Elizabeth Y. Anthony

Multiple inflation and deflation events at Kenyan volcanoes, East African Rift

The presence or absence of magma exerts a fundamental control on the distribution of strain in continental rift zones, yet the time scales of magma intrusion remain unconstrained. Using more than a decade of measurements from interferometric synthetic aperture radar (InSAR), we detected geodetic activity at four of the eleven central rift volcanoes in the Kenyan sector of the East African Rift. Subsidence of 2–5 cm occurred at Suswa and Menengai over the period 1997–2000, ~9 cm of uplift was recorded at Longonot in 2004–2006, and ~21 cm of uplift occurred at Paka during 2006–2007. The deformation is episodic, and no deformation was observed at these volcanoes during other time periods. Preceding the infl ation at Paka, we observed transient uplift and subsidence of a second, nearby source, likely associated with fl ow through a complex plumbing system. The best-fi tting source models for each episode include infl ation or defl ation of a horizontal penny-shaped crack at a depth of 2–5 km. The episodic nature of the activity, its lack of correlation with seasons, and the preferred source geometry are all consistent with activity in the volatile-rich cap to a crystal-rich magma chamber beneath each of the four volcanoes. The presence of active magmatic systems beneath more than 40% of the volcanoes, and the 2007–2008 explosive eruptions at nearby Oldoinyo Lengai volcano, have implications for models of continental rifting, caldera volcanoes, geothermal resources, and volcanic and seismic hazard in rift zones.

Potential field evidence for a volcanic rifted margin along the Texas Gulf Coast

Potential fi eld data along the Texas portion of the Gulf of Mexico indicate a large-amplitude coast-parallel magnetic maximum and a smaller Bouguer gravity high. Models constrained by seismic-refraction data indicate that these maxima manifest a deeply buried volcanic rifted passive margin or other magnetic high in the outer transitional crust. Buried 12‐15 km, the source is 220 km wide, similar to the Voring Plateau in Norway and the U.S. East Coast. This margin, which formed during the opening of the Gulf of Mexico, differs in origin from the transform boundary of the northeast Mexico margin (Tehuantepec transform), and we infer a Jurassic triple junction related to the Borderland rift system, which is traceable as far as southeast California.

3HE SURFACE EXPOSURE DATING AND ITS IMPLICATIONS FOR MAGMA EVOLUTION IN THE POTRILLO VOLCANIC FIELD, RIO GRANDE RIFT, NEW MEXICO, USA

Dating of very young igneous activity has been hampered by lack of suitable chronometers. We report here the results of 3He surface exposure dating (Kurz, 1986a,b; Craig and Poreda, 1986) for lavas from the Potrillo volcanic field. This field is particularly suited to the technique because it has well-preserved, young (<700 ka) surfaces. The study applies 3He dating in a systematic manner to a well-characterized volcanic field and shows the power of the technique in deciphering both relative and absolute chronologies. Dates are reproducible to within 14% for different samples from the same surface and are similar to within 8% for samples which, based on field relations, represent synchronous events. Cosmogenic 3He21Ne confirms that 3He is retained by the samples. Comparison of the relative chronology with compositional trends of the lavas suggests that eruptive centers within the volcanic field are fed by individual magma chambers. Ages range from 80 ka to 17 ka, consistent with estimates from geomorphology, soil profile development, and some K/Ar determinations.

Architecture of a 1.38-1.34 Ga granite-rhyolite complex as revealed by geochronology and isotopic and elemental geochemistry of subsurface samples from west Texas, USA

Abstract During the evolution of Laurentia, a Mesoproterozoic felsic igneous belt extended from Fennoscandia through Canada to the southwestern United States. This belt, referred to as the granite–rhyolite province in North America, forms much of the west Texas and eastern New Mexico basement. We present data from 41 closely spaced wells in west Texas that penetrated several hundred meters into the southern granite–rhyolite province and provide the first opportunity to develop a three-dimensional view of the basement from subsurface samples. The felsic rock types include hornblende-bearing quartz monzonite, ignimbritic rhyolite, and comagmatic granite with eutectic textures. These rocks plot in the high-K to ultra-high-K field. Quartz syenite, which plots exclusively in the ultra-high-K field, is the final felsic phase of magmatism. All of the felsic magmas were variably oxidizing based on biotite compositions. UPb chronology supports the intrusive relations established by petrography: the quartz monzonite is oldest (∼1380 Ma) and is followed by the granite–rhyolite sequence (∼1360 Ma) and the quartz syenite (∼1340 Ma). Nd model ages range from 1520 to 1740 Ma, indicating involvement of older Proterozoic crust. In many wells, thick mafic sills intrude the granite–rhyolite sequence. Nd model ages for the mafic rocks range from 1560 to 1440 Ma, implying that they are Mesoproterozoic in age. The mafic rocks include an alkaline, OIB-like suite, which is not supportive of a subduction origin. UPb ages are also reported for the Mescalero Well #1 in eastern New Mexico. In this well, metasedimentary and metavolcanic rocks of the Debaca sequence unconformably overlie a quartz syenite similar to the quartz syenite from the west Texas wells. Detrital zircons from the basal meta-arkose of the sequence are ∼1690 and ∼1320 Ma, indicating a maximum age of ∼1300 Ma for this sequence.

Characteristics of Mantle Fabrics beneath the South-Central United States: Constraints from Shear-Wave Splitting Measurements

New shear-wave splitting measurements at permanent broadband seismic stations in the south-central United States reveal the orien- tation and degree of polarization of mantle fabrics, and provide constraints on models for the formation of these fabrics. For stations on the stable North American craton, correspon- dence between observed polarization direc- tion of the fast wave and the trend of Protero- zoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin for the observed anisotropy. The larg- est splitting times (up to 1.6 s) are observed at stations located in the ocean-continent transi- tion zone, in which the fast directions are par- allel to the Gulf of Mexico continental margin. The parallelism and the geometry of the keel of the craton beneath the study area suggest that asthenospheric fl ow around the keel of the North American craton, lithospheric fab- rics developed during Mesozoic rifting, or a combination of these factors are responsible for the observed anisotropy on stations above the transitional crust.

History of Science--With Labs.

We describe here an interdisciplinary lab science course for non-majors using the history of science as a curricular guide. Our experience with diverse instructors underscores the importance of the teachers and classroom dynamics, beyond the curriculum. Moreover, the institutional political context is central: are courses for non-majors valued and is support given to instructors to innovate? Two sample projects are profiled.

Monazite occurrence, chemistry, and chronology in the granitoid rocks of the Lachlan Fold Belt, Australia: An electron microprobe study

Abstract In-situ electron-microprobe dating of monazite holds the promise of being an effective technique for obtaining chronologic data. Our research focuses on I- and S-type granitoids of the Lachlan Fold Belt, Australia, whose petrology and zircon chronology have been thoroughly characterized. This study documents the textural relationships, morphology, zoning, and ages of monazite in these granitoid rocks. The I-type granitoids that lack monazite usually contain other mineral phases enriched in rare earth elements, such as allanite, titanite, and bastnasite. Only silica-rich, highly evolved I-type granitoids contain monazite. This preference in I-type rocks for phases other than monazite to host the REE and Th limit the applicability of monazite dating for this group of igneous rocks. On the other hand, monazite is ubiquitous in S-type rocks, both as interstitial and included grains. High-resolution X-ray maps of individual monazites reveal complex patterns of chemical zoning. Weighted averages of multiple analyses for individual chemical domains show small but systematic differences in age. These weighted averages of the chemical domains are considered the best estimate of the age of the monazite. Monazites from 8 different S-type samples range in age from 405 to 759 Ma, with the majority being 490 Ma and older. These are premagmatic ages for these granitoids, which have crystallization ages of 400 to 430 Ma. These premagmatic ages are similar to Cambro-Ordovician ages obtained from inherited zircon cores in the same granitoids, indicating that monazite can survive anatexis in peraluminous rocks. Thus, monazite dating in peraluminous rocks may illuminate characteristics (composition and age) of source rocks and anatectic processes.

Compositional diversity in late Cenozoic mafic lavas in the Rio Grande rift and Basin and Range province, southern New Mexico

This study reports major- and trace-element compositions of late Cenozoic (5 Ma and younger) mafic volcanic rocks from southern New Mexico. Stratigraphic sampling is reported for two of the volcanic fields, the Potrillo and the Jornado del Muerto volcanic fields; reconnaissance sampling, for an additional seven fields. These are among the first chemical analyses for some of these volcanic fields, and they provide insight into the dynamics of melt generation in this continental rift zone. The lavas fall into two groups: (1) an alkaline suite of basanite, alkali basalt, and trachybasalt with low SiO2 and high abundances of incompatible elements (TiO2, Nb, Rb, and Sr); (2) a subalkaline suite of subalkali basalts with high SiO2 and low abundances of the incompatible elements. Compositions of volcanic rocks from the Geronimo volcanic field, the Taos Plateau volcanic field, and the transition zone in central New Mexico are similar to those reported in this study, confirming that these are general characteristics of lavas throughout the region. For many of the large fields, the volcanic rocks belong exclusively to one compositional group or the other. Furthermore, there appear to be no patterns to the distribution of these compositional groups; for example, the Potrillo volcanic field, which is composed exclusively of alkaline lavas, and the Jornado del Muerto volcanic field, which is dominantly subalkaline, both occur in the axis of the Rio Grande rift. We see no evidence for evolution in melt chemistry during the past 5 m.y. In the Potrillo volcanic field, for which we have the best documentation of stratigraphic changes in composition, incompatible elements increase upsection and are correlated with decreasing Mg number, suggesting that magmas underwent differentiation before their final ascent. Phenocrystic olivine and plagioclase, which are liquidus phases at low pressure, imply a shallow depth for this crystallization event. Processes responsible for chemical divergence between the alkaline and subalkaline groups occurred at an early stage of magmatic evolution. Correlation between incompatible-element enrichment and silica undersaturation suggests that variable degrees of partial melting played a significant role in the generation of the two lava types.

Southern Louisiana salt dome xenoliths: First glimpse of Jurassic (ca. 160 Ma) Gulf of Mexico crust

No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because the igneous rocks are deeply buried beneath sediments. Three salt diapirs from southern Louisiana exhume samples of alkalic igneous rocks; these salt domes rise from the base of the sedimentary pile and overlie an isolated magnetic high, which may mark the position of an ancient volcano. Three samples from two domes were studied; they are altered but preserve relict igneous minerals including strongly zoned clinopyroxene (diopside to Ti-augite) and Cr-rich spinel rimmed with titanite. 40 Ar/ 39 Ar ages of 158.6 ± 0.2 Ma and 160.1 ± 0.7 Ma for Ti-rich biotite and kaersutite from two different salt domes are interpreted to represent the time the igneous rock solidifi ed. Trace element compositions are strongly enriched in incompatible trace elements, indicating that the igneous rocks are low-degree melts of metasomatized upper mantle. Isotopic compositions of Nd and Hf indicate derivation from depleted mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern fl ank of the Gulf of Mexico ca. 160 Ma. These results also confi rm that some magnetic highs mark accumulations of mafi c igneous rocks buried beneath thick sediments around the Gulf of Mexico margins.

Isotopic and elemental chemistry of subsurface Precambrian igneous rocks, west Texas and eastern New Mexico

We present major element, trace element, and Nd isotopic analyses from cuttings and core samples for three subsurface terranes in west Texas and eastern New Mexico. The most northerly is the Panhandle volcanic terrane, which represents a large part of the Mesoproterozoic southern granite-rhyolite province. This terrane is comprised of undeformed rhyolite, ignimbritic tuff, granite, and diabase. The Panhandle terrane is split by the Debaca terrane, which consists of intercalated metasedimentary and metavolcanic rocks intruded by olivine gabbro, ferrogabbro, and diabase. Mildly to strongly deformed intermediate and felsic intrusive rocks of unknown affinity make up the third terrane, called here the crystalline terrane; it is located south and southeast of the Panhandle and Debaca terranes. Intermediate-to-felsic rocks of the terranes can be subdivided on the basis of their geochemistry into those with: (1) K 2 O/Na 2 O > 1 and A-type trace element characteristics; and (2) K 2 O/Na 2 O 1.7-Ga crust. The southern edge of Laurentia, therefore, is farther south than previously inferred. A diabase from the Panhandle terrane has a T DM of 1.44 Ga. If this model age is close to the crystallization age, then diabase in the Panhandle terrane is approximately coeval with the granite and rhyolite. The model age for the gabbro from the Debaca terrane is distinctly younger at 1.26 Ga, and is the same as crystallization ages of felsic tuffs associated with shelf carbonates in the Franklin Mountains and Van Horn area. In the crystalline terrane, both A- and I-type granites are present. Model ages for the I-type granites are 1.40–1.47 Ga. These are distinctly younger than the model age for the Panhandle terrane, and an A-type granite has a T DM of 1.35 Ga. These data indicate that granites in the crystalline terrane are not part of the granite-rhyolite province; rather, they constitute a separate group.