ALBERT

Notes: Abstract Arc magmas ranging in composition from basaltic andesites to rhyolites and intrusive equivalents were emplaced into the western margin of the North American craton starting in Late Triassic time giving way to rift0related sedimentation in the Late Jurassic. The region of this study cuts across Proterozoic basements of contrasting Nd model ages, 1.7–1.8 Ga (average ɛNd∼−11) in eastern Arizona and 2.0 to 2.3 GA (average ɛNd∼−18) in western Arizona and eastern California (Bennett and DePaolo 1987). The Mesozoic rocks have initial ɛNd of -3.4 to-6.4 in the eastern part of the study area and -7.1 to -9.2 in the western part. All of the rocks have elevated 87Sr/87Sr initial ratios (〉0.706). Trends in initial ɛNd values of Mesozoic arc rocks are directly correlated with the Nd model ages of the basement through which they passed. Simple two-component mixing calculations indicate that recycled continental crust in the arc magmas represents on average about 65%. A minimum of 35% mantle input into continental arc magmas, as recent as the Mesozoic, represents a significant contribution to the growth of the continental crust, in the absence of a return flow of continental material into the mantle of similar magnitude. In a detailed study in the Santa Rita Mountains. Arizona, there is a pattern of increase of ɛNd with time: early basaltic andesites have more negative ɛNd than later felsic rocks. A correlated pattern of depletion with time is also observed with trace element and major element data. We attribute this either to progressive hybridization of the lower crust by repeated injection of mantle magmas, or the progressive thinning of the continental crust during prolonged arc magmatism. The present data do not allow distinction between the two models. Progressive decrease in crustal contribution to arc magmas with time may be an important feature of continental arc evolution. Hybridization of the lower crust due to repeated injection of mantle melts during arc magmatism may help contribute to small-scale heterogeneities in lower crust inferred from seismic and xenolith data. Similarly, whether there is a well defined MOHO or sharp crust-mantle boundary in any given segment of the continental crust may in part depend on the extent of crust modification as a result of continental arc magmatism.

Notes: Abstract 14C abundance on the Earth can be modulated by both the solar wind and irradiance components of the solar cycle. The magnetic field component of the solar wind modulates 14C production whereas the irradiance component can result in a change in the exchange rate between the various reservoirs of the carbon biogeochemical cycle. The effects would be nearly synchronous and difficult to separate. The 0.1% amplitude of irradiance variation during the two most recent solar cycles is well known. A 22-yr cycle exists also in the measured global temperature record. We have divided the University of Washington high-precision data onΔ14C in tree rings into three 91-yr intervals: AD 1540–1630, 1630–1720 and 1715–1805, before, during and after the Maunder Minimum. Unfortunately the AD 1540–1630 interval includes part of the Spörer Minimum as well as the intermediate interval of high solar activity. These data were analyzed by the DFT, MEM and MTM methods of spectral time series analysis. The ca. 22-yr cycle is prominent during the Maunder Minimum, whereas the 11-yr cycle is most prominent after the Maunder Minimum but totally suppressed during the Maunder Minimum. The lesser amplitude of the 11-yr cycle before the Maunder Minimum is most probably due to overlap with the Spörer Minimum. Vasiliev and Kocharov VK83 have previously suggested that the 22-yr cycle persists through the Maunder Minimum whereas the 11-yr cycle is suppressed. Our calculations show that irradiance forcing of the carbon cycle during the 11-yr cycle is negligible, so the observed 11-yr cycle inΔ14 C must be the result of production rate changes. The presence of the 22-yr cycle and suppression of the 11-yr cycle during the Maunder Minimum is in accord with a model by Jokipii Jok91.