ANOMALOUS WET AND DRY SPELLS IN THE CENTRAL UNITED STATES, 1931–75

The Palmer Index (PI) was employed to compare anomalous dry spells with anomalous wet spells for 76 climatic divisions in the central United States. Comparisons were made in terms of the frequency, severity and persistence of wet and dry climatic episodes for the period 1931–75. The results indicated more frequent dry anomalies in the southwest (New Mexico and west Texas) portion of the study area and more frequent wet anomalies in the northeast (Illinois and Iowa). Not surprisingly, these are the most arid and most humid portions respectively. The results of the analysis for persistence were less clear. Typically dry anomalies lasted longer than wet anomalies, especially as the minimum length was increased. This was especially apparent when comparing the very longest dry spell with the very longest wet spell during the 45-year study period. Thus there are more but shorter wet spells and less frequent, but typically longer lasting dry spells. This final relationship posits the question of feedback processes between surface soil moisture and the persistence of atmospheric precipitation anomalies. [Key Words: Palmer Index, drought, moisture anomalies, wet spells, dry spells.]     

Sphene (titanite)

Few people have heard of the mineral sphene (also called titanite), but it is nevertheless extremely widespread, occurring in most common rocks. It can be useful to the Earth scientist as it contains information on the timing, temperature and pressure of formation of many common rocks. It is also a source of titanium—and can sometimes be used as a gemstone.     

Lithium minerals

Lithium is a critical element in modern technology, and lithium minerals will play a key role in the fight against climate change. However, the demand for lithium-ion batteries is dependent on an expanding supply of primary resources. Lithium occurs in limited amounts on the Earth in a surprising diversity of mineral species, from pyroxenes to amphiboles, phyllosilicates to phosphates. This article examines the principal mineral groups likely to be a target for future exploitation.     

Granitoid events in space and time: Constraints from igneous and detrital zircon age spectra

The goal of this study is to evaluate the global age distribution of granitoid magmatism and juvenile continental crust production with U/Pb isotopic ages from igneous and detrital zircons, and with Nd isotopic data. Granitoid age peaks, which are largely defined by TIMS data, are narrow and precise in contrast to detrital peaks that are often broad and hump-shaped due to the larger uncertainties of SHRIMP and LAM-ICPMS data. Granitic age peaks do not always have detrital counterparts and vice versa. Possible contributing factors to this mismatch are removal of crustal sources by erosion, inadequate sampling of granitoids because of cover by younger rocks, or small age peaks hidden by large age peaks in detrital spectra.Seven igneous peaks are found on five or more cratons or continents (3300, 2700, 2680, 2500, 2100, 1900 and 1100 Ma) and seven detrital peaks occur on three or more continents (2785, 2700, 2600, 2500, 1900, 1650 and 1200 Ma). Nd isotope distributions suggest important additions of juvenile continental crust at 2700, 2500, 2120, 1900, 1700, 1650, 800, 570 and 450 Ma. Tight clusters of craton ages occur for Superior–Karelia, Sao Francisco–Nain, and Kaapvaal–Siberia in the early Archean and for Wyoming–Kaapvaal–Slave, Superior–Nain, and West Africa–Amazonia in the late Archean. The global 2700-Ma peak is not a simple spike, but involves several peaks between 2760 and 2650 Ma. Events older than 3700 Ma are limited to the Yilgarn, Slave, Nain and North China cratons, and events between 2600 and 2500 Ma are widespread only in East Asia, Central and East Africa, and India.Single, short-lived mantle plume events at 2700 and 1900 Ga (or any other time) cannot easily account for prolonged episodes of granitoid magmatism during the Precambrian. The causes of geographically widespread and geographically restricted events are probably not the same.     

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.     

Effects of Jpeg Compression on the Accuracy of Digital Terrain Models Automatically Derived from Digital Aerial Images

It is well recognised that data volume represents a huge overhead for softcopy photogrammetry. For example, a file size of 100 Mbytes will be generated from a black and white aerial photograph if digitised with a resolution of 20μm. Large data volumes not only create storage problems but also affect the speed of image processing. As a consequence, data compression of image data is a matter of great significance. This paper describes an investigation into the effects of image compression on the accuracy of digital terrain models (DTMs) extracted from the compressed images. The JPEG system implemented in the Z/I Imaging ImageStation digital photogrammetric workstation (DPW) was used in the study. A systematic test has been carried out on the effect of different levels of JPEG compression (with Q-factors from 1 to 100) on the resulting DTM, which is automatically generated by the DPW using Match-T software. An analysis of the results from the two sites tested shows that image compression tends to cause more significant degradation when the image texture is richer, but that recommendations on Q-factors for use with the ImageStation appear to err on the side of caution. This analysis leads to some tentative conclusions and recommendations both for future investigation and for photogrammetric practice.     

Rare earth element distributions in a proto-stratiform ultramafic intrusion

Rare earth element (REE) abundances are reported for ten whole rock and eight mineral samples from the Preacher Creek ultramafic intrusion of southeastern Wyoming. Chondrite-normalized distribution patterns for the whole rocks exhibit a broad maximum between Sm and Gd and reflect the REE pattern of clinopyroxene, the major REE-bearing phase. Alteration of the primary mineral assemblages to actinolite and chlorite, which is generally minor, does not appear to have significantly affected the REE distributions. Absolute abundances of the REE in the rocks and constituent minerals increase as a function of differentiation, and relative abundances suggest an accompanying light REE enrichment. Trapped-liquid phases, which may be relatively enriched in REE, possibly account for some or all of the observed REE trends. The REE data, interpreted in terms of crystal-melt fractionation, suggest derivation of the intrusion by crystallization from a gabbroic magma having a REE distribution pattern similar to the parent magma of the Skaergaard stratiform complex. The results of this study are in accord with and complement a previous proposal that the Preacher Creek body formed in a manner analogous to major stratiform intrusions.