Use of ultrasonically modulated electron resonance to study S-state ions in mineral crystals: Mn2+, Fe3+, in tremolite     

Dennis G. McGavin  Ross A. Palmer  W. Craighead Tennant  Sean D. Devine 《Physics and Chemistry of Minerals》1982,8(5):200-205

The use of ultrasonically modulated electron resonance (UMER) to study S-state ions in substitutional sites of mineral single crystals is discussed. Mn²⁺ and Fe³⁺ in natural single crystals of tremolite are used as examples. Combined electron paramagnetic resonance (EPR) and UMER measurements establish almost certainly that Mn²⁺ enters predominantly into the distorted M4 sites occupied by Ca²⁺ in the ideal tremolite structure and only to a minor extent into the M1, M2 and M3 sites normally occupied by Mg²⁺. Fe³⁺ in tremolite gives rise to the well known high spin resonance with g _eff?4.3 but there is considerable uncertainty as to the site of the impurity ion.  相似文献   

Tetrapyrrole pigments in United States humic coals     

S.E. Palmer  E.W. Baker  L.S. Charney  J.W. Louda 《Geochimica et cosmochimica acta》1982,46(7):1233-1241

Analyses of forty-two United States humic coals have revealed a striking divergence between chlorophyll diagenesis in coals when compared to the more widely studied marine sediments, shales, asphalts and petroleums. Porphyrins of humic coals have been found to be dominated by the ETIO-series, to lack members above C-32, and, in lower ranked samples (e.g. Sub-bituminous-B, high volatile C) to exhibit mass spectral envelopes with unique even-carbon number predominances. The weighted average mass, as well as the carbon number maximum (viz. mode), of these coal porphyrin homologies has been found to decrease as rank increases. The generation of porphyrins of the ETIO-series is suggested as occurring both during early coal formation, including oxidative scission of the isocyclic ring at the phorbide stage, and later during the catagenic alteration of surviving DPEP-series porphyrins. A preliminary study of chlorophyll diagenesis in a South Florida peat partially substantiates this suggestion and has further shown that the coal porphyrins can be derived from bacterial, as well as higher plant, chlorophylls.  相似文献   

KAr isochron study of the Tudor Gabbro,Grenville Province,Ontario     

A. Hayatsu  H.C. Palmer 《Earth and Planetary Science Letters》1975,25(2):208-212

Ten whole-rock samples from the Tudor Gabbro, Grenville Province, Ontario, Canada have been dated by the KAr method. The ages calculated by the conventional method range from 900 m.y. to 2040 m.y. On an isochron plot, three samples from a sampling site near the northern border of the gabbro lie along a 670-m.y. isochron with an initial⁴⁰Ar/³⁶Ar ratio of about 17,300 whereas all other samples lie along another 670-m.y. isochron with an initial ratio of about 5000. Although it is not certain yet as to what geological event the isochron age represents, the results clearly demonstrate that the effect of initial argon can be significant even on old samples such as these.  相似文献   

Downstream effects of restored freshwater inflow to Rincon Bayou, Nueces Delta, Texas, USA     

Terry A. Palmer  Paul A. Montagna  Richard D. Kalke 《Estuaries and Coasts》2002,25(6):1448-1456

Construction of two dams in 1958 and 1982 decreased freshwater inflow to the Nueces River and Nueces Delta marsh, which has become a reverse estuary where salinity increases upstream rather than downstream as would occur in a normal estuary. In 1995, an overflow channel was dug to breach the banks of the Nueces River to restore inundation of the marsh via Rincon Bayou, which is the main stem channel of the delta. Previous studies demonstrated a restoration of a normal salinity gradient and positive affects on benthos in the upper reaches of Rincon Bayou. The present study was performed to determine how far downstream the overflow channel had beneficial effects. A transect of eight stations was established and sampled quarterly between October 1998 and October 1999. Benthic characteristics were measured to assess ecological change. There were 12 precipitation and freshwater inflow events between the month preceding and ending sampling. The largest were in fall and restored normal salinity patterns. The macrobenthic community was different in three zones. The upper four stations (<6.4 km from the overflow channel) were highly variable in water inundation and salinity. The broad salinity range caused the lowest diversity and allowed short-lived pioneer speciesStreblospio benedicti, Laeonereis culveri, and Chirominid larvae to dominate. Biomass blooms occurred during fall inflow events in the upper reaches. The central stations (11.4 to 14.3 km from the overflow channel) were brackish, had more narrow salinity ranges, and were more diverse.Mulinia lateralis, Mediomastus ambiseta, Cerapus tubularis, andAmpelisca abdita were dominant species. The third zone, the lower portion of Nueces Bay (27 km from the overflow channel), was distinct from other stations in the transect, because it had the greatest marine influence. It is in a later successional stage, with a more diverse community of larger organisms, e.g.,Polydora caulleryi, Tharyx setigera, andMysella planulata, which were dominant species and contributed to the larger biomass there. Overall, the overflow channel restored normal salinity patterns in the upper reaches (<14 km) of Rincon Bayou and freshwater pulses (>10⁶ m³) in fall increased benthic productivity indicating ecological functions were restored as well. This restoration however, only occurs intermittently for short time periods because the freshwater events are isolated and inflow volumes during the current study were too small to affect the lower Rincon Bayou or Nueces Bay.  相似文献   

The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras     

S. L. Donoghue  V. E. Neall  A. S. Palmer  R. B. Stewart 《Bulletin of Volcanology》1997,59(2):136-146

 Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre, New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P. and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated for the most widespread members are all less than 0.1 km³. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every 100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members (Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000 years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events. Received: 14 September 1996 / Accepted: 6 June 1997  相似文献   

Shallow seismic reflection profiles and geological structure in the Benton Hills, southeast Missouri   总被引：2，自引：0，他引：2  

J.R. Palmer  D. Hoffman  W.J. Stephenson  J.K. Odum  R.A. Williams 《Engineering Geology》1997,46(3-4):217-233

During late May and early June of 1993, we conducted two shallow, high-resolution seismic reflection surveys (Mini-Sosie method) across the southern escarpment of the Benton Hills segment of Crowleys Ridge. The reflection profiles imaged numerous post-late Cretaceous faults and folds. We believe these faults may represent a significant earthquake source zone.

The stratigraphy of the Benton Hills consists of a thin, less than about 130 m, sequence of mostly unconsolidated Cretaceous, Tertiary and Quaternary sediments which uncomfortably overlie a much thicker section of Paleozoic carbonate rocks. The survey did not resolve reflectors within the upper 75–100 ms of two-way travel time (about 60–100 m), which would include all of the Tertiary and Quaternary and most of the Cretaceous. However, the Paleozoic-Cretaceous unconformity (Pz) produced an excellent reflection, and locally a shallower reflector within the Cretaceous (K) was resolved. No coherent reflections below about 200 ms of two-way travel time were identified.

Numerous faults and folds, which clearly offset the Paleozoic-Cretaceous unconformity reflector, were imaged on both seismic reflection profiles. Many structures imaged by the reflection data are coincident with the surface mapped locations of faults within the Cretaceous and Tertiary succession. Two locations show important structures that are clearly complex fault zones. The English Hill fault zone, striking N30°–35°E, is present along Line 1 and is important because earlier workers indicated it has Pleistocene Loess faulted against Eocene sands. The Commerce fault zone striking N50°E, overlies a major regional basement geophysical lineament, and is present on both seismic lines at the southern margin of the escarpment.

The fault zones imaged by these surveys are 30 km from the area of intense microseismicity in the New Madrid seismic zone (NMSZ). If these are northeast and north-northeast oriented fault zones like those at Thebes Gap they are favorably oriented in the modern stress field to be reactivated as right-lateral strike slip faults. Currently, earthquake hazards assessments are most dependent upon historical seismicity, and there are little geological data available to evaluate the earthquake potential of fault zones outside of the NMSZ. We anticipate that future studies will provide evidence that seismicity has migrated between fault zones well beyond the middle Mississippi Valley. The potential earthquake hazards represented by faults outside the NMSZ may be significant.  相似文献   

Flow directions in ash-flow tuffs: a comparison of geological and magnetic susceptibility measurements,Tshirege member (upper Bandelier Tuff), Valles caldera,New Mexico,USA     

W D MacDonadl  H C Palmer 《Bulletin of Volcanology》1990,53(1):45-59

This study concludes that the elongation axis (K ₁) of the ellipsoid of anisotropic magnetic susceptibility (AMS) is a suitable proxy for flow axis in ashflow tuffs. 153 oriented samples (176 specimens) were studied from 18 sites in the 1.1 Ma Tshirege member of the Bandelier Tuff. These sites are distributed around the Valles caldera at distances of 5–25 km outside of the rim.K ₁ axes correlate well with postulated radial flow axes at 13 sites.K ₁ also agrees with measured geological flow indicators, mainly imbricated larger clasts, at 7 sites. At 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredK ₁ axes, theK ₁ axes correspond well with geological flow indicators, indicating that the divergence of flow from the predicted radial flow pattern is real. Two major topographic buttresses are suggested as the cause of flow divergence for the Tshirege ash flows: the San Pedro buttress northwest of the caldera, and the San Miguel buttress in the southeast. In situK ₁ axes plunge about 7° toward the source at two-thirds of the sites; therefore the plunge ofK ₁ is a plausible in situ indicator for thedirection of flow. Multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash-flow emplacement. These observations raisre the question of how best to represent ‘mean’ flow directions in ash-flow sheets: by eigenvector methods, by vector-sum methods, or by modes. A method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined. Imbrication, determined in this way at one-third of the sites, dips toward the source, i.e., up-flow. The minimum (K ₃) axis of the AMS ellipsoid correlates with the flow foliation rather than with the larger clast imbrication. The flow axes of ash flows correspond with theK ₁ axes, not with the declination ofK ₃ axes as suggested by some authors. Initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction, which is reversed with a mean ofD=173.5°,I=-38.4°, α₉₅=3.4°N=18. This mean is not different at the 95% confidence level from that of earlier workers. The mean pole, at 098.0°E, 74.8°N,A ₉₅=3.3°,N=18, is about 15° far-sided relative to the expected time-averaged geomagnetic pole, suggesting a history of emplacement too short to adequately average secular variation.  相似文献   

Phase transitions in leucite: X-ray diffraction studies     

David C. Palmer  Ekhard K. H. Salje  Wolfgang W. Schmahl 《Physics and Chemistry of Minerals》1989,16(7):714-719

Lattice parameters, and intensities of selected X-ray reflexions, have been measured as a function of temperature for natural leucite, to characterise the phase transformation behaviour. At low temperatures leucite has a large ferroelastic distortion, but the temperature evolution of lattice parameters cannot be explained in terms of a purely ferroelastic phase transition; in particular, the considerable change in volume with temperature implies an additional transition mechanism, which we correlate with off-centring of K-ions in the low-temperature phase, and a collapse of the 111 structural channels. The transition behavior can therefore be rationalised in terms of two competing mechanisms: (I) Ferroelastic (consistent with the change m3m 4/mmm); (II) Volume-changing (consistent with m3m 4/m). Coupling of the two order parameters Q_I, and Q_II gives rise to the intermediate 4/mmm tetragonal phase.Our results confirm the existence of an I4 ₁/aI4 ₁/acd transition, but the non-disappearance of the 200 reflexion at high temperatures implies that the expected transition from I4₁/acd to Ia3d (cubic) symmetry does not occur. We attribute this to a residual strain field conjugated to the order parameter, due to defects (with possible Al/Si order). Nevertheless, within our experimental resolution, the lattice becomes metrically cubic at 665° C.  相似文献   

Correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the Mulcahy Gabbro, north-west Ontario     

M. G. Gala  H. C. Palmer 《Geophysical Journal International》1994,118(3):485-498

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Raman spectroscopic study of high-pressure phase transitions in cristobalite     

David C. Palmer  Russell J. Hemley  Charles T. Prewitt 《Physics and Chemistry of Minerals》1994,21(8):481-488

The pressure dependence of the cristobalite Raman spectrum has been investigated to 22 GPa at room temperature, using single-crystal Raman spectroscopy with a diamond-anvil cell. We observe a rapid, first-order phase transition on increasing pressure, consistent with the cristobalite I?II transition revealed in previous x-ray diffraction experiments. The phase transition has been bracketed at 1.2±0.1 GPa on increasing pressure and 0.2±0.1 GPa on decreasing pressure. The pressure shifts II) of 11 Raman bands in the high-pressure phase (cristobalite have been measured. Evidence for an unusual hybridization of modes at 490–500 cm^?1 is found. Changes in the Raman spectra also reveal an additional phase transition to a new phase at P ≈ 11 GPa, which remains to be fully characterized.  相似文献