Lois M. Jones

Sulfur- and strontium-isotopic geochemistry of celestite, barite and gypsum from the Mesozoic basins of northeastern Mexico☆

Abstract The Mesozoic sedimentary basin of northern Mexico contains large limestone-hosted stratiform “mantos” of celestite and barite that appear to have formed by carbonate replacement. We have measured the isotopic compositions of sulfur and strontium in these mantos in order to determine whether they formed by diagenetic processes and, if so, what fraction of the diagenetically released Sr is fixed in celestite deposits. S- and Sr-isotopic analyses of gypsum from layers in the limestone confirm its Cretaceous sedimentary origin. Isotopic compositions of most celestite and barite differ from values observed for the gypsum. Division of the deposits into two geologically and geochemically distinct groups shows that there is a general positive correlation between δ 34S and 87 Sr 86 Sr compositions of celestite and barite in the two systems, with celestite containing lighter S and less radiogenic Sr in both groups of deposits. Less radiogenic Sr in the celestite probably came from nearby Cretaceous limestones. More radiogenic Sr, which is found largely in the barite, probably came from basement-derived arkoses in the Cretaceous basin. Sulfur was derived from evaporites in one deposit group and from some other lighter source, perhaps coal or petroleum, in the other. These isotopic constraints, considered in the light of available solubility data, require that at least two distinct solutions existed and mixed locally during formation of the celestite and barite mantos. No more than 10% of the diagenetically-released Sr is concentrated in the celestite deposits.

Rubidium-strontium dating of ore deposits hosted by Rb-rich rocks, using calcite and other common Sr-bearing minerals

The authors have tested a Rb-Sr technique that permits ore deposits to be dated using common gangue minerals such as calcite and fluorite. The only conditions the deposit must meet are that (1) it have minerals with a low Rb/Sr ratio and (2) it be enclosed by wall rock with a high Rb/Sr ratio. Because hydrothermal minerals acquire a strontium-isotope composition that is usually similar to that of the wall rock, minerals with low Rb/Sr ratio should record and retain the isotopic composition that the wall rock had at the time of mineralization. The difference between that ratio and that of the wall rock at present is a function of time and the Rb/Sr composition of the wall rock. The technique was tested fusing fluorite and calcite from three deposits ranging in age from Tertiary to Precambrian. In all cases the age determined here closely resembles that obtained by conventional K-Ar and Rb-Sr dating methods. The precision, however, can be poor and depends chiefly on the strontium-isotope heterogeneity of the wall rock and its Rb/Sr enrichment. 36 references, 1 figure, 1 table.

Petrogenesis of the Kirkpatrick Basalt, Solo Nunatak, northern Victoria Land, Antarctica, based on isotopic compositions of strontium, oxygen and sulfur

Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial 87Sr/86Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ18O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial 87Sr/86Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial 87Sr/86Sr ratios and δ18O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ34S= -4.01 to +3.41‰ Variations in (δ34S probably resulted from outgassing of SO2 under varying oxygen fugacities.

Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt, north Victoria Land, Antarctica: implications for low-temperature alteration

Chemical and isotopic (Sr, O, H) variations have been examined in an iron-rich lava flow of the Kirkpatrick Basalt from the Mesa Range in north Victoria Land, Antarctica. The flow is homogeneous with respect to the less mobile elements, whereas variations observed in K, Na, Si, Fe, and Rb result largely from alteration of glassy matrix material. Whole-rock Rb−Sr isotope data fall along a poorly-defined 103 Ma array attributed to secondary mobilization of Rb during the mid-Cretaceous. Alteration at that time is suggested by paleomagnetic data and would also account for discordant K−Ar dates. Whole-rock δ18O values vary from +5.8 to +8.2‰ and a plagioclase separate has a δ18O value of +5.6‰, reflecting the original composition of the magma. The range of δ18O values for the whole-rock samples results from low-temperature alteration occurring primarily in the Jurassic and/or mid-Cretaceous. Whole-rock δD values (-201 to -243‰) are markedly depleted, approaching equilibrium with modern meteoric water. In light of these data, variable Sr and O isotopic ratios in the underlying sequence of flows, previously interpreted in terms of an assimilation-fractionation model, may largely reflect post-magmatic alteration.

The Sr isotopic composition of early Jurassic mafic rocks of Atlantic Canada: Implications for assimilation and injection mechanisms affecting mafic dykes

Abstract Early Mesozoic rift-related mafic rocks of Atlantic Canada (Caraquet, Minister Island, Shelburne and Avalon dykes and North Mountain Basalt) show high initial 87Sr/86Sr ratios (0.705–0.709) suggesting derivation from a LILE-enriched subcontinental mantle or assimilation of continental crust during ascent. Despite wide geographic distribution, Rb, Sr and Sr isotopic data for each body tend to overlap on variation diagrams, implying a comagmatic origin and/or similar petrogenetic processes. Modelling of these data supports assimilation of average continental crust by a fractionally crystallizing normal MORB magma. Rb/Sr ratios show a slightly positive or negative correlation with 87Sr/86Sr (depending on igneous body). Apparently assimilation involved early absorption of a Rb-rich partial melt with progressive assimilation of residual material enriched in radiogenic Sr. This Rb/Sr 87Sr/86Sr relationship is difficult to explain with a LILE-enriched subcontinental mantle model. Chilled contact samples generally display lower 87Sr/86Sr ratios than dyke interiors, implying that early pulses of magma through dyke systems were less evolved than later pulses. Assimilation occurred prior to injection or at deep crustal levels. Once in the dyke fracture system magma was quenched against cool upper-crustal rocks preventing further assimilation.

Anomalous isotopic compositions of Sr, Ar and O in the Mesozoic diabase dikes of Liberia, West Africa

The Mesozoic diabase dikes of Liberia are tholeiites whose 87Sr/86Sr and 87Rb/86Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial 87Sr/86Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using λ(87Rb)=1.42 × 10−11y−1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous 40Ar after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous 40Ar. However, dikes from all three age provinces of Liberia have elevated initial 87Sr/86Sr ratios. These results indicate that contamination with radiogenic 87Sr occurred primarily before intrusion of the magma whereas the addition of extraneous 40Ar occurred after emplacement and reflects the age and mineral composition of the country rock.The δ18O values of the Liberian diabase range from +5.6/% to +9.10/% and correlate positively with initial 87Sr/86Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in 18O and enriched in 87Sr by interactions with groundwater at high temperature.

Strontium-isotopic geochemistry of fluorite mineralization (Coahuila, Mexico)

Abstract The widespread fluorite deposits of Coahuila, Mexico, include limestone replacement deposits formed at the contact with Tertiary rhyolites and microsyenites, as well as mantos and veins that are not clearly associated with any known igneous body. In recent studies, it was concluded that most of the calcium in these deposits came from the limestone, whereas the fluorine was derived largely from the Tertiary igneous rocks. We have tested this model using strontium-isotopic data and simultaneously placed constraints on the mechanisms of fluorite deposition. Our results indicate that the 87Sr/86Sr ratio of the host-limestones ranges from 0.7075 to 0.7078, whereas that of the Tertiary rocks, at the time of mineralization, was 0.7074–0.7118. The 87Sr/86Sr ratios of the fluorite range from 0.7076 to 0.7083. The fluorites fall on two distinct mixing lines between Cretaceous limestone and Tertiary igneous rock. Between 70% and 98% of the Sr in the fluorite came from limestones. Relative abundances of Sr and F in the limestone and the igneous rocks suggest that between 70% and 96% of the fluorine came from the igneous rocks.

Evidence for the existence of the gondwana ice sheet in the 18O depletion of carbonate rocks in the permian formations of the transantarctic mountains

Abstract New measurements of 87 Sr/ 86 Sr ratios of calcite lenses in the Permian formations of the Beacon Supergroup in the Queen Alexandra and Queen Elizabeth Ranges of the Transantarctic Mountains confirm the results of a previous study and indicate that the calcite lenses are probably of diagenetic origin. The δ 18 O-values of the calcites range from −30.1 to −15.5% (PDB) whereas δ 13 C-values range from −23.1 to + 2.6‰ (PDB). The depletion in 18 O requires that the calcite precipitated from meteoric water which was also strongly depleted in 18 O, such as glacial meltwater of the Gondwana ice sheet. The depletion in 13 C indicates that the carbon in the calcite is partly of biogenic origin. These results therefore suggest that in late Carboniferous to early Permian time the MacKellar Basin in the Queen Alexandra and Queen Elizabeth Ranges received discharge of glacial meltwater from the Gondwana ice sheet and that vegetation was growing on the land surrounding the basin and within the water.

The origin of salts on Mount Erebus and along the coast of Ross Island, Antarctica*

Abstract Salt deposits in the form of crusts and efflorescences are a common feature of rock exposures along the coast of Ross Island and on the summit of Mt. Erebus, an active volcano that dominates the island. The 87Sr/86Sr ratios of salts from coastal sites decrease with increasing elevation from 0.70861 at Cape Royds to 0.70344 at the top of Castle Rock on Hut Point peninsula, only 413 m above sea level, but may also vary along the coast in response to the seasonal duration of open water. The variation of 87Sr/86Sr ratios can be attributed to mixing of marine Sr with Sr derived from the volcanic rocks and confirms the importance of chemical weathering as a source of Sr in these secondary salts. Yellow salts of complex mineralogical composition from the summit of Mt. Erebus have 87Sr/86Sr ratios whose average is 0.70345. This value is consistent with the hypothesis that these salts formed both from the plume of volcanic gases emanating from a convect-ing lava lake of phonolitic composition and by chemical weathering of the volcanic rocks. One sample of white salt from ice caves at the summit has an anomalously high 87Sr/86Sr ratio of 0.70460 perhaps because it contains a component of marine Sr released by melting of large quantities of snow in the ice caves.

Strontium isotopic geochemistry of intrusive rocks, Puerto Rico, Greater Antilles

The strontium isotopic geochemistry is given for three Puerto Rican intrusive rocks: the granodioritic Morovis and San Lorenzo plutons and the Rio Blanco stock of quartz dioritic composition. The average calculated initial 87Sr/86Sr ratios are 0.70370, 0.70355 and 0.70408, respectively. In addition, the San Lorenzo data establish a whole-rock isochron of 71 +- 2 m.y., which agrees with the previously reported K-At age of 73 m.y. Similarity of most of the intrusive rocks in the Greater Antilles with respect to their strontium isotopic geochemistry regardless of their major element composition indicates that intrusive magmas with a wide range of composition can be derived from a single source material. The most likely source material, in view of the available isotopic data, is the mantle wedge overlying the subduction zone.