Neoproterozoic ice sheets of the Siberian Platform: U-Pb-LA-ICP-MS ages of detrital zircons from the Bol’shoi Patom formation and the geotectonic position of its provenance

The centers of many Neoproterozoic and Phanerozoic glaciations were located on cratons and crystalline shields not necessarily, as some researchers believe, associated with rifts. An example is the Vendian Bol’shoi Patom glacial formation (Lena River, Yakutia). The U-Pb ages of detrital zircons from this formation imply that it was deposited by glaciers located on the pre-Riphean crystalline shields of the Siberian Platform. Glaciation in this region occurred after cessation of rifting and could not be related to the latter. The Bol’shoi Patom Formation is an element of the Middle Siberian glacial horizon that was deposited during the Marino Glaciation and is traceable along the southern periphery of the Siberian Platform from the Aldan Shield to the Baikal and Sayany regions. This indicates that the Marinoan Glaciation in Siberia involved at least the entire southern part of the Siberian Platform. Rifting was not the single, or even a principal, factor responsible for glaciations, although it could accompany and enhance them simultaneously providing space for sedimentation and conditions for preserving glacial deposits from erosion.     

Case studies on the origin of basalt

The simplified model of basalt genesis described in Part I of this series, equilibrium partial melting followed by Rayleigh-type fractional crystallization, is applied to a stratigraphically controlled sequence of basalt flows from Kohala volcano. Major-element compositions were determined for 52 samples and show a time-stratigraphic progression from tholeiites through transitional basalts to alkali basalts. Twenty-six of these samples were analyzed by isotope dilution for K, Rb, Cs, Sr, Ba and the REE, 13 for⁸⁷Sr/⁸⁶Sr, and 19 for Co, Cr, Ni and V by atomic absorption. After a simple, first-order correction for the effects of fractional crystallization (involving mostly olivine and aluminous clinopyroxene), the major element concentrations cluster tightly, and the incompatible trace elements show monotonic increases in concentration as a function of stratigraphic height. The process identification plot shows that all the (fractionation corrected) melt compositions can be explained by equilibrium partial melting of compositionally identical batches of source material. The REE and Sr are fractionated because of the presence of residual clinopyroxene. Garnet may also be present but in much smaller amounts. In this respect our results differ significantly from those of Leeman et al. (1980). The calculated chondrite-normalized REE patterns of the source are nearly flat to slightly convex upward. Therefore there is no need to invoke special mechanisms, such as metasomatic REE preenrichment of the source, in order to explain the petrogenesis of the suite of lavas. Specifically, Ce concentrations ranging from 20 to 250 times chondritic are all explained by the same calculated source pattern having a chondrite-normalized ratio of Ce/Sm=0.9±0.2. However, the normalized ratio Ce/Ba?2 shows that the source is not simply primitive mantle.     

Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust

The average chemical compositions of the continental crust and the oceanic crust (represented by MORB), normalized to primitive mantle values and plotted as functions of the apparent bulk partition coefficient of each element, form surprisingly simple, complementary concentration patterns. In the continental crust, the maximum concentrations are on the order of 50 to 100 times the primitive-mantle values, and these are attained by the most highly incompatible elements Cs, Rb, Ba, and Th. In the average oceanic crust, the maximum concentrations are only about 10 times the primitive mantle values, and they are attained by the moderately incompatible elements Na, Ti, Zr, Hf, Y and the intermediate to heavy REE.This relationship is explained by a simple, two-stage model of extracting first continental and then oceanic crust from the initially primitive mantle. This model reproduces the characteristic concentration maximum in MORB. It yields quantitative constraints about the effective aggregate melt fractions extracted during both stages. These amount to about 1.5% for the continental crust and about 8–10% for the oceanic crust.The comparatively low degrees of melting inferred for average MORB are consistent with the correlation of Na₂O concentration with depth of extrusion [1], and with the normalized concentrations of Ca, Sc, and Al ( 3) in MORB, which are much lower than those of Zr, Hf, and the HREE ( 10). Ca, Al and Sc are compatible with clinopyroxene and are preferentially retained in the residual mantle by this mineral. This is possible only if the aggregate melt fraction is low enough for the clinopyroxene not to be consumed.A sequence of increasing compatibility of lithophile elements may be defined in two independent ways: (1) the order of decreasing normalized concentrations in the continental crust; or (2) by concentration correlations in oceanic basalts. The results are surprisingly similar except for Nb, Ta, and Pb, which yield inconsistent bulk partition coefficients as well as anomalous concentrations and standard deviations.The anomalies can be explained if Nb and Ta have relatively large partition coefficients during continental crust production and smaller coefficients during oceanic crust production. In contrast, Pb has a very small coefficient during continental crust production and a larger coefficient during oceanic crust production. This is the reason why these elements are useful in geochemical discrimination diagrams for distinguishing MORB and OIB on the one hand from island arc and most intracontinental volcanics on the other.The results are consistent with the crust-mantle differentiation model proposed previously [2]. Nb and Ta are preferentially retained and enriched in the residual mantle during formation of continental crust. After separation of the bulk of the continental crust, the residual portion of the mantle was rehomogenized, and the present-day internal heterogeneities between MORB and OIB sources were generated subsequently by processes involving only oceanic crust and mantle. During this second stage, Nb and Ta are highly incompatible, and their abundances are anomalously high in both OIB and MORB.The anomalous behavior of Pb causes the so-called “lead paradox”, namely the elevated U/Pb and Th/Pb ratios (inferred from Pb isotopes) in the present-day, depleted mantle, even though U and Th are more incompatible than Pb in oceanic basalts. This is explained if Pb is in fact more incompatible than U and Th during formation of the continental crust, and less incompatible than U and Th during formation of oceanic crust.     

High-Precision Trace Element Data for the USGS Reference Materials BCR-1, BCR-2, BHVO-1, BHVO-2, AGV-1, AGV-2, DTS-1, DTS-2, GSP-1 and GSP-2 by ID-TIMS and MIC-SSMS

Different batches of the new US Geological Survey (USGS) reference materials (RMs) BCR-2, BHVO-2, AGV-2, DTS-2 and GSP-2 and the original USGS RMs BCR-1, BHVO-1, AGV-1, DTS-1 and GSP-1 have been analysed by isotope dilution using thermal ionisation mass spectrometry (ID-TIMS) and by multi-ion counting spark source mass spectrometry (MIC-SSMS). The concentrations of K, Rb, Sr, Ba and the rare earth elements were determined with overall analytical uncertainties of better than 1% (ID-TIMS) and 3% (MIC-SSMS). The analyses of different aliquots and batches of BCR-2, BHVO-2, AGV-2 and GSP-2, respectively, agree within 1%, i.e. approximately the analytical uncertainties of the data. This indicates an homogeneous distribution of the trace elements in these RMs. Differences in element concentrations of up to 17% in different aliquots of the depleted RM DTS-2 are outside the analytical uncertainty of our data. They may be attributed to a slightly heterogeneous distribution of trace elements in this dunite sample. Our trace element data for BCR-2, BHVO-2, AGV-2 and GSP-2 agree within about 3% with preliminary reference values published by the USGS. They also agree within 1-6% with those of the original RMs BCR-1, BHVO-1, AGV-1 and GSP-1. Large compositional differences are found between DTS-2 and DTS-1, where the concentrations of K, Rb, Sr and the light REE differ by factors of 2 to 24.