摘要:

A distinctive pattern of small‐scale marine magnetic anomalies (25–100 nT amplitude, 8–25 km wavelength: tiny wiggles) is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the Indian Ocean. By normalizing and stacking multiple profiles, it is demonstrated that this pattern of tiny wiggles is a high‐resolution recording of paleodipole field behavior between chrons C24 and C27. The pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading (82 mm/yr half rate) profile from the southeast Pacific where a similar signal is observed, confirming the paleodipole field origin of the anomalies. Two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations. We conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles. The implications of this study are that (1) tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo; (2) the small‐scale anomalies observed in the Jurassic quiet zones may be due to paleointensity fluctuations; (3) tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the Cretaceous quiet zones; and (4) much of the variance in anomaly profiles normally attributed to crustal emplacement processes, particularly at fast and ultrafast spreading rates, is actually due to intensity variations in the paleomagne

ISSN:0148-0227    

DOI:10.1029/92JB01090

年代:1992

数据来源: WILEY

摘要:

A horst block was drilled in the center of the Mariana forearc near 20°N during leg 125 of the Ocean Drilling Program. At this site 781, the drill penetrated a Pleistocene vesicular, porphyritic basalt at 72 m below the seafloor, and the top of the basalt corresponds to a high‐amplitude reflection on seismic reflection profiles across the site. The thickness of the basalt unit can only be estimated to be between 13 and 25 m because of poor core recovery (28% to 55%). The presence of an upper glassy chilled zone and a lower, fine‐grained margin suggest that the basalt unit is either a single lava flow or a near‐surface sill. The basalt is an island‐arc tholeiite (IAT) enriched in large‐ion‐lithophile elements relative to high‐field‐strength elements, similar to the submarine lavas of the southern Mariana Arc seamounts. The basalt layer, the youngest in situ igneous layer reported from the Izu‐Bonin and Mariana forearcs, is enigmatic because of its location more than 100 km from the active volcanic arc. The sediment layers above and below the basalt unit are late Pliocene (about 2.5 Ma) and normally magnetized. The basalt has schlieren ‐ like structures, reverse magnetization, and a K‐Ar age of 1.68 ± 0.37 Ma. Thus, the basalt layer is probably a sill fed by magma intruded along a fault zone bounding the horst and graben in the forearc. The geochemistry of the basalt is consistent with a magma source similar to that of rocks from the magmatic axis (or volcanic front) of the island arc, and derived from a mantle source above the s

ISSN:0148-0227    

DOI:10.1029/92JB01079

年代:1992

数据来源: WILEY

摘要:

In a companion paper (Egbert et al., this issue) we describe the estimation of very long period (0.16

ISSN:0148-0227    

DOI:10.1029/92JB01251

年代:1992

数据来源: WILEY

摘要:

Eleven years (1932–1942) of electric potential and magnetic measurements at the Tucson observatory represent a unique very long period magnetotelluric (MT) data set. We report here on a careful reanalysis of this data using modern processing techniques. We have developed and used novel methods for separating out the quasi‐periodic daily variation fields and for cleaning up outliers and filling in missing data in the time domain. MT impedance tensors, estimated using the cleaned and filled data and using robust frequency domain methods, are well determined and smoothly varying for periods between 4 hours and 10 days. At longer periods the electric field data are swamped by large‐amplitude incoherent noise, particularly after the third year of the experiment. Although we find no evidence for contamination of any field components by oceanic motional induction at tidal periods, the MT impedance estimates do show evidence of small systematic biases due to finite spatial scale geomagnetic sources at harmonies of the daily variation period. These periods are thus removed from the time series and not used in further analysis. We show that the resulting impedance tensor is well modeled by a real, frequency‐independent distortion of a scalar impedance, which is consistent with non‐inductive distortion of the electric fields by local surface geology. To estimate the undetermined static shift of the MT impedance, we compare the long‐period MT results to equivalent MT impedances determined from 46 years of geomagnetic data. Combining the geomagnetic and undistorted MT impedances results in scalar impedance estimates for periods 0.17

ISSN:0148-0227    

DOI:10.1029/92JB01252

年代:1992

数据来源: WILEY

摘要:

The Bishop Tuff (BT), erupted from the Long Valley Caldera at 0.74 Ma, is composed of a Plinian tephra and ignimbrite (Hildreth, 1979). Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz and sanidine phenocrysts, a strong H2O gradient was present in the upper portion of the magma chamber, mainly in the first 120 km3of erupted material. The H2O content of the magma which formed the Plinian tephra drops from 6 wt % to 3.5 wt %. In contrast, the magma which formed the ignimbrite contained a relatively constant amount of H2O, between 2 and 4 wt %. The strong drop in H2O content of the magma which formed the Plinian tephra suggests that only in its extreme upper part, if any, was thePH20in the magma close toPtotal. The halogen content of the BT magma was low and relatively constant at ∼700 ppm Cl and ∼ 500 ppm F. The trace and major element composition of MI from the Plinian and first‐erupted ash flow lobes of the BT are similar to that of bulk tephra. The range in the trace element concentrations of inclusions suggests that fractional crystallization may have affected magmatic composition. However, in addition to fractional crystallization, the composition of MI, phenocrysts and bulk pumice lumps suggests that magma mixing may have been an important process in establishing the final trace element zonation within the Bishop magma chamber and may be responsible for some of the most dramatic observed trace element variations. The BT eruption appears to have removed sequential stratigraphic compositional layers from the magma chamber, whereas fhe Lower Bandelier Tuff (LBT) appears to have erupted chaotically, although both magma chambers are characterized by essentially the same volatile and therefore density zonation. However, the H2O content of the Plinian:ignimbrite transition is different for the two (∼3 wt % for the BT, 4–5 wt % for the LBT), suggesting that the LBT Plinian eruption may have ended prematurely, possibly due to caldera collapse. Therefore, the magma withdrawal dynamics of these eruptions may be more strongly controlled by external factors, such as vent configuration, rather that the volatile gradient of

ISSN:0148-0227    

DOI:10.1029/92JB00764

年代:1992

数据来源: WILEY

摘要:

The Lower Bandelier Tuff, erupted from the Valles Caldera at 1.51 Ma, is composed of a Plinian tephra and associated ignimbrite. Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz, sanidine, and pyroxene phenocrysts, a strong volatile gradient was present in the upper portion of the magma chamber. In the 20 km3of magma that produced the Plinian tephra, most H2O and F contents of MI are between 4 to 5 wt % and 0.18 to 0.20 wt%, respectively, and between 3 to 4 wt% and 0.12 and 0.14 wt% in the first‐deposited basal ignimbrite. Measurements of the H2O and F contents of the 380 km3of magma which produced the bulk of the ignimbrite are concentrated between 2 to 3 wt% and 0.05 to 0.14 wt% respectively. The invariance of H2O (and possibly Cl and B) relative to F in the magma which formed the Plinian tephra suggests that this portion of the magma chamber was saturated with respect to an H2O‐rich vapor phase. The trace element composition of MI are varied, and overlap with composition of pumice lumps. A group of MI are rich in Ti, Sr, Ba and low in B, Cl, Rb, Y, Nb, and Th, and are compositionally similar to analyses of matrix glass from some ignimbrite pumice lumps. These represent the mixing of a second rhyolite into the base of the Lower Bandelier magma chamber. MI influenced by the second magma have not been found in Plinian samples, but occur in ignimbrite samples from base to top of the deposit. When these MI are removed from the data set, the remainder show strong linear correlations between Nb and Rb, Y, Zr, and Th. These correlations can be most easily explained by fractionation of approximately ∼40% quartz and alkali feldspar (with trace amounts of chevkinite). MI from the Plinian tephra are similar to bulk Plinian pumice composition, suggesting that magmatic evolution was well‐progressed at the time that the MI were trapped. However, fractional crystallization is difficult to reconcile with the observed distribution of some other elements, including H, B, Li, F and Cl. The enrichment of H2O and F in the upper portion of the magma chamber, along with their lack of correlation with Nb, Rb, Zr, Y, and Th, suggest that these sets of elements were decoupled during magmatic evolution. The upward enrichment of H2O and F must have occurred at a faster rate than other trace elements. The observed volatile gradient may have influenced the eruption dynamics of the magma, initially causing discrete layers of magma to be removed when the gradient was large, followed by chaotic eruption of the bulk of the ignimbrite when the gradient was small. Comparison with the Bishop Tuff eruption, however, suggests that the chaotic eruption style of the LBT may be related to vent geometry and conduit evolution, promoting high discharge rates, not differences in the absolute concentration of vo

ISSN:0148-0227    

DOI:10.1029/92JB01340

年代:1992

数据来源: WILEY

摘要:

Microtopographic profiles were measured and power spectra calculated for dated lava flow surfaces at Cima volcanic field in the eastern Mojave Desert of California in order to quantify changes in centimeter‐to meter‐scale roughness as a function of age. For lava flows younger than about 0.8 m.y., roughness over all spatial scales decreases with age, with meter‐scale roughness decreasing slightly more than centimeter scales. Flows older than about 0.8 m.y. show a reversal of this trend, becoming as rough as young flows at these scales. Modeling indicates that eolian deposition can explain most of the change observed in the offset, or roughness amplitude, of power spectra of flow surface profiles up to 0.8 m.y. Other processes, such as rubbling and stone pavement development, appear to have a minor effect in this age range. Changes in power spectra of surfaces older than about 0.8 m.y. are consistent with roughening due to fluvial dissection. These results agree qualitatively with a process‐response model that attributes systematic changes in flow surface morphology to cyclic changes in the rates of eolian, soil formation, and fluvial processes. Identification of active surficial processes and estimation of the extent of their effects, or stage of surficial evolution, through measurement of surface roughness will help put the correlation of surficial units on a quantitative basis. This may form the basis for the use of radar remote sensing data to help in regional correlations of surficia

ISSN:0148-0227    

DOI:10.1029/92JB01592

年代:1992

数据来源: WILEY

摘要:

Melting temperatures of the assemblage pyroxene + garnet on the enstatite‐pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split‐sphere anvil apparatus (USSA‐2000). The compositions of pyroxene, garnet, and melt coexisting along the solidus were also determined. Comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene. It is proposed that the observed variation in the garnet composition is caused by disorder in garnet. The obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80–110 kbar contributed to an improvement of the garnet peridotite thermobarometry. An internally consistent set of thermodynamic parameters was derived, which allows calculation of the temperature‐pressure phase diagram for the enstatite‐pyrope join at

ISSN:0148-0227    

DOI:10.1029/92JB01465

年代:1992

数据来源: WILEY

摘要:

Five differences are noted between the compositions of minerals in oceanic cumulates and minerals that crystallize from mid‐ocean ridge basalts (MORBs) at 1 atm. These differences are (1) Mg numbers of clinopyroxene (CPX) in oceanic cumulates are often higher (>88) than found in experimental studies of MORBs at 1 atm; (2) Mg numbers of orthopyroxene (OPX) in oceanic cumulates are often higher (>81) than found in experimental studies of MORBs at 1 atm; (3) the low‐Ca pyroxene in oceanic cumulates (with some notable exceptions) is OPX rather than (inverted) pigeonite; (4) Na2O contents of CPX in oceanic cumulates are often higher (0.3 to 0.8% Na2O) than in 1‐atm experiments or phenocrysts in MORBs (typically 0.15 to 0.35% Na2O); and (5) TiO2contents of CPX in oceanic cumulates are often higher than in 1‐atm experiments. These characteristics are consistent with crystallization of many oceanic gabbros at moderate pressures (3–10 kbar). The Na2O and TiO2contents of liquids calculated to be in equilibrium with CPXs from oceanic cumulates generally are comparable to the abundances of these oxides in MORBs. The notable exception is the suite of cumulates from Deep Sea Drilling Project site 334, which appears to have crystallized at low pressures from strongly depleted Na2O‐poor, TiO2‐poor, SiO2‐rich basalts. We suggest that such strongly depleted basaltic liquids are a widespread, but minor, component in mid‐ocean ridge petrogenesis even though such liquids have never been

ISSN:0148-0227    

DOI:10.1029/92JB01187

年代:1992

数据来源: WILEY

摘要:

We have investigated the transitional, semibrittle deformation of a mica schist (∼75 % biotite) by shortening cylinders cored at 0°, 45°, and 90° to foliation to varying strains, at confining pressuresPcto 500 MPa, constant strain rates ε from 1.5 × 10−7to 1.6 × 10−4s−1and temperaturesTfrom 25° to 400°C. Deformation is concentrated within one or more throughgoing, millimeter‐wide shear zones at all conditions; these localize at low strains (ε<2%) through the nucleation and coalescence of dense sets of intragranular microkink bands (MKBs). Despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions, the differential stresses required for shear zone development vary little with fabric orientation. Biotite schist undergoes a transition from strain‐softening to steady strength mechanical response at confining pressures in the range 75 to 150 MPa. The pressure sensitivity of strength (characterized by the slope μ of the Mohr envelope) decreases from μ ∼0.5 (atPc<100 MPa) to μ<0.1 at pressures greater than 200 MPa, reflecting the increasing contribution of glide and kinking in biotite at higher pressures. However, dilatancy associated with microcracking and void formation along MKB boundaries persists to at least 500 MPa. Within the pressure‐insensitive regime (200 ≤Pc≤ 500 MPa), temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law. Samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed alongTdecreasing or ε increasing paths. Path‐dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments. By assuming an exponential relationship between differential stress σdand strain rate ε of the form ε = C exp(ασd) exp (−Q/RT), we fit the data with two end‐member flow laws with a single activation energyQ= 89 kJ/mol, and exponential constants αss= 0.15 ± 0.01 MPa−1and αws=0.55 ± 0.04 MPa−1that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive toTand ε, respectively. Application of the results to crustal deformation suggests that mica‐rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range, supporting the inference that retrograde reaction to phyllosilicates may be important in localizing cr

ISSN:0148-0227    

DOI:10.1029/92JB00620

年代:1992

数据来源: WILEY