Diffusion–reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering

Earth and Planetary Science Letters 237 (2005) 829 – 854 www.elsevier.com/locate/epsl

Diffusion–reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering Craig Lundstrom a,*, Alan Boudreau b, Maik Pertermann a,1 b

a Department of Geology, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA Nicolas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA

Received 3 November 2004; received in revised form 24 January 2005; accepted 15 June 2005 Available online 10 August 2005 Editor: B. Wood

Abstract Piston-cylinder experiments investigating the interaction between basaltic andesite melt and partially molten gabbro in a thermal gradient provide insight into melt-rock reaction processes occurring during magma differentiation in the crust. In two experiments juxtaposing basaltic andesite and gabbro at 0.5 GPa pressure for durations of either 13 or 26 days, diffusive chemical exchange between the two materials results in mineral layering and notable mineral compositions such as anorthitic plagioclase. Specifically, the basaltic andesite gains Al2O3, MgO and CaO from the gabbro and loses Na2O, K2O, SiO2 and FeO to it with a plagioclase-rich layer developing at the interface between the two materials in a process termed diffusion–reaction. The percent crystallinity of the basaltic andesite increases during the process and the plagioclase crystals within the interface region develop anorthitic cores (up to An90) that abruptly shift in composition to thin rims that are in Na–Ca exchange equilibrium with the co-existing melt. Both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion–reaction. Isotopic tracers (45Ca, 6Li, 84 Sr and 136Ba) initially deposited at the basaltic andesite–gabbro interface in the 13-day experiment were detected in the cores of the anorthitic plagioclase after the experiment, demonstrating that the melt chemically communicates with the plagioclase cores over the duration of the diffusion–reaction experiment. The formation of anorthitic plagioclase during diffusion–reaction may explain its widespread occurrence in terrestrial volcanic rocks without requiring the presence of ultra-calcic melts. Textures and mineralogical changes in the gabbro indicate that chemical transport occurs throughout the experiments despite temperatures at the cold end of the experimental capsule approaching 500 8C. For instance, apatite, FeNiS, olivine and almost pure albite occur at distinct, specific horizons in the gabbro within the 26-day experiment. Because the bulk element profiles indicating chemical transport reflect analyses of almost completely solid gabbro, equilibration between minerals and fluids/ melts must be rapid. The overall effect of the diffusion–reaction process is to make an ascending magma more primitive in

* Corresponding author. Tel.: +1 217 244 6293; fax: +1 217 244 4996. E-mail address: [email protected] (C. Lundstrom). 1 Now at Washington University, St. Louis, MO. 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.06.026

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composition (and in this case, produce anorthitic plagioclase) while making surrounding crustal wall rocks more evolved. Several observations within igneous rocks support the occurrence of this process, suggesting that the genesis of porphyritic high alumina basalt, ubiquitously observed at convergent margins, could reflect a diffusion–reaction process in the crust. D 2005 Elsevier B.V. All rights reserved. Keywords: diffusion; plagioclase; gabbro; basalt

1. Introduction Despite great progress in determining the phase equilibria relevant to magma differentiation, many details including the physical mechanics of the process remain poorly understood, particularly for magma evolution in convergent margin settings. For example, the relationship between observed basaltic andesites, dparentalT high alumina basalts (HAB) and the process producing these magmas remains widely debated e.g. [1–4]. Whether or not porphyritic low MgO HAB are important to discerning arc magma genesis is disputed because these rocks could simply reflect accumulation of plagioclase [1]. Nevertheless, their ubiquitous presence in this environment, even if reflecting mineral accumulation, begs the question of how these rocks are formed. Furthermore, the origin of anorthitic plagioclase, commonly observed in both convergent and divergent margin volcanic rocks, remains a major unresolved problem in igneous petrology. Indeed, no mid-ocean ridge basalt glasses have ever been observed that are in equilibrium with plagioclase more calcic than An85 [5]. Despite numerous attempts to create anorthitic plagioclase in the laboratory by variation in temperature, pressure, f(O2) or melt composition, no clear explanation for its occurrence in anhydrous magmas like MORB exists [5–7]. Identifying the processes that result in these rock and mineral compositions is critical to understanding magma differentiation and possibly crustal evolution. Volcanic rock suites often follow well-defined trends in major element composition that have long been interpreted to reflect differentiation by fractional crystallization. Volca´n Arenal (Costa Rica) provides an excellent example as bulk rock compositions (porphyritic high alumina basaltic andesite) have subtly changed over the course of the ongoing eruption from 1968 to present [8,9]. Although simple fractional

crystallization reproduces bulk composition variations in the eruption [8], mineral compositional heterogeneity within a single rock sample or an individual mineral grain argue for more complex, open system processes [9]. Large, highly anorthitic (An90–95) plagioclase frequently occur at Arenal despite quenched melt inclusions and matrix glasses indicating equilibrium with bAn70 plagioclase [8– 12]. Thus, understanding the relationship between bulk and mineral compositional variations at Arenal could provide general insight into the genesis of porphyritic HAB. Here, based on a combination of experiments and thermodynamic models, we examine the interaction between low MgO HAB and partially molten gabbro by diffusion and subsequent reaction. Specifically, we propose that diffusion–reaction between magmas and surrounding crust within a thermal gradient may be an important process within those constituting magma differentiation. We show that this process results in significant increase in the plagioclase mode of a HAB and provides a mechanism for the creation of anorthitic plagioclase.

Table 1 Starting material compositions

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2 O P2O5 wt.% oxides. All Fe as FeO.

AR-8

Stillwater Gabbro

54.61 0.64 18.90 7.59 0.16 5.08 9.21 3.04 0.62 0.16

49.43 0.17 19.74 4.36 na 9.95 14.42 1.79 0.15 na

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

831

[8]) and a leucocratic gabbro from the Lower Banded Series of the Stillwater Complex (located just below the JM reef) (Table 1). In both experiments, the basaltic andesite was held in the hotspot of the piston cylinder at 1200 8C, juxtaposed against a column of gabbro that extended down into the thermal gradient of the piston cylinder (Fig. 1). Experiment #1 lasted 26 days while experiment #2 was 13 days in duration.

2. Experimental methods Two diffusion–reaction experiments were performed at 0.5 GPa in a piston cylinder apparatus at the University of Illinois at Urbana Champaign (UIUC). Details of the experiments and analytical procedures are given in Appendix A. Starting materials consisted of fine-grained powders (b 30 Am grain size) of a basaltic andesite from Volca´n Arenal (AR-8;

EXPT #1

EXPT #2

-5 mm

Stainless Steel base plug

AR-8

A

B

+ NaCl

C

Plag rich

D

FeNiS Olivine

A

AR-8

Thermocouple

Plag rich

Stillwater Gabbro

Domain 1 basaltic andesite powder

MgO

Domain 2

18 mm

1080°C

Graphite furnace

Albite

Ti capsule with Pt lining

1200°°C

Graphite capsule

Stillwater Gabbro

gabbro powder

-1mm

D2O + NaCl

Pyrex

NaCl

~500°C

A

Piston

Domain 3 (porous)

B

-1mm

C

Fig. 1. A) Schematic diagram of the 3/4W experimental assembly used in these experiments. Approximate temperatures at different locations are shown. B) The graphite capsule used in experiment #1 showing the geometry of the AR-8-gabbro couple. Domains refer to regions of distinct textures within the gabbro. Mineral names indicate location of visual observation (see text). Squares with letters indicate location of backscattered electron images shown in Fig. 3. C) Geometry of Ti–Pt–graphite capsule used in experiment #2. Squares denote locations of images shown in Fig. 6.

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3. Results 3.1. Phase equilibria experiments Phase equilibria experiments at the same pressure and temperature provide needed background for the diffusion–reaction experiments. Pre-synthesis of AR8 for experiment #2 (without NaCl; 1200 8C, 0.5 GPa, 12 h) resulted in a homogeneous glass containing 23% plagioclase (PLAG) and a trace of orthopyroxene (OPX). Backscattered electron (BSE) images indicate some heterogeneity in the plagioclase with darker areas (constituting N60% of the plagioclase) having An66–67 composition and lighter areas having An73–78 compositions (15 analyses total). A second experiment of AR 8 + 1% NaCl (1200 8C, 0.5 GPa, 6 h) consisted of glass and 19% PLAG with the glass containing ~0.2 wt.% H2O. Phase relation experiments at 0.5 GPa using

a synthetic AR-8 composition powder indicate that plagioclase is the liquidus phase, persisting to N 1250 8C with small amounts of H2O (b0.5 wt.%) present while orthopyroxene does not become a stable phase until ~1200 8C (Pertermann and Lundstrom, unpublished data). A single experiment on the Stillwater gabbro (1200 8C, 0.5 GPa, 6 h) produced ~10% melt and ~5% olivine (OL) coexisting with the original PLAG, OPX and clinopyroxene (CPX). These results are in good agreement with the prediction of MELTS [13] for the respective compositions at these pressure and temperature conditions. 3.2. Experiment #1 Modes of both the basaltic andesite and the gabbro in experiment #1 (Table 2) significantly changed after diffusion–reaction (Fig. 2). The crystallinity of the basaltic andesite increases relative to its mode (81%

Table 2 Compositions of representative minerals in experiment #1 Sample mineral

Gab-IF PLAG

BA-rim PLAG

BA-core PLAG

Sample mineral

Gabbro OL

Gabbro OPX

Gabbro CPX

SiO2 Al2O3 FeO MgO CaO Na2O K2O

57.25 27.49 0.41 na 9.9 4.52 0.43

57.05 27.71 0.3 na 9.86 4.64 0.45

49.87 31.68 0.37 na 15.42 2.53 0.13

SiO2 Al2O3 FeO MgO CaO Na2O TiO2

37.78

55.4

25.36 36.52 0.34

15.25 28.68 0.67

51.95 3.94 8.79 16.45 17.6 0.89 0.38

Cations: Si Al Fe2 Ca Na K Total Cat Anorthite

2.56 1.45 0.02 0.48 0.39 0.03 4.92 53

2.55 1.46 0.01 0.47 0.40 0.03 4.93 52

2.28 1.71 0.01 0.76 0.22 0.01 4.99 77

Si Al Fe2 Mg Ca Na K Ti Mg# Total Cat

1.00

1.99

0.56 1.44 0.01

0.46 1.54 0.03

SiO2 Al2O3 FeO MgO CaO Na2O K2O TiO2

VG-2

Lake County

50.27 14.50 12.07 7.54 11.21 2.50 0.16 1.75

51.47 30.98 0.4 13.78 3.25 0.13

72 3.0

77 4.0

1.91 0.17 0.27 0.90 0.69 0.06 0.00 0.01 77 4.0

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

833

Wt % SiO2 50

52

A

B

Al2O3

TiO2

Cl

CaO

BASALTIC ANDESITE

C

D

-2

Plag MELT CPX

SiO2

K 2O

Na 2O

2

MgO

4

OL

Distance (mm)

0

Plag rich band

FeO

6

GABBRO

18

20

Wt % Al2O3

22

6

10

14

1

Wt. % Oxide

2

3

OPX

10

30

50

Mode

Fig. 2. Bulk compositional (A–C) and modal (D) changes as a function of position in experiment #1 relative to the original basaltic andesite– gabbro interface. Dashed lines leading to symbols on figure edge showing initial compositions. The element profiles indicate transport of higher temperature components (Al, Mg, Ca) toward the basaltic andesite and lower temperature components (Na, K, Si, Fe) toward the gabbro. The presence of large voids limits the analyses to domains 1 and 2. The compositional variations reflect the integrated change in mineralogy and phase composition at a given location; hence, the spikes in Al2O3, FeO and MgO content near the interface reflect the increase in plagioclase mode at this point. Note that the behavior of Fe in this experiment likely depends on the f(O2) which will be bQFM based on the use of graphite capsules [63]. Error bars reflect 1r variation on the average of 2–3 analyses.

glass, 19% PLAG, trace OPX) under the same P-T conditions in the phase relation experiments becoming 50% PLAG, 5% OPX, 5% CPX, and 40% glass with a trace of OL. The clinopyroxene present has low and variable CaO content of 12–15 wt.%. These low CaO contents are also found in pyroxenes in phase equilibria experiments using the synthetic AR-8 starting material (Pertermann and Lundstrom, unpublished data). Significant modal changes in the gabbro occur within 2 mm of the interface leading us to distinguish three domains in this experiment. Domain 1 is a plagioclase-rich zone (mode: 66% PLAG, 10% OPX, 20% melt, only a trace of CPX; Fig. 3A) occurring within 0.5 mm from the interface. The glass abundance rapidly decreases and the mode returns to the starting material mode by 2 mm distance from the interface. Domain 2 (N2 but b 7.5 mm from the interface) represents a zone where newly precipitated OL (1%–2%) occur as small grains between PLAG, CPX and OPX, along with traces of apatite and a sharp onset in the occurrence of small FeNiS blebs (Fig. 3B). The mineral texture in domain 3 (N 7.5 mm), a fractured appearance of the

original starting powder, indicates that overall little reaction has occurred. Nonetheless, small grains of essentially pure albite (An4) occur along the sides of voids in this coldest portion of the experiment. A notable feature of almost all of the large plagioclase crystals in AR-8 after reaction with the gabbro is their systematic morphology of homogeneous anorthitic cores (An75–87) that abruptly shift to thin, less calcic rims (~10 Am in width). The Na–Ca exchange coefficient (Ca–NaK D = [xtalX Ca] [meltX Na] / [xtalX Na] [meltX Ca]) between the rim and the coexisting melt is ~1, in agreement with previous studies involving anhydrous melts [2,14]. The boundary between core and rim is planar and often contains melt inclusions and trapped CPX (Fig. 3C, D). Bulk compositional profiles across the experiment resemble a melt–melt diffusion couple despite the fact that these are melt-poor, highly crystalline materials (Fig. 2A–C). Comparison of the AR-8 composition with the starting material (dashed lines) indicates that concentrations of CaO and Al2O3 increase (by ~1 wt.%) and concentrations of SiO2 and FeO decrease

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C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854 CPX

OPX CPX OL

PLAG

PLAG OPX

Melt

A

100 µm

100 µm

B

OPX

CPX

Plagioclase Core

An50 An54

Glass inclusions: CPX MELT

An76

An An77 80 An An75 79 An54

Plagioclase Rim

Melt channel?

CPX

Plagioclase core rim

C

Glass

20 µm

D

10 µm

Fig. 3. A–D show backscattered electron (BSE) images of various locations in experiment #1. A) 200 view of the plagioclase-rich zone near the interface between AR-8 and the gabbro. The dashed line at the top reflects a boundary between the plagioclase-rich zone having no clinopyroxene and areas of the gabbro farther from the interface having clinopyroxene present. B) 200 view of small grains of olivine (bright in BSE image) between plagioclase, orthopyroxene and clinopyroxene, interpreted to reflect reaction with gabbro producing more sodic plagioclase and olivine (see text). FeNiS are also present in this location although they are too small to be clearly distinguished in this image. C) 1000 magnification of a plagioclase grain within AR-8; note the relatively homogenous core of anorthitic plagioclase, which abruptly changes to less calcic plagioclase at the rim. Numerous melt inclusions occur at the boundary between the An75 core and the ~An50 rim. D) 2000 view of a different plagioclase in AR-8 showing a prominent melt channel through the plagioclase rim. Such channels may exist throughout the plagioclase allowing fast equilibration between melt and growing core.

(by ~1 wt.%) during the experiment. Compositional changes of other components in AR-8 are not as readily apparent reflecting the volume of the AR-8 reservoir compared to the gabbro (Fig. 1). However, the concentration profiles in the gabbro indicate that the gabbro has gained FeO, SiO2, Na2O, K2O, Cl, and TiO2 from and lost MgO, CaO and Al2O3 to AR-8. Glass compositions in the basaltic andesite and top of the gabbro change little across the interface (Electronic supplement Fig. 1A). The AR-8 glass contains 0.8 (F 0.2) wt.% H2O, consistent with the observed crystallinity of AR-8 and the initial water content of 0.2 wt.%. Bulk trace element concentrations of the gabbro and basaltic andesite systematically change as a function of distance from the interface, directly following

mineralogical changes (Fig. 4). For instance, Sr concentration reaches a maximum in the plagioclase-rich boundary layer at the interface indicating Sr diffusion from both sides of the experiment toward the interface. P concentration decreases from basaltic andesite to gabbro but shows a notable second peak in concentration at the location where apatite is positively observed. Ni concentrations sharply drop off between 1 and 2 mm from the interface, corresponding to the visual disappearance of FeNiS. Cr concentrations also abruptly drop in the gabbro near the location where clinopyroxene disappears from the gabbro and are elevated in the basaltic andesite compared to the AR-8 starting material. The elongated Li and K content profiles decreasing from basaltic andesite to gab-

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

Sr, Cr (PPM) 200

P (PPM)

600

200

835

K (PPM)

600

1000

1000

3000

5000

BASALTIC ANDESITE Cr

Ni

Sr

-2

P K Domain 1 Onset of FeNiS

Domain 2

Apatite observed

0

2

Domain 3 4

Li

Distance (mm)

PLAG-rich band

GABBRO 6

A

B

C 100

200

Ni (PPM)

0

4

8

Li (PPM)

Fig. 4. Variation in trace element concentration as a function of distance from the interface in experiment #1. Each point represents the average of two 250 Am by 250 Am raster SIMS analyses (see Appendix A for method details). Domains 1, 2, and 3 are marked. A) Cr and Sr vary directly with the major minerals hosting these elements. Sr shows a pronounced peak at the location of the plagioclase-rich zone near the interface while Cr is depleted in the plagioclase-rich region because clinopyroxene is not present; Cr returns to high concentrations once clinopyroxene again appears in the gabbro at lower temperature. Cr contents of the AR-8 bulk are higher than expected based on the starting mineral powder composition. B) Ni contents abruptly drop off approaching the interface, consistent with the disappearance of FeNiS at this location. P contents decrease going from the basaltic andesite to the gabbro but form a small peak in elevated P concentration at the location that apatite was visually observed in the experiment. C) Alkali elements Li and K produce smooth and slightly elongated profiles of decreasing concentration from the basaltic andesite to the gabbro.

bro are consistent with the relatively higher rates of diffusion expected for these elements [15]. 3.3. Experiment #2 Mineral modes follow those in experiment #1 with a prominent increase in plagioclase occurring at the interface at the expense of clinopyroxene (Fig. 5). However, unlike experiment #1, the plagioclase-rich zone occurs within the basaltic andesite. The basaltic andesite is distinctly layered with the plagioclase-rich OPX-bearing zone near the interface shifting to glass plus PLAG only assemblage farther from the interface (Fig. 6A). The gabbro texture in experiment #2 differs from experiment #1, presumably due to water added to this experiment. Whereas most of experiment #1 was noncompacted gabbro, the experiment #2 gabbro has no porosity and a well-equilibrated texture and contains a small amount of olivine (except in gabbro adjacent to the interface; Fig. 6). Despite the addition of water to

this experiment, no visible melt occurs in the gabbro. Notably, greater plucking along grain boundaries occurs during polishing at the cold end of the experiment relative to the hot end, possibly indicating the presence of small amounts of fluid along these grain boundaries (Fig. 6D). Apatite or FeNiS are not observed in experiment #2. Because FeNiS readily shows up in BSE images, this must reflect an intrinsic difference between experiments #1 and #2. Experiment #2 may have been initially more oxidizing due to the addition of water [16], consistent with a few small titanomagnetites observed at the coldest end of the experiment. Bulk compositional profiles across the couple follow those of experiment #1 with a prominent peak in Al2O3 occurring at the plagioclase-rich zone (Fig. 7). Comparison of the profiles with the starting material compositions shows that the basaltic andesite has gained MgO, CaO, Al2O3 and lost Na2O and FeO during the course of the experiment. The increase in Na2O content in the cold end of the gabbro probably

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C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

modes, experiment #2 70 60

PLAG

Melt 50

Mode

40

CPX

30 20

B2 OPX

10

B1 G1

OL G2 G3

0

2

G4 4

G5 6

8

G6 10

distance (mm) Fig. 5. Mineral modes as a function of distance for experiment #2. Note the dramatic increase in the mode of plagioclase at the interface. In contrast to experiment #1, the plagioclase-rich zone occurs on the basaltic andesite side of the interface. Olivine is present throughout the colder end of the gabbro, likely as a result of the presence of water added to the gabbro, but is not present at the hottest region near the interface.

reflects the incorporation of added NaCl to the plagioclase. Note that Cl, however, has shifted its distribution so that its peak concentration occurs closer to the interface, indicating its transport toward the interface. Glass compositions in the basaltic andesite systematically change approaching the interface becoming less rich in SiO2, Na2O, K2O and Al2O3 and more rich in CaO, FeO, TiO2, and Cl (Fig. 8). SIMS water analyses of the AR-8 glass indicate 0.5 (F0.1) wt.%

H2O but only 0.02 (F0.007) wt.% D2O (despite the fact that D2O was doped into the gabbro starting material). Mineral compositions change with distance along the gabbro profile (Fig. 9, Table 3). The Mg# (molar Mg / (Mg + Fe+ 2) of olivine increases slightly approaching the interface while OPX and CPX compositions remain constant except near the interface. Mg# of OL, OPX and CPX are notably offset. CaO contents of olivine and orthopyroxene steadily increase approaching the interface. Like experiment #1, plagioclase in AR-8 has highly anorthitic cores (An90 in Fig. 6E) abruptly mantled by lower An67 rims that are in exchange equilibrium with the coexisting glass (K D = 1.1). Fig. 6E shows that the core– rim boundary almost exactly follows the shape of the crystal rim with the detail of a rim indentation reproduced in the core–rim compositional boundary. The Na-rich rim is usually ~10 Am thick. However, thinner rims sometimes (~1 Am; see Fig. 6B caption) mantle the anorthitic cores and these rims always face toward the interface with the gabbro. The consistency of the general rim thickness and the occurrence of thin rims facing the interface in both experiments suggest that rim width and orientation reflect the process creating the anorthitic core. Isotopic analyses indicate near full chemical communication between plagioclase core and melt over the 13-day experiment duration. The homogeneous distribution of 45Ca h tracks across the plagioclase-

Fig. 6. Backscattered electron images and h track maps from experiment #2. A) 250 view of basaltic andesite located ~2 mm from the interface. Note the high melt fraction, moderate plagioclase content and lack of any pyroxene. B) 250 view of basaltic andesite located in the plagioclase-rich zone ~1 mm from the interface. Note the crystal rich texture with a high proportion of strongly zoned plagioclase, abundant small orthopyroxenes and much lower melt abundance than in A. The difference between A and B corresponds to the mineral layering within the experiment, interpreted to reflect the role of the thermal gradient. The box shows the region enlarged in E. Thinner sodic rims are observed on crystal faces of plagioclase that face the interface. C) 250 view of gabbro 4 mm from the interface. In upper right corner can be seen larger mineral grains and the end of one trail of Au blebs penetrating into the gabbro (see Appendix A). Note the abrupt switch in mineral texture becoming finer grained in the area where the Au disappears. D) 250 view of gabbro farthest from the interface, 10 mm away. Here, the highest reflectance phase is olivine but a few Fe oxides also occur. Note the large amount of black plucks in this area of the experiment, occurring at grain boundaries. These plucks may reflect the existence of fluid remaining between grains after the experiment. E). 1200 view of plagioclase grain in the plagioclase rich region near the interface in experiment #2. Note the prominent anorthite rich core with well-defined boundary with rim. On the left side can be seen an indentation in the mineral grain face and a corresponding indentation in the rim–core boundary. Measured plagioclase compositions indicate a relatively homogeneous anorthite-rich core which abruptly shifts to a less anorthite-rich rim that is in Na–Ca exchange equilibrium with the melt. Inset shows an image of the presynthesized starting material with measured An contents, identical in composition to the rim compositions after the diffusion–reaction experiment. F) h track map of area taken at approximately the same location as E) showing the homogeneous distribution of the 45Ca radiotracer. This distribution is similar to that of Ca X-ray maps of the sample indicating that the radiotracer was incorporated quickly into the cores of the plagioclase. Thus, the mechanism producing the anorthite-rich cores in plagioclase does not involve component interdiffusion [18] but rather must involve some form of dissolution–reprecipitation. This conclusion is also reinforced by isotopic analyses that indicate a fast approach to isotopic equilibrium between the host melt and the anorthitic core (Table 4).

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

rich layer indicates that 45Ca must have been incorporated into the cores of the plagioclase crystals during diffusion–reaction (Fig. 6F). h particles have a resolution of a few micrometers, much smaller than the size of the anorthitic plagioclase, providing irrefutable evidence for the core incorporating 45 Ca from the melt. The cores of two different AR-8: 2 mm from interface

plagioclase grains have 84Sr/88Sr, 136Ba/138Ba and 6 Li/7Li (Table 4) that are either isotopically indistinct from the surrounding melt or have isotopic compositions distinct from their original value also indicating uptake of these elements from the melt (Fig. 10). Plagioclase #2 (Table 4), whose isotopic composition has not yet reached that of the melt, is located farther AR-8: 1 mm from interface Melt OPX

Plagioclase

Melt

Note thin rim Plagioclase

A toward interface

100 µm

Gabbro: 3 mm from interface

100 µm

B toward interface

Gabbro: 8 mm from interface larger grai n size

OPX

OPX

AU CPX

Plagioclase

CPX Plagioclase

OL

100 µm

C

D

OL

100 µm

Beta Track map-same location as E An67 Melt An66 20 µm

Plagioclase An59

An67

Melt

An85 An90 An90 An88 An85

An67

An68 OPX

E

20 µ m

837

F

25 µm

838

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

SiO2 49

53

Wt. % oxide

57

61

4 Bas. Andesite

-2

8

12

Wt. % oxide 16

0.4

0

Na2O

0

CaO

K 2O

distance (mm)

2 4

TiO 2

Gabbro

6

Cl 8

SiO2

Al 2O3

MgO

10

FeO 14

18

22

2.4

Al2O3

4

5.6

Na2O

Fig. 7. Profiles of the bulk composition as a function of distance from the interface for raster analyses of experiment #2. Dashed lines leading to symbols on figure edge show initial compositions. Like experiment #1, the profiles of the bulk element profiles indicate transport of higher temperature components toward the basaltic andesite and lower temperature components toward the gabbro. The compositions reflect the integrated change in mineralogy and phase composition at a given location as can be seen by the spikes in Al2O3, near the interface reflecting the increase in plagioclase mode. The basaltic andesite clearly gains CaO and Al2O3 during the experiment while MgO content increases slightly.

from the initial tracer source (in Fig. 6A) than plagioclase #1, which is closer (Fig. 6B) and isotopically identical. These results, indicating nearly full chemical communication between plagioclase cores and melt, are demonstrably robust for two reasons. First, the inferred partitioning for Sr, Ba and Li agree well with experimentally determined plagioclaseArenal melt partitioning (Table 4, [17]). Second, to check that isobaric interferences during SIMS analysis did not artificially elevate 84Sr/88Sr, we measured the glass from experiment #1 and found it to be an order of magnitude lower than that measured in experiment #2, consistent with the expected natural 84Sr/88Sr. 3.4. Summary of experiment observations We emphasize several important observations from the experiments. First, reaction between silicate minerals and fluids or melts is fast, implying solidstate diffusion is not significant in controlling the rate and efficiency of the diffusion–reaction process. Second, diffusion–reaction between the two partially molten materials produces mineral layering at the boundary, creating anorthitic plagioclase. Third, min-

eral changes away from the lithologic interface emphasize that elemental transport may occur without observable volumes of silicate melt present and that supercritical fluid transport could play a critical role in Earth’s magmatic processes.

4. Discussion 4.1. Interpretations of processes within the experiments Both experiments indicate that the crystallinity of AR-8 increases after diffusion–reaction, reflecting compositional exchange between the two materials and not experimental artifact. For instance, the fact that AR-8 in experiment #2 starts out as a presynthesized aggregate of 77% glass demonstrates that the crystallinity does not reflect undissolved starting material minerals. Flotation of plagioclase from the gabbro cannot explain the increased plagioclase content because the gabbro remains essentially fully crystalline in both experiments. Temperature inaccuracy cannot explain the result because the output power was held constant at a similar value in all

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839

1.4

TiO2

2.8

1.0

2.6

K2O 2.4

0.6

Cl

Na2O Wt. %

Cl, TiO2, K2O Wt. %

Na 2O

2.2

0.2

9.5

MgO FeO

4.5

MgO Wt. %

CaO, FeO Wt. %

5.5

CaO

8.5

3.5

57.0

SiO2

15.5

Al2O3

15.0

SiO2 Wt. %

56.0 16.0

Interface

Al2O3 Wt. %

16.5

55.0

54.0 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

distance (mm)

into Basaltic Andesite Fig. 8. Glass compositions with distance from the interface for the 13 day duration experiment #2. Note that no glass compositions in the gabbro were measured due to the lack of any observable melt in this portion of the experiment. SiO2 and Al2O3 contents steadily decrease approaching the interface while CaO and FeO increase approaching the interface.

the 1200 8C experiments. Finally, models of diffusion–reaction discussed below show that the crystallinity and plagioclase-rich band are fully consistent with prediction.

To summarize the elemental exchange process, both experiments show bulk composition profiles across the interface indicating exchange of chemical components leading to increases in the CaO, Al2O3

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0.016 0.20

Ca in OL

0.15

0.10

0.012

Gabbro

0.008

0.05

Basaltic Andesite

0.010

Ca in OPX

Ca in OL

0.014

0.00

Ca in OPX

85

Mg#

80

75

70

-2

Mg# Olivine Mg# CPX Mg# OPX 0

2

4

6

8

10

12

distance (mm) Fig. 9. Variation in mineral compositions as a function of distance from the interface in experiment #2. A) Olivine and orthopyroxene increase in CaO content approaching the interface, consistent with increasing temperature. B) Mg# of olivine, clinopyroxene and orthopyroxene. No clear trend in orthopyroxene or clinopyroxene are apparent except for a drop in Mg# in the mineral closest to the interface. Olivine shows a slight increase in Mg# approaching the interface. Note the offset of N10 in Mg# in coexisting olivine and clinopyroxene and N7 between orthopyroxene and olivine.

and MgO contents of the basaltic andesite. The smoothly changing element profiles in the almost fully crystalline gabbro indicate that elements are incorporated into the solid minerals essentially instantaneously. These profiles are inconsistent with solely infiltration of a melt into the gabbro followed by reaction. For instance, such a mechanism cannot explain the marked increase in Al2O3 occurring at the interface. Mass balance within both experiments is satisfied within the uncertainties of the raster bulk composition analyses. Plagioclase grains with cores much more anorthitic than expected for equilibrium with any melt in the experiment occur at the basaltic andesite–gabbro interface of both experiments. Although the origin of

the anorthitic plagioclase in experiment #1 could be debated due to the use of the rock powder starting material, its occurrence in experiment #2 after presynthesis indicates that the anorthitic cores result from the diffusion–reaction process. The results of the isotopic tracer (Table 4; Fig. 10) and 45Ca h-track analysis (Fig. 6F) further indicate that the melt was in chemical communication with the plagioclase cores throughout the diffusion–reaction process. Because interdiffusion of albite–anorthite components in plagioclase is very slow [18], the re-equilibration of the core cannot reflect solid-state diffusion. Although melting experiments indicate that dissolution–reprecipitation controls plagioclase-melt reequilibration [19], it is difficult to envision how this

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841

Table 3 Analyses of OPX and olivine in experiment #2 Sample mineral

B2

B2

B1

B1

G1

G1

G2

G2

G3

G3

G4

G4

G5

G5

G6

G6

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

opx

SiO2 FeO MgO CaO

54.6 14.6 26.1 4.8

54.5 14.6 25.8 5.1

55.5 13.3 29.1 2.1

55.5 13.1 28.8 2.5

55.2 14.7 28.9 1.2

54.8 14.1 27.7 3.4

55.1 14.6 28.3 2.0

54.8 14.1 27.7 3.4

54.8 16.6 27.8 0.7

55.0 13.9 28.5 2.6

54.9 15.0 29.0 1.0

55.1 14.9 29.0 1.0

55.1 14.9 28.5 1.5

55.2 14.7 28.6 1.5

55.1 14.9 28.9 1.1

55.1 15.1 28.8 1.0

Cations: Si Fe2 Mg Ca Mg#

2.0 0.4 1.4 0.2 76

2.0 0.4 1.4 0.2 76

2.0 0.4 1.6 0.1 80

2.0 0.4 1.5 0.1 80

2.0 0.4 1.5 0.0 78

2.0 0.4 1.5 0.1 78

2.0 0.4 1.5 0.1 78

2.0 0.4 1.5 0.1 78

2.0 0.5 1.5 0.0 75

2.0 0.4 1.5 0.1 79

2.0 0.5 1.6 0.0 77

2.0 0.4 1.6 0.0 78

2.0 0.4 1.5 0.1 77

2.0 0.4 1.5 0.1 78

2.0 0.4 1.5 0.0 78

2.0 0.5 1.5 0.0 77

Sample mineral

G2

G2

G3

G3

G4

G4

G5

G5

G6

G6

OL

OL

OL

OL

OL

OL

OL

OL

OL

OL

SiO2 FeO MgO CaO

38.1 24.1 37.3 0.6

38.0 23.8 37.7 0.5

38.7 25.8 35.5 0.0

39.7 25.5 34.8 0.0

37.3 27.0 35.2 0.5

37.1 27.6 34.9 0.4

37.6 25.5 36.4 0.5

37.9 25.7 36.1 0.3

37.3 28.1 34.4 0.3

37.8 25.6 36.2 0.4

Cations: Si Fe+ 2 Mg Ca Mg#

1.0 0.5 1.5 0.0 73

1.0 0.5 1.5 0.0 74

1.0 0.6 1.4 0.0 71

1.0 0.6 1.4 0.0 71

1.0 0.6 1.4 0.0 70

1.0 0.6 1.4 0.0 69

1.0 0.6 1.4 0.0 72

1.0 0.6 1.4 0.0 71

1.0 0.6 1.4 0.0 69

1.0 0.6 1.4 0.0 72

Sample mineral

G1

G2

G3

G4

G5

G6

cpx

cpx

cpx

cpx

cpx

cpx

SiO2 Al2O3 FeO MgO CaO Na2O TiO2

52.34 4.26 7.84 16.46 17.86 0.68 0.38

49.8 3.48 6.36 16.64 22.54 0.66 0.36

51.08 3.52 6.87 17.11 20.29 0.67 0.34

49.59 3.86 7.11 17.16 20.98 0.71 0.37

51.88 3.18 5.99 15.46 22.23 0.73 0.36

48.25 3.15 6.26 16.92 24.15 0.74 0.38

Cations: Si Al Fe+ 2 Mg Ca Na Ti Mg#

1.919 0.184 0.24 0.9 0.701 0.048 0.01 79

1.854 0.153 0.198 0.923 0.899 0.048 0.01 82

1.886 0.153 0.212 0.942 0.803 0.048 0.009 82

1.845 0.169 0.221 0.952 0.837 0.052 0.01 81

1.915 0.139 0.185 0.851 0.879 0.053 0.01 82

1.813 0.14 0.197 0.948 0.972 0.054 0.011 83

(continued on next page)

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Table 3 (continued) Sample mineral

B1

G1

G3

G4

G5

G6A

PLAG

PLAG

PLAG

PLAG

PLAG

PLAG

PLAG

SiO2 Al2O3 FeO CaO Na2O K2O

47.91 32.78 0.51 16.61 2.13 0.06

47.48 33.34 0.31 16.81 2.03 0.04

47.81 32.89 0.5 16.58 2.15 0.08

48.15 32.82 0.48 16.25 2.26 0.04

47.51 33.26 0.5 16.59 2.11 0.04

46.96 33.55 0.93 16.2 2.3 0.06

59.67 26.08 0.41 7.73 5.94 0.17

Cations: Si Al Fe+ 2 Ca Na K Total cations

2.202 1.775 0.02 0.818 0.19 0.004 5.007

2.181 1.805 0.012 0.827 0.18 0.002 5.008

2.197 1.781 0.019 0.817 0.192 0.004 5.011

process produces the observed changing core compositions while maintaining equilibrium rim compositions. Instead, we argue that plagioclase cores communicate with the melt through small intracrystalline melt pathways in the crystal, as indicated by the SEM image in Fig. 3D. Such microchannels have been observed previously by Transmission Electron Microscopy (TEM) and were postulated to be fast paths for element transport through plagioclase [20]. We recently examined a plagioclase extracted from experiment #1 by TEM finding patchy dark spots in bright field view [21]; these spots are identical in size and appearance to those observed in bdustyQ plagioclases in subduction zone volcanic rocks and interpreted to reflect melt globules [22]. Notably, this latter TEM study found small chlorine enriched glass veins throughout a plagioclase of moderately high anorthite content [22], further reinforcing the connection between our experimental melt composition, anorthitic plagioclase observed in volcanic rocks, and Cl

2.209 1.775 0.018 0.799 0.201 0.002 5.005

2.183 1.802 0.019 0.817 0.188 0.003 5.011

G6B

2.164 1.822 0.036 0.8 0.205 0.004 5.03

2.653 1.367 0.015 0.368 0.512 0.01 4.925

enriched melt inclusions found in anorthitic plagioclase [23]. Finally, we note that similarly fast equilibration of minerals and melt has been inferred between chromite and basalt within the Kilauea Iki lava lake [24]. Notably, diffusion–reaction experiments between partially crystalline basanite and partially molten peridotite show that olivine behaves similarly to plagioclase [15]. During basanite–peridotite diffusion– reaction, Mg diffused from the peridotite into the basanite during a 2-h long experiment increasing the Mg# (molar Mg/(Mg + Fe+ 2)) of the basanite melt. As this occurred, the rims of all olivines throughout the basanite maintained Fe–Mg exchange equilibrium with the melt. However, the olivines closest to the interface developed overly forsteritic cores analogous to the anorthitic plagioclase observed here while the olivines farthest from the interface had cores of lower Mg# than the rims, consistent with their initial composition before diffusion–reaction (see Fig. 11 of

Table 4 SIMS measurements of isotopic compositions in experiment #2 7

Li/30Si 88 Sr/30Si 138 Ba/30Si 7 Li/6Li 88 Sr/84Sr 138 Ba/136Ba

Glass 1 Glass 2 Glass 3 Average 1r

Fig. 5B. plag. 1 Fig. 5A. plag. 2 Average 1r

0.0030 0.0289 0.0117 1.83 4.27 5.30

0.0016 0.048 0.005 1.63 5.54 5.20

0.0026 0.0330 0.0110 1.79 5.04 5.46

0.0025 0.0348 0.0092 1.78 5.56 5.41

*(Tepley et al., in preparation [17]).

0.0027 0.032 0.011 1.80 4.96 5.39

0.0002 0.003 0.001 0.02 0.65 0.08

0.0008 0.072 0.003 1.43 14.13 6.18

0.0012 0.060 0.004 1.53 9.83 5.69

plag/glass

0.0006 0.45 0.017 1.87 0.002 0.39 0.14 6.07 0.69

D

plag/glass

0.30 1.94 0.32

D*

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160 7Li/ 6Li

140

138Ba/136Ba

PLAG #2 beyond interface

88Sr/ 84Sr

10 8 6

120 100

Glass Analyses PLAG #1 interface

4

80

natural ratios in starting PLAG

60

88Sr/ 84Sr

7Li/ 6Li 138Ba/136Ba

12

843

40 20

2

0 0

Individual Analysis Fig. 10. Isotopic ratios of Sr, Li and Ba for plagioclase and glass. On right are shown the natural values of these ratios for the original plagioclase grains before diffusion–reaction.

[15]). In the present experiments, the plagioclase similarly adjusts to the continuously increasing Ca/Na of the AR-8 melt by developing anorthitic cores (Fig. 3C) as rims remain in Na–Ca exchange equilibrium with the melt. However, these two examples differ in the olivine grains having smooth diffusion profiles while the plagioclase grains have an abrupt core to thin rim transition. To generalize, solid solutions appear to compensate for a dynamically changing melt composition by component exchange. In this way, these minerals buffer the composition of the melt in which they are immersed by expelling elements lost from the melt by diffusion–reaction and incorporating elements gained in the melt by diffusion–reaction. The buffering of Ca/Na and Mg/Fe in a melt by the coexisting minerals would have important implications for igneous processes in general. It is important to emphasize that the more sodic rims on the plagioclase crystals in these experiments are not quench features. First, initial quench rates in the piston cylinder are several hundred degrees per second, resulting in usually no quench crystallization of plagioclase. When observed in 1 atmosphere experiments, plagioclase quench crystals are skeletal and not euhedral as observed here. Finally, it is unlikely that quench rims would be in Na–Ca exchange equilibrium as observed in both experiments. Why plagioclase behaves in this manner to a changing melt composition is more difficult to say; however, we speculate that by maintaining rim–melt equilibri-

um but disequilibrium around the channels in the crystal, the melt–mineral system maintains a lower energy condition than a situation where rim–melt disequilibrium were maintained. Despite the last quantifiable melt being b2 mm into the gabbro from the interface, chemical transport occurs throughout the gabbro in experiment #1. Olivine, apatite and FeNiS occur at specific spatial horizons in the experiment, with positions reinforced by trace element concentration profiles (Fig. 2). We interpret the locations of apatite and FeNiS appearance to reflect the saturation temperature of each phase with mass transport in a supercritical hydrous fluid distributing elemental components within the colder portion of the experiment. In this regard, the composition of apatite and the presence of sulfides within several layered mafic intrusions have been similarly attributed to reflect fluid transport processes [25–27]. Crossing redox boundaries do not explain the location of the FeNiS horizon. Similarly, the appearance of olivine in experiment #1 likely reflects the reaction 2Na+ + 2Mg2 Si2 O6 + 3 Ca Al2 Si2 O8 Y 2Mg2SiO4 + 2NaAlSi3O8 + 2CaAl2SiO6 + Ca+ 2; here Na+ and Ca+ 2 are depicted as ions reflecting either fluid phase transport as chlorides or diffusing entities in a melt. The anorthite to albite conversion releases alumina, resulting in either increased Tschermak’s component in augite or transport of aluminum toward the interface. Because both Na and water will destabilize pyroxene relative to olivine [28], olivine in the experiment #2 gabbro likely reflects the

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addition of saline water to this experiment, similar to the proposed origin of olivine bearing zones in the Minneapolis Adit section of the Stillwater Complex [29]. Finally, the extended profiles of Na2O and Cl and the presence of albitic plagioclase in the coldest end of experiment #1 both indicate that chemical transport, at least of Na, has occurred through the full 18 mm column of gabbro in experiment #1. Because melt did not exist at the cold end of this experiment, we conclude that element transport reflects a combination of diffusion through the melt in the hot end of the experiment which grades into a hydrous supercritical fluid within the colder end. The inability to determine the water content of the gabbro makes assessment of the role of water in these experiments challenging. We can only infer the presence of water through anhydrous mineral changes, textures (i.e., plucking along grain boundaries in the cold end of experiment #2) or transport of other elements such as Cl. Analysis of the AR-8 glass in experiment #2 indicates a hydrogen–deuterium ratio of 10, far exceeding that expected (~0.1) based on the amount of deuterium added and the nominal water present in AR-8. This result suggests that either the juxtaposed capsules did not seal or were not impermeable to hydrogen exchange. Pt capsules alone do not provide hydrogen impermeability at temperatures greater than 1100 8C [30]; however, the hydrogen permeability of Pt lined Ti capsules, used successfully for fluid–mineral partitioning studies at lower temperatures [31], is not known at higher temperatures. Because isotopic mixing of D2O and H2O should be nearly instantaneous, the similarity of the melt H2O concentrations in the two experiments suggests a bsteady-stateQ concentration of water dissolved in the melt. This concentration is maintained by exchange through the capsule walls, after loss of the initial wt.% levels of added D2O. This is consistent with the observed high H/D because continuous exchange with hydrogen outside the capsule after loss of D2O will replace remaining D with H over time. Despite temperatures and bulk compositions that should result in a significant melt mode within the gabbro, we observe much less melt than predicted from either MELTS or the phase equilibrium experiment. MELTS suggests the gabbro with 0.1 wt.% H2O will have 13% melt present at 1200 8C [13], consistent with the amount observed in the phase relation exper-

iment. Lithologic partitioning is unlikely to explain the lack of melt in the gabbro given melt partitioning observed between peridotite and orthopyroxenite [32]. Rather, this behavior probably reflects the role of thermal gradients in the experiments. In bthermal migrationQ experiments with temperature gradients (5–10 8C/cm) much smaller than our experiments, Walker et al. [33] showed that orthocumulate texture of olivines separated from melt could be produced in a day and adcumulate texture of almost completely separate olivine and melt could be produced in two weeks. Piston cylinder thermal migration experiments similarly showed that a MORB melt segregated from its liquidus minerals, plagioclase and clinopyroxene, and compositions of these minerals evolved to lower temperature endmembers with decreasing temperature in the experiment [34]. Experiments simulating metasomatism of peridotite in a thermal gradient also indicate little melt in the cold end of the experiment [35,36], consistent with thermal migration effects. Finally, although soret diffusion [37] will affect the overlying melt composition during thermal migration, its effects on crystal compositions are limited [33]. 4.2. Models of diffusion–reaction using IRIDIUM The results of these experiments have application to a number of igneous phenomena, including the characteristic mineral rims that commonly occur at the contact between partially molten rocks of different bulk composition. For example, foundered basaltic blocks in the Skaergaard intrusion can begin to remelt and develop magnetite-rich rinds during reaction with Skaergaard magmas [38]. The number of variables in such systems makes it unrealistic to recreate each observation experimentally. However, computer programs simulating infiltration–diffusion–reaction in igneous systems are now available and the experiments described here present a useful test to validate these numerical predictions. We used IRIDIUM [39] to model the results of these experiments. Briefly, the IRIDIUM program links a phase equilibration routine based on the MELTS program [13] with mass and heat diffusion transport equations. Mass diffusion uses constant diffusion coefficients (i.e., no thermal dSoretT or compositional dependencies are included) and the program does not include off-diagonal diffusion terms. One

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

component (e.g., SiO2) is found by mass balance, allowing buphillQ diffusion phenomenon to occur. In the simulation, the total distance across the compositional couple was 2 cm, with gabbro comprising the bottom 1 cm and basaltic andesite the top 1 cm. The thermal diffusion coefficient was 0.01 cm2 s 1 and mass diffusion coefficient for all elements beside SiO2 was set at 10 7 cm2 s 1. The imposed initial thermal condition consisted of the basaltic andesite at 12008C and the gabbro at 1100 8C. Under these conditions, the basaltic andesite contains ~30% plagioclase, slightly more than found experimentally, and a few percent orthopyroxene. Similarly, the gabbro at 1100 8C is essentially all solid with the solid assemblage including ~10% olivine (also indicated by MELTS). In the simulation, however, we suppress olivine crystallization providing more melt in the gabbro and better illustration of the changes in melt and solid abundance with time. The bulk compositions are allowed to equilibrate at the initial thermal conditions; the dashed lines in Fig. 11 show the initial mineral modes and concentration profiles. Although the initial conditions have the top and bottom held at 1200 8C and 1100 8C, respectively, the thermal profile rapidly diffuses to a uniform thermal gradient across the simulation. Thus, because the temperature profile in the experiment is imposed, there is a slight difference between the model’s and the experiment’s temperature profile. Compositional gradients occur in the liquid as a consequence of both the initial compositional differences across the couple and change as the solid phases equilibrate to the new thermal profile. The resulting diffusive mass transport then leads to local regions of mineral growth or dissolution. The 21-day simulation predicts that the hot end of the experiment will have decreasing solid over time such that the basaltic liquid becomes 100% liquid at the top. This gradation in percent melt present is observed in experiment #2 (Fig. 10A–B), and as explained above, probably reflect the effects of thermal migration [33,34]. In the simple case of a single mineral phase such as olivine in a silicate melt [33], the concentration of the solid component in the melt will be higher in the hot liquid than in the cooler liquid for a melt saturated in a solid at both temperatures. The resulting concentration gradient in the melt leads to diffusion of these components to the cold end.

845

As liquid components are lost from the hot end, the solid continually dissolves to keep the melt at saturation levels. Conversely, the increase in liquid concentrations at the cold end leads to the melt precipitating more solid. In summary, thermal migration in the monomineralic case leads to high temperature components migrating from the hot end to the cold end of the experiment. The experiments here add a twist to the experiments of [33,34] in that there are two, not one, solid assemblages and the solid assemblages are polymineralic. Because of this, for many elements, the compositional gradients in the liquid do not monotonically change from one side of the contact to the other but develop a local maximum or minimum at the boundary. This implies that these elements do not simply migrate from one side of the contact to the other but either migrate to, or away from, the contact from both sides simultaneously. A similar behavior for Cl was observed in an experiment discussed in [15]. In general, model predictions agree well with observed variations in composition and mineralogy as a function of position. For instance, the simulation predicts the dramatic increase in plagioclase mode and decrease in clinopyroxene mode that is observed at the interface in both experiments. Changes in Al2O3, FeO, TiO2 and MgO contents of the basaltic andesite melt in experiment #2 (Fig. 8) are well matched by the model. However, opposite to the prediction of the model, CaO becomes enriched in the melt approaching the interface while Na2O concentration decreases slightly. This behavior could reflect the inability of the model to account for the creation of anorthitic plagioclase at the interface (although Al2O3 does not follow this explanation). Because the model does not account for the strong coupling of Na diffusion to melt silica content (i.e. significant off diagonal diffusion term seen in [40]) nor for the behavior of plagioclase cores to buffer the melt content of Ca and Na, the model is unlikely to reproduce all of the subtle changes in melt composition observed. Simulations indicate that the plagioclase enrichment occurs within the basaltic andesite (experiment #2) and not in the gabbro (experiment #1). Although not shown, simulations were also run under a variety of different conditions, including a run with constant temperature of 1150 8C across the system and one

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Fig. 11. A simulation of diffusion–reaction using IRIDIUM. See text for model details. Dashed lines indicate initial conditions while solid lines are after 21 days. A) mineral mode in weight %. Note the peak in plagioclase mode at the expense of CPX occurring on the basaltic andesite side of the interface; B) temperature; C) Al2O3 concentration in the liquid; D) bulk Al2O3 concentration. Note the correspondence with the observed bulk composition in the experiments; E) Na2O concentration in the liquid; F) bulk Na2O; G) K2O concentration in the liquid; H) bulk K2O concentration; I) MgO concentration in the liquid; J) bulk MgO concentration; K) CaO concentration in the liquid; L) bulk CaO concentration.

C. Lundstrom et al. / Earth and Planetary Science Letters 237 (2005) 829–854

with changing diffusion coefficients (the alkalis were allowed to diffuse an order of magnitude faster and Al an order of magnitude slower). In all cases, a plagioclase-rich band forms at the contact and mainly within the basaltic andesite. The observation of the plagioclase-rich zone occurring within the gabbro in experiment #1 could reflect the uneven geometry (the basaltic andesite having a much larger cross sectional area than the gabbro) or reflect advection of melt into the gabbro powder. If the basaltic andesite liquid infiltrates a short distance into the gabbro prior to the start of the simulation, a plagioclase-rich layer develops in the gabbro. Thus, the IRIDIUM models give insight into a number of important phenomena within the experiment. First, the simulations reinforce the observation from both experiments that mineral layering, in this case of plagioclase, results from diffusion–reaction between partially molten materials of distinct lithology. Second, the model results show that melt compositions can become unusually enriched or depleted in certain elements at the boundary because of migration of these elements either toward or away from the boundary from both sides simultaneously. Third, although the simulation does require some liquid to be present (to allow for diffusive transport), the simulation is consistent with the experiments in that the amount of liquid in the gabbro does not increase over time. This observation is in agreement with [33] in showing that interconnected melt within a thermal gradient will lead to more melt in the hotter areas and almost fully solid materials in colder areas. 4.3. Relating experimental observations to features in igneous rocks At the outset of applying these results to real rocks, we must emphasize that the thermal gradient imposed by the piston cylinder dictates a gradient orders of magnitude greater than thermal gradients possible in nature. However, although chemical transport rates from the experiments cannot be directly applied to nature, the observed changes in composition and mineralogy do provide a template for how the process might operate in the natural setting. Indeed, the occurrence of sulfide, apatite and olivine at specific horizons in experiment #1 does provide a food-forthought reproduction of the occurrence of these

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phases at meter to kilometer length scale in layered mafic intrusions. Furthermore, results of IRIDIUM models show that the general features of mineral layering are produced in a variety of different initial thermal gradients. The production of anorthitic plagioclase during the diffusion–reaction experiments provides critical evidence linking this process to crustal depth magma differentiation in both convergent margin and midocean ridge settings. The fact that diffusion–reaction can produce anorthitic plagioclase without requiring the existence of ultra-calcic melts nor the extended time scale of solid-state diffusion makes this process attractive for explaining the existence of anorthitic plagioclase in both environments. As an example from convergent margins, mineral compositional variations in Arenal lavas mimic the experiments in several ways. For instance, while Arenal plagioclase can have extremely anorthitic cores, thin rims of decreasing anorthite content always mantle these [8]. OPX remains comparatively homogeneous (En70-76) but CPX can vary widely in Cr2O3 content (0.7 to b 0.04 wt.%; [9,12]). Finally, wide variation in 87Sr/86Sr (0.7038–0.7052) occurs within single plagioclase crystals at Arenal despite the homogeneity of 87Sr/86Sr in whole rock compositions over the course of the eruption (Tepley, unpublished data). Diffusion–reaction could account for all of these features. Along with the ability to create anorthitic plagioclase, the experiments show that OPX in the basaltic andesite varies little in Mg# (cores and rims are Mg# 75–76; in Mg–Fe exchange equilibrium with the melt) and that Cr and Sr contents are dramatically affected by chemical transport and mineralogy at the interface. The widely varying Cr contents and 87Sr/86Sr observed at the microscale in minerals in Arenal lavas could reflect diffusion– reaction occurring at the edge of a magma reaction zone with erupted rocks incorporating the plagioclase-rich boundary layer (Fig. 12). This is consistent with the fact that the variations in 87Sr/86Sr require the presence of at least two sources for Sr: in our model these components are ascending magmas and the reacting shallow crust. In detail, this reaction zone might be very complicated given the observed mineral variations at Arenal [12]. While our simple model may not explain all the observed complexity, it at least provides a possible spatial context for

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A Map view: Crustal Mush Zone

800

C

1200 C

Olivine gabbro rite

Channel of Ascending Melt

Dio

ascending magmagabbro interface

s cla gio a a l N P

horizontal element transport by diffusion

increasing temperature

higher porosity channel of melt ascent

e

surrounding crustal gabbro

Ca Na, Si Ca, Al

B

ascending basaltic andesite

C

Fig. 12. Schematic diagram illustrating the processes involved in the diffusion–reaction hypothesis for creation of porphyritic high alumina basalt. A) magmas beneath a volcano ascend through a magma–crust reaction zone, which develops a thermal profile as shown. This reaction zone could represent a system of channels as shown. At the contact between the ascending magma and surrounding gabbroic crust, the plagioclase-rich diffusion reaction boundary layer develops. This boundary layer is incorporated into erupted lavas producing plagioclase rich basaltic andesites. Inset shows map view of mush zone and possible mineralogy after reaction. B) A porous network of melt connected to higher porosity melt channels allows reactive exchange between the resident crustal rock and the ascending magmas. C) A close up of the processes within the high porosity conduit showing the development of anorthite rich cores in the plagioclase. As the conduit melt loses Na to the surrounding gabbro and gains Ca from it, the core of the plagioclase responds by taking up Ca and expelling Na; this occurs by micro-channels extending through the crystal rim, resulting in the mineral zonation observed.

explaining the four environments of crystallization identified by Streck et al. [12]. Furthermore, gabbroic xenoliths in Arenal lavas reinforce this hypothesis as they often contain plagioclase with homogeneous anorthitic cores (~An92) and rims of lower ~An78 content [6,11,41]. Notably, mafic inclusions often develop a fine-grained rim of plagioclase at the lava-inclusion interface, possibly representing a natural example of the experiment and simulation result [41]. Our conclusion is that the porphyritic low MgO HAB from Arenal, and possibly many locations, may reflect a commonly occurring diffusion–reaction process between ascending magmas and surrounding

crust that operates in conjunction with other differentiation processes. As such, these rocks never reflect liquid compositions, consistent with both Arenal phase equilibria results (Pertermann and Lundstrom, unpublished data) and comparison of mineral and bulk rock compositions at Arenal [12]. This conclusion suggests that the excellent fit of fractional crystallization models (e.g., [8]) simply reflects the incorporation of relatively constant composition boundary layer material produced by diffusion–reaction. In this way, the subtle but monotonous highly porphyritic basaltic andesite bulk composition at Arenal reflects a steady state flux balance between ascending magmas and components

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enriched by diffusion–reaction from the surrounding crust (Ca, Al, Mg). The solid residue of this process, the reacted gabbro surrounding the conduit feeding an overlying volcano, could be quite distinct in its compositional structure, particularly if the process included supercritical fluid transport in the colder periphery. Notably, repeated lithologic sequences within the Smartville Complex (CA), interpreted to reflect a dissected magma chamber beneath a subduction zone volcano [42], could represent such a complementary residue. Here, several kilometer scale concentric sequences of quartz diorites surrounding gabbronorites surrounding central cores of olivine gabbro are observed. Although these sequences are interpreted to reflect repeat injections of progressively less fractionated magmas, mineral compositions do not abruptly change at each lithologic boundary but instead continuously change across the entire sequence [42]. Reinterpreted here, these concentric sequences could be analogous to the cold end of our experiments, reflecting fluid-based diffusive transport across the thermal gradient extending out from a conduit for melt ascent (Fig. 12). The hot center of the conduit becomes the location of concentration of higher temperature components producing olivine gabbro while surrounding lithologies reflect modification of the colder preexisting crust. The scale of the Smartville lithologic sequences is orders of magnitude larger than is allowably extrapolated from our results. However, we note that measurements of the Si diffusion coefficient through supercritical hydrous fluid approaches that of heat at temperatures of 1000 8C [43]; thus, the diffusive length scale (MDt) by transport through an interconnected hydrous fluid could produce lithologic sequences of kilometer length scale in ~1 million years. Anorthitic plagioclase also provides evidence for diffusion–reaction occurring in the mid-ocean ridge basalt (MORB) environment. Notably, MORB bearing anorthitic plagioclase are often found in environments of steep thermal gradient. In the Pacific basin, for instance, anorthitic plagioclase is found in samples from several transform settings (Garrett; [44]; Siquieros; [45,46]) and from off-axis seamounts (Lamont: [47]), locations where hot magmas arguably ascend through cold mantle and crust.

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Texturally, these plagioclase often have homogeneous anorthitic cores with thin rims of lower An content [45]; Allan et al. [47] show 4 analytical traverses (their Fig. 7) of plagioclases from Lamont Seamount lavas, each showing a distinct drop to lower An content rims like that seen in our experiments. A cumulate xenolith from the East Pacific Rise also appears to reproduce features of the plagioclase-rich layer of the experiments; its mode of 70% plagioclase consisting of homogeneous anorthitic cores (An90–94) mantled by lower An70 rims [48] parallels features of diffusion–reaction. If this process occurs extensively, it could explain anomalous observations associated with the composition of the lower ocean crust observed at the Atlantis Bank (Southwest Indian Ridge). Here, evolved gabbros have been drilled rather than the expected primitive cumulates at the base of the ocean crust (bulk Mg# = ~69; [49,50]. The majority of MORB overlying this location are significantly less differentiated than implied by the lower crust gabbros [51]. Diffusion–reaction whereby ascending melts (as represented by the overlying basalts) remove mafic components from the lower crust gabbros could explain this observation and might be promoted by the steep thermal gradients within the slow spreading mid-ocean ridge setting. If diffusion–reaction occurs enough to affect the composition of the lower oceanic crust, the process might have broad implications for surface geochemical cycles. By transferring CaO from relatively sequestered locations in the plutonic crust to overlying lavas more available to weathering, it could play a role in atmospheric CO2 regulation through the Urey reaction. Even a modest effect of adding 0.5 wt.% CaO to erupted MORB would probably have impact on calcium availability for exchange with ocean water during basalt spilitization. This is because the process not only adds CaO to the basalt but also makes the CaO already in the basalt more available by increasing its presence in easily weathered anorthitic plagioclase [52].

5. Conclusions Diffusion–reaction between basaltic andesite and partially molten gabbro in a thermal gradient results

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in mineral layering and notable mineral compositions such as anorthitic plagioclase. The process results in Al2O3, MgO and CaO diffusing from the gabbro into the basaltic andesite and Na2O, K2O, SiO2 and FeO diffusing in the opposite direction. Because of this, a plagioclase-rich layer develops at the interface between the two materials. Plagioclase crystals within the interface region develop anorthitic cores (up to An90) that abruptly shift in composition to thin rims that are in Na–Ca exchange equilibrium with the co-existing melt. Both the mineralogical layering and bulk compositional change occurring at the interface are reproduced in model simulations of diffusion–reaction. The overall effect of the diffusion–reaction process is to make an ascending magma more primitive in composition while making surrounding crustal wall rocks more evolved. The hypothesis that diffusion–reaction plays a major role in magma differentiation is testable, given high-precision isotope measurements available today. Laboratory experiments show diffusion-based processes can create per mil fractionations of isotope ratios [53]. Observations of Li isotope variations within mantle samples provide evidence for fractionation of isotope ratios during magmatic diffusion processes [54]. Observations of analytically significant isotopic variations in Mg [55] and Fe [56] in mantle minerals have also been observed with no clear explanation for their origin. We suggest that analyses of several isotopic systems (Mg, Ca, Li) across lithologic sequences such as layered mafic intrusions or the Smartville Complex may provide a fruitful direction of future study.

Acknowledgements We thank J. Brophy and J. Allan for helpful reviews, S. Marshak, J. Kirkpatrick, T. Johnson and M. Stewart for comments and editor B. Wood for editorial handling. We thank E. Hauri for providing the SIMS water standards and J. Lee and J. Baker for technical assistance with the ion probe. Ion probe analyses took place in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by DOE grant DEFG02-91-ER45439. This work was supported by NSF EAR0207761.

Appendix A Both experiments used graphite capsules and 3/4W NaCl-pyrex assemblies. In experiment #1, the two starting material powders were juxtaposed with the basaltic andesite on top of the gabbro (Fig. 1). This experiment was nominally anhydrous (oven dried at 140 8C for 1 day prior to the experiment) although SIMS analyses indicate the melt contained some water. Because quenched glasses in Arenal rocks contain 0.2 to 0.78 wt.% Cl [10], we added 1 wt.% NaCl to the AR-8 powder in experiment #1 to assess the role of Cl during diffusion–reaction. Experiment #2 used a presynthesized mineral-glass aggregate of AR-8 for the top half of the couple. Synthesis reflected a separate piston cylinder experiment at 0.5 GPa and 1200 8C for 12 h producing a quenched glass coexisting with plagioclase and trace orthopyroxene. After removing a portion of this material for characterization, the exposed end of this material was polished and a solution containing 10 nCi of 45Ca (t 1/2 = 45 days), 1 Ag of 136Ba, 0.5 Ag of 84Sr and 1 Ag of 6Li evaporated onto its surface. This presynthesized half thus served as the blidQ to the lower capsule that contained the gabbro powder and 5 wt.% saline water (D2O containing 5% by wt. NaCl). The couple of these two materials (experiment #2) was first pressurized to seal in the water, then brought to 1200 8C in the hotspot quickly (300 8C/m) and held there for 13 days. Analyses of experiment #2 indicate that Fe loss to the Pt capsule is minimal if it occurred. The long column of gabbro extending through the imposed thermal gradient of the piston cylinder resulted in estimated temperatures at the base of the gabbro of ~5008C (experiment #1) and ~800 8C (experiment #2) [57]. The hotspot in the assembly of this piston cylinder was located using five separate double thermocouple experiments with temperature varying by b 28 8C over the central 5 mm of the assembly (consistent with results of [58]). Both experiments included a gold wire placed along side or embedded within the graphite capsule to provide constraint on the temperature profile [59]; however, as explained below, this procedure only worked for experiment #1. Whereas the gold wire along the side of the experiment #1 capsule changed texturally, indicating where it melted (Fig. 1), the gold wire

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embedded within the experiment #2 graphite capsule disappeared from the capsule and Au found its way into the silicate material in the experiment. Small blebs of Au occur up to 4 mm from the interface, likely reflecting transport by the fluid present in this experiment. Because gold is relatively inert, it is not believed to have affected the phase equilibria of the experiment and analysis of the gold indicates no other constituents. Notably, the distribution of the gold follows a somewhat sinuous path through the gabbro with a clear increase in mineral grain size in areas containing the gold blebs (Fig. 6C). Because of the duration of these experiments and likelihood of thermocouple drift, both experiments were run using a W-Re (type D) control thermocouple for 1 day until the output power had stabilized, and were then switched to constant output power thereafter. Over this time period, no large deviations in pressure occurred indicating the temperature in the hotspot remained near 1200 8C. The temperature uncertainty is estimated as F 15 8C. After quenching, both experiments were sliced along their length and examined using backscattered electron imaging on the JEOL 840A scanning electron microscope located in the Department of Geology at the University of Illinois at Urbana Champaign (UIUC). Modal analysis followed methods described in [15]. Quantitative analysis of major elements took place by standards-based EDS analysis using a 4Pi analysis system on the JEOL840A. A 15 kV accelerating voltage and 10 nA current was used for analysis. Standards included anorthite glass, Kakanui hornblende, rutile, Kakanui augite, omphacite, and San Carlos olivine; ZAF corrections were performed. Minerals were analyzed in spot mode, glasses were analyzed by small ~10 Am raster (to reduce alkali volatilization), and bulk compositions were measured by rastering over 250 Am squares. Each of the bulk and mineral analyses shown in the figures reflects the average of 2–3 analyses with error bars reflecting the 1r variation on the population. Although the raster analyses may have small accuracy biases due to the details of X-ray interactions, the trends of compositional change as a function of position should be robust. Analyses of VG-2 basaltic glass, Lake County Plagioclase and Mn Hortonolite olivine were used to assess accuracy (Table 2). The measured glass compositions have

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been corrected for the slight deviations between observed and true compositions of VG-2. No correction was made to olivine, pyroxene, plagioclase or bulk analyses. Water and trace element contents were measured using the Cameca IMS-5f secondary ion mass spectrometer (SIMS) at the Center for Microanalysis of Material at UIUC. Analysis of Li, Sr, P, Ni, Cr, and K used a primary O-beam of ~20 nA and a 12.5 kV accelerating voltage. Energy filtering used an energy offset of 50 V and a window of F19 eV [60]. Trace element concentrations reflect the average of two 250 Am square rasters at a given horizon in the charge and are calculated based on the M+ / 30Si ratios for unknowns relative to values for NIST 610 in the same analysis session. Concentrations should be accurate to F 20% but could be greater due to the uncertainty of analysis of mixed phase materials. However, the major goal, to discern compositional changes as a function of position in the charge, will be attained with the measured gradients. Analyses of Sr, Ba and Li isotope ratios in the isotopically doped experiment #2 also used 50V F 19 eV energy filtering. H analyses used a O- ion beam of ~12 nA and 12.5 kV accelerating voltage generally following methods in [61]; an offset energy of 100V was used but the energy window was slightly wider (F19 eV). Two natural basaltic glass standards (GLO7 D30-1 and 519-4-1; [62]) were used for calibration; a fit of the H/30Si versus H2O (wt.%) data (forced through the intercept) resulted in a calibration line having an R 2 of 0.99. There are considerable uncertainties with the water analysis, particularly in these mixed phase experiments and accuracy may only be F30%. For the water and isotope analyses, which required tight spatial resolution, a Mg ion image was used to locate either glass pools or crystals; the sample was then moved so that the desired sample spot lay at a location where only ions passing through the smallest field aperture were detected, providing spatial resolution of ~10 Am.

Appendix B. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl. 2005.06.026.

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