Lina C. Patino

Volcanism and Geochemistry in Central America: Progress and Problems

Most Central American volcanoes occur in an impressive volcanic front that trends parallel to the strike of the subducting Cocos Plate. The volcanic front is a chain, made of right-stepping, linear segments, 100 to 300 Km in length. Volcanoes cluster into centers, whose spacing is random but averages about 27 Km. These closely spaced, easily accessible volcanic centers allow mapping of geochemical variations along the volcanic front. Abundant back-arc volcanoes in southeast Guatemala and central Honduras allow two cross-arc transects. Several element and isotope ratios (e.g. Ba/La, U/Th, B/La, 10 Be/ 9 Be, 87 Sr/ 86 Sr) that are thought to signal subducted marine sediments or altered MORB consistently define a chevron pattern along the arc, with its maximum in Nicaragua. Ba/La, a particularly sensitive signal, is 130 at the maximum in Nicaragua but decreases out on the limbs to 40 in Guatemala and 20 in Costa Rica, which is just above the nominal mantle value of 15. This high amplitude regional variation, roughly symmetrical about Nicaragua, contrasts with the near constancy, or small gradient, in several plate tectonic parameters such as convergence rate, age of the subducting Cocos Plate, and thickness and type of subducted sediment. The large geochemical changes over relatively short distances make Central America an important margin for seeking the tectonic causes of geochemical variations; the regional variation has both a high amplitude and structure, including flat areas and gradients. The geochemical database continues to improve and is already adequate to compare to tectonic models with length scales of 100 Km or longer.

Flux versus decompression melting at stratovolcanoes in southeastern Guatemala

Abstract The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental arc setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua–Pacaya and Tecuamburro–Moyuta. The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral–aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)N ratios of ∼1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting.

Slab control over HFSE depletions in central Nicaragua

Abstract Mafic lavas from the central Nicaraguan portion of the Central American volcanic front exhibit considerable variability in the magnitude of high-field-strength element (HFSE) depletions. This variability cannot be attributed to variable magmatic differentiation, or more significantly, to variable depletion originating in the mantle wedge. Instead the HFSE depletions are thought to be the product of variable contributions from the subducting Cocos plate. Both subducted hemipelagic sediment and subducted oceanic crust are identifiable contributors to the overall signal. The latter could exert important control over HFSE depletions, but only if the dehydrating crust is rutile-saturated. Slab control over HFSE depletions is possible throughout the Central American subduction zone, even where slab contributions to magma generation are thought to be minimal, as in central Costa Rica. In some other subduction zones, slab control over HFSE depletions is apparent, but in others, such as the Marianas and the Aleutians, HFSE depletions are probably wedge-based.

Rates and time scales of clay-mineral formation by weathering in saprolitic regoliths of the southern Appalachians from geochemical mass balance

Rates of clay formation in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina, have been determined from solute flux-based mass balance methods. A system of mass balance equations with enough equations and unknowns to allow calculation of secondary-mineral formation rates as well as the more commonly determined primary-mineral dissolution rates was achieved by including rare earth elements (REE) in the mass balance. Rates of clay-mineral formation determined by mass balance methods have been used to calculate the time needed for a 5% (50 g kg−1) change in relative clay abundance in the saprolite at Coweeta; this corresponds to the “response time” of the clay mineral to, for example, a change in climate. The 5% change in relative clay abundance is the smallest change that can generally be detected using X-ray diffraction (XRD). Response times range from tens of thousands to hundreds of thousands of years. Extrapolating the Coweeta clay formation rates to other southern Appalachian regoliths, the time required to form measured clay abundances (“production times”) in eastern Blue Ridge and Inner Piedmont regolith have been calculated. The production times of clay-mineral assemblages range from 2 k.y. to 2 m.y., with mean values ranging from 50 k.y. to 1 m.y. The results of this study are consistent with the arguments of [Thiry (2000)][1] that the best resolution of the paleoclimatic record in marine clay-rich sediments and mudrocks is ∼1 or 2 m.y. [1]: #ref-107

Abrupt change in magma generation processes across the Central American arc in southeastern Guatemala: flux-dominated melting near the base of the wedge to decompression melting near the top of the wedge

Lavas erupted behind the volcanic front in southeastern Guatemala have many important distinctions from lavas erupted on the volcanic front. These include: generally higher MgO, Nb, Sr, TiO2, and rare earth element concentrations; higher La/Yb and Nb/Y ratios; and lower Ba/La, La/Nb, Ba/Zr and Zr/Nb ratios. These major and trace element distinctions are caused by reduced fractionation during ascent and storage in the crust, lower degrees of melting in the source, and greatly reduced contributions from the subducted Cocos plate in the source. In addition, because all of these important distinctions are even borne in lavas erupted within 20 km of the front, there is little apparent petrogenetic continuity between front and behind-the-front magmas. What little geochemical continuity exists is in radiogenic isotopes: 143Nd/144Nd falls across the arc, Pb isotopic ratios (except 206Pb/204Pb) rise across the arc, and 87Sr/86Sr rise across the arc after an initial discontinuity within 20 km of the front. These continuous across-arc changes in radiogenic isotopes are caused by increased contamination with older, more isotopically disparate rocks, away from the front. Once the effects of crustal contamination are removed, the remaining isotopic variability behind the front is non-systematic and reflects the inherent isotopic heterogeneity of the source, the mantle wedge. Geochemical disconnection in southeastern Guatemala suggests that behind-the-front magmas are produced by decompression melting near the top of the wedge, not by flux-dominated melting near the base of the wedge.

Calcic cores of plagioclase phenocrysts in andesite from Karymsky volcano: Evidence for rapid introduction by basaltic replenishment

Calcic cores in plagioclase of Karymsky andesite of the 1996–2000 eruptive cycle texturally and compositionally (both trace and major elements) mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle. These observations support the view that simultaneous eruption of andesite and basalt at Karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir. Homogeneity of andesitic output occurred within two months. This suggests to us that blending of injected basalt into reservoir magma was thorough and rapid.

Constraints on partial melting imposed by rare earth element variations in Mauna Kea basalts

Thirty-one basalts from Mauna Kea collected through the Hawaii Scientific Drilling Project [1994] have been analyzed by inductively coupled plasma (ICP) mass spectrometry for rare earth element (REE) concentrations. The systematic variations in REE abundance provide constraints for theoretical models of mantle partial melting. Lavas are given a first-order correction for the effects of clinopyroxene and olivine fractionation or accumulation so that all lavas can be considered direct mantle melts. Failure to account for fractionation generates errors in calculated source mineralogies and REE patterns. Incremental models of melt generation (including fractional, aggregated fractional, and continuous melting) are found to reproduce the observed lava compositions only under special conditions: a garnet-free source, a fractionated heavy rare earth element (HREE) source pattern, and aggregation of melt fractions. These conditions are not consistent with the high-pressure phase equilibria for Mauna Kea lavas. Equilibrium batch melting provides a significantly better match to the observed lava chemistry and is less sensitive to changes in initial conditions. The equations for batch melting can be inverted to compute initial source REE patterns and partitioning behavior and are internally consistent with the calculated forward models. Successful batch melting models include several with an approximately flat chondrite-normalized source REE pattern, degrees of melting between about 1 and 18%, and a mantle residue containing garnet and clinopyroxene.

Sr and Nd isotopic compositions, age and petrogenesis of A-type granitoids of the Vernon Supersuite, New Jersey Highlands, USA

Abstract Voluminous late Mesoproterozoic monzonite through granite of the Vernon Supersuite underlies an area of approximately 1300 km2 in the Highlands of northern New Jersey. The Vernon Supersuite consists of hastingsite±biotite-bearing granitoids of the Byram Intrusive Suite (BIS) and hedenbergite-bearing granitoids of the Lake Hopatcong Intrusive Suite (LHIS). These rocks have similar major and trace element abundances over a range of SiO2 from 58 to 75 wt.%, are metaluminous to weakly peraluminous, and have a distinctive A-type chemistry characterized by high contents of Y, Nb, Zr, LREE, and Ga/Al ratios, and low MgO, CaO, Sr and HREE. Whole-rock Rb–Sr isochrons of BIS granite yield an age of 1116±41 Ma and initial 87 Sr / 86 Sr ratio of 0.70389, and of LHIS granite an age of 1095±9 Ma and initial 87 Sr / 86 Sr ratio of 0.70520. Both suites have similar initial 143 Nd / 144 Nd ratios of 0.511267 to 0.511345 (BIS) and 0.511359 to 0.511395 (LHIS). Values of eNd are moderately high and range from +1.21 to +2.74 in the BIS and +2.24 to +2.95 in the LHIS. Petrographic evidence, field relationships, geochemistry, and isotopic data support an interpretation of comagmatism and the derivation of both suites from a mantle-derived or a juvenile lower crustal parent with little crustal assimilation. Both suites crystallized under overlapping conditions controlled by P–T–fH2O. Lake Hopatcong magma crystallized at a liquidus temperature that approached 900°C and a pressure of about 6 kbar, and remained relatively anhydrous throughout its evolution. Initial P–T conditions of the Byram magma were ≥850°C and about 5.5 kbar. BIS magma was emplaced contemporaneous with, or slightly preceding LHIS magma, and both magmas were emplaced during a compressional tectonic event prior to granulite facies metamorphism that occurred in the Highlands between 1080 and 1030 Ma.

Allanite and epidote weathering at the Coweeta Hydrologic Laboratory, western North Carolina, U.S.A.

Abstract Allanite and epidote occur in the parent rocks of weathered regolith at the Coweeta Hydrologic Laboratory in North Carolina and exhibit different responses to weathering. Petrographically, epidote and allanite are identical at Coweeta, and only with additional analytical techniques (e.g., EDS or LAICP- MS) can the two be distinguished. Allanite is more abundant in unweathered bedrock but weathers readily below the weathering front. In contrast, the much less abundant epidote persists into the solum with only minimal evidence of weathering. The rapid dissolution of allanite is likely influenced by its metamict nature. Both epidote and allanite at Coweeta dissolve by interface-controlled dissolution kinetics, evidenced by etch pits on grain surfaces. Etch pits appear to be either large “negative crystals,” or small, shallow, and elongated. The incipient stage of allanite weathering is characterized by Al mobility and Fe precipitation as goethite. During the initial stage of allanite weathering, carbonate precipitates, but with progressive weathering the carbonate is dissolved. Based on electron microprobe analyses and point-count data of the Ca-bearing phases in the Coweeta bedrock, accessory (<1%) allanite contains a minimum of approximately 25% of the total bedrock Ca by volume, whereas garnet and plagioclase contain 5% and 70%, respectively. Although allanite and epidote are Ca-hosts, only allanite is present in significant quantities, and is weathering sufficiently rapidly, to serve as an important source of Ca in pore and stream waters at Coweeta. Allanite weathering should, therefore, be evaluated in weathering studies of crystalline silicate bedrock, especially where other lines of evidence indicate the need to invoke additional Ca sources.

Tectonic insights provided by Mesoproterozoic mafic rocks of the St. Francois Mountains, southeastern Missouri

Abstract Although Mesoproterozoic silicic rocks are predominant in the St. Francois Mountains of southeastern Missouri, basalts, basaltic andesites and their plutonic equivalents are not uncommon. These mafic rocks fall into two distinct petrologic suites as first discerned by Sylvester (Ph.D. thesis (1984) 588). One, the Silver Mines suite, consists of mafic rocks formed contemporaneously with the voluminous silicic rocks. The second suite, the Skrainka suite, originated from mafic magmatism that may have postdated silicic activity. The rocks of both suites have a number of incompatible element indices typically associated with subduction zone environments. This suggests that the voluminous silicic magmatism of the St. Francois Mountains, and contemporaneous portions of the Granite–Rhyolite Provinces, originated during subduction along an active continental margin or during post-subduction (orogenic) extensional collapse. We favor the former tectonic setting, but an active margin in which extensional stresses were prevalent in the overriding plate.