V11B-0583
Modeling dike cooling time to obtain plagioclase growth rate
Finite element modeling indicates that the cooling/crystallization time interval for a diabase porphyry dike, 12.2 meters thick, intruded into lower Paleozoic limestones in the Shenandoah Valley of Virginia, was from 5.5 to 8 years. The limestone does not show contact metamorphism, yet fine grained chilled margins exist locally in the dike. In the model we assumed an ambient limestone temperature of 100 degrees Celsius and a depth of 1 kilometer at the time of intrusion, and magma liquidus and solidus temperatures of 1100 and 600 degrees respectively. The wide range between the highest and lowest thermal diffusivities for limestone as obtained from published tables accounts for the range in the cooling time span above. The latent heat of crystallization was modeled over the cooling interval and added approximately 15 percent to the cooling time of the dike. The groundmass (GM) plagioclase crystals show a linear crystal size distribution which would suggest a constant growth rate as a first approximation. Since the largest GM plagioclase was 0.08 cm, this would indicate a plagioclase growth rate from 0.317 E -9 to .46 E -9. This rate is faster than that reported for plagioclase in Makaopuhi lava lake (E -11) but within the range of other reported plagioclase growth rates (E-8 to E -11) .
V11B-0584
Clinopyroxene Compositions from Belingwe Komatiites: Constraints on Alteration and Komatiite Genesis
Previous studies have demonstrated that chemical analysis of relict primary igneous clinopyroxene crystals from komatiite lavas provide a means to investigate pre-metamorphic magmatic compositions and to assess similarities between komatiites and mafic magmas from modern plume or subduction zone environments. For this study we have analyzed major and trace element from two komatiite samples from the Belingwe greenstone belt (2.7 Ga), Zimbabwe, for major and trace element composition using the electron microprobe and laser- ablation ICP-MS. Clinopyroxene crystals within komatiites are typically small (generally between 0.03-0.7 mm) subhedral and euhedral bladed crystals located interstitially between larger polyhedral olivine. We interpret these textures to indicate crystallization of pyroxene relatively late in the crystallization history. Although groundmass and some olivine show indications of serpentinization clinopyroxenes typically appear unaltered, and narrow iron-rich rims most likely relate to late crystallization. Clinopyroxenes are relatively homogenous in composition with Mg# 71-80 and average composition of Ca0.8Mg0.9Fe0.30Si2O6 and coexist with olivine of Fo90-92. Comparison between measured pyroxene trace element contents and a hypothetical pyroxene calculated to be in equilibrium with whole rock compositions suggest that mid and heavy REE, Ti, Zr and Y have remained largely immobile during alteration and metamorphism, whereas Sr is depleted. Interestingly there are indications of gain of LREE during alteration, although this might also reflect late crustal contamination. Belingwe komatiites do not show the low Ti/Zr ratios recorded in Barberton komatiite samples and inferred to resemble modern boninites.
V11B-0585
Mineralogy and melt-mineral reactions during transport of xenocrysts in the Thjorsa lava, Iceland
The Thjorsa lava is a voluminous tholeiite fissure lava flow originating within the Bardarbunga volcanic system of the Eastern Rift Zone, Iceland. The lava contains a high content of xenocrysts of plagioclase and minor olivine. Detailed analytical work using electron microprobe reveals a very thin reaction rim surrounding the xenocryst. Individual plagioclase crystals have high Anorthite uniform core with a thin rim of plagioclase with a significantly lower An composition, identical with the groundmass plagioclase. The olivine also has a high Fo cores but have normal zoning. Short residence time for the xenocryst in the host magma is inferred from diffusion modeling of olivine xenocryst rims, indicating that the xenocrysts originate by disintegration of plutonic assemblages by the ascending host magmas. The mineral analyses and results from an isotope study of seperated groundmass and xenocrysts can be used to construct a time scale for the disintegration and remobilization of earlier crustal cumulates, most likely analogous to the layered gabbros in ophiolites. The primitive mineralogy of the xenocrysts (An83-90 and Fo80-86) formed from highly primitive liquids deep in the crust as gabbro consisting of oli+pl+cpx(+sp). The disintegration of the gabbro caused assimilation of cpx, which left a chemical signature reflected in the concentration of cpx-compatible elements in the groundmass and minor but unusually Cr-rich, partially melted cpx can be found throughout the lava flow. The normal zoning pattern of the olivine xenocrysts can be used to estimate the time the olivine was in contact with the host magma, assuming the zoning is due to diffusion resulting in estimates of only a few months. The final fissure eruption is, therefore, a mixture of liquid (groundmass) with a minor amount of equilibrium microphenocrysts and xenocrysts. The high plagioclase could reflect density differences of the gabbro minerals. The Thjorsa lava is a remarkable example of a large-scale remobilization of crustal cumulates only shortly before, or during eruption. The occurrence of the large plutonic assemblages hosted by the Thjorsa lava (1.25-2.5 km3) shows rapid material transport processes taking place within the Icelandic rift system. The length of the lava flow (aprox. 120 km) and the apparently rapid emplacement show that the incorporation of the cumulates did not adversely affect the fluidity of the lava.
V11B-0586
Olivine-Hosted Melt Inclusions: Record of High-Pressure or Late-Stage Processes?
Melt inclusions in olivine have been thought to be high fidelity recorders of equilibrium melt compositions for incompatible trace elements because partition coefficients are very low, and incompatible elements are very large atoms that do not substitute in the olivine structure. This view has recently been shaken by the Spandler et al. (Nature, 2007) results on very fast REE diffusion in olivine: heavy REE in olivine would entirely re-equilibrate in less than a decade, and the light REE in a century. Spandler et al. conclude that all olivine-hosted melt inclusions record only shallow and very recent processes. If this conclusion holds up, all previous conclusions about mantle processes from olivine-hosted inclusions are in doubt. The new diffusion data also provide predictions for observations in a well constrained set of olivine-hosted melt inclusions: for time scales >100 years, all inclusions should be similar, and for <100 years there should be systematic changes with partition coefficient and inclusion size. These predictions can be addressed using samples along the FAMOUS segment (Mid- Atlantic Ridge, MAR). Lava samples from this region were the original data set from which dynamic melting was proposed (Langmuir et al., EPSL, 1977), and major elements have a very well developed "local vector", that has been ascribed to melting column effects (Langmuir et al., Geophys. Mon., 1992; Asimow et al., G3, 2004). Such effects have been subsequently inferred elsewhere from a single sample on the MAR from melt inclusion data (Sobolev, Petrology, 1996). Ni-zoning profiles (Nabelek and Langmuir, Contrib. Mineral. Petrol., 1986) provide an independent assessment of time scales for the olivines; melt inclusions provide REE data. Would a large set of inclusion data support melting column, crustal processes, or diffusive re-equilibration effects for the REE? Existing data on olivine-hosted melt inclusions (Fo87-92) from the FAMOUS area display a wide range of trace element compositions (Kamenetsky, EPSL, 1996; Shimizu, Physics Earth Planet. Int., 1998; Laubier et al., Chem. Geol., 2007; and new data). Ni-zoning profiles imply time scales of <10 years. While data quantity for all published melt inclusion data sets is limited, melt inclusions analyzed thus far do not show clear evidence for diffusive re-equilibration for the REE. The ratio of the variability in LREE and in HREE in the melt inclusions from a single sample is large and consistent with that observed in the suite of lavas and with mantle melting partition coefficients. Furthermore no correlation between the size of the melt inclusions and the trace element variability is observed. Based on the recent diffusion rates, these results would suggest that these melt inclusions have been formed at shallow depth and record late-stage processes. However the calculation of the liquid lines of descent from the melt inclusions is consistent with the major element variability and trends of the FAMOUS glasses, and the behavior of Sr does not suggest plagioclase, a mineral that would be present at shallow levels. Some lavas suggest high-pressure fractionation (5 kbar) which implies that the formation of the melt inclusions occurs at high pressures, and that time scales from depth to the surface are fast. Therefore the major and trace elements compositions of the melt inclusions may be interpreted in terms of deep petrogenetic processes and can be used to constrain the melting column models as well as transport rates. This may not be the case for liquids with longer crustal residence times, likely also to be found at FAMOUS and elsewhere.
V11B-0587
Are Variations in TiO2 Contents in Anorthite-Hosted MORB Melt Inclusions Controlled by Magma Compositions or Boundary Layer Phenomena?
Anorthitic, AN greater than 90, plagioclase-phyric pillow basalts have been recovered from almost every Mid Ocean Ridge (MOR). These phenocrysts typically contain bands of primitive (high MgO) melt inclusions (MI's). Previous workers have described large variations in the abundance of Ti in melt inclusions within a single anorthite crystal. The origin of this anomalous Ti is uncertain. It has been ascribed to both variations in Ti (and by inference other elements) in melts from which plagioclase crystallize or to processes relating specifically to the formation of melt inclusions (e.g. diffusion controlled boundary layer effects). The goal of this study was to discriminate between these possibilities by comparing the Ti contents of melt inclusions and co-existing plagioclase. If it can be shown that the Ti content of plagioclase can be related to the Ti content of MI's through simple partitioning relationships, then this implies that inclusions represent melts with variable Ti contents. In this case, MI's potentially preserve a range of "unmixed" magma compositions that occur in the MOR magma environment. Data collected from MI's and high anorthite phenocrysts illustrate the same results for both Gorda Ridge and Southeast Indian Ridge. The range of Ti concentrations in MI's within individual phenocrysts varies from near zero (e.g. homogeneous) up to a factor of two at a given MI Mg number and constant anorthite value of plagioclase. Host feldspar analyzed next to the inclusion is characterized by a greater range of Ti concentration than that measured in the MI band. Calculated Ti distribution coefficients, based on the MI/host feldspar pairs are constant as a function of MI TiO2 and melt inclusion size. We interpret this to show that MI's represent real trapped melts, from which the plagioclase crystallized. Furthermore, MI compositions were not affected by diffusion or crystal nucleation and growth processes.
V11B-0588
Spinels from the "Green Sand" Bed of the 3.3 Ga Mendon Formation: Implications for a Possible new Archean Impact Layer
A distinctive 3-4 m thick unit of greenish sandstone, here termed the "Green Sand" bed, occurs at the top of a 10-12 m thick layer of chert between komatiitic flow units in the 3,334-3,260 Ma Mendon Formation. This sequence has yet to be correlated with other parts of the Mendon Formation because of its structural isolation by two major faults. The chert containing the "Green Sand" also includes a previously unreported impact layer. This unit is unique in our study of Barberton greenstone belt (BGB) zircons in both diversity and antiquity (at least five different zircon populations, including the oldest (>3.7 Ga) zircons found in the Kaapvaal Craton). Much of the data presented here is based on the composition of spinels, which have been shown to have a high potential for preservation in greenschist rocks of the BGB. Spinels in komatiitic volcanic rocks are high-Cr varieties produced by high degrees of partial melting and/or melting of a previously depleted source. Spinel compositions, as with bulk rock and pyroxene compositions (when preserved) accurately reflect variation in komatiitic petrogenesis to yield Type 1 (chondritic alumina, spinels Cr# 75-85), Type 2 (alumina-depleted, spinels Cr# >85) or Type 3 (alumina-enriched, spinels Cr# <75) lavas. Spinels from the "Green Sand" and associated komatiitic lava flows are compositionally distinctive from those in other parts of the Mendon Formation, but closely resemble those of komatiitic rocks in the Weltevreden Formation, previously known only from the northernmost parts of the greenstone belt. Another suite of spinels, associated with meteorite impacts in the BGB, are characterized by high nickel contents, though nickel-rich spinels are yet to be found in the "Green Sand". It is unclear how the "Green Sand" is related to the newly discovered impact layer. Although separated from the impact layer of spherules by 1-2m of chert, it is possible that the unusual components in the "Green Sand" reflect the wide distribution of distantly derived minerals excavated by a large meteorite impact about 3.3 Ga and or the subsequent uplift of near-target crust.
V11B-0589
Sr Isotope Zoning in Plagioclase Phenocrysts from Giant Plagioclase Basalts: Mixing Processes in the Oldest Deccan Formations.
The older Deccan Formations of the Western Ghats are each capped by highly evolved ultraphyric flows that contain "giant" (up to 5 cm long) plagioclase phenocrysts, termed Giant Plagioclase basalts (GPB). We investigate the origin of GPB from the two lowermost Deccan formations and are able to deduce magmatic events that occurred during plagioclase growth. We carried out in situ LA-ICPMS analysis of selected trace elements and 87Sr/86Sr on giant plagioclase crystals in the Thalghat GPB from the Jawhar Formation (oldest) and Keshele GPB from the Igatpuri Formation (second oldest). The Thalghat GPB has two populations of crystals, indistinguishable with respect to size: unzoned crystals and a few zoned crystals with sharp Sr isotopic zoning between core and rim. In these, the 87Sr/86Sr of the cores is 0.70960, while the rims are isotopically similar to the unzoned crystals with ratios of 0.71060. Neither population shows significant zoning in An or trace element contents other, and they are devoid of visible (optically or with BSE) resorption features. The zoned crystals record a single mixing event between two magma compositions, and two lava flows that underlie the Thalghat GPB have MgO contents and 87Sr/86Sr that would be appropriate for the mixing end-members. Furthermore, mixing between the same end-members (followed by variable fractionation) can also account for the compositions of the lava flows that comprise the overlying Igatpuri Formation. Keshele GPB crystals show multiple 87Sr/86Sr zones, and a core to rim transect encompasses the variation in the underlying Igatpuri flows: 0.71049 - 0.71120. The zone boundaries in Thalghat crystals are sharp, about 500 microns, whereas in Keshele GPB, ratios vary gradually over 3 mm in some cases. These variations are not due to diffusion, but by growth from varying mixed melt compositions. Based on growth rate calculations, we can assign a growth timescale of 780 years to the largest Keshele crystals. If the crystals have "seen" magmas represented by all the underlying lavas, then the same timescale can be applied to the 220m thick Igatpuri Formation. This lends further credence to a short duration of Western Ghats Deccan activity. The large plagioclase phenocrysts of the Deccan could therefore have originated by growth during rapid mixing events, rather than from a dormant magma and the growth probably took place in a feeder conduit that was constantly flushed with mixed magma.
V11B-0590
Incipient Dissolution Features of Corestone Pyroxenes in Guatemalan Basalts
The earliest stages in the chemical weathering of volcanic rocks are characterized by the redistribution of rare- earth elements (REE). Corestones are ideal for studying the onset of weathering both morphologically and chemically. The fresh core surrounded by adjacent spheroidally weathered rind provides a natural setting in which to study incipient pyroxene alteration. Thin sections of spheroidally weathered basaltic-andesite core-rind pairs from the Tecuamburro volcano in Guatemala were studied using a scanning electron microscope with and energy dispersive spectrometer, microprobe, and a laser ablation inductively coupled mass spectrometer. Backscattered images show the presence of sub-micron to micron en echelon etch pits forming within augite- diopside and enstatite. The etch pits appear to be associated with nearby fractures within the pyroxenes. Sub- micron lamellae, identified in backscattered mode on the scanning electron microscope, are oriented in rows parallel to, and contiguous with, the strings of en echelon etch pits. This suggests that the lamellae are preferentially susceptible to weathering. Electron microprobe data of the augite show a slight increase of Ti in the lamellae, with little difference in major and minor elements when compared to the host. Trace element data for La, Ce, Sm, and Nd show REE enrichment relative to unweathered augites, with several showing a negative Ce anomaly. The greatest degree of enrichment of REE appears to be associated with the most extensive etching of the lamellae. Fine-scale petrologic features of the primary minerals influence weathering textures of volcanic pyroxenes, and influence the contributions of those primary minerals to REE mobilization during incipient weathering.
V11B-0591
(226Ra)/(230Th) Disequilibrium and Amphibole Crystallization in Rhyodacite From Ilopango Caldera, El Salvador