J.D. Macdougall

Origin of the Deccan Trap flows at Mahabaleshwar inferred from Nd and Sr isotopic and chemical evidence

Abstract The Deccan flows at Mahabaleshwar are divisible into a lower and an upper group, based on Nd and Sr isotopic ratios, which define two correlated trends. This distinction is supported by incompatible element ratios and bulk compositions. The data reflect contamination in a dynamic system of magmas from an LIL-depleted, e JUV ≥ +8 mantle by two different negative e JUV endmembers, one undoubtedly continental crust, the other either continental crust or enriched mantle. The depleted mantle source, anomalously high in ( 87 Sr/ 86 Sr), may have been in the subcontinental lithosphere or a region of rising Indian Ocean MORB mantle.

Petrology of the axial ridge of the Mariana Trough backarc spreading center

Abstract The axial ridge of the Mariana Trough backarc basin, between 17°40′N and 18°30′N rises as much as 1 km above the floor of a 10–15 km wide rift valley. Physiographic segmentation, with minor ridge offsets and overlaps, coincides with a petrologic segmentation seen in trace element and isotope chemistry. Analyses of 239 glass and 40 aphyric basalt samples, collected with alvin and by dredging, show that the axial ridge is formed largely of (olivine) hypersthene-normative tholeiitic basalt. About half of these are enriched in both LIL elements and volatiles, but are depleted in HFS elements like other rocks found throughout much of the Mariana Trough. The LIL enrichments distinguish these rocks from N-MORB even though Nd and Sr isotope ratios indicate that much of the crust formed from a source similar to that for N-MORB. In addition to LIL-enriched basalt there is LIL depleted basalts even more closely resembling N-MORB in major and trace elements as well as Sr, Nd and Pb isotopes. Both basalt varieties have higher Al and lower total Fe than MORB at equivalent Mg level. Mg# ranges from relatively “primitive” (e.g. Mg# 65–70) to more highly fractionated (e.g. Mg# 45–50). Highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite (e.g. 52–56%) SiO 2 . These are due to extreme fractional crystallization of basalts forming the axial ridge. Active hydrothermal vents with chimneys and mats of opaline silica, barite, sphalerite and lesser amounts of pyrite, chalcopyrite and galena formed near these silicic rocks. The vents are surrounded by distinctive vent animals, polychaete worms, crabs and barnacles. Isotope data indicate that the Mariana Trough crust was derived from a heterogeneous source including mantle resembling the MORB-source and an “arc-source” component. The latter was depleted in HFS elements in previous melting events and later modified by addition of H 2 O and LIL elements.

Sm-Nd evidence for 3.3 Ga old rocks in Rajasthan, northwestern India

Abstract Tonalitic to granodioritic gneisses from the basement gneiss of the Aravalli Region, northwestern India, define a Sm-Nd isochron corresponding to an age of 3.31 ± 0.07 Ga and an initial Nd isotopic ratio that is close to chondritic. Lenticular bodies of mafic amphibolites within the gneisses, however, define an isochron corresponding to a younger age of 2.83 ± 0.05 Ga and small positive initial ϵNd. This age is similar to that reported for a large intrusive granite (the Untala granite) in this area and, combined with field evidence, suggests that mafic volcanism may have been associated with formation of the granite. A highly HREE enriched garnet separate from one of the tonalitic gneisses defines a two-point garnet-wholerock isochron age of 2.45 ± 0.02 Ga, which also is similar to the age of a known thermal event in the region: intrusion of the Berach and Bundelkhand granites at approximately 2.5 Ga. These results document in broad outline Archean crustal formation events in the northwestern segment of the Indian subcontinent, and demonstrate the need for more detailed studies of the complex terranes mapped by Heron (1953) as a single unit, the Banded Gneiss Complex.

Sr and Nd isotopes in basalts from the East Pacific Rise: significance for mantle heterogeneity

Isotopic data for Sr and Nd from fresh glassy East Pacific Rise basalts suggest that this part of the suboceanic mantle is characterized by subtle but distinct large-scale regional isotopic variability which may reflect differences between cells of the convecting mantle. In spite of a systematic N—S change in spreading rate of a factor of three along the sampled portion of the EPR, no correlation is observed between spreading rate and range of isotopic composition, indicating that the regional variations override homogenization effects which may be correlated with rate of magma generation and hence spreading rate. There is no clear signature in our data of effects from the postulated global “Dupal Anomaly” [30,31]. However, for a restricted ridge segment at the latitude of Easter Island, anomalously high87Sr/86Sr and low143Nd/144Nd occur, coupled with high incompatible element concentrations. These features are most easily understood as being the result of inclusion of a “plume” component in these ridge basalts.

230Th-238U disequilibrium systematics in oceanic tholeiites from 21°N on the East Pacific Rise

Significant disequilibrium occurs between ^(230)Th and its parent, ^(238)U, in a suite of fresh basalt glasses from the RISE Project study area at 21°N on the East Pacific Rise. The (^(230)Th/^(232)Th) activity ratios observed for eight of nine samples from the crest of the axis at this site are constant within analytical uncertainty, with a value of 1.22. This observed homogeneity of (^(230)Th/^(232)Th) has two possible interpretations. First, the measured (^(230)Th/^(232)Th) can be considered to indicate a mantle-source for the RISE basalts with Th/U of 2.5. This interpretation, however, conflicts with the proposed correlation between (^(230)Th/^(232)Th) and ^(87)Sr/^(86)Sr [1] which predicts that (^(230)Th/^(232)Th) should equal 1.33 at the RISE site. A second possible interpretation is that radioactive decay of ^(230)Th, in the basalts themselves or in a magma chamber, has decreased (^(230)Th/^(232)Th) from 1.33 to the observed values. The required time span is 11,000 to > 100,000 years. However, petrologic arguments rule against long residence time in a magma chamber, and the spreading rate of this section of the East Pacific Rise (6 cm/yr) predicts that the maximum age for axis basalts is 27,000 years. Both interpretations of the measured (^(230)Th/^(232)Th) imply a low Th/U ratio for the RISE basalt source and suggest that the MORB source at this location is depleted in Th with respect to U relative to primitive mantle or bulk earth. In spite of their constant (^(230)Th/^(232)Th), the basalts from 21°N have wide ranges of measured Th/U and thorium and uranium concentrations, in apparent conflict with the common assumption that these two elements have very small, similar crystal-liquid partition coefficients. Participation of an accessory phase with high Th and U concentrations during partial melting or fractional crystallization appears to be required to explain this anomaly. Major and other trace element compositions provide more information about petrogenetic processes. Although fractional crystallization can explain the variations in major element composition, a more complex process such as continuous melting is required to account for the observed trends in trace element compositions. In addition to its use as a tracer, the ^(230)Th-^(238)U disequilibrium technique is capable of putting some constraints on the timing of fractionation events, and it may eventually lead to a method for dating very young mid-ocean ridge basalts.

Chemistry of hydrothermal solutions from Pele's Vents, Loihi Seamount, Hawaii

Abstract Hydrothermal fluids were sampled from Peles Vents on the summit of Loihi Seamount, an intraplate, hotspot volcano, on four occasions from February 1987 to September 1990. The warm (≤ 31°C) vent solutions are enriched in dissolved Si, CO 2 , H 2 S, alkalinity, K + , Li + , Rb + , Ca 2+ , Sr 2+ , Ba 2+ , Fe 2+ , Mn 2+ , NH 4 + , and possibly Ni 2+ , and depleted in SO 4 2− , O 2 , Mg 2+ , 87 Sr 86 Sr , NO 3 − , and sometimes Cl − and Na + (calculated), relative to ambient seawater. Dissolved Si correlates linearly with sample temperature, suggesting that the solutions sampled from numerous vents in the ~ 20 m diameter field have a common source and that Si can be used as a conservative tracer for mixing of the vent fluids with ambient seawater. There are general similarities of the vent waters with ridge-axis warm springs on the Galapagos Rift and Axial Seamount, but also striking differences: very high total dissolved CO 2 (> 200 mmol/kg), high alkalinity (> 8 meq/kg) and dissolved Fe 2+ (almost 1 mmol/kg), and relatively low pH (~ 4.2–4.4 estimated, in situ, sws) and dissolved H 2 S (several μmol/kg). The Mg 2+ and SO 4 2− data are inconsistent with the “Galapagos model” proposed for the warm springs at 86°W, Galapagos Rift, whereby the warm fluids result from sub-seafloor mixing of a high-temperature (~ 350°C) hydrothermal endmember with essentially unaltered seawater. The variable Cl − depletions in the vent fluids, however, suggest that the warm vent fluids do contain a high-temperature (> 200°C) component. The fluid history can be qualitatively described by a modified “Galapagos model” which includes the overprint of reactions resulting from the addition of juvenile CO 2 and SO 2 to the circulating fluids; the CO 2 attacks the basalt releasing metal cations, HCO 3 − and possibly Si into solution, and the SO 2 is hydrolyzed to SO 4 2− . These juvenile inputs likely reflect the shallow, hotspot setting of this hydrothermal system. A simple quantitative fluidhistory model is considered and shown to be consistent with mass-balance constraints and saturationstate calculations, which suggest that the Si concentration of the fluids may be controlled by amorphous silica saturation at ~ 31°C. Observed temporal variations in fluid composition between expeditions—specifically, in Cl − , A T , C T , Na + (calculated), Mg 2+ , Ca 2+ , Sr 2+ , 87 Sr 86 Sr , Fe 2+ , Mn 2+ and perhaps NH 4 + , relative to Si—are, excepting Mg 2+ , 87 Sr 86 Sr , and Mn 2+ , consistent with the effects of variable phase segregation at the proposed high-temperature endmember.

Dating of neovolcanic MORB using (226Ra/230Th) disequilibrium

Abstract A method for determining ages of Mid Ocean Ridge Basalts (MORB) erupted within the last 8000–10,000 years is presented and assessed. This technique employs radioactive disequilibrium between the U-series parent/daughter isotopes 230 Th and 226 Ra. Ba is used as a surrogate stable Ra isotope in order to constrain the amount of Ra—Th fractionation during generation of the basalts. Ages are determined by comparing the present day ( 226 Ra/ 230 Th) activity ratio to an inferred initial value. Measurements on a number of MORB from the East Pacific Rise (EPR) show that there is a strong correlation between Ba/Th and ( 226 Ra/ 230 Th) for the youngest basalts, giving confidence in this approach. The ages derived for dredged and submersible-collected basalts from the axial zone span a range from 0 to about 8000 years, and provide direct information about the time scale and possible episodicity of eruptions in this environment.

Refractory-element-rich inclusions in CM meteorites

Refractory-element-rich inclusions of a variety of types occur in CM meteorites. Based on detailed study of Murchison and more limited investigations of Murray, Nogoya, Cold Bokkeveld and Mighei, the most common of these inclusions are small spinel-hibonite bodies rimmed with diopside. Bulk chemical compositions are estimated to be approximately 67% Al2O3, 21% MgO, 5.5% CaO, 5.5% TiO2, and 1% SiO2. Although this chemical composition does not agree closely with current theoretically predicted compositions of early condensate assemblages, the inclusions have many features which suggest that they are indeed early condensates. These include texture, mineralogical composition, sequence of minerals and magnesium isotopic composition of hibonite. The CM refractory inclusions exhibit several differences from those observed in Allende and other CV meteorites. These probably record variations in time, place or physical conditions of origin for the two cases.

Seawater Sr isotopes at the Cretaceous/Tertiary boundary

Abstract Seawater 87 Sr/ 86 Sr values increase abruptly by 28 × 10 −6 across the Cretaceous/Tertiary boundary (KTB). This small, but rapid shift is superimposed on the larger scale structure of the seawater Sr isotope curve. The time scale of radiogenic Sr addition appears to be too rapid to reconcile with sources associated with volcanism, and we show that the amount of Sr required to produce even this small increase is too large to be derived from: (1) a KT bolide of the size constrained by the Ir anomaly, (2) continental crust ejecta from the impact of such a bolide, (3) soot from global wildfires initiated by an impact, or (4) any combination of these sources. The probable source of the radiogenic Sr is enhanced continental weathering, but the high rate of increase appears to rule out processes such as sea level regression, glaciation or tectonism. A plausible mechanism for rapid addition of radiogenic Sr to the oceans is enhanced weathering associated with globally distributed acid rain (pH ∼ 1) which is a proposed by-product of a bolide impact [51, EPSL Vol. 83].

Origin of contemporaneous tholeiitic and K-rich alkalic lavas: a case study from the northern Deccan Plateau, India

Concurrently erupted, alternating Deccan Trap flows of tholeiitic and potassic alkalic basalt outcrop along both banks of the Narmada River near Navgam. Nearby, around Rajpipla, early tholeiites are overlain by K-rich alkalic flows and intrusives, which are themselves cut by later tholeiitic dikes. Nd and Sr isotopic ratios of a wide variety of rocks from both areas form a single correlated array, which reflects mixing between positive eJUV and negative eJUV endmembers. There is an almost complete overlap between values for tholeiitic and alkalic samples; thus, both alkalic and tholeiitic primary magmas must have been produced that were isotopically much alike. A Rajpipla rhyolite also falls on the array, near the midpoint. For positive values of eJUV(T) the array is indistinguishable from that defined by the lower group of tholeiites at Mahabaleshwar, some 450 km to the south, implying a similar or identical high eJUV mantle source. The negative eJUV component, apparently different from either of the two at Mahabaleshwar, may have been continental crust or enriched mantle. Both alkalic and tholeiitic groups display wide overlapping ranges in incompatible elements other than K, Rb, and Ba—particularly in Sr and Nb. This partial decoupling of incompatible elements, in conjunction with the isotopic similarity between the two classes of rocks, is strongly suggestive of an enrichment event in portions of the mantle source shortly before magmatism.