Sm-Nd and Rb-Sr Isotopic Systematics of Tektites and Other Impactites, Appalachian Mafic Rocks, and Marine Carbonates and Phosphates

Citation

Shaw, Henry Francis, III (1984) Sm-Nd and Rb-Sr Isotopic Systematics of Tektites and Other Impactites, Appalachian Mafic Rocks, and Marine Carbonates and Phosphates. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/x0zc-7w22. https://resolver.caltech.edu/CaltechTHESIS:12142018-091056975

Abstract

This thesis is made up of three separate studies, each using the Sm-Nd and Rb-Sr isotopic systems to solve a problem of geologic interest.

In the first study it is shown that Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of the target materials which were impact melted to form these objects. Tektites have large negative values of ε_Nd(0) which are uniform within each tektite group, while the ε_Sr(0) values are large positive and show considerable variation within each group. The chemical, trace element, and isotopic compositions of tektites are consistent with their production by melting of sediments derived from old continental crust. Each tektite group is characterized by a uniform Nd model age, T^Nd_CHUR, interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ~1.15AE for Australasian tektites; (2) ~1.9AE for Ivory Coast tektites; (3) ~0.9AE for moldavites; (4) ~0.65AE for North American tektites; and (5) ~0.9AE for high-Si irghizites. Sr model ages, T^Sr_UR, are variable within each group, reflecting Rb-Sr fractionation during weathering and sedimentation. In the favorable limit of very high Rb/Sr ratios T^Sr_UR approaches the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ~0.25AE sediments, moldavites from ~0.0AE sediments, and Ivory Coast tektites from ~0.95AE sediments. The parent sediments of the other tektite groups have poorly constrained ages. The isotopic data on the moldavites and Ivory Coast tektites are consistent with their derivation from the Ries and Bosumtwi Craters, respectively. Irghizites are isotopically distinct from the Australasian tektites and are probably not related. Sanidine spherules from an iridium-rich Cretaceous-Tertiary boundary clay were heavily overprinted with seawater-derived Sr and Nd during diagenesis. The inferred initial isotopic composition of the sanidine itself is ε_Nd(T) = +2 and ε_Sr(T) = +5. These results show that the spherules were not derived from old continental crust or meteoritic potassium feldspar. These objects may represent an impact melt of a mixture of basaltic oceanic crust and overlying sediments and are consistent with an oceanic impact at the Cretaceous-Tertiary boundary. The isotopic data are also consistent with an origin by authigenic growth of the spherules from young detrital material.

The second study in this thesis uses the Sm-Nd and Rb-Sr isotopic systematics of mafic rocks from the Appalachians to place constraints on their origin. Isotopic analyses of modern oceanic basalts and ophiolites have shown that both modern and ancient oceanic crust have a characteristic Nd and Sr isotopic signature indicative of derivation from a depleted mantle reservoir. It also appears that the Nd isotopic system is not appreciably disturbed by metamorphism. These isotopic characteristics have been extended to the Pt. Sal, Kings-Kaweah, and Josephine Ophiolites of California. These characteristics are used in an attempt to identify pieces of proto-Atlantic oceanic crust among the mafic and ultramafic rocks of the Appalachians. Sm-Nd mineral isochrons for the Baltimore Mafic Complex, Md (BMC) yield an age of 490±20 My which is interpreted as the igneous crystallization age. BMC whole rock samples do not define isochrones and have initial isotopic compositions of -6.4 < ε_Nd(T) < -2.2, +51 < ε_Sr(T) < +115. ε_Nd(T) and ε_Sr(T) are anti-correlated. This is not the signature of depleted mantle and oceanic crust, but is similar to old continental crust. It is proposed that the BMC is a mafic continental intrusion, possibly subduction related, which was contaminated with old continental crust during emplacement. Whole rock samples from the Thetford Mines Complex, Qe (TMC) do not define isochrons and have -1.5 < ε_Nd(T) < +4.2, +2.6 < ε_Sr(T) < +114. These data do not in any way reflect the signature of normal oceanic crust. These results are in contrast with geologic relationships which show the TMC to have the characteristics of an ophiolite complex. The TMC is chemically and isotopically similar to a class of other ophiolites which have affinities to modern boninites. The TMC may therefore represent an ophiolite formed under an arc complex. The Chunky Gal Amphibolite, N.C., Lake Chatuge complex, N.C., and Hazen's Notch Amphibolite, Vt., were found to have a depleted mantle signature with +5 < ε_Nd(T) < +8 and may be fragments of oceanic crust. The Webster-Addie body, N.C., has ε_Nd(T) ~-1, ε_Sr(T) ~+30 and is not isotopically similar to oceanic crust or the other North Carolina mafic bodies analyzed. From these isotopic results it is clear that Appalachian mafic rocks have diverse origins, some are continental intrusives (BMC), others are probably fragments of oceanic crust (Vermont and N. Carolina amphibolites). Future models for the development of the Appalachians must allow for these various origins. The possibility that some ophiolites are not normal oceanic crust but have an origin in a partially continental setting or as anomalous oceanic crust will require further attention.

The final study is an exploration of the possibility of establishing the Nd isotopic variations in seawater over geologic time by analysing a marine sedimentary phase which records and preserves the ε_Nd(T) value of the seawater in which it formed. Apatite and CaCO₃ (calcite and aragonite) are examined as possible such phases. Modern biogenic and inorganic calcite and aragonite were found to have low REE concentrations: Nd = 0.2 to 65 ppb. The ε_Nd(0) values of Atlantic (-8.3 to -9.6) and Pacific (-0.1 to -1.3) carbonates are distinctly different and reflect the isotopic composition of Nd in the seawater in which they formed. The high concentrations of REE measured in limestones and carbonate fossils cannot be primary but must be due to the presence of other phases in the carbonate of the introduction of REE during diagenesis. Modern biogenic apatite also has a low REE content (<150 ppb Nd), but appears to quickly scavenge REE from seawater. Levels up to 1000 ppm Nd can be reached by this process. Inorganically precipitated apatite from phosphorites also has high concentrations of seawater-derived REE. A seawater-like REE pattern with a characteristic negative Ce-anomaly is often preserved by sedimentary apatite and apatite samples of the same age from different localities bordering a common sea record a common value of ε_Nd(T). These characteristics suggest that apatite can be used to trace the evolution of ε_Nd(T) in ancient seawater. The values of ε_Nd(T) in seawater as inferred from analyses of conodonts and phosphorite apatite range between -1.7 and -8.9 over the last 700My. These values lie in the range of modern seawater values and show no evidence for drastic changes in the sources for Nd in seawater during this time. High values of seawater ε_Nd(T) in the Triassic and latest Precambrian may correlate with the breakup of large continental landmasses. The initial ε_Nd(T) = -15.0 of the 2AE old Rum Jungle phosphorite requires the presence of -1.5AE old continental crust at 2AE ago. This demonstrates how the ε_Nd value of ancient seawater can be used to constrain the age of the exposed crust as a function of time.

Thesis Files