. There is no demonstrable difference between lead isotopes in high-alumina basalt and andesite at particular volcanos, although the lead data are more variable than the strontium isotope data. Strontium in crystalline basement rocks from northern Washington is substantially more radiogenic than that in the calcic and calc-alkalic volcanic rocks of the Cascade Mountains. The same is true of lead in some, though not all, of the basement rocks. The isotopic data are utilized, along with other trace-element data to test models of andesite genesis. The isotopic compositions of lead and strontium in andesite and high-alumina basalt from the Cascade Mountains are consistent with derivation of the two lava types from a common source, or with derivation of andesite by differentiation of high-alumina basalt magma. The data do not support models in which the Cascade calcic and calc-alkalic magmas were formed by melting of Oregon coastal eugeosynclinal sedimentary rocks, or by anatexis of basement rocks similar to those now exposed near volcanic centers in northern Washington. Various subduction zone melting models are also considered. Mixing models, whereby radiogenic strontium and lead are added to magmas containing lead and strontium with isotopic compositions similar to those in oceanic ridge tholeiite, do not fit the observed data satisfactorily because values of Pb206/Pb204 in Pacific Ocean sediments are too low to account for the Pb206/Pb204 values in the Cascade lavas. Mixtures of eugeosynclinal sedimentary rock and oceanic ridge tholeiite can probably account for the Cascade volcanic rock isotopic data in a general way, but the uniform isotopic composition of strontium in the lavas over a wide range of concentrations is hard to understand in terms of such a model. The same is true for strontium in the oceanic sediment mixing model. The uniform isotopic composition of strontium in the Cascade lavas, irrespective of strontium concentration and geologic setting, argues against contamination of magma with radiogenic strontium during ascent. A similar argument can be made for lead. The best explanation of the isotopic and trace-element data appears to be a multistage model in which orogenic andesite is derived from three or more stages of partial melting of mantle material in which crustal materials play an insignificant role. This model best explains the uniform Sr87/Sr86 values in the lavas over a wide range of strontium concentrations. It can also explain why strontium in lavas extruded at continental margins, such as in the Cascade Mountains and in Japan, has the same isotopic composition as strontium in lavas from the Mariana arc system in the Pacific Ocean. Pb206/Pb204 and Pb208/Pb204 values in Cascade lavas may show a correlation with crustal thickness, but this does not prove that crustal contamination is responsible for the trend. The lead and strontium data exhibit no correlation with the Quartz Diorite Line of Moore.

Distribution of rubidium and strontium in the potassium feldspars of two granite batholiths

The abundances of rubidium and strontium have been determined in potassium feldspar separates and some whole rock samples from two Precambrian batholiths, the San Isabel batholith of the Wet Mountains, Colorado, U.S.A., and the Buchanan massif (Lone Grove pluton) of central Texas, U.S.A. Absolute abundances of the two elements in the feldspars vary widely, but the Rb/Sr ratio is relatively constant within mapped zones or facies. Selective precipitation of potassium over rubidium into early-crystallizing potassium feldspar leaves late-stage magmatic fluids enriched in rubidium and thus causes the absolute rubidium contents and the Rb/Sr ratios of feldspars and whole rocks to be highly dependent on the activity of volatiles. Escape of volatile-rich, end-stage fluids is apparently responsible for decrease in Rb/Sr ratios toward later-crystallizing rocks. The preservation of widely different Rb/Sr ratios in minerals in different facies of a comagmatic rock assemblage provides, through later partial melting, one method of generating non-zero initial apparent isochrons in whole-rock strontium isotope studies of igneous suites.

Time differences within a calc-alkaline association

A Strontium isotope study of a high-K diorite complex at Yeoval, N.S.W. (Australia) points to clear time differences (40 m.y.) between the diorite suite (gabbro-diorite-grano-diorite) and surrounding granites. Such a time interval, in association with other data, precludes a genetic relationship between these rock groups whereas the data support a common origin for the diorite suite and associated andesitic volcanics. The low (0.705) initial Sr ratios for the diorite suite and volcanics are similar to those of other diorite and andesite occurrences.

Strontium isotope studies on ultramafic inclusions from Dreiser Weiher, Eifel, Germany

The isotopic composition of strontium, and rubidium and strontium concentrations in eight ultramafic inclusions and two host basalts from Dreiser Weiher, Eifel, Germany are presented. Comparison of the 87Sr/86Sr ratios of the inclusions with the basaltic hosts indicates that the lherzolite inclusions are exotic in nature but the wehrlite-clino-pyroxenites are probably cognate with the host rocks. An apparent age of 550 m.y. (±a large uncertainty) shown by the lherzolites is believed to reflect a mantle event.

The isotopic composition of strontium in some South African kimberlites

Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr87/Sr86 ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr87/Sr86 ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr87/Sr86 ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr87/Sr86 ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr87/Sr86 ratio or (ii) selective diffusion of material of high Sr87/Sr86 ratio into kimberlitic fluids.